Question

How many equivalents of CH₃MgBr are required to make 2-methylpropan-2-ol from ethyl ethanoate?

Answer 1

Hint: To convert ethyl ethanoate into 2-methylpropan-2-ol, two extra C-C bonds must be created. Therefore, a Grignard’s reagent such as \[C{H_3}MgBr\] is used.

Complete Step by Step Solution:
In this question, we are asked the number of equivalents\[C{H_3}MgBr\]required to convert ethyl ethanoate (more commonly known as ethyl acetate) into 2-methylpropan-2-ol. So, let’s look at the conversion itself first.

Image: Conversion of ethyl acetate to 2-methylpropan-2-ol

It can be seen above that 2-methylpropan-2-ol has three carbon atoms attached to the blue-coloured carbon atom while ethyl acetate has only one. Thus, we must use a reagent that can construct C-C bonds in esters. Grignard’s reagents are fit for this purpose. Also, notice that the groups attached to the blue-coloured carbon atom are methyl groups (\[ - C{H_3}\]). So, we require a Grignard’s reagent that can supply methyl groups as well as form C-C bonds in esters. Therefore, methyl magnesium bromide (\[C{H_3}MgBr\]) is used as Grignard’s reagent here.

Grignard’s reagents have the general formula \[R - MgBr\]in which the R group may be a methyl group, ethyl group, propyl group etc. Since magnesium is a metal, it possesses a partial positive charge in Grignard’s reagent. To balance this partial positive charge, the R group carries a partial negative charge and thus, behaves like a nucleophile (\[{R^ - }\]).

When ethyl acetate reacts with one equivalent \[C{H_3}MgBr\], the \[C{H_3}\] group acts as a nucleophile and attacks the electron-deficient carbonyl carbon. The \[ - OC{H_2}C{H_3}\]group leaves and the product formed is acetone.


Image: Reaction of Ethyl Acetate with equation (1) with CH₃MgBr
Acetone must be reacted with another equivalent\[C{H_3}MgBr\]to obtain 2-methylpropan-2-ol. The reaction proceeds comparably as shown earlier except, unlike the previous reaction, acid hydrolysis yields alcohol instead of a ketone.


Image: Formation of 2-methylpropan-2-ol
Thus, to convert ethyl ethanoate to 2-methylpropan-2-ol, two equivalents \[C{H_3}MgBr\]must be used

Note: The base catalysis of an ester is a reaction irreversible as the end products are alcohol and carboxylate ion which is resonance stabilised and thus stable and does not react with alcohol.