Esterification: Methyl benzoate

Objectives:

1. To produce methyl benzoate by esterification

2. To learn the reaction mechanism involved in esterification

3. To demonstrate how an ester can be made by the interaction of a carboxylic acid and an alcohol with the presence of a sulfuric acid catalyst.

Introduction:

The ester group is an important functional group that can be synthesized in a number of different ways. The low molecular-weight esters have very pleasant odours and indeed are major components of the flavour and odour aspects of a number of fruits. Although the natural flavour may contain nearly a hundred different compounds, single esters approximate the natural odours and are often used in the food industry for artificial flavours and fragrances.

The esterification is a reaction between an alcohol and a carboxylic acid or a carboxylic acid derivative, water and ester will be formed as products in this process under reflux. In the chemical structure of carboxylic acid, R-COOR’, where R and R' are either alkyl or aryl groups. As shown in the diagram below, the esterification is also known as condensation process which water is produced.

In this case, each of the carboxylic acid will contributes hydroxyl group while each alcohol will contribute hydrogen atom to form a water molecule. So, the reaction involves the removal of water once the ester is formed. The ester linkage will appear as the bond that connected the one carboxylic acid and one alcohol in the ester molecule as shown as the following:

Generally, an acid will be used to act as a catalyst in esterification process.

The esterification is a reversible process which the equilibrium between reactants and products will be reached. The opposite of the esterification reaction is called hydrolysis. The addition of water to the ester link will cause breaking apart of the ester into their parent carboxylic acid and alcohol. The hydrolysis also requires the presence of a catalyst (either acid or base). The esterification is a slow process. The main reason is due to the water produced in the esterification will be used back in the hydrolysis which converts the ester to form parent alcohol and carboxylic acid in the reaction. As the ester is start to form in the reaction but at the same time the hydrolysis start to begin. An equilibrium is finally attained, all of the related reactants and products will present in the mixture formed via esterification and hydrolysis.

The mechanism of the formation of ester under acidic condition might be follows this steps below:

1)

However, the esterification only can be applied by using simple alcohol and carboxylic acid in acidic condition. In another word, the long chain alcohol cannot be used to react with carboxylic acid. We can use another method to produce ester by using carboxylic acid reacts with haloalkane in a basic condition. But the same problem may be encountered at the end, this method is only limited to the primary alkyl halides. So, in order to utilize the maximum ester, an alternative for the preparation of esters is to treat the alcohol with a reactive carboxylic acid derivative. For example, carboxylic acid anhydrides or chloride can be used.

(RCO)₂-O + R’’OH -----> RCOOR’’ + RCO₂H

RCOCl + R’’OH -------> RCOOR’’ +HCl

These reaction is irreversible and they react rapidly especially when catalyzed by a strong acid.

Apparatus:

Round bottomed flask (250ml), Liebig condenser, separating funnel, Bunsen burner. Thermometer

Materials:

Chloroform (trichloromethane), anhydrous sodium sulphate, benzoic acid, methanol, conc. sulphuric acid, anti bumping granules

Procedure:

1. Benzoic acid is placed into a 250ml beaker and then methanol is added .

2. Concentrated sulphuric acid is added whilst swirling the contents and washed with methanol.

3. Two or three anti-bumping granules are added to the mixture and fit in to the reflux set up.

4. The mixture is being refluxed for 1 hour, the mixture is poured into a separating funnel with 150ml of water after cooling.

5. The reaction flask is rinsed with chloroform two times and is added into the funnel.

6. The aqueous layer is being removed and water is used to wash the organic layer.

7. Anhydrous sodium sulphate is used to dry the solution and then is filtered out.

8. The organic solvent is distilled at three different ranges, which are 30-90°C, 91-190°C and 190°C above.

9. The ester is collected in a weighed flask and the distillation temperature range is noted.

10. The percentage of theoretical yield is calculated.

Results:

Weight of benzoic acid = 12.2017g

Weight of conical flask = 49.1914g

Weight of conical flask + weight of ester = 62.5712g

Weight of ester = 13.3798g

C₆H₅COOH + CH₃OH <-----> C₆H₅COOCH₃ + H₂O

Number of moles of C₆H₅COOH = 12.2017g / 122.118 g mol^-1

= 0.09992 mole

0.09992 mole of benzoic acid will produce 0.09992 mole of ester

Weight of C₆H₅COOCH₃ = 0.09992 mole x 136.144 g/mol

= 13.6035 g

Percentage yield = 13.3798g/ 13.6035g x 100%

= 98.3556 %

Discussion:

Benzoic acid and methanol are used the reactants with the presence of sulphuric acid in this experiment. The acid catalysed reaction between benzoic acid and methanol may be represented as:

This esterification using the benzoic acid and methanol is known as Fisher esterification. The concentrated sulphuric acid is added as a catalyzed in this experiment. The purpose of using catalyze is to speed up the esterification because it is a slow process. Concentrated sulphuric acid also serve as another function which is used to protonates the carboxylic acid and hence this will initiate the reaction to start. The esterification between benzoic acid and methanol is favourable in acidic condition; as a result more ester can be formed in this experiment.

After added with the concentrated sulphuric acid, another portion of methanol is introduced into the mixture. The adding of methanol is to ensure all the carboxylic acid could be reacted completely during reflux. Furthermore, anti-bumping granules are added to promote smooth boiling and to prevent bumping of the solvent. As mention at the above, the esterification is slow and reversible process so it is distilled for one hour and hence more ester could be generated. The esterification mechanism is take place as the diagram 1 below:

Diagram 1

The dissociation of sulphuric acid will produce hydrogen ion which can be used to protonates the carbonyl group in benzoic acid. The carbonyl group is protonated reversibly and caused the positive charge of carbonyl group to be increased. Thus this increases the reactivity of carbonyl group towards nucleophile. The C-O double bond is broken in order to stabilize the OH⁺ group to form hydroxyl group in the benzoic acid molecule. The methanol acts as a nucleophile attacks the benzoic acid.

