Preparation of cis-bis(glycinato)copper(II) monohydrate & tran-bis(glycinato)copper(II) monohydrate

Objectives:

1. To prepare both cis- and trans­ copper glycine complexes

2. To verify the complex is kinetic product or thermodynamic product

3. To characterize both cis- and trans­ copper glycine complexes

Introduction:

Glycine is one of the biologically important compounds in the group of amino acids. Among the twenty one natural amino acids, glycine is the simplest amino aicd. Amino acid has both the functional group of amine (-NH₂) and carboxylic acid (-COOH). They are the basic units of the proteins in which the building block of every single living cell. Proteins are polypeptides or polyamide that formed by joining the –NH₂ group of one amino acid to the –COOH group of another one and therefore a long and complicated peptide chain is formed.

 

Glycine is the simplest model for peptide coordination and its complexes with various metal ions have been thoroughly studied. Elzbieta (2008) claimed that glycine can even forms more essentially stable complex with copper (II) compared to other amino acids. The structure and stability of the complexes are determined by nature of metal, the nature of the ligands and environment. The environment is controlled by the factors such as temperature, the type of solvent, the interaction enthalpies, entropies, and Gibbs energies.

 

The binding modes of glycine ligand can be varied since it has at least two donor atoms. The glycinate ion is able to adopt a η³-coordinated mode via its amino, -NH₂ and carboxylate, -COOH groups to chelate one copper ion and bridge to another copper ion. The variation of glycinate binding modes is shown in the picture below:

Picture 2. Five coordination modes of amino acid. a) η1-coordination mode; b)η2-coordination mode; c) η3-coordination mode with strong Cu-O bond; d)η3-coordination mode with weak Cu-O bond beyond 2.5 Å; e)η4-coordination mode (M is Na+ ion or lanthanide ion)

 

In this experiment, a pair of geometric isomers of copper(II) glycine complex are prepared. Deprotonated glycine, or known as glycinate ion, NH₂CH₂COO^- is capable to form two coordination bonds to copper metal through the lone pair electrons of nitrogen and oxygen atoms. Hence, it functions as a chelating ligand or more specifically it is known as bidentate ligand and favors the formation of bis(glycinato)copper(II) complex. The reaction between copper(II) acetate monohydrate and glycine can produces mixture of both isomers in an equilibrium mixture. However, the cis isomer precipitates much more quickly then trans isomer and hence leading to a shift in equilibrium away from trans with producing only cis isomer. Cis isomer is the kinectically favoured product whereas trans isomer is thermodynamically favoured. In order to produce trans isomer, the cis isomer can be converted into another isomer by supplying heat energy at 180 °C for time of 15 minutes.

Picture 3. a) cis-Cu(gly)2 b) trans-Cu(gly)2

Materials:

Copper(II) acetate monohydrate, glycine, ethanol 95%

Apparatus:

Magnetic stirring hot plate, magnetic stirring bar, Erlenmeyer flask, Hirsch funnel, test tube

Procedure:

Part A: Preparation of cis_-bis(glycinato)copper(II) monohydrate

Part B: Preparation of tran-bis(glycinato)copper(II)

Results and calculations:

Table 1 Amount of reactants used and amount of products obtained

Reactants
Weight of copper(II) acetate monohydrate	0.3007g
Weight of glycine	0.2328g
Products
Weight of filter paper I	0.3233g
Weight of filter paper I + cis-product	0.6061g
Weight of cis-product	0.2828g
Weight of filter paper II	0.3001g
Weight of filter paper II + trans-product	0.3684g
Weight of trans-product	0.0683g

Chemical reaction:

(CH₃COO)₂Cu.H₂O + 2 H₂NCH₂COOH àcis-(H₂NCH₂COO)₂Cu.H₂O + 2 CH₃COOH

Determination of limiting agent

Molecular weight of (CH₃COO)₂Cu.H₂O = 199.55 g mol^-1

Number of mole of (CH₃COO)₂Cu.H₂O = 0.3007g / 199.55 g mol^-1

= 0.0015 mol

Molecular weight of H₂NCH₂COOH = 75.00 g mol^-1

Number of mole of H₂NCH₂COOH = 0.2328g / 75.00 g mol^-1

= 0.0031 mol

Thus, copper(II) acetate monohydrate is the limiting agent in this reaction.

