We have looked at a whole series of interconversions between carboxylic acid derivatives and, after this next section, we shall summarize what you need to understand. We said that it is always easy to move down the series of acid derivatives we listed early in the chapter, and so far, that is all we have done. But some reactions of carboxylic acids also enable us to move upwards in the series. What we need is a reagent that changes the bad leaving group HO− into a good leaving group. Strong acid does this by protonating the OH−, allowing it to leave as H₂O. In this section we look at two more reagents, SOCl₂ and PCl₅, which convert the OH group of a carboxylic acid and also turn it into a good leaving group. Thionyl chloride, SOCl₂, reacts with carboxylic acids to make acyl chlorides.

This volatile liquid with a choking smell is electrophilic at the sulfur atom (as you might expect with two chlorine atoms and an oxygen atom attached) and is attacked by carboxylic acids to give an unstable, and highly electrophilic, intermediate.

Reprotonation of the unstable intermediate (by the HCl just produced, i.e. reversal of the last step above) gives an electrophile powerful enough to react even with the weak nucleophile Cl⁻ (HCl is a strong acid, so Cl⁻ is a poor nucleophile). The tetrahedral intermediate collapses to the acyl chloride, sulfur dioxide, and hydrogen chloride. This step is irreversible because SO₂ and HCl are gases that are lost from the reaction mixture.

Although HCl is involved in this reaction, it cannot be used as the sole reagent for making acid chlorides. It is necessary to have a sulfur or phosphorus compound to remove the oxygen. An alternative reagent for converting RCO₂H into RCOCl is phosphorus pentachloride, PCl₅. The mechanism is similar—try writing it out before looking at the scheme below.

The mechanism is closely related to the previous one, except that the formation of a very stable P=O bond is the vital factor rather than the loss of two gaseous reagents.

These conversions of acids into acid chlorides complete all the methods we need to convert acids into any acid derivatives. You can convert acids directly to esters and now to acid chlorides, the most reactive of acid derivatives, and can make any other derivative from them. The chart below adds reaction conditions, relevant pKas, and infrared stretching frequencies to the reactivity order we met earlier.

●Interconversion of carboxylic acid derivatives

All these acid derivatives can, of course, be hydrolysed to the acid itself with water alone or with various levels of acid or base catalysis depending on the reactivity of the derivative.

To climb the reactivity order therefore, the simplest method is to hydrolyse to the acid and convert the acid into the acid chloride. You are now at the top of the reactivity order and can go down to whatever level you require.

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

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