Diagram 2

In the diagram 2, the methanol successful attacked the carbonyl group to form a new C-O bond to the carboxyl group in the benzoic acid to form a tetrahedral intermediate. This is called nucleophilic addition. The oxygen atom in the carboxyl group in benzoic acid is more electronegative due to its lone pair electron. The lone pair electron in the particular electron attracts the hydrogen atom from the methanol to form oxonium ion. Now, the oxygen atom in the methanol becomes unstable and hence the C-H bond will tend to be broken down. The electron between the C-H bond will delocalise to the oxygen atom of the methanol. The formation of oxonium ion in the carboxyl group in benzoic acid tends to be released from the intermediate to form water. Eventually, another hydroxyl group will donate the lone pair electron to the attached oxygen atom to form a more stable intermediate.

Diagram 3

The hydrogen atom in the ester intermediate will be attacked the acid in diagram 3 and the acid catalyze is regenerated. Thus, finally the methyl benzoate is formed.

During esterification, the hydrolysis process start to begin once the ester is being produced. The water produced in esterification is used back in the hydrolysis to hydrolyze the ester to form the carboxylic acid and alcohol. The process is a continuous reversible process until the equilibrium is reached. The hydrolysis process would be the following equation:

The hydrolysis process must under acidic or basic condition in order to break down the stable ester molecule. To avoid the hydrolysis process, the water could be removed from the mixture. According to Le Chatelier’s principle, the equilibrium position will shift to the product side if water is being removed. Another method to increase yield of ester is by adding more alcohol into the mixture. Hence, more ester could be generated when the amount of alcohol increased which shift the equilibrium to product side. The mechanism of hydrolysis is shown in the diagram 4 below:

Diagram 4

Water molecule acts as nucleophile to attack the carbonyl carbon of ester reversiblely. The C=O will be break and the oxygen atom attached to the carbonyl carbon form negative oxygen atom. The methoxy group tends to leave the tetrahedral intermediate to form a stable methoxide, so the negative oxygen atom donates the electron back to the carbonyl carbon. Finally, the methoxide acts as a nucleophile which attack the hydroxyl group bonded to carbonyl carbon and hence to form methanol. The oxygen atom now is negatively charge and it tends to get a proton from the environment. Thus benzoic acid is formed.

After reflux for one hour, the reaction mixture is introduced into the separating funnel with distilled water to carry out extraction. This is because the unreacted methanol can be dissolved in distilled water and hence it could be removed together with the aqueous layer. Small amount of the benzoic acid present in the mixture will dissolve in the water although it is highly insoluble in water. Distilled water can extracts the leftover of methanol and small amount of benzoic acid. Chloroform is added to extract the ester by dissolving the ester in the chloroform. After the aqueous layer is being removed, the anhydrous sodium sulphate is added as drying agent which will absorb water droplet and hence residual water can be removed completely.

Now, the residual ester still contains some methanol, benzoic acid and other side products. So, we use distillation to obtain the pure ester from the mixture. During distillation, the temperature of the distillate is kept constant at 63°C. This is because some of the methanol is still left in the mixture and its boiling point is around 65°C. So, the temperature remains at 63°C until all the methanol are completely distilled. For the second time of distillation, the temperature of distillate is keep changing in the range of 140°C to 180°C. This might be due to the mixture contain some impurities, so the temperature may fluctuate in the wider range of temperature. Another reason for the fluctuation of temperature may be side product formed during reflux is exist in the mixture. So, this contribute to the temperature to be fluctuated. The ester is being synthesized is 98.3556% but this figure is not reliable. The weight of the solution is very high due to the impurities present in the solution. The actual figure that an ester could be form is 60-70%, which according to the estimation of scientist.

 

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Synthesis of n-Butyl Ethyl Ether from 1-Butanol

Objectives:

1. To synthesize n-butyl ethyl ether from 1-butanol
2. To understand mechanism involved in the reaction

Introduction:

In this experiment, the procedure to generate n-butyl ethyl ether from 1-butanol is divided into two parts. The first part involves the formation of n-butyl bromide from 1-butanol. Alkyl halides are very useful intermediates in organic syntheses. The most common synthetic preparation of alky halides is the replacement of the hydroxyl group, OH of an alcohol by a halogen, HX. The displacement of a hydroxyl group by halide ion is successful only in the presence of a strong acid. 1-butanol is used to be converted into 1-bromobutane with adding of sodium bromide and sulphuric acid. The nucleophile for the reaction is Br^- ions. The nucleophile in this lab is generated from an aqueous solution of sodium bromide. The sulfuric acid acts as a catalyst in this reaction. The sulphuric acid protonates 1-butanol to produce suitable leaving group, OH, in S_N2 reaction. The chemical reaction is shown as below:

If this displacement reaction is attempted in the absence of an acid it is unsuccessful because leaving group would be a hydroxide ion which is a poor leaving group and a strong base.

In the second part of the experiment, the n-butyl bromide produced in the first part is being converted into n-butyl ethyl ether by using methanol and sodium hydroxide. The chemical reaction is shown as below:

C₄H₉Br CH₃(CH₂)₃-O-C₂H₅

The mechanism of synthesis of ether is also known as mechanism of Williamson ether synthesis. This mechanism involves one alkoxide reacts with alkyl bromide to form ether with two alkyl groups by using a strong base. This reaction can be used to produce both symmetrical or unsymmetrical ethers and also cyclic ethers. The following diagram is the mechanism of the Williamson ether synthesis.

 

The alkoxide ion functions as nucleophile and attacks the electrophilic C of the alkyl halide, displacing the bromide and creating the new C-O bond.

For the synthesis of n-butyl ethyl ether, both reactions are undergoes S_N2. S_N2 is known as second order nucleophilic substitution which is for bimolecular process. The kinetic rate of S_N2 is defined as

Rate = k [R-X][Nu^-]

The more the alkyl groups attached to the reacting carbon, the slower the reaction. The order os reactivity in S_N2 is shown in the following:

Tertiary alkyl > secondary alkyl > primary alkyl > methyl

--------------------------------->

Reactivity increasing

S_N2 involves the inversion of configuration (rearrangement of atoms in molecule) to form a transition state which is different with S_N1. First order nucleophilic substitution is a unimolecular process which form carbocation during the reaction.

