Reactions and uses of carboxylic acids

The reactions at a glance

The picture has one acid in the centre and seven arrows. We take them in three groups: reactions of the O-H, reactions at the carbonyl carbon, and reactions of the rest of the molecule.

The O-H group: salts

Carboxylic acids react with active metals, with bases and with carbonates. All three give the carboxylate salt.

$_{2}\text{RCOOH} + _{2}\text{Na} \longrightarrow _{2}\text{RCOONa} + \text{H}_{2}$

$\text{RCOOH} + \text{NaOH} \longrightarrow \text{RCOONa} + \text{H}_{2}\text{O}$

$\text{RCOOH} + \text{NaHCO}_{3} \longrightarrow \text{RCOONa} + \text{H}_{2}\text{O} + \text{CO}_{2}$

At the carbonyl carbon: replacing the OH

A nucleophile attacks the carbonyl carbon and the OH group leaves. This is nucleophilic acyl substitution. It is different from addition, because a group leaves.

Esterification. A carboxylic acid and an alcohol, with a little concentrated sulphuric acid as catalyst and heat, give an ester and water.

$\text{RCOOH} + \text{R}'\text{OH} \rightleftharpoons \text{RCOOR}' + \text{H}_{2}\text{O}$

The reaction is an equilibrium, with $K \approx 4$ for ethanoic acid and ethanol. To push it forward, use an excess of the alcohol or remove the water. Labelling the alcohol with oxygen-18 shows that the label ends up in the ester. So the acid loses its O-H, and the alcohol loses only its H.

Acid chlorides. Thionyl chloride, $\text{SOCl}_2$, is the best reagent, because the by-products are gases:

$\text{RCOOH} + \text{SOCl}_{2} \longrightarrow \text{RCOCl} + \text{SO}_{2} + \text{HCl}$

Phosphorus pentachloride gives $\text{POCl}_3$ and HCl. Phosphorus trichloride gives $\text{H}_3\text{PO}_3$, from $3\,\text{RCOOH} + \text{PCl}_3$.

Anhydrides. Heating two molecules of an acid with a dehydrating agent, such as phosphorus pentoxide, removes water.

$_{2}\text{RCOOH} \xrightarrow{\text{P}_{2}\text{O}_{5}\ \text{heat}} (\text{RCO})_{2}\text{O} + \text{H}_{2}\text{O}$

Succinic and phthalic acids form cyclic anhydrides on heating, because a ring of five atoms can close.

Amides. Ammonia makes the ammonium salt. Heating the salt removes water.

$\text{RCOOH} + \text{NH}_{3} \longrightarrow \text{RCOO}^{-} \text{NH}_{4}^{+} \xrightarrow{\text{heat}} \text{RCONH}_{2} + \text{H}_{2}\text{O}$

Reduction. Lithium aluminium hydride in dry ether, then dilute acid, reduces the acid to a primary alcohol. Diborane does so too, and it spares an ester group. Sodium borohydride does not reduce a carboxylic acid.

$\text{RCOOH} \xrightarrow{1.\ \text{LiAlH}_{4}\ 2.\ \text{H}_{3}\text{O}+} \text{RCH}_{2}\text{OH}$

With 1 mol of ethanol the ester is two thirds of the acid. With a tenfold excess it reaches about 97 %. The curve levels off, because water is also made.

Practise with new numbers

The rest of the molecule

Hell-Volhard-Zelinsky reaction. An acid with an alpha hydrogen reacts with bromine or chlorine and a little red phosphorus. The alpha hydrogen is replaced by the halogen.

$\text{CH}_{3}\text{CH}_{2}\text{COOH} \xrightarrow{1.\ \text{Br}_{2}\ \text{red}\ \text{P}\ 2.\ \text{H}_{2}\text{O}} \text{CH}_{3}\text{CHBrCOOH}$

The product is a 2-halo acid. It gives a 2-hydroxy acid with water and a 2-amino acid with ammonia. The method is a route to amino acids.

Decarboxylation. The sodium salt of an acid, heated with soda lime (a mixture of sodium hydroxide and calcium oxide), gives the alkane with one carbon fewer.

$\text{CH}_{3}\text{COONa} + \text{NaOH} \xrightarrow{\text{CaO}\ \text{heat}} \text{CH}_{4} + \text{Na}_{2}\text{CO}_{3}$

Kolbe electrolysis. The electrolysis of the sodium salt in water gives an alkane with twice the carbons of the alkyl group, carbon dioxide and hydrogen.

$_{2}\text{CH}_{3}\text{COO}^{-} \longrightarrow \text{CH}_{3}\text{CH}_{3} + _{2}\text{CO}_{2} + _{2}\text{e}^{-}$

The benzene ring. The carboxyl group deactivates the ring and directs to the meta position. Nitration of benzoic acid gives 3-nitrobenzoic acid. Friedel-Crafts reactions do not work with benzoic acid.

Where they are used

• Methanoic acid: coagulating rubber latex, leather and textile finishing, and descaling.
• Ethanoic acid: vinegar, a solvent, and making cellulose acetate and ethyl ethanoate.
• Benzoic acid and its sodium salt: food preservatives.
• Hexanedioic acid: making nylon-6,6.
• Esters: flavours, scents and solvents. Aspirin is made from 2-hydroxybenzoic acid and ethanoic anhydride.
• Long-chain acids: salts of fatty acids make soap.

Try it in Python

The program solves the esterification equilibrium for a range of amounts of alcohol. It uses the quadratic formula.

Common mistakes

• Expecting sodium borohydride to reduce an acid. Use lithium aluminium hydride.
• Thinking that esterification goes to completion. It is an equilibrium.
• Mixing up decarboxylation and Kolbe electrolysis. Soda lime gives one carbon fewer. Kolbe gives a doubled chain.
• Using the Hell-Volhard-Zelinsky reaction on an acid with no alpha hydrogen. Methanoic acid and benzoic acid cannot do it.
• Reading the oxygen label wrongly. The ester oxygen comes from the alcohol.

Remember these

• Acid, alcohol, conc. H$_2$SO$_4$: ester. SOCl$_2$: acid chloride. P$_2$O$_5$ and heat: anhydride. NH$_3$ then heat: amide.
• LiAlH$_4$ then H$_3$O$^+$: primary alcohol. Br$_2$ and red P: 2-bromo acid.
• Soda lime: the alkane with one carbon fewer. Kolbe: the alkane with double the carbons.
• The carboxyl group is meta-directing in the benzene ring.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is made when ethanoic acid is heated with ethanol and a little concentrated sulphuric acid?

2. Why is thionyl chloride the best reagent for making an acid chloride?

3. What does sodium propanoate give when it is heated with soda lime?

4. Which of methanoic acid and propanoic acid can take part in the Hell-Volhard-Zelinsky reaction?

References

• OpenStax, Chemistry 2e, Chapter 20, section 20.3 Aldehydes, Ketones, Carboxylic Acids, and Esters.
• NCERT, Chemistry Part II for Class XII, Chapter 8, the section on chemical reactions of carboxylic acids and the uses.