You can’t make esters from carboxylic acids and alcohols under basic conditions because the base deprotonates the carboxylic acid (see p. 207). However, you can reverse that reaction and hydrolyse an ester to a carboxylic acid (more accurately, a carboxylate salt) and an alcohol.

This time the ester is, of course, not protonated fi rst as it would be in acid, but the unprotonated ester is a good enough electrophile because OH⁻, and not water, is the nucleophile. The tetrahedral intermediate can collapse either way, giving back ester or going forward to acid plus alcohol.

The backward reaction is impossible because the basic conditions straightaway deprotonate the acid to make a carboxylate salt (which, incidentally, consumes the base, making at least one equivalent of base necessary in the reaction). Carboxylate salts do not usually react with nucleophiles, even those a good deal stronger than alcohols.

How do we know this is the mechanism?

Ester hydrolysis is such an important reaction that chemists have spent a lot of time and effort finding out exactly how it works. Many of the experiments that tell us about the mechanism involve oxygen-18 labelling. The starting material is an ester enriched in the heavy oxygen isotope ¹⁸O. By knowing where the heavy oxygen atoms start off, and following (by mass spectrometry—Chapter 3) where they end up, the mechanism can be established.

One further labelling experiment showed that a tetrahedral intermediate must be formed: an ester labelled with ¹⁸O in its carbonyl oxygen atom passes some of its ¹⁸O label to the water. We discussed this on p. 201. There is more on the mechanism of ester hydrolysis in Chapter 12.

The saturated fatty acid tetradecanoic acid (also known as myristic acid) is manufactured commercially from coconut oil by hydrolysis in base. You may be surprised to learn that coconut oil contains more saturated fat than butter, lard, or beef dripping: much of it is the trimyristate ester of glycerol. Hydrolysis with aqueous sodium hydroxide, followed by reprotonation of the sodium carboxylate salt with acid, gives myristic acid. Notice how much longer it takes to hydrolyse this branched ester than it did to hydrolyse a methyl ester (p. 210).

Saponification

The alkaline hydrolysis of esters to give carboxylate salts is known as saponification because it is the process used to make soap. Traditionally, beef tallow (the Tri stearate ester of glycerol—stearic acid is octadecanoic acid, C₁₇H₃₅CO₂H) was hydrolysed with sodium hydroxide to give sodium stearate, C₁₇H₃₅CO₂Na, the principal component of soap. Finer soaps are made from palm oil and contain a higher proportion of sodium palmitate, C₁₅H₃₁CO₂Na. Hydrolysis with KOH gives potassium carboxylates, which are used in liquid soaps. Soaps like these owe their detergent properties to the combination of polar (carboxylate group) and non-polar (long alkyl chain) properties.

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

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

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