Topics

Acid–base reactions of carboxylic acids and amines

Two families that behave as acid and base

Carboxylic acids (–COOH) are weak acids — they donate a proton, $H^+$ .
Primary amines (–NH₂) are weak bases — they accept a proton.
These are the only two families in this standard with acid–base chemistry, and they are opposites.

Family	Behaves as	What it does to $H^+$	Ion formed
carboxylic acid	acid	donates it	negative (carboxylate)
primary amine	base	accepts it	positive

Carboxylic acids

The –COOH group loses its hydrogen as $H^+$ , leaving a negatively charged carboxylate ion:

$CH_3COOH \rightarrow CH_3COO^- + H^+$

The ion is named by changing -oic acid to -oate: ethanoic acid → the ethanoate ion.
They are weak acids: only a small fraction of the molecules donate their proton at any moment.

They give all the ordinary reactions of an acid:

With a reactive metal → salt + hydrogen.
- $2CH_3COOH + Mg \rightarrow (CH_3COO)_2Mg + H_2$
- Observation: fizzing; the gas pops with a lit splint.
With a carbonate → salt + water + carbon dioxide.
- $2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2$
- Observation: fizzing; the gas turns limewater milky.
- This is the standard test that tells a carboxylic acid from an alcohol.
With an alkali → salt + water (neutralisation).
- $CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$
- Observation: no gas.

Primary amines

The nitrogen of the –NH₂ group accepts a proton, giving a positively charged ion:

$CH_3NH_2 + H^+ \rightarrow CH_3NH_3^+$

With an acid, an amine forms a salt:

$CH_3NH_2 + HCl \rightarrow CH_3NH_3^+Cl^-$

They are weak bases, and their solutions in water are basic — pH above 7.

An acid and an amine together

Put a carboxylic acid and an amine together and a straightforward acid–base reaction occurs: the acid donates $H^+$ and the amine accepts it.

$CH_3COOH + CH_3NH_2 \rightarrow CH_3COO^- + CH_3NH_3^+$

Both ions are formed, and the product is a salt.

Distinguishing the families by their acid–base behaviour

Test	Carboxylic acid	Alcohol	Amine
pH of aqueous solution	below 7	about 7	above 7
Add sodium carbonate	fizzes ( $CO_2$ )	no reaction	no reaction
Add hydrochloric acid	no visible reaction	no reaction	forms a salt

Tips and Tricks

When you write the products of an acid–base reaction, show the charges. The carboxylate ion is $CH_3COO^-$ with a negative charge and the protonated amine is $CH_3NH_3^+$ with a positive one; leaving the charges off usually costs the mark.

Count the hydrogens on the amine carefully. Methylamine is $CH_3NH_2$ ; once it accepts a proton it becomes $CH_3NH_3^{+}$ — three hydrogens on the nitrogen, not two. Students frequently write $CH_3NH_2^+$ , which does not balance.

And remember the carbonate test is the way to distinguish a carboxylic acid from an alcohol. Both contain an –OH, but only the acid fizzes. If a question gives you two compounds that both contain oxygen and asks you to tell them apart, this is very often the answer.

Worked ExampleAn acid and a base reacting

(a) Write an equation for the reaction of propanoic acid with sodium carbonate, and state what would be observed. (b) Write an equation for the reaction of ethanamine ( $CH_3CH_2NH_2$ ) with hydrochloric acid, and explain why ethanamine behaves as a base.

(a) Propanoic acid + sodium carbonate

Step 1 — Identify the reaction type. Propanoic acid, $CH_3CH_2COOH$ , contains the –COOH group, so it is an acid. Acid + carbonate is a standard acid–base reaction.

Step 2 — Write the products. Acid + carbonate always gives salt + water + carbon dioxide. The salt is the sodium propanoate, $CH_3CH_2COONa$ .

Step 3 — Balance it. The carbonate needs two acid molecules to supply two $H^+$ :

$2CH_3CH_2COOH + Na_2CO_3 \rightarrow 2CH_3CH_2COONa + H_2O + CO_2$

Step 4 — State the observation. Fizzing / effervescence, and the gas given off turns limewater milky, confirming carbon dioxide.

(b) Ethanamine + hydrochloric acid

Step 1 — Identify the reaction type. Ethanamine contains the –NH₂ group, so it is a base and will accept a proton from the acid.