Apparatus: Round bottomed flasks (50cm³ and 250cm³), Bunsen burner, condenser, thermometer, separating funnel

Materials: Sodium bromide, 1-butanol, conc. sulphuric acid, anti-bumping granules, 5% aqueous sodium bisulphate, distilled water, 10% aqueous sodium carbonate, anhydrous calcium chloride, sodium hydroxide, 95% ethanol, anhydrous magnesium sulphate

Procedure:

i) n-Butyl bromide

1. 27g of sodium bromide, 30cm³ of water and 20cm³ of 1-butanol are placed into a

250cm³ round bottom flask.

2. The mixture is cooled in an ice bath and 25cm³ conc. sulphuric acid is added with continuous swirling.

3. Two or three anti-bumping granules are added and attached with a gas trap to prevent HBr escaping, the round bottom flask is heated vigorously under reflux for 1.5 hours as shown in diagram 1 below.

Diagram 1

4. Distill the two layered mixture until the temperature reaches the boiling point of water.

5. The distillate is transferred to a separating funnel and shakes with an equal volume of 5% aqueous sodium bisulphate.

6. Allow the two layers to separate and wash the organic layer twice with 25cm³ water followed by 10% aqueous sodium carbonate (25cm³).

7. The product is dried with 5g calcium chloride and is filtered into 50cm³ round bottom flask.

8. Anti bumping granules are added and distilled, the material which boiled between 90-105 ̊C is collected.

9. The appearance of product is noted and weight is measured.

ii) Ethyl n-butyl ether

1. 4g of sodium hydroxide and 12cm³ 95% of ethanol are added into round bottom flask and is heated under reflux for 20 minutes.

2. 10cm³ n-butyl bromide is added into the mixture through the top of the condenser and the reaction is heated under reflux for 90 minutes.

3. After cooling, the mixture is transferred to a separating funnel and 50cm³ of water is added which has been used to rinse the reaction flask.

4. The mixture is shaked and the lower layer is removed.

5. The washing is repeated for two times with 20cm³ of water.

6. The organic layer is dried with anhydrous magnesium sulphate and the liquid is filtered into a 50cm³ round bottom flask.

7. The product is distilled slowly; the material which boiled in the range 90-96 ̊C is collected in a pre-weighed flask.

8. The density of the pure ethyl n-butyl ether is determined by pipetting 1cm³ liquid into a pre-weighed measuring cylinder and noting the weight difference.

Results and calculation:

Part I

Weight of conical flask = 78.2260g

Weight of conical flask + weight of n-butyl bromide = 89.8416g

Weight of n-butyl bromide = 11.6156g

Density = Mass/Volume

Mass = (Volume x Density) / Molecular mass

Number of mole of 1-butanol = (20cm³ x 0.81g/cm³) / 74.08g mol^-1

= 0.2187 mole

1 mole of NaBr reacts with 1 mole of 1-butanol to produce 1 mole of butyl bromide.

Thus, 0.2187 mole of butyl bromide is formed.

Theoretical weight of butyl bromide = 0.2187 mole X 136.972 g/mol

= 29.9558g

Experimental weight of butyl bromide = 11.6156g

Yield percentage of n-butyl bromide = 11.6156g/29.9558g X 100% = 38.78%

 

Part II

Weight of conical flask = 51.1825g

Weight of conical flask + weight of n-butyl ethyl ether = 58.0435g

Weight of n-butyl ethyl ether = 6.8610g

Weight of 5ml measuring cylinder = 14.7446g

Weight of 1ml n-butyl ethyl ether + weight of 5ml measuring cylinder = 15.6545g

Weight of 1ml n-butyl ethyl ether = 0.9099g

CH₃CH₂CH₂CH₂Br + NaOH + CH₃CH₂OH ---->

CH₃CH₂CH₂CH₂-O-CH₂CH₃ + NaBr + H₂O

Number of mole of n-butyl bromide = (10 cm³ x 1.2686 g cm^-3)/ 136.972g mol^-1

= 0.0926 mole

Theoretical weight of n-butyl ethyl ether = 0.0926 mole x 102.112g/mol

= 9.4556g

Experiment weight of n-butyl ethyl ether = 6.8610g

Yield percentage of n-butyl ethyl ether = 6.8610g/9.4556g x 100% = 72.08%

Density = mass/volume

Density of n-butyl ethyl ether = mass of 1ml of n-butyl ether / volume of 1ml of n-butyl ether

                                           = 0.9099g/1cm³

                                           =0.9099g/cm³

Discussion:

Alkyl halides can be prepared from alcohols by reacting them with a hydrogen halide, HX (X = Cl, Br, I). The mechanisms of acid-catalyzed substitution of alcohols are termed S_N1 and S_N2, where “S” stands for substitution, the “N” stands for nucleophilic, and the “1” or “2” for unimolecular or bimolecular, respectively. The purpose of this experiment is to synthesize n-butyl ethyl ether via an S_N2 reaction and to purify it using simple distillation where substances with different volatility and boiling points are separated from each other.

In the experiment, the primary alkyl halide n-butyl bromide can be prepared easily by allowing 1-butanol to react with sodium bromide and sulphuric acid. The sodium bromide reacts with sulphuric acid under reflux to produce hydrogen halides. The chemical reaction as shown in below indicates that the hydrogen halide is produced from the reaction.

2 NaBr + H₂SO₄ ------> 2 HBr + Na₂SO₄

The hydrogen halide produced is used to convert 1-butanol to become butyl bromide by undergoes nucleophilic substitution. Excess sulphuric acid serves to shift the equilibrium and thus to speed up the reaction by producing a higher concentration of hydrobromic acid. The sulphuric acid also protonates the hydroxyl group of 1-butanol so that water is displaced rather than the hydroxide ion OH^-. The acid also protonates the water as it is produced in the reaction and deacticvates it(water) as a nucleophile, hence the water keeps the butyl bromide from being converted back into the alcohol by nucleophilic attack of water.

