Substitution of the OR group of an ester by an R group would give us a ketone. You might therefore think that reaction of an ester with an organolithium or Grignard reagent would be a good way of making ketones. However, if we try the reaction, something else happens, as you saw at the start of this chapter.

Two molecules of Grignard have been incorporated and we get an alcohol! If we look at the mechanism, we can understand why this should be so. First, as you would expect, the nucleophilic Grignard reagent attacks the carbonyl group to give a tetrahedral intermediate. The only reasonable leaving group is RO−, so it leaves to give us the ketone we set out to make.

Now, the next molecule of Grignard reagent has a choice. It can react with either the ester starting material or the newly formed ketone. Ketones are more electrophilic than esters so the Grignard reagent prefers to react with the ketone in the manner you saw in Chapter 9. A stable alkoxide anion is formed, which gives the tertiary alcohol on acid work-up.

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 alcohols instead of ketones

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