Grignard Reaction Lab Report

Topics: Diethyl ether, Magnesium, Solvent Pages: 8 (2774 words) Published: April 19, 2012
The Grignard Reaction

Through the use of the Grignard reaction, a carbon-carbon bond was formed, thereby resulting in the formation of triphenylmethanol from phenyl magnesium bromide and benzophenone. A recrystallization was performed to purify the Grignard product by dissolving the product in methanol. From here, a melting point range of 147.0 °C to 150.8 °C was obtained. The purified product yielded an IR spectrum with major peaks of 3471.82 cm-1, 3060.90 cm-1, 1597.38 cm-1, and 1489.64 cm-1, which helped to testify whether the identity of the product matched the expected triphenylmethanol. The identity of the product being correct was further confirmed by way of both proton and carbon-13 NMR spectra. This is due to the fact that both spectra yielded peaks that contained the main structural residues, alcohol and aromatic groups, of triphenylmethanol. The final yield of pure triphenylmethanol was 8.04 grams, resulting in a percentage recovery of 67.7%. Introduction

When faced with the task of needing to form a new carbon-carbon bond in organic chemistry, chemists often turn towards organometallic reactions. One particular reaction, the Grignard reaction, allows for a magnesium halide to add to a carbonyl group at the double-bonded oxygen, thus resulting in the converting of the carbonyl containing molecule into an alcohol, as observed in Mechanism 1. However, the carbonyl compound must be chosen carefully, for different carbonyls yield different types of alcohols. Because of the structure of the carbonyl compound, either a primary, secondary, or tertiary alcohol may be synthesized. This is seen in Figure 1.

But before the actual Grignard reaction occurs, another mechanism must be taken into consideration – the formation of the Grignard reagent. The Grignard reagent is synthesized by adding an organic halide, alkyl or aryl, to magnesium in an ethereal solvent. Common halides used are iodides and bromides, although occasionally chlorides may also be used. Fluorides are generally never seen due to their relatively unreactive nature. As seen in Mechanism 2, a single electron transfer occurs as both the halide and the magnesium donate an electron to form a magnesium halide. Then, a second single electron transfer occurs through the two radicals. Common solvents used to create the Grignard reagent include diethyl ether, tetrahydrofuran (THF), and 1,4-dioxane. Such ethereal solvents are used because the oxygen of the ether group is able to provide ligands to help stabilize the Grignard reagent, as shown in Figure 2.

Reagent Table|
Name/Structure| MW (g/mol)| Amt. Used (mL or g and # of mol)| Concentration| Density (g/mL)| Mpt./Bpt.| Safety| MagnesiumMg| 24.31| 1.53 g0.0629 mol| N/A| 1.74| 648.0 °C1,090.0 °C| Flammable; Skin/eye irritant; May be harmful if swallowed/inhaled/absorbed| IodineI-I| 253.81| N/A| N/A| 4.93| 113.0 °C184.0 °C| Causes skin/eye burns; Toxic if inhaled; May be harmful if swallowed| Anhydrous ethyl ether (diethyl ether)| 74.12| 205.0 mL1.95 mol| N/A| 0.706| -116.0 °C34.6 °C| Skin/eye irritant; May be harmful if swallowed/inhaled; Highly flammable| Water| 18.02| N/A| N/A| 1.00| 0.0 °C100.0 °C| May cause irritation to skin/eyes; May be harmful if swallowed/inhaled| Benzophenone | 182.22| 9.1 g0.050 mol| N/A| N/A| 47.0-51.0 °C305.0 °C| Skin/eye irritant; May be harmful if swallowed/inhaled/absorbed| Sulfuric acid| 98.08| 4.5 mL0.084 mol| N/A| 1.84| 3.0 °C290.0 °C| Causes eye burns; May be harmful if swallowed/inhaled/absorbed| Bromobenzene| 157.01| 5.3 mL0.050 mol| N/A| 1.49| -31.0 °C156.0 °C| May cause eye irritation; May be harmful if swallowed/inhaled/absorbed| Methanol| 32.04| N/A| N/A| 0.791| -98.0 °C64.7 °C| Highly flammable; Eye/skin irritant; Toxic if swallowed/inhaled/absorbed|


1.5 g of magnesium (Mg) turnings were collected into a dry 250 mL round bottom flask and stirred with a...
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