TOP

Carboxylic Acids and Derivatives

Key Terminology
Term Definition
Carboxylic Acid Organic compound with the group COOH, formed by oxidation of aldehydes
Ester Organic compound made by replacing the hydrogen of an acid by an alkyl group
Glycerol Propane-1,2,3-triol; a component of triglycerides
Biodiesel A mixture of methyl esters of long-chain carboxylic acids used as a renewable fuel
Soap Sodium salts of long-chain carboxylic acids which act as cleaning agents
Acid Derivatives Compounds containing an acyl group (RCO), such as acid chlorides and anhydrides
• Carboxylic acids have the functional group COOH, where the OH group is acidic. Carboxylic acids
are weak acids but will liberate CO2 from carbonates. The salts formed have the name carboxylates.
The carboxylate ion is stable as the negative charge is delocalised across the entire COOgroup.
• In the presence of an acid catalyst, carboxylic acids react with alcohols to form esters. Esters have
the general formula RCOOR’, and are named based on the parent acid, with an alkyl group which
originated from the alcohol e.g. methyl ethanoate is formed from methanol and ethanoic acid. The
acid parent will appear next to the CO bond.
• Short chain esters are volatile and have pleasant fruit smells and so are used in flavouring and
perfumes. Esters are also used as solvents and plasticisers.
Esters can be hydrolysed in acid to form alcohols and carboxylic acids. Basic NaOH also causes the
hydrolysis of esters, but produces sodium salts of the carboxylic acid instead.
RCOOR’ ⇌ RCOOH + R’OH RCOOH +NaOH → Na+RCOO- + H2O + R’OH
Vegetable oils and animal fats are esters of glycerol. Vegetable oils and animal fats are hydrolysed in
alkaline conditions, by addition of sodium hydroxide, to give soap (salts of long-chain carboxylic acids)
and glycerol (propane-1,2,3-triol). Glycerol is used in pharmaceuticals and cosmetics due to its ability
to attract water.
Triglyceride + 3NaOH → Glycerol + 3 Soap (Na+RCOO-
)
Biodiesel is a renewable fuel made from oil from crops such as rapeseed. Biodiesel is a mixture of methyl
esters of long-chain carboxylic acids. It is produced by reacting oils with methanol in the presence of a
KOH catalyst.
Triglyceride + 3CH3OH → Glycerol + 3 Biodiesel (RCOOCH3)
Acylation
Acylation is the addition of an RCO acyl group into another molecule. This is done by reacting acid
derivatives with other molecules. Two examples are acid chlorides and acid anhydrides.
Acid chlorides have
general formula RCOCl and
are more reactive than
acid anhydrides
Acid anhydrides, formula
RCOOCOR’ are produced
by reacting two carboxylic
acids together.
Despite acid chlorides being more reactive, acid anhydrides are often used in industry. For example,
salicylic acid is reacted with ethanoic anhydride to form aspirin. Anhydrides are used because:
• Cheaper • Does not react with water
• Safer, as the byproduct is a carboxylic acid
rather than fumes of HCl
• Less corrosive
The following acylation mechanisms are nucleophilic addition-elimination reactions. The nucleophile
used is in bold

Core Practical 9: Preparation of a) a pure organic solid and test of its purity; b) a
pure organic liquid.
Aspirin (2-ethanoylhydroxybenzoic acid) is an analgesic (pain killer), and is made using salicylic acid.
1. Working in a fume cupboard, swirl 2g of 2-hydroxybenzoic acid (salicylic acid)
with 4cm3 of ethanoic anhydride in a conical flask
2. Add five drops of concentrated sulfuric acid, which acts as a catalyst. Agitate
the flask for 10 minutes. Crystals of aspirin will appear and soon the whole
solution will form a crystalline mixture
3. Stir in 4cm3 of cold ethanoic acid to dilute the mixture and cool by placing in a
water bath containing crushed ice. Add distilled water
4. Filter off the crystals using a Hirsch funnel by vacuum filtration, washing once with cold water
5. Dry the purified sample on filter paper and weigh it
6. Purity can be tested by TLC (see 3.3.16), or melting point determination (1°C range for pure)
An example of an organic liquid is propanal, produced from propan-1-ol
1. Set up simple distillation apparatus, including anti-bumping granules – this ensures smooth boiling
2. Pour acidified potassium dichromate into a pear-shaped flask. Add propan-1-ol and mix contents
3. Heat the flask gently to slowly distil off the liquid distillate into a conical flask immersed ice to
prevent loss of the volatile aldehyde produced
4. Use the Tollen’s reagent test to confirm that an aldehyde has been produced

Comments are closed.