Grignard reaction with bromobenzene and magnesium

Grignard reagent THE GRIGNARD REAGENT-PREPARATION AND REACTION In this experiment, you will prepare a Grignard reagent, phenyl magnesium bromide, from bromobenzene and magnesium metal. The Grignard reagent will then react with methyl benzoate to form triphenylmethanol. Introduction Grignard reagents, such as organomagnesium halides, were discovered in 1910 by French chemist Victor Grignard. The Grignard reaction is one of the most general methods for carbon- carbon bond formation in all of organic chemistry. In the first stage of this procedure, an organic halide reacts with magnesium metal to form an organomagnesium compound, which is known as a Grignard reagent. It is important to recognize that this is yet another example of an organic redox reaction. More specifically, the carbon atom that was initially bonded to the halide is reduced by two electrons, and the magnesium is oxidized by two electrons. By forming the Grignard reagent, we have effectively reversed the polarity of the bond to carbon. Since carbon has a higher electronegativity than the metal magnesium, the C-Mg sigma bond is highly polarized, placing a partial negative charge on carbon. As a result, Grignard reagents act as carbanion equivalents, and they are both strong nucleophiles and strong bases. This means that great care must be taken to exclude even relatively weak acids (such as water or alcohols) from the reaction mixture in order to avoid the formation of byproducts arising from proton transfer. Grignard reagents are almost always synthesized in ethereal solvents such as diethyl ether or tetrahydrofuran (THF) for two very important reasons: (1) ethers do not generally react with Grignard reagents (i.e. they are stable to strong bases and nucleophiles) and (2) the lone pairs on oxygen help to stabilize the partial positive charge on magnesium and facilitate formation of the Grignard reagent. Once formed, Grignard reagents can react with a wide variety of carbonyl-containing compounds to form new carbon-carbon bonds in a general process known as nucleophilic addition. The C-O bond is highly polarized, making carbonyl compounds electrophilic at carbon. If the Grignard reagent reacts with an aldehyde, ketone or ester, the ultimate product is an alcohol. Overall Reaction Me PartA Part B Pre-lab questions (15 points) Show the mechanism of the reaction of bromobenzene with magnesium metal in anhydrous diethyl ether 1. 2. There are many inert solvents that could be used but diethyl ether was chosen because it can assist the preparation of a Grignard reagent. How so?
Grignard reagent 56 3. The reaction to form a Grignard (and use it) requires "anhydrous" conditions. List one thing that is done before this lab that ensures "anhydrous" conditions: 4. Why do we use anhydrous diethyl ether in Part A of this reaction? Why does the procedure call for dry glassware? Think about what would happen if water was present. Write equation. 5. How many equivalents of Grignard reagent are used in this reaction compared to the limiting reagent? 6. What is the fate of the methoxy group from the methyl benzoate in this reaction? 7. Draw the setup of a simple distillation apparatus. Include the names of all glassware. 8. What is the molecular weight and melting point of triphenylmethanol? Chemicals Week 1: Mg turnings (1.2 g), anhydrous diethyl ether (-40 mL.), bromobenzene (4.7 mL), 1,2- dibromoethane (1 drop), methyl benzoate (2.5 mL), 6M sulfuric acid (25 mL) Week 2: MTBE (-20 ml), 3M sulfuric acid (25 mL), 80% Ethanol in water (-20 mL) Procedure During week one you will generate the Grignard B). During the second week, you will protonate the alkoxide ion, isolate the alcohol, and purify reagent (Part A) and react it with the ester (Part the alcohol by recrystallization. Week 1: Part A- Preparation of the Grignard reagent: phenylmagnesium bromide. Br MgBr phenylmagnesium bromide aka phenyl Grignard Make sure that all glassware is clean and free of water. Impurities and water will interfere with the reaction. All glassware (Claisen adapter, condenser, addition funnel, and RB flask) must be dried in an oven overnight. You can ensure the removal of water from the glassware by rinsing it with acetone and then hexane and allow the solvent to evaporate. Use anhydrous diethyl ether that was dried using molecular sieves for one week. A coat of magnesium oxide may be formed on the top of magnesium turnings that have been exposed to air. This oxide film will prevent the magnesium from being in contact with the bromobenzene. See step three below on how to overcome this setback