Basic Properties (AQA A-Level Chemistry): Revision Notes

7.5.2 Basic Properties

Properties of Amines

Basicity of Amines

• Amines as Weak Bases: Amines act as weak bases due to the lone pair of electrons on the nitrogen atom, which can accept protons ($H+$ ions). This lone pair allows amines to accept protons, behaving similarly to ammonia.
• Reaction with Acids: All amines, regardless of their structure, react with strong acids to form alkylammonium salts, a reaction similar to ammonia.

Reactions of Amines with Acids

• When amines react with acids, they follow a general pattern, accepting a proton to form a positive alkylammonium ion paired with the acid's conjugate base: Ammonia:

$${NH}_3 + \text{HCl} \rightarrow \text{NH}_4\text{Cl}$$ $${NH}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{(NH}_4\text{)}_2\text{SO}_4$$

Primary Amines:

$${R-NH}_2$$ $${R-NH}_2 + \text{HCl} \rightarrow \text{R-NH}_3^+\text{Cl}^-$$ $$2\text{R-NH}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{(R-NH}_3^+\text{)}_2\text{SO}_4^{2-}$$

Secondary Amines:

$${R}_1\text{R}_2\text{NH}$$ $${R}_1\text{R}_2\text{NH} + \text{HCl} \rightarrow \text{R}_1\text{R}_2\text{NH}_2^+\text{Cl}^-$$ $$2\text{R}_1\text{R}_2\text{NH} + \text{H}_2\text{SO}_4 \rightarrow \text{(R}_1\text{R}_2\text{NH}_2^+\text{)}_2\text{SO}_4^{2-}$$

Tertiary Amines

$${R}_1\text{R}_2\text{R}_3\text{N}$$ $${R}_1\text{R}_2\text{R}_3\text{N} + \text{HCl} \rightarrow \text{R}_1\text{R}_2\text{R}_3\text{N}^+\text{HCl}^-$$ $$2\text{R}_1\text{R}_2\text{R}_3\text{N} + \text{H}_2\text{SO}_4 \rightarrow \text{(R}_1\text{R}_2\text{R}3\text{N}\text{H}^+\text{)}_2\text{SO}_4^{2-}$$

Note on Charges: Charges on ions are often explicitly shown, as in ${R}_1\text{R}_2\text{NH}_2^+\text{Cl}^-$ to clearly indicate the formation of an ammonium salt.

Comparative Basic Strength of Amines

• Lone Pair Availability and Basicity: The basicity of ammonia and amines depends on how readily the nitrogen atom's lone pair can be donated. Greater availability of this lone pair results in a stronger base.

• Influence of Alkyl Groups (Positive Inductive Effect): The electron-donating effect of alkyl groups attached to the nitrogen increases electron density around the nitrogen atom, making the lone pair more available for bonding with protons.

  • Primary Amines: Primary amines have one alkyl group attached to nitrogen. Alkyl groups donate electron density through a "positive inductive effect," which raises the electron density around nitrogen, strengthening the base.
  • Secondary Amines: Secondary amines have two alkyl groups, leading to a stronger positive inductive effect and thus greater electron density around nitrogen compared to primary amines. Consequently, secondary amines are stronger bases than primary amines.
  • Tertiary Amines: Tertiary amines, with three alkyl groups, exhibit an even higher electron-donating effect, making them generally the strongest bases among ammonia, primary, and secondary amines. Ammonia and the amines are weak bases (proton acceptors).

• They are weak bases because the lone pair of electrons on the N atom is able to accept a$H+$.

• The ability of amines to act as bases depends on the availability of the lone pair of electrons on the N atom.

• Increased electron density of the lone pair on the N atom ➜ increased ability to accept $H+$ ➜ stronger bases

  

Explaining Base Strength of Amines

Primary, Secondary, and Tertiary Amines Compared to Ammonia

• Alkyl groups are more electron releasing than $H$ atoms (have a greater 'inductive effect').
• The more alkyl groups substituted on to the $N$ atom in place of H atoms, the more electron density is pushed on to the $N$ atom.
• Therefore, the more alkyl groups attached to the $N$atom, the greater the electron density of the lone pair on the N atom, the stronger a base the amine is.
• This means in order of increases base strength: $NH3$< 1° < 2° < 3°

Aromatic Amines Compared to Aliphatic Amines

• The lone pair on the N atom is partially delocalised into the benzene ring.
• This means the electron density of the lone pair on the N atom is reduced meaning that aromatic amines are weaker bases than aliphatic amines.
• Therefore in order of increased base strength: Aromatic < $NH3$ < 1° < 2° < 3°

Nucleophilic Properties

Ammonia and the amines are nucleophiles (electron-pair donors). They are nucleophiles because they have a lone pair of electrons on the N atom.