Synthesis of butyl bromide from the 1-butanol is undergoes S_N2 mechanism. S_N2 is known as second order nucleophilic substitution for bimolecule. The essential feature of the S_N2 mechanism is that take place in a single step without intermediates when the incoming nucleophile, hydrogen bromide reacts with the 1-butanol from a direction opposite the group that leaves. As the bromide ion, Br^- comes in on one side and bonds to the carbon, the OH^- departs from the other side, thereby inverting the stereochemical configuration. The mechanism is shown in the figure 1 as below.

Figure 1

Since the hydrogen halide is polar molecule, the bromide ion is partial negative and the hydrogen is partial positive. The highly partial negative bromide ion acts as a nucleophile to attack the 1-butanol at the opposite side of the departing OH group. This leads to a transition state in which the new Br-C bond is partially forming at the same time the hold C-OH bond is partially breaking, in which the partial negative charge by both the incoming nucleophile and the leaving hydroxyl ions. The transition state for this inversion has the remaining three bonds to the carbon in a planar arrangement as shown in figure 1. The water is formed after the bromide successfully becomes part of the molecule.

After the reflux process, the distillate is introduced with same approximate same amount of sodium bisulphate, NaHSO₄. The purpose of adding of sodium bisulphate is used to absorb water from, the organic solvent since it has high hydrophobic property. Sodium carbonate (mild acid) is added to neutralize the acidic solution. Anhydrous calcium chloride is added as drying agent to absorb water droplets in order to purify the organic layer. An excess drying agent should be used to ensure that all the water in solvent is removed. If the water remains in the materials collected, it could interfere with the analysis. After filter out the drying agent, several anti bumping granules (boiling chips) are added to prevent over boiling during distillation. Distillation process is carried out to purify the butyl bromide in the range of temperature 90°C to 105°C. The pure n-butyl bromide is obtained in the distillation process.

The n-butyl bromide formed after complete distillation in the first part is used as the materials in the latter part of this experiment. Now, the n-butyl bromide is used to synthesis n-butyl ethyl ether. The synonym of n-butyl ethyl ether is known as ethoxy butane which has the molecular formula with C₆H₁₄O. This molecule is categories in the ether group which has the general formula of R-O-R’. Sodium hydroxide and ethanol are introduced together and is heated under reflux. The purpose of introduction sodium hydroxide and ethanol is to produce ethoxide ions. The chemical reaction is shown as the following equation:

Figure 2

During reflux, the ethoxide ions react with n-butyl bromide under the second time reflux to form n-butyl ethyl ether as product. In this reaction, the bromide ions are escaped from the alkyl bromide and combine with the sodium ions to form sodium bromide to achieve stable molecule.

Figure 3

The butyl bromide undergoes second order nucleophilic substitution, S_N2. The ethoxide functions as nucleophile which attacks the electrophilic C of the butyl bromide by displacing the bromide and creating a new C-O bond between ethoxide and butyl bromide. The bromide ion is being displaced and leaves the butyl group. As a result, an ether with butyl ethyl groups is formed which is known as n-butyl ethyl ether. The mechanism is shown in the figure 4 as below.

Figure 4

After the reflux, the mixture is transferred into a separatory funnel and water is introduced to rinse the mixture. This is because water is used to dissolve some unreacted NaOH and hence the NaOH can be removed by removing the water. In order to remove the water droplets, anhydrous magnesium sulphate is added. Distillation of the organic layer is carried out to purify the product. Pure n butyl ethyl ether is obtained through the distillation at temperature 90°C-96°C.The overall chemical equation for the synthesis of n-butyl ethyl ether is

CH₃CH₂CH₂CH₂Br + NaOH + CH₃CH₂OH  ----->

CH₃CH₂CH₂CH₂-O-CH₂CH₃ + NaBr + H₂O

In the experiment, the but-1-ene is being produced via E₂ elimination mechanism. E₂ elimination reaction also can be occurred because the ethoxide ions are strong base which initiate elimination reaction to compete with the substitution reaction. The mechanism of elimination of butyl bromide is shown in figure 5.

Figure 5

In figure 5, the mechanism shows that the bromide atom in butyl bromide attracts the electron from the C-Br bond to form partial negative and this causes the C₁ lack of electron. The hydroxide ion acts as nucleophile and attack the electrophile C₁ via E₂ mechanism to form a transition state. As the OH^- start to attack a neighboring H and begins to remove the H at the same time as the C-C double bond starts to form and the Br group start to leave. The water and bromide ion are leave and hence but-1-ene are formed via elimination. However, dibutyl ether also can be formed due to the strong sulphuric acid used. The strong acid causes the side reaction to the butyl alcohol which is dehydration and ether formation which is shown in the figure 6 below:

Figure 6

 

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Physical and chemical properties of alcohols

Objectives:

1. To study the physical and chemical properties of alcohols

2. To identify the two unknown liquids from their experimental observations

3. To study the difference between primary alcohols, secondary alcohols and tertiary alcohols.

Introduction:

The hydrocarbon chains that attached with a hydroxyl group, OH^- to a carbon atom are known as alcohols. If the carbon atom is bonded to three hydrogen in addition to the OH^-, the alcohol is called methanol. Methanol, CH₃OH is the most simple alcohol molecule. The category of the alcohol is classified as three groups which are primary (1 ) alcohol, secondary (2 ) alcohols and tertiary (3 ) alcohol. If the alcohol bonded to one alkyl group, the alcohol is primary alcohol. The secondary alcohol is defined as the alcohol which one of the carbons is bonded to two alkyl groups and one hydrogen atom. If one of the carbons in alcohol is bonded to three alkyl groups is called tertiary alcohol.

