Esterification process and mechanism

 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.

 

Flickr Tags: Esterification: Methyl benzoate,mechanism of the ester formation,acid catalyzed esterification,benzoic acid and methanol

Alkyl group reactivity

 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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