Percentage yield calculation

Molecular weight of (H₂NCH₂COO)₂Cu.H₂O = 229.55 g mol^-1

Theoretical weight of cis-(H₂NCH₂COO)₂Cu.H₂O = 0.0015 mol x 229.55 g mol^-1

= 0.3443 g

Percentage yield of cis-(H₂NCH₂COO)₂Cu.H₂O = 0.2828g / 0.3443g x 100%

= 82.14%

Theoretical weight of trans-(H₂NCH₂COO)₂Cu. = 70 mg

Percentage yield of tran-(H₂NCH₂COO)₂Cu = 0.0683g / 0.070 g x 100%

= 97.57%

Discussion:

From this experiment, the weights of both isomer cis-Cu(gly)₂.H₂O and trans-Cu(gly)₂ obtained are 0.3443g and 0.0683g respectively. Each isomer contributed the percentage yield of 82.14% for cis-Cu(gly)₂.H₂O and 97.57% for trans-Cu(gly)₂ respectively.

 

The dissociation of glycine molecule produces a glycinate anion, NH₂CH₂COO^- in which it replaces the position of acetate ion, CH₃COO^- in the copper complex. The dissociated proton from glycintate ion is accepted by acetate ion and hence acetic acid is produced in the reaction between copper(II) acetate monohydrate and glycine.

(CH₃COO)₂Cu.H₂O + 2 H₂NCH₂COOH à(H₂NCH₂COO)₂Cu.H₂O + 2 CH₃COOH

The reaction was takes place in a hot 95% ethanol. Ethanol did not participate in the reaction but it acts as a medium to allow the copper(II) complex to form at 70°C. When cooled down, the copper(II) complex crystallize out from the ethanol.

The solid cis-monohydrate was heated at 200 °C by using an aluminium block in order to convert cis-monohydrate to trans­-complex. The temperature of aluminium block is approximately measured by using hexane with a thermometer. The dehydration of cis-bis(glycinato) copper(II) monohydrate, cis-Cu(gly)₂.H₂O at sufficiently high temperature (approximately at 200 °C)leads to the formation of mainly anhydrous trans-complex, which is readily to be re-hydrated to give trans-bis(glycinato) copper(II), trans-Cu(gly)₂.H₂O if present in solution.

Precaution steps:

1. Handle copper(II) acetate monohydrate carefully. It is harmful if swallowed, inhaled or absorbed through the skin.

2. Keep a distance from the hot plate when heating aluminium block.

 

Technorati Tags: cis- and trans­ copper glycine complexes,cis-bis(glycinato)copper(II) monohydrate & tran-bis(glycinato)copper(II) monohydrate

Reduction of 1-phenyl-1,2-propanedione to 1-phenyl-1,2-propanediol

Asymmetric Synthesis with Baker’s Yeast:

Objectives:

1. To reduce 1-phenyl-1,2-propanedione to 1-phenyl-1,2-propanediol

2. To monitor the course of reaction by using TLC

3. To characterize the produce by FT-IR spectroscopy and gas chromatography-mass spectroscopy (GC-MS)

Introduction:

Lithium aluminium hydride and sodium borohydride are good reducing agent, in which it can reduce a ketone to an alcohol. However, they are not able to generate a chiral alcohol because they can nucleophilic attack both sides of carbonyl group, hence producing racemic mixture. In order to obtain a chiral alcohol, baker’s yeast is used.

Asymmetric reduction of 1-phenyl-1,2-propanedione by using baker’s yeast in this experiment in order to produce (-)-(1R,2S)-1-phenyl-1,2-propanediol with the enantiomeric excess of 98% or more. The completeness of reaction is determined by using thin layer chromatography (TLC). Pure 1-phenyl-1,2-propanediol is characterized by using infrared (IR) spectroscopy and mass gas chromatography-mass spectroscopy (GC-MS).

Apparatus:

Erlenmeyer flask, hotplate-stirrer, magnetic stirring bar, TLC tank, micropipette, separatory funnel, UV lamp, rotary evaporator

Materials:

1-phenyl-1,2-propanedione, freeze-dried Baker’s yeast, TLC plate, tert­-butyl methyl ether (BME), magnesium sulphate anhydrous, cylohexane

Instruments:

IR spectroscopy, gas chromatography-mass spectrometer

Procedures:

Result and calculations:
Observations on TLC plate:

Descriptions: At 0th minute, two spots present on the TLC plate. Reactant and product present in the reaction mixture. At 20th minute, two spots present on the TLC plate. Reactant and product present in the reaction mixture. At 40th minute, only one spot presents on the TLC plate. Reactants have been used up shows complete reaction. At 60th minute, only one spot present on the TLC plate. Reactants have been used up shows complete reaction.