All these alcohols share some similar characteristics but other characteristics are different owing to their different molecular structures. For physical properties, the size of alcohol determines its boiling point. Usually, the larger the size of the alcohol, the higher the boiling point. This is because the bigger the size of the molecules, the stronger the Van der Waals force between the alcohol molecules. So, more heat energy is needed to be absorbed in order to break down the intermolecular force between each alcohol molecules. Hence, the boiling point of the alcohols increases with the size of the alcohols. The solubility of the alcohol is depends on the size of molecules. Small alcohols are water soluble because the hydroxyl group can form hydrogen bond with water molecules. But, as the size of the alkyl group increases, the solubility of alcohol in water decreases as the hydrophobic property of alcohol increases. For example, if the carbon molecules in alcohol more than six per molecule, the particular alcohol definitely are not soluble in water. This is the result of the alkyl group disrupting the hydrogen bond among the water molecules. If the disruption becomes larger enough, the water molecules will repel the alcohol molecules effectively to reestablish hydrogen bonding.

The classification of an alcohol as primary, secondary or tertiary (see above) affects the chemical properties of the alcohols. Due to their different classes, the alcohols may give out different chemical properties when they react with the same compounds. Based on their chemical properties, we are able to differentiate among the classes of alcohols. Generally, Lucas test and chromic acid test is the two common tests that we always use to distinguish and categorize the classes of alcohols.

Lucas test

Water soluble alcohols are always tested by using the Lucas reagent to differentiate among the primary, secondary, and tertiary alcohols. This test depends on the appearance of an alkyl chloride as an insoluble second layer. Lucas reagent is the mixture of zinc chloride and hydrochloric acid. Zinc chloride is a Lewis acid which is added with hydrochloric acid to make its property becomes more acidic. The tertiary alcohol (water soluble) reacts with Lucas reagent almost immediately to form an alkyl chloride which is insoluble in the aqueous solution. The formation of a second layer liquid phase in the test tube almost as soon as the alcohol initially dissolves is indicative of a tertiary alcohol. The secondary alcohol reacts slowly with Lucas reagent, and it gives a second phase after heating for 10 minutes whereas the primary alcohol and methanol do not react with Lucas reagent under normal condition. The chemical equation is as shown below (if the reaction takes place)

ZnCl2

R-OH + HCl     ------------------>    R-Cl + H₂O

Chromic acid test

Chromic acid is a strong oxidizing agent which uses to oxidize the alcohols. This test is based on the reduction of chromium (VI) ions to chromium (III) ion. When chromic acid reacts with alcohols, the change in colour of the solution from red-brown to green is a positive test. Primary alcohols are oxidized to carboxylic acid by chromic acid. The Cr⁶⁺ in the chromic acid which is red-brown is reduced to green Cr³⁺; secondary alcohols are oxidized to ketones by the reagent with the same colour change. Tertiary alcohols are not oxidized at all by the chromic acid. Hence, this reaction can be used to distinguish tertiary alcohols form primary and secondary alcohols.For tertiary alcohols, the alcohols would not be oxidized by the reagent. Hence, this test is used to distinguish the tertiary alcohols from primary and secondary alcohols.

 

Reaction with sodium metal

The acidic properties of alcohol can be shown by adding the sodium metal into alcohol. The alcohols are weak acids when they react with sodium metal. The hydroxyl group can act as a porton donor to form an alkoxide ion. Alkoxide ions dissolved in alcohol are strong bases which can be prepared by the reaction of an alcohol with sodium metal. Hydrogen gas is released by the reaction.

2 R-OH + 2 Na  -------> 2 R-O^-Na⁺ + H₂

The hydrogen gas can be collected and tested by using a burning wooden splinter. A pop sound will be produced.

Apparatus: test tube, measuring cylinder, droppers

Materials: ethanol (C₂H₅OH), isopropyl alcohol (C₃H₇OH), t-butyl alcohol (C₄H₉OH), Lucas reagent (mixture of concentrated HCl and ZnCl), chromic acid (H₂CrO₄), sodium metal

Procedure:

A. Solubility of alcohols

1. In three separate dry test tubes, ethanol, isopropyl alcohol and t-butyl alcohol are added into each test tube.

2. Water is added to each tubes, the contents are mixed and observed.

3. The results are recorded in a table.

4. The above procedures are repeated with two unknown liquids and observations are being made.

B. Lucas test

1. Ethanol, isopropyl alcohol and t-butyl alcohol are added into separate dry test tubes and Lucas reagent is added at room temperature.

2. The tubes are closed with a cork, the tubes are shaken and the length of time it takes for the mixture to become cloudy or separate two layers.

3. The results are recorded.

4. The above procedures are repeated with the two unknown liquids and observations are being made.

C. Chromic acid test

1. Ethanol, isopropyl alcohol and t-butyl alcohol are added into separate dry test tubes.

2. A small piece of sodium metal is added and any reactions occur are noted.

3. The step 1 and step 2 are repeated with two unknown liquids and any observations are made.

Results:

Part A

Table 1 The solubility of alcohols and their observations

Alcohols	Observation	Reaction equations(if any)	Deduction/conclusion/discussion
Ethanol	Soluble in water to form colourless solution	C2H5OH (aq) + H2O (l) ---> C2H5O- (aq)+ H+(aq)	Soluble in water
Isopropyl alcohol	Soluble in water to form colourless solution	C3H7OH (aq) + H2O (l)  ---> C3H7O- (aq) + H+ (aq)	Soluble in water
t-butyl alcohol	Soluble in water to form colourless solution	C4H9OH (aq) + H2O (l) -----> C4H9O- (aq) + H+ (aq)	Soluble in water
Unknown A	Soluble in water to form colourless solution	-	Soluble in water
Unknown B	Soluble in water to form colourless solution	-	Soluble in water

Part B

Table 2 The reaction between alcohols and Lucas Reagent

Alcohols	Observation	Reaction equations(if any)	Deduction/conclusion/discussion
Ethanol	Yellowish solution remains unchanged after heating.		No reaction between ethanol and Lucas reagent.
Isopropyl alcohol	Yellowish solution remains unchanged within 10 minutes, but turns into cloudy solution after heating.	C3H7OH (aq) + HCl (aq)  -----> C3H7Cl (aq) + H2O (l)	The reaction between isopropyl alcohol and Lucas reagent occur after heating for 10 minutes.
t-butyl alcohol	Cloudy solution is formed immediately and two layers are formed. Upper layer is clear while lower layer is cloudy.	C4H9OH (aq) + HCl (aq) C4H9Cl (aq) + H2O (l)	The reaction between t-buytl alcohol and Lucas reagent is instant reaction.
Unknown A	Yellowish solution remains unchanged within 10 minutes, but turns into cloudy solution after heating.		Unknown A is isopropyl alcohol because it has the similar reaction with it.
Unknown B	Cloudy solution is formed immediately and two layers are formed. Upper layer is clear while lower layer is cloudy.		Unknown B is t-butyl alcohol because it has the same reaction with it.

