Understanding Basicity in the Gas Phase

The strength of a base is related to its ability to accept a proton ($\text{H}^+$). When considering basicity in the gas phase, solvation effects are minimal or absent. Therefore, the basicity is primarily determined by the electronic factors influencing the stability of the conjugate acid formed after the amine accepts a proton.

Amines ($\text{R}_3\text{N}$) act as bases by donating the lone pair of electrons on the nitrogen atom to a proton, forming a conjugate acid ($\text{R}_3\text{NH}^+$).

\(\text{R}_3\text{N} + \text{H}^+ \rightarrow \text{R}_3\text{NH}^+\)

A stronger base forms a more stable conjugate acid. The stability of the conjugate acid is influenced by the electron-donating or withdrawing effects of the groups attached to the nitrogen atom.

Inductive Effect and Basicity of Amines

Alkyl groups (like ethyl, \(\text{CH}_3\text{CH}_2\text{-}\)) are electron-donating groups. They stabilize the positive charge on the nitrogen atom in the conjugate acid through the positive inductive effect (+\(\text{I}\) effect). The greater the number of alkyl groups attached to the nitrogen atom, the stronger is the positive inductive effect, leading to greater stabilization of the positive charge on the nitrogen in the conjugate acid, and thus a stronger base.

Comparing the Given Amines and Ammonia

Let's look at the structures and the number of ethyl groups in each option:

• Ammonia (\(\text{NH}_3\)): Contains no alkyl groups.
• Ethylamine (\(\text{CH}_3\text{CH}_2\text{NH}_2\)): Contains one ethyl group.
• Diethylamine (\((\text{CH}_3\text{CH}_2)_2\text{NH}\)): Contains two ethyl groups.
• Triethylamine (\((\text{CH}_3\text{CH}_2)_3\text{N}\)): Contains three ethyl groups.

Determining the Strongest Base in Gas Phase

Based on the positive inductive effect in the gas phase:

• Triethylamine has three electron-donating ethyl groups, providing the maximum stabilization to the positive charge in its conjugate acid (\((\text{CH}_3\text{CH}_2)_3\text{NH}^+\)).
• Diethylamine has two ethyl groups, providing less stabilization than triethylamine but more than ethylamine.
• Ethylamine has one ethyl group, providing some stabilization but less than diethylamine and triethylamine.
• Ammonia has no alkyl groups, so there