740 CHAPTER 18 Carbonyl Compounds II
Tertiary alcohols are also formed from the reaction of two equivalents of a Grignard
reagent with an acyl halide.In theory, we should be able to stop this reaction at the ketone stage because a ketone
is less reactive than an acyl halide. However, the Grignard reagent is so reactive that it
can be prevented from reacting with the ketone only under very carefully controlled
conditions. There are better ways to synthesize ketones (Appendix IV).PROBLEM 7What product would be obtained from the reaction of one equivalent of a carboxylic acid
with one equivalent of a Grignard reagent?PROBLEM 8 SOLVEDa. Which of the following tertiary alcohols cannot be prepared from the reaction of an
ester with excess Grignard reagent?b. For those alcohols that can be prepared by the reaction of an ester with excess Grignard
reagent, what ester and what Grignard reagent should be used?SOLUTION TO 8a A tertiary alcohol is obtained from the reaction of an ester with two
equivalents of a Grignard reagent. Therefore, tertiary alcohols prepared in this way must
have two identical substituents on the carbon to which the OH is bonded, because twoOHCH 3COHCH 3CH 3 CH 2 CCH 2 CH 3OHCH 3CH 3 CCH 2 CH 3OHCH 2 CH 3CH 3 CCH 2 CH 2 CH 2 CH 3OHCH 3CH 3 CH 2 CCH 2 CH 2 CH 3OHCH 3CH 3 CCH 3butyryl chloride
3-ethyl-3-hexanol- 2 CH 3 CH 2 MgBr
CH 2.H^3 O+
3 CH 2 CH 2 Cl
OHCH 3 CH 2 CH 2 CCH 2 CH 3CH 2 CH 3COMovie:
Reaction of a Grignard
reagent with an esterH 3 O++an ester a ketonea tertiary alcoholCH 3 MgBr + CH 3 O−OCH 3MgBrCH 3 CH 2 C OCH 3− +OCH 3MgBrCH 3 CH 2 CCH 3OHCH 3CH 3 CH 2 CCH 3− +CH 3 MgBrmechanism for the reaction of an ester with a Grignard reagentCOCH 3 CH 2 OCH 3COCH 3 CH 2 CH 3product of
nucleophilic
additionproduct of
nucleophilic acyl
substitutionTutorial:
Grignard reagents in synthesis