Three gaseous alkylamines A, B and C or primary, secondary and tertiary respectively. What will be the correct order of their basicity?

B > C > A

The correct answer is option 3. B > C > A.

Basicity in amines is determined by the availability of the lone pair of electrons on the nitrogen atom for protonation. The factors influencing this availability include:

Inductive Effect:

Alkyl groups are electron-donating via the inductive effect (+I effect). This increases the electron density on the nitrogen atom, making it more basic.

Steric Hindrance:

Tertiary amines have three alkyl groups attached to the nitrogen. This can create steric hindrance, making it more difficult for the lone pair on nitrogen to be accessible for protonation.

Solvation Effect:

In the gaseous state, solvation effects are absent. Solvation typically stabilizes the protonated form of amines in a solution. Hence, in the gas phase, we primarily consider the inductive effect and steric hindrance.

Order of Basicity in Gaseous State

Primary Amine (A):

Structure: \(RNH_2\)

Effect: Has one alkyl group donating electron density to nitrogen.

Steric Hindrance: Minimal steric hindrance.

Lower than secondary and tertiary amines due to fewer electron-donating groups.

2. Secondary Amine (B):

Structure: \(R_2NH\)

Effect: Has two alkyl groups donating electron density to nitrogen, significantly increasing electron density on nitrogen.

Steric Hindrance: More than primary amines but less than tertiary amines.

Generally the highest due to a balance between inductive effect and steric hindrance.

3. Tertiary Amine (C):

Structure: \(R_3N\)

Effect: Has three alkyl groups donating electron density to nitrogen.

Steric Hindrance: Highest among the three types, which can somewhat hinder the nitrogen's lone pair from accepting a proton.

Higher than primary amines due to strong inductive effect but often lower than secondary amines due to steric hindrance.

Conclusion

In the gaseous state, where solvation effects are not a factor, the order of basicity is primarily influenced by the inductive effects and steric hindrance:

Secondary Amines (B): Strongest inductive effect with moderate steric hindrance.

Tertiary Amines (C): Strong inductive effect but higher steric hindrance than secondary amines.

Primary Amines (A): Least inductive effect with minimal steric hindrance.

Thus, the correct order of basicity in the gaseous state is: B > C > A

This means the secondary amine (B) is the most basic, followed by the tertiary amine (C), and then the primary amine (A).