In other words, the problem here lies in the fact that the ketone product is more reactive than the ester starting material. We shall meet more examples of this general problem later (in Chapter 23, for example): in the next section we shall look at ways of overcoming it. Meanwhile, why not see it as a useful reaction? This compound, for example, was needed by some chemists in the course of research into explosives. It is a tertiary alcohol with the hydroxyl group flanked by two identical R (= butyl) groups. The chemists who wanted to make the compound knew that an ester would react twice with the same organolithium reagent, so they made it from this unsaturated ester (known as methyl methacrylate) and butyllithium.

●Tertiary alcohol synthesis Tertiary alcohols with two identical R₂ groups can be made from ester R¹CO₂R plus two equivalents of organolithium R₂Li or Grignard reagent R2MgBr.

This reaction works in reduction too if we use lithium aluminium hydride, LiAlH₄. This is a powerful reducing agent that readily attacks the carbonyl group of an ester. Again, collapse of the tetrahedral intermediate gives a compound, this time an aldehyde, which is more reactive than the ester starting material, so a second reaction takes place and the ester is converted (reduced) into an alcohol. Sodium borohydride, often used for the reduction of ketones, does not usually reduce esters.

This is an extremely important reaction, and one of the best ways of making alcohols from esters. Stopping the reaction at the aldehyde stage is more difficult: we shall discuss this in Chapter 23.

مواضيع ذات صلة

Aldehydes can react with alcohols to form hemiacetals

Making the products less reactive

Making the starting materials more reactive

A bit of shorthand

Making ketones from esters: the problem

Acid chlorides can be made from carboxylic acids using SOCl2 or PCl5

Hydrolysing nitriles: how to make the almond extract, mandelic acid

Amides can be hydrolysed under acidic or basic conditions too

Base-catalysed hydrolysis of esters is irreversible

Acid-catalysed ester hydrolysis and transesterification

Ester formation is reversible: how to control an equilibrium

Acid catalysts can make bad leaving groups into good ones