Question

How would you prepare 2-methyl-2-propanol via a grignard with dimethyl carbonate as your carbonyl source? show all reagents?

Answer 1

Answer:

React it with CH₃MgBr and work up the product with saturated ammonium chloride solution

Explanation:

Grignard reagents convert esters into tertiary alcohols.

The general equation is

$\text{RCOOR}' \xrightarrow[\text{2. H}^{+}]{\text{1. R ^{\prime \prime} MgBr}}\text{RR _{2}^{\prime \prime} C-OH}$

The Grignard reagent in this synthesis is methylmagnesium bromide. You prepare it by reacting a solution methyl bromide in anhydrous ether with magnesium and a few crystals of iodine.

The reaction consumes 3 mol of CH₃MgBr per mole of dimethyl carbonate, and everything happens in the same pot.

Acid workup of the product usually involves the addition of a saturated aqueous solution of ammonium chloride and extraction with a low-boiling organic solvent.

The mechanism involves:

Step 1. Nucleophilic attack and loss of leaving group

(a) The Grignard reagent attacks the carbonyl of dimethyl carbonate, followed by (b) the loss of a methoxide leaving group.

Step 2. Nucleophilic attack and loss of leaving group

(a) A second mole of the Grignard reagent attacks the carbonyl of methyl acetate, followed by (b) the loss of a methoxide leaving group.

Step 3. Nucleophilic attack and protonation of the adduct.  

(a) A third mole of the Grignard reagent attacks the carbonyl of acetone, followed by (b) protonation of the alkoxide to form 2-methylpropan-2-ol.