Part C

Table 3 The reaction of alcohols with chromic acid

Alcohols	Observation	Reaction equations(if any)	Deduction/conclusion/discussion
Ethanol	The solution turns from colourless to green	3 CH3CH2OH + 2 CrO4- + 10 H+ à 3 CH3CHO + 2 Cr3+ +8 H2O	Ethanol is oxidized by the chromic acid.
Isopropyl alcohol	Two layers are formed. Upper layer is green while lower layer is black.	C3H7OH (aq) + H2CrO4(aq) -----> C2H6CO (aq) + Cr2(SO4)3(aq) + H2O(l)3 (CH3)2CHOH + 2 CrO4- + 10 H+ à3 (CH3)2CO + 2 Cr3+ +8 H2O	Isopropyl alcohol is oxidized by the chromic acid.
t-butyl alcohol	Two layers are formed. Then, the precipitate dissolves in solution to become reddish brown solution.	-	No reaction.
Unknown A	Two layers are formed. Upper layer is green while lower layer is black.		Unknown A is isopropyl alcohol because it has the similar reaction with isopropyl alcohol.
Unknown B	Two layers are formed. Then, the precipitate dissolves in solution to become reddish brown solution.		Unknown B is t-butyl alcohol because it has the similar reaction with t-butyl alcohol.

Part D

Table 4 The reaction between alcohols and sodium metal

Alcohols	Observation	Reaction equations(if any)	Deduction/conclusion/discussion
Ethanol	Bubbles of colourless gas released quickly. A pop sound is produced when tested with burning wooden splinter.	2C2H5OH (aq) + 2Na(s) ------> 2C2H5ONa (aq) + H2(g)	Many of hydrogen gas is being produced from the reaction between ethanol and sodium metal.
Isopropyl alcohol	Bubbles of colourless gas released slowly. A pop sound is produced when tested with burning wooden splinter.	2C3H7OH (aq) + 2Na(s) -----> 2C3H7ONa (aq) + H2(g)	Small amount of hydrogen gas is being produced from the reaction between isopropyl alcohol and sodium metal.
t-butyl alcohol	Bubbles of colourless gas released very slowly. A pop sound is produced when tested with burning wooden splinter.	2C4H9OH (aq) + 2Na (s) ------> 2C4H9Ona (aq) + H2 (g)	Less hydrogen gas is being produced from the reaction.
Unknown A	Bubbles of colourless gas released slowly. A pop sound is produced when tested with burning wooden splinter.		Unknown A is isopropyl alcohol because it has the similar reaction with isopropyl alcohol.
Unknown B	Bubbles of colourless gas released very slowly. A pop sound is produced when tested with burning wooden splinter.		Unknown B is t-butyl alcohol because it has the similar reaction with t-butyl alcohol.

Discussion:

This is because all of them, ethanol, isopropyl alcohol and t-butyl alcohol are short alkyl chain alcohols. Alcohols can soluble in water is because of the presence of the hydroxyl group (-OH) in the compounds. The hydroxyl group can form hydrogen bond with the water molecules and thus make it soluble in water. The solubility of alcohols in water are always depends on their structure and size. When the sizes of alcohols increase, the solubility of alcohols in water will decrease. This is because the bulky groups are highly hydrophobic and tend to block the water molecule from nearing alcohol and stabilize it. This is the result of the alkyl group disrupting the hydrogen bond among the water molecules. If the disruption becomes larger enough, the water molecules will repel the alcohol molecules effectively to reestablish hydrogen bonding. Usually, the number of carbon per molecule is more than six are not soluble in the water.

Based on the properties of solubility of alcohols in water, this information is not enough for us to differentiate clearly the classes of alcohols which are being used in the experiment. So, Lucas test is used to distinguish among the primary, secondary, and tertiary alcohols. For Lucas test, the formation of two layers (aqueous layer and cloudy layer) is known as a positive test. The second layer (cloudy layer) formed is alkyl chloride which is insoluble in the aqueous solution because all the alkyl halides molecules are insoluble in the water. The alkyl chloride produced from the reaction is not water soluble and causes cloudiness (emulsion) to form in the aqueous solution. When ethanol is added with Lucas reagent, the yellowish solution is still remains the same before and after heating. This is because primary alcohol does not react with Lucas reagent. Besides, isopropyl alcohol does not react with Lucas reagent before heating but it does turns to cloudy solution after heating for 10 minutes. The reason is the reaction between Lucas reagent and secondary alcohol is slow. For tertiary alcohol (t-butyl alcohol), the reaction of alcohol with Lucas reagent is very fast which can be known as an instant reaction. The reaction that takes place on the Lucas test is a SN₁ nucleophilic substitution. The alcohols with the properties of generating a stable carbocation intermediates will undergo the particular reaction. The OH group of the alcohol attracts the H in hydrochloric acid to form oxonium ion and leave the group (form water). The carbocation intermediate is formed and tends to react with Cl^- (nucleophile) to produce the alkyl halide product. The mechanism of SN₁ nucleophilic substitution is shown as diagram below:

