Carboxylic Acids: Quick Revision

A brief note on Carboxylic Acids. Read it before a test. Try each question at the end before you open its answer.

Names and structure

• The carboxyl group $\text{-}\text{COOH}$ has a flat $sp^2$ carbon. Its carbon is carbon 1. Names end in -oic acid, -dioic acid or -carboxylic acid.
• Common names: formic, acetic, propionic, butyric, oxalic, malonic, succinic, adipic and benzoic acid.

Preparation

• Oxidation of primary alcohols and aldehydes. Hot alkaline KMnO$_4$, then acid, on an alkylbenzene (a benzylic hydrogen is needed).
• Hydrolysis of nitriles, amides and esters. RMgX and CO$_2$, then dilute acid, gives an acid with one more carbon.

Physical properties

• Two hydrogen bonds join two molecules in a dimer. So the boiling point is higher than that of an alcohol of similar molar mass, and the molar mass in benzene is near double.
• The first four members mix with water. The solubility falls as the chain grows.

Acidity

• $K_a = 10^{-\text{p}K_a}$. A smaller pKa is a stronger acid. The carboxylate ion shares its charge between two oxygens.
• Withdrawing groups (Cl, F, NO$_2$) strengthen the acid, and the effect falls with distance. Alkyl groups weaken it.
• Carboxylic acids give CO$_2$ with NaHCO$_3$. Phenols and alcohols do not.

Reactions

• ROH and conc. H$_2$SO$_4$: ester (an equilibrium). 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: alkane with one carbon fewer. Kolbe electrolysis: R-R. The carboxyl group directs meta.

A worked example

Find the pH of a $0.10\ \text{mol/L}$ solution of an acid with $\text{p}K_a = 4.76$.

1. Ka. $10^{-4.76} = 1.74 \times 10^{-5}$.
2. Hydrogen ions. $\sqrt{1.74 \times 10^{-5} \times 0.10} = 1.32 \times 10^{-3}\ \text{mol/L}$.
3. pH. $-\log(1.32 \times 10^{-3}) = 2.88$.

Practice with new numbers

A program to try

The program compares two pairs of acids from their pKa values.

Mistakes to avoid

• A bigger pKa as a stronger acid. It is weaker.
• Phenol with NaHCO$_3$. No carbon dioxide.
• NaBH$_4$ for the reduction of an acid. Use LiAlH$_4$.
• An alpha hydrogen in benzoic acid. There is none, so the HVZ reaction does not work.

Check yourself

1. What does the Grignard reagent C$_6$H$_5$MgBr give with CO$_2$ and then dilute acid?

2. Which is stronger, 2-chlorobutanoic or 4-chlorobutanoic acid?

3. What does sodium ethanoate give with soda lime?

4. Why does ethanoic acid boil higher than propan-1-ol?

Sources

• OpenStax, Chemistry 2e, Chapter 20, section 20.3 Aldehydes, Ketones, Carboxylic Acids, and Esters.
• NCERT, Chemistry Part II for Class XII, Chapter 8.