Question

Briefly, explain why your 3.0 mL reaction vial and diethyl ether solvent need to be completely dry (free of water)? Support your argument with a reaction equation between your Grignard reagent and water. What does the water do to the phenyl magnesium bromide? Briefly, explain why 3 M HCI (aq.) = [H_3O^+/CI-] hydrochloric acid used instead of a base for the work-up procedure? What did you observe when a low pH was obtained? What was the purpose of NaHSO_3 (sodium bisulfite)?

Answer 1

1) The Grignard reaction is an organometallic chemical reaction in which alkyl, vinyl, or aryl-magnesium halides (Grignard reagents) add to a carbonyl group in an aldehyde or ketone.This reaction is an important tool for the formation of carbon–carbon bonds. The reaction of an organic halide with magnesium is not a Grignard reaction, but provides a Grignard reagent.

MgBr H/H2O он 2 R-MgBr AR R-R R-R

Reaction conditions:

In reactions involving Grignard reagents, it is important to exclude water and air, which rapidly destroy the reagent by protonolysis or oxidation. Since most Grignard reactions are conducted in anhydrous diethyl ether or tetrahydrofuran, side-reactions with air are limited by the protective blanket provided by solvent vapors. Small-scale or quantitative preparations should be conducted under nitrogen or argon atmospheres, using air-free techniques. Although the reagents still need to be dry, ultrasound can allow Grignard reagents to form in wet solvents by activating the magnesium such that it consumes the water.

2)

• The reaction mixture containing the Grignard reagent is allowed to warm to room temperature in a water bath to allow excess dry ice to evaporate
• Any remaining Grignard reagent is quenched by the addition of water
• Dilute hydrochloric acid is added to the reaction mixture to protonate the benzoate salts, as well as to dissolve the magnesium salts. White solids of impure benzoic acid are obtained.
• The benzoic acid is decanted to remove the aqueous solution of impurities, more water is added, and the mixture is brought to a boil with more water added to give a homogeneous solution.
• The solution is allowed to cool slowly to room temperature, then in an ice bath to recrystallize benzoic acid.
• The recrystallized benzoic acid crystals are collected on a Buchner funnel and are allowed to air-dry to give pure benzoic acid.