Step 2 — Write the equation. The nitrogen accepts $H^+$ from the HCl, going from $NH_2$ to $NH_3^+$ , and the chloride balances the charge:

$CH_3CH_2NH_2 + HCl \rightarrow CH_3CH_2NH_3^+Cl^-$

Step 3 — Explain the basicity. The nitrogen atom of the amino group accepts a proton ( $H^+$ ). Accepting a proton is the definition of a base, so ethanamine is a base — a weak one.

Acid–base reactions of carboxylic acids and amines

Covers AS91165

Two families that behave as acid and base

Carboxylic acids (–COOH) are weak acids — they donate a proton, $H^+$ .
Primary amines (–NH₂) are weak bases — they accept a proton.
These are the only two families in this standard with acid–base chemistry, and they are opposites.

Family	Behaves as	What it does to $H^+$	Ion formed
carboxylic acid	acid	donates it	negative (carboxylate)
primary amine	base	accepts it	positive

Carboxylic acids

The –COOH group loses its hydrogen as $H^+$ , leaving a negatively charged carboxylate ion:

$CH_3COOH \rightarrow CH_3COO^- + H^+$

The ion is named by changing -oic acid to -oate: ethanoic acid → the ethanoate ion.
They are weak acids: only a small fraction of the molecules donate their proton at any moment.

They give all the ordinary reactions of an acid:

With a reactive metal → salt + hydrogen.
- $2CH_3COOH + Mg \rightarrow (CH_3COO)_2Mg + H_2$
- Observation: fizzing; the gas pops with a lit splint.
With a carbonate → salt + water + carbon dioxide.
- $2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2$
- Observation: fizzing; the gas turns limewater milky.
- This is the standard test that tells a carboxylic acid from an alcohol.
With an alkali → salt + water (neutralisation).
- $CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$
- Observation: no gas.

Primary amines

The nitrogen of the –NH₂ group accepts a proton, giving a positively charged ion:

$CH_3NH_2 + H^+ \rightarrow CH_3NH_3^+$

With an acid, an amine forms a salt:

$CH_3NH_2 + HCl \rightarrow CH_3NH_3^+Cl^-$

They are weak bases, and their solutions in water are basic — pH above 7.

An acid and an amine together

Put a carboxylic acid and an amine together and a straightforward acid–base reaction occurs: the acid donates $H^+$ and the amine accepts it.

$CH_3COOH + CH_3NH_2 \rightarrow CH_3COO^- + CH_3NH_3^+$

Both ions are formed, and the product is a salt.

Distinguishing the families by their acid–base behaviour

Test	Carboxylic acid	Alcohol	Amine
pH of aqueous solution	below 7	about 7	above 7
Add sodium carbonate	fizzes ( $CO_2$ )	no reaction	no reaction
Add hydrochloric acid	no visible reaction	no reaction	forms a salt

Tips and Tricks

Worked ExampleAn acid and a base reacting

(a) Propanoic acid + sodium carbonate

Step 1 — Identify the reaction type. Propanoic acid, $CH_3CH_2COOH$ , contains the –COOH group, so it is an acid. Acid + carbonate is a standard acid–base reaction.

Step 2 — Write the products. Acid + carbonate always gives salt + water + carbon dioxide. The salt is the sodium propanoate, $CH_3CH_2COONa$ .

Step 3 — Balance it. The carbonate needs two acid molecules to supply two $H^+$ :

$2CH_3CH_2COOH + Na_2CO_3 \rightarrow 2CH_3CH_2COONa + H_2O + CO_2$

Step 4 — State the observation. Fizzing / effervescence, and the gas given off turns limewater milky, confirming carbon dioxide.

(b) Ethanamine + hydrochloric acid

Step 1 — Identify the reaction type. Ethanamine contains the –NH₂ group, so it is a base and will accept a proton from the acid.

Step 2 — Write the equation. The nitrogen accepts $H^+$ from the HCl, going from $NH_2$ to $NH_3^+$ , and the chloride balances the charge:

$CH_3CH_2NH_2 + HCl \rightarrow CH_3CH_2NH_3^+Cl^-$

Step 3 — Explain the basicity. The nitrogen atom of the amino group accepts a proton ( $H^+$ ). Accepting a proton is the definition of a base, so ethanamine is a base — a weak one.