Names, preparation and physical properties of carboxylic acids

A familiar group

A carboxylic acid has the group $\text{-}\text{COOH}$, called the carboxyl group. The name joins carbonyl and hydroxyl. The carbon is $sp^2$, flat, and bonded to three atoms at angles close to $120^\circ$. One of them is a carbonyl oxygen and one is an $\text{OH}$ group.

You know some of them already. Vinegar has ethanoic acid. Ants and nettles have methanoic acid. Curd has lactic acid. A lemon has citric acid. Benzoic acid keeps food from spoiling.

Names

1. The carboxyl carbon is carbon 1 of the chain, so it needs no number. Replace the final -e of the alkane by -oic acid: ethanoic acid, 3-methylbutanoic acid.
2. An acid with two carboxyl groups ends in -dioic acid: ethanedioic acid, hexanedioic acid. The final -e of the alkane stays.
3. When the carboxyl group is on a ring, the name ends in -carboxylic acid: cyclohexanecarboxylic acid. The simplest is benzoic acid, also called benzenecarboxylic acid.
4. If a carboxyl group is present, it takes priority over aldehydes, ketones and alcohols. So $\text{HOCH}_{2}\text{COOH}$ is hydroxyethanoic acid.

The common names are in daily use, and you must know them:

• $\text{HCOOH}$: formic acid, IUPAC methanoic acid.
• $\text{CH}_{3}\text{COOH}$: acetic acid, ethanoic acid.
• $\text{CH}_{3}\text{CH}_{2}\text{COOH}$: propionic acid, propanoic acid.
• $\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{COOH}$: butyric acid, butanoic acid.
• $\text{HOOC}\text{-}\text{COOH}$: oxalic acid, ethanedioic acid.
• $\text{HOOC}\text{-}\text{CH}_{2}\text{-}\text{COOH}$: malonic acid, propanedioic acid.
• $\text{HOOC}\text{-}(\text{CH}_{2})_{2}\text{-}\text{COOH}$: succinic acid, butanedioic acid.
• $\text{HOOC}\text{-}(\text{CH}_{2})_{4}\text{-}\text{COOH}$: adipic acid, hexanedioic acid.
• $\text{C}_{6}\text{H}_{5}\text{COOH}$: benzoic acid.

How they are made

1. Oxidation of primary alcohols and aldehydes. Potassium permanganate (acidic, alkaline or neutral) or acidified potassium dichromate takes a primary alcohol or an aldehyde to the acid with the same number of carbons.

$\text{CH}_{3}\text{CH}_{2}\text{OH} \xrightarrow{\text{KMnO}_{4}\ \text{or}\ \text{K}_{2}\text{Cr}_{2}\text{O}_{7}\ \text{H}+} \text{CH}_{3}\text{COOH}$

2. Oxidation of alkylbenzenes. Hot alkaline potassium permanganate, then acid, turns any alkyl side chain with at least one hydrogen on the carbon next to the ring into $\text{-}\text{COOH}$. Toluene, ethylbenzene and cumene all give benzoic acid. A tert-butyl group has no such hydrogen, so it is not attacked.

$\text{C}_{6}\text{H}_{5}\text{CH}_{3} \xrightarrow{1.\ \text{KMnO}_{4}\ \text{KOH}\ \text{heat}\ 2.\ \text{H}_{3}\text{O}+} \text{C}_{6}\text{H}_{5}\text{COOH}$

3. Hydrolysis of nitriles and amides. Heating a nitrile with dilute acid or alkali gives an amide first, then the acid. The acid has the same carbon count as the nitrile. If the nitrile came from an alkyl halide and cyanide, the acid has one more carbon than the halide.

$\text{RCN} + _{2}\text{H}_{2}\text{O} \xrightarrow{\text{H}_{3}\text{O}+\ \text{heat}} \text{RCOOH} + \text{NH}_{4}^{+}$

4. Grignard reagent and carbon dioxide. The Grignard reagent adds to solid carbon dioxide (dry ice). Dilute acid then gives the acid. It has one more carbon than the Grignard reagent.

$\text{RMgX} + \text{CO}_{2} \longrightarrow \text{RCOOMgX} \xrightarrow{\text{H}_{3}\text{O}+} \text{RCOOH}$

5. From acid derivatives. Acyl chlorides and anhydrides react with water. Esters are hydrolysed by dilute acid, or by alkali and then acid. The alkali route is called saponification when the ester is a fat.

Industrial routes are worth knowing. Methanoic acid comes from carbon monoxide and sodium hydroxide at high pressure, which gives sodium methanoate. Ethanoic acid comes from the carbonylation of methanol, and vinegar comes from the fermentation of ethanol by bacteria.

Why the molecules pair up

A carboxylic acid has both a hydrogen bond donor (the O-H) and a hydrogen bond acceptor (the C=O). Two molecules meet and form two hydrogen bonds, making a ring of eight atoms. The pair is a dimer. It exists in the pure liquid, in the vapour and in non-polar solvents such as benzene.

The effects are easy to see.

• Boiling points are higher than those of alcohols with the same molar mass. Ethanoic acid boils at $118\ ^\circ\text{C}$, and propan-1-ol at $97\ ^\circ\text{C}$.
• In benzene, a freezing-point measurement gives a molar mass near double the true value for a lower acid. The molecules are pairs.
• The first four members mix with water in every ratio, because they form hydrogen bonds with water. Above that, the hydrocarbon chain makes the acid less soluble. Benzoic acid is only slightly soluble in cold water, and dissolves well in hot water.

Move from 5 carbons to 10. Each extra carbon lowers the solubility, and the measured dots fall almost along a line on the log scale. The long carbon chain does not suit water.

Practise with new numbers

Try it in Python

The program takes four side chains on a benzene ring and says what hot alkaline permanganate does to each. It uses the one rule that matters: is there a hydrogen on the carbon next to the ring?

Common mistakes

• Numbering the carboxyl carbon with something other than 1. It is always carbon 1.
• Saying that the acid made from a Grignard reagent has the same carbons. It has one more.
• Oxidising tert-butylbenzene with permanganate and expecting benzoic acid. There is no benzylic hydrogen.
• Thinking that the dimer is a covalent compound. It is two molecules held by hydrogen bonds.

Remember these

• Carbon 1 is the carboxyl carbon. -oic acid, -dioic acid, -carboxylic acid.
• Oxidation (primary alcohol, aldehyde, side chain), hydrolysis (nitrile, amide, ester) and Grignard with CO$_2$ make acids.
• A pair of hydrogen bonds makes a dimer. So acids boil higher than alcohols of similar molar mass.

Check yourself

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

1. Name $\text{CH}_{3}\text{CH}(\text{CH}_{3})\text{CH}_{2}\text{COOH}$.

2. A Grignard reagent $\text{C}_{2}\text{H}_{5}\text{MgBr}$ is poured over dry ice and then acidified. What is the product?

3. Why does ethanoic acid boil at a higher temperature than propan-1-ol?

4. What does hot alkaline KMnO$_4$, then acid, do to ethylbenzene?

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 sections on carboxylic acids: nomenclature, preparation and physical properties.