Reactions
Alternatively, the reflux is done with aqueous alkali. The salt of the carboxylic acid is
produced. This latter process is called ’saponification’ because when fats are hydrolysed in
this way, their salts are useful as soap.
90.3 Anhydrides
See acid anhydride^1.
90.4 Amides
Conceptually, an amide is formed by reacting an acid (an electrophile^2 ) with an amine
compound (a nucleophile^3 ), releasing water.
RCOOH + H 2 NR’→RCONHR’ + H 2 O
However, the acid-base reaction is much faster, which yields the non-electrophilic carboxy-
late and the non-nucleophilic ammonium, and no further reaction takes place.
RCOOH + H 2 NR’→RCOO-+ H 3 NR’+
To get around this, a variety of coupling reagents have been developed that first react
with the acid or carboxylate to form an active acyl compound, which is basic enough to
deprotonate an ammonium and electrophilic enough to react with the free base of the amine.
A common coupling agent is dicyclohexylcarbodiimide^4 , or DCC, which is very toxic.
90.5 Acid Decarboxylation
On heating with sodalime (NaOH/CaO solid mix) carboxylic acids lose their –COOH group
and produce a small alkane plus sodium carbonate:
CH 3 CH 2 COOH + 2 NaOH→CH 3 CH 3 + Na 2 CO 3 + H 2 O
Note how a carbon is lost from the main chain. The product of the reaction may be easier
to identify than the original acid, helping us to find the structure.
90.6 Ethanoic anhydride
Industrially, ethanoic anhydride is used as a less costly and reactive alternative to ethanoyl
chloride. It forms esters and can be hydrolysed in very similar ways, but yields a second
ethanoic acid molecule, not HCl The structure is formed from two ethanoic acid molecules...
1 https://en.wikipedia.org/wiki/Acid%20anhydride
2 http://en.wikipedia.org/wiki/Electrophile
3 http://en.wikipedia.org/wiki/Nucleophile
4 http://en.wikipedia.org/wiki/Dicyclohexylcarbodiimide