The Grignard Reaction - Synthesis of 1,2-Diphenyl-1,2-Propanediol Via a Diastereoselective Reaction

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Objective:

The purpose of this experiment was to prepare the Grignard reagent methylmagnesium iodide and react it with benzoin to form the 3o alcohol 1,2-diphenyl-1,2-propanediol, through an addition reaction pathway.

Introduction:
Grignard reagents are alkyl or aryl-magnesium halides that act as the nucleophile in Grignard reactions, where ketones are reacted with the reagent, then treated with acid to produce an alcohol. In the case of this experiment, methylmagnesium iodide was created from methyl iodide and magnesium metal. It acted as the Grignard Reagent in Part B upon which it was reacted with benzoin to create 1,2-diphenyl-1,2-propanediol. A racemic mixture of benzoin was used, and such theoretically both diasteriomers of the diol could have be formed. However the methylmagnesium iodide is diasterioselective as it generally prefers to attack one particular face of the benzoin molecule. By comparing the obtained melting points and comparing them to literature value, the diastereomer formed in the isolated purifeed product can be determined

Reaction Equation:

Fig 1: Reaction equation for the formation of the Grignard Reagent

Table 1: Reagent Properties and Amounts used
Chemical| Structure| Formula| Mass or Volume Required (g or mL)| M.W. (g/mol)| Density (g/cm3)| # Moles| m.p./b.p. (°C)| Actual Mass (g)| Methyl Iodide| | CH3I| 24 mL| 141.94| 2.28| 0.3855| 42.5| 52.72 g (24 mL)| Magnesium Metal| | Mg(s)| 0.58g| 24.31| N/A| 0.0239*| 648| 0.58g| Methyl-magnesium iodide| | CH3MgI| 3.97g†| 166.24| ~1.26| 0.0239†| N/A| 3.97g†(~3mL)| * Limiting Reagant

† Theoretical yield, assumes 100% recovery
=nCH3MgI = nLimiting reagent = nMg
=0.0239 mols
=3.97g

Fig. 2: Reaction equation for the Grignard Reaction

Table2: Reagent Properties and Amounts Used
Chemical| Structure| Formula| Mass or Volume Required (g or mL)| M.W. (g/mol)| Density (g/cm3)| # Moles| m.p./b.p. (°C)| Actual Mass (g)| Methyl-magnesium iodide| | CH3MgI| 3.97g†| 166.24| ~1.26| 0.0239†| N/A| 3.97g†(~3mL)| Benzoin| | C14H12O2| 1.0| 212.24| N/A| 0.00476 *| 134-138| 1.0067| Sulfuric Acid| | H2SO4| 50mL| 98.08| 1.84| 0.9380| ~290| 92g (50mL)| 1,2-diphenyl-1,2-propanediol| | C15H16O2| 1.086‡| 228.28| N/A| 0.00476‡| | 1.086‡| * Limiting Reagant

† Theoretical yield from Part A, assumes 100% recovery
‡ Theoretical Yield, assumes 100% recovery
=n1,2-diphenyl-1,2-propanediol = nLimiting reagent = nbenzoin =0.00476 mols
=1.086g

Procedure:
Please refer to the 2012 Chem 2283 Lab Manual.[2] Please note that due to time constraints, the crude product was only recrystallized once

Results:
a. Yield
Crude Yield: 0.7427g

% yield = ExperimentalTheoretical x 100
= 0.7427g1.086g x 100
= 68.39%

Recrystallized Yield:0.2003g

% yield = ExperimentalTheoretical x 100
= 0.2003g1.086g x 100
= 18.44%

b. Physical Properties
1. Appearance
Grignard Reagant
* Cloudy, white, translucent liquid
Crude Product:
* Orange, dark brown, clumped crystalline solid, small crystals Recrystallized Product”
* Fine, pale, creamy orange, small crystals, powder

2. Melting Points

Table 3. Melting Point Data for Products
Purity| Crude| Recrystallized| Literature Values for 1,2-diphenyl-1,2-propanediol| Melting Point (°C)| 76-86| 112-130| (R,R): 94| (R,S): 104|

3. Thin Layer Chromatography (TLC)

Crude Product
Chemical| Benzoin| Mixture| Crude|
Distance (cm)| 2.6| A: 3.75| A: 3.75|
| | B: 2.6| B: 2.6|
| | C: 2.0| C: 2.0|
Rf Value| 0.59| A: 0.85 | A: 0.85 |
| | B: 0.59| B: 0.59|
| | C: 0.45| C: 0.45|
2.0 cm
2.0 cm
3.75cm
3.75cm
2.6 cm
2.6 cm
2.6 cm
B M C
Solvent Front: 4.4 cm

Recrystallized Product

Chemical| Benzoin| Mixture| Product|
Distance (cm)| 1.7...
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