The purpose of chromic acid test used in this experiment is to distinguish the primary and secondary alcohols from the alcohols group. In the reaction between alcohols and chromic acid, the chromic acid is being reduced which the chromium (VI) ions, Cr⁶⁺ reduced to become chromium (III) ion, Cr³⁺. The positive test for chromic acid is represented by the change in colour from orange to green-blue. In the test tube with ethanol, the colourless alcohol is turned to green solution because the chromium (IV) ions, Cr⁶⁺ are being reduced by ethanol. In the test tube containing isopropyl alcohol, two layers of colour are formed. The upper layer is green whereas the lower layer is black. This is shows that the isopropyl reacted with the chromic acid due to the secondary alcohol is readily to be oxidized. The black layer actually is the dark blue colour, it is hard to differentiate because the light is not enough in the laboratory. The t-butyl alcohol would not oxidized by the chromic acid since the tertiary alcohol is a highly oxidized alcohol. This is shown by the formation of reddish brown precipitate. The chemical equation for the reaction is shown as below:

Ethanol: 3 CH₃CH₂OH + 2 CrO₄^- + 10 H⁺ à 3 CH₃CHO + 2 Cr³⁺ +8 H₂O

Isopropyl alcohol: 3 (CH₃)₂CHOH + 2 CrO₄^- + 10 H⁺ à3 (CH₃)₂CO + 2 Cr³⁺ +8 H₂O

 

The general mechanism for the reaction is shown below:

Hydrogen ions will be attracted to one of the oxygen atom in the chromic acid that is double bonded to chromium. The lone pair electrons from hydroxyl group of alcohols will then attack the chromium ion and form a chromate intermediate. The hydrogen on the hydroxyl group of alcohols will then leave as hydrogen ion and combine with the oxygen atom that is previously attacked by a hydrogen ion. A water molecule will come and attack α-hydrogen on the alcohol and produce hydronium ion, forming C=O double bond. A water molecule will leave the chromate intermediate and the carbonyl product is formed. Since the oxidation of alcohol requires at least one hydrogen atom to be presence on α-carbon, thus tertiary alcohol cannot be oxidized because it does not have the α-hydrogen. This explains why t-butyl alcohol does not change the color of the chromic acid which is red brown to green. The precipitate formed at the beginning may be because of the formation of chromium trioxide precipitate.

The last test in this experiment is the reaction of sodium metal with alcohols. All the different classed of the alcohol are able to react with the sodium metal since all of them have OH group. According to the Lewis Bronsted Theory, a Bronted acid is a proton donor. In this case, alcohol acts as a proton donor which donates H⁺ into the solution while the sodium metal acts as a strong base. A strong base can deprotonates the alcohol to yield an alkoxide ion (R-O). The H in OH group will be substituted by sodium ion to form sodium alkoxide, R-O^-Na⁺. The reaction between an acid and an active metal in group I element definitely will produce hydrogen gas as the product as the metallic sodium reduces proton to form hydrogen gas. The evidence is shown by the release of bubbles of colourless gas from the reaction between sodium metal and alcohol. The release of hydrogen gas can be collected and tested with a burning wooden splinter. A pop sound will be produced as the hydrogen gas is an explosive gas. The chemical equation of alcohols and sodium metal is shown as the diagram 1 below:

2C₂H₅OH (aq) + 2Na(s) 2C₂H₅ONa (aq) + H₂ (g)

2C₃H₇OH (aq) + 2Na(s) 2C₃H₇Ona (aq) + H₂ (g)

2C₄H₉OH (aq) + 2Na (s) 2C₄H₉Ona (aq) + H₂ (g)

Diagram 1 The reaction between alcohols with different classes with sodium metal

The acidity of the alcohol can be determined by the rate of gas evolution in this experiment. The more the gas released from the reaction, the more acidic the properties of alcohol. The acidity of alcohols decreases as the number of carbon in bulky alkyl group that bonded to OH group increases. Since alkyl group is electron releasing group, the more bulky the alkyl group, the stronger the electron donor effect. Thus, the negative charge density on the O atom in and proton are less readily to be released. The second reason is the bulky groups are highly hydrophobic and tend to block the water molecule from nearing alcohol and stabilize it. The alcohols are arranged in order of increasing acidity of alcohol as below:

t-butyl alcohol < isopropyl alcohol < ethanol

Acidity increasing

 

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Dehydration of An Alcohol: Cyclohexanol and Cyclohexene

 

Objectives:

1. To produce cyclohexene through the acid catalyzed elimination of water from cyclohexanol

2. To understand mechanism involved in the reaction

3. To learn the technique of distillation

Introduction:

Dehydration is defined as a process of removing water from a substance. The loss of water from a molecule is called dehydration which is exactly opposite with the process of hydrolysis. Dehydration is an elimination reaction of an alcohol involves the loss of an OH from one carbon and an H from an adjacent carbon. Overall, this amounts to the elimination of a molecule of water, resulting in a pi-bond formation of an alkene or alkyne. In most of the dehydration of alcohol, heat and catalyze are needed in the reaction. Sulphuric acid (H₂SO₄) and phosphoric acid (H₃PO₄) are the most commonly used acid catalysts.

The dehydration process can be carried out by in two ways. The first way is heating a mixture of alcohol and dehydrating agent in a distilling flask and collecting the olefin (also known as alkene) from the mixture. The second way is passing the alcohol vapour through a heated tube packed with the dehydrating agent at 350°C. The collecting the olefin as it emerges from the tube. The chemical equation for dehydration of alcohol to form alkene and water is shown as below diagram:

For the dehydration of alcohol, the alkene is formed in the reaction. At the same time, the side products are produced from the reaction such as dicyclohexyl ether, polymer, mono and dicyclohexyl sulphate abd degradation products (carbon and carbon dioxide). In many cases, the phosphoric acid is used in the dehydration of alcohol instead of using sulphuric acid due to two reasons. The one of the reason is the lost of organic compound can be minimize through oxidation of phosphoric acid. In addition, the product is being produced without contamination with volatile decomposition products such as sulphurous acid.

Apparatus: Round-bottomed flask(50ml), take off distillation adapter, condenser, thermometer, electric flask heater

Materials: boiling chips, cyclohexanol, cyclohexene, 85% conc. Phosphoric acid, anhydrous magnesium sulpahte

Procedure:

1. Cyclohexanol and conc.(85%) phosphoric acid are added in a round bottomed flask and is mixed together by swirling.

2. Several boiling chips are added, the flask is clamped to a ring stand at electric flask heater height attached with a take off distillation adapter, a thermometer, a condenser and a small receiving flask as shown in the diagram below.

3. The mixture is heated and distillate boiling in the range 85°C-90°Cis obtained.

4. When the distillate is exhausted, the heat is increased gradually. Using the same receiver, the distillate boiling in the range of 90°C - 100°C is collected.

5. The two layers in the receiving flask are tested by adding the distilled water. With the aid of a 9-disposable pipette, the aqueous layer is drawn off and is being discarded.

6. The organic layer is dried up with anhydrous magnesium sulphate.

7. The drying agent is removed by filtering the mixture through a cotton wool plug wedged into the constricted part of a small funnel.

8. The filtrate is collected round bottom flask or small distilling flask. Boiling chip is added to the dried product and distil it through a take off distillation adapter packed with a few small wads of coarse steel wool.

9. The product boiling is collected in the range 3 below and 2 above the boiling point of cyclohexene (83°C) in a tarred bottle.

10. The yield in grams is calculated and the product is submitted to instructor.

Result and calculation:

Weight of cyclohexanol = 10.0060g

Weight of dry conical flask = 51.2460g

Weight of dry conical flask + weight of cyclohexene = 56.2281g

Experimental weight of cyclohexene = 4.9821g

Number of mole of C₆H₁₁OH =

= 0.09996 mole

1 mole of C₆H₁₁OH produces 1 mole of C₆H₁₂

0.09996 mole of C₆H₁₁OH produces 0.09996 mole of C₆H₁₂

Theoretical weight of C₆H₁₂ = 0.09996 mole X 84.096 g/mol

= 8.4066g

Yield percentage =

 

= 59.26%

Discussion:

In this experiment, the cyclohexanol solution is being used in the dehydration process. The cyclohexanol is a six carbon aromatic hydrocarbon which one of the hydrogen atoms, H is substituted by one hydroxyl group, OH^-. Due to the low melting point, the cyclohexanol appear in liquid form at room temperature. The dehydration process of alcohol will convert cyclohexanol which the hydroxyl group, OH^- will be removed to become cyclohexene. Cyclohexene is a six carbon aromatic hydrocarbon with a single double bond in the molecule.

The solution is added with concentrated phosphoric acid in a round bottomed flask and is mixed together by swirling. The phosphoric acid is added as catalyze as such increase the rate of reaction in dehydration without affects the particular chemical reaction. The boiling chips are added into the solution in order to prevent over boiling of the solution. The boiling chips are small, insoluble, and porous stones made of calcium carbonate or silicon carbide. There are a lot of pores inside the boiling chips which provide cavities both to trap air and to provide spaces to allow bubbles of solvent can be form. When boiling chips are heated, it will release tiny bubbles which can prevent boiling over. Boiling over of solvent will cause lost of solution which may lead to inaccurate result to be obtained.

The heating of mixture is carried out in a fractional distillation apparatus. As the mixture is heated, the alkene and water are produced as the products in the reaction. Besides, the side product and impurities of the reaction will be produced at the same time. The temperature of mixture when heating is fluctuated. During the temperature 80°C, the temperature of mixture drops suddenly by 2°C and the temperature remained at 78°C constantly for few seconds. This is because some impurities or side products are being produced in the mixture which may have the boiling point of 78°C. The temperature of mixture drops suddenly because the heat being is absorbed which used to break down the bonding of the side products. When the boiling point is reached, then temperature of mixture remains constant as the state of the side products are converted from liquid form to gaseous form. The temperature of mixture is increases until 107°C after all the side products are being converted.

The temperature of mixture is reached to maximum at 107°C. This temperature is known as the activation temperature which the cyclohexanol start to be dehydrated. The temperature of mixture drops to 83°C and remains constant. This phenomenon takes place because the cyclohexene with lowest boiling point will tends to be distilled first before the higher one. The temperature remains unchanged because the heat is being absorbed to break down the bond between cyclohexene molecules. In all the distillation process, some of the product will be lost since it is hold up in the apparatus which reduce the product yield. In order to maximize the yield, the mixture is continued to be heated at higher temperature range which more than the boiling point of cyclohexene. When the mixture is heated at 90°C-100°C, the water in the mixture will push over the products into the receiving flask along the condenser. The products produced are collected in the same receiving flask.

Then, the receiving flask containing cyclohexene, water and small amount of the impurities. Two layers liquid are present in the receiving flask, one drop of distilled water is added into flask in order to determine the location of aqueous layer. Since the water droplet mix with the lower layer, so the upper layer is determined as cloudy solution while the lower layer is aqueous layer. The upper cloudy layer is cyclohexene with some impurities and water inside. The lower aqueous layer is removed and discarded. But, that is not easy to remove all the water in the receiving flask. So, anhydrous magnesium sulphate is being added. The purpose of adding of anhydrous magnesium sulphate is used to remove residual water in the organic solvent. The magnesium sulphate in the granular form will be preferable. It is known as drying agent in the organic solvent which are not dissolves in the solvent but drying the solvent. The magnesium sulpahte clump together with the water droplets as it solidified them. In another words, it is reacts with water to form hydrates which is their preferred form when water is available. An excess drying agent should be used to ensure that all the water in solvent is removed. If the water remains in the materials collected, it could interfere with the analysis.

Water has been successfully removed from the organic compound mixture, so it is very important to not reintroduce water into the mixture. The final distillation of unpurified cyclohexene must be done very carefully in order to obtain purified products. The temperature of mixture is heated until approximate 85.5°C since this temperature is the boiling point of cyclohexene and hence the pure cyclohexene could be obtained rapidly. The weight of the cyclohenexe is 4.23g.

Error of source includes the condenser and Liebig tubes are not rinsed with a little of distilled water, a little of ethanol and acetone to speed up drying process. So that, the water in the desired products would not presents. This is because some of the water droplets is hold up and stick on the wall of condenser and the second distillation will produce the contaminated cyclohexene.

 

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