Table 1: R_f value of each spots on TLC plate

Time (minutes)	0	20	40	60
Distance travelled by each aliquots (cm)	1st spot	2.90	2.90	2.90	2.90
2nd spot	5.20	5.20	-	-
Retardation factor, Rf	1st spot	0.41	0.41	0.41	0.41
2nd spot	0.74	0.74	-	-

*TLC is performed by using a mixture of cyclohexane:BME (3:2)

*solvent front is 7cm.

Table 2: Weight of 1-phenyl-1,2-propanediol

Weight of round bottom flask	97.6703g
Weight of (round bottom flask + 1-phenyl-1,2-propanediol)	97.9018g
Weight of 1-phenyl-1,2-propanediol	0.2315g

Table 3: Significant peaks of starting material and product in IR spectrum

Functional group	Wavenumber of Compound, v (cm-1)
1-phenyl-1,2-propanedione	1-phenyl-1,2-propanediol
C=O stretch	1715, 1676	absent
O-H stretch	absent	3369

Molecular weight of 1-phenyl-1,2,-propanedione = 148 g /mol

Molecular weight of 1-phenyl-1,2-propanediol = 152 g / mol

Density of 1-phenyl-1,2,-propanedione = 1.101 g ml^-1

Mass of 1-phenyl-1,2,-propanedione = density x volume

= 1.101 g ml^-1 x 0.23 ml

= 0.2532g

1 mole of 1-phenyl-1,2,-propanedione produces 1 mole of 1-phenyl-1,2-propanediol

Mole number of 1-phenyl-1,2,-propanedione = 0.2532g / 148 g mol^-1

= 0.0017 mol

Theoretical mass of 1-phenyl-1,2-propanediol = 0.0017 mol x 152 g mol^-1

= 0.2584g

Percentage yield = 0.2315g / 0.2584g x 100%

= 89.59%

Discussion:

In this experiment, 1-phenyl-1,2-propanedione was reduced to 1-phenyl-1,2-propanediol by using baker’s yeast. The mass of 1-phenyl-1,2-propanediol obtained experimentally is 0.2315g and its percentage yield is 89.59%.

Thin layer chromatography was used to monitor the reduction of 1-phenyl-1,2-propanedione in this experiment. Based on the observation of TLC plate, the reaction was completed at 40^th minutes since the TLC plate only shows one spot is present. After the reaction completed, the product was extracted with BME and was dried with drying agent to remove water in the organic layer.

The following mechanism shows that how ketone was reduced into alcohol by yeast.

Firstly, the hydride from baker’s yeast nucleophilic attack to carbonyl carbon (labeled as carbon 2) and caused the reduction of the carbonyl group. Oxygen of the particular carbonyl group was bearing with partial negative charge in which it can abstract a proton from water molecules to form hydroxyl group.

Then, another carbonyl group (labeled as carbon 1) was attacked by hydride from back side and hence produced an alcohol that has two carbons with different chirality. The alcohol obtained is named (1R,2S)- 1-phenyl-1,2-propanediol. However, (1S,2R)- 1-phenyl-1,2-propanediol might present with a very low yield.

From the IR spectrum, 1-phenyl-1,2-propanedione shows the presence of C=O stretch at 1715cm^-1 and 1676cm^-1. Besides, this ketone compound did not show any O-H stretch signal in the IR spectrum. From the alcohol spectrum, there shows a broad O-H stretch signal at 3369cm^-1 and it did not have C=O stretch signal. According to IR spectrum, the ketone compound has been successfully reduced to form an alcohol compound as obtained in the experiment because IR spectrum indicated the presence of O-H group.

Precaution steps:

1. Do not use separatory funnel point to anybody when releasing the vapour.

2. Do not shake the separatory funnel vigorously to avoid emulsion formation.

Technorati Tags: Asymmetric Synthesis with Baker’s Yeast,Reduction of 1-phenyl-1,2-propanedione,IR of 1-phenyl-1