Transcript 10.6 Carboxylic Acids Learning Outcomes (a) describe the

10.6 Carboxylic Acids
Learning Outcomes
(a) describe the formation of carboxylic acids from alcohols,
aldehydes and nitriles
(b) describe the reactions of carboxylic acids in the formation of
(i) salts
(ii) esters
Describe the formation of carboxylic acids from alcohols and aldehydes
You need to use excess of the oxidizing agent and make sure the
aldehyde formed half-way stays in the reaction mixture. Again we see a colour
change in the chromate when the alcohol is oxidized to the aldehyde and/ or
the aldehyde is oxidized to the acid. So both types of compounds can be used
to begin with in concentrated acids and put in reflux apparatus with chromate
(VII). Concentrated HCl is preferred.
Making Carboxylic Acids by hydrolysing nitriles
1. Prepare nitriles
Nucleophilic substitution
Nucleophilic addition
Two ways to do the hydrolysis:
Acid hydrolysis
Alkaline hydrolysis
Acid hydrolysis
The nitrile is heated under reflux with a dilute acid such as dilute
hydrochloric acid. A carboxylic acid is formed. For example, starting from
ethanenitrile you would get ethanoic acid. The ethanoic acid could be
distilled off the mixture.
Hydroxynitriles (cyanohydrins can also be used) RCH(CN)OH.
The carboxylic acid is formed via the amide, but this is not isolated.
RCH2CN
H+
RCONH2
H+
RCOOH
Alkaline hydrolysis
The nitrile is heated under reflux with an alkali such as sodium hydroxide
solution.
This time you wouldn't, of course, get a carboxylic acid produced - any acid
formed would react with the sodium hydroxide present to give a salt. You also
wouldn't get ammonium ions because they would react with sodium hydroxide to
produce ammonia.
Acidification of this mixture would produce the carboxylic acid
Simple reactions of Carboxylic acids
Carboxylic acids are weak acids, as they ionize to a small degree in
water; pH about 2-3.
Carboxylate ions, e.g. sodium ethanoate stabilized by delocalisation of
negative charge over two oxygen atoms making it more spread out and
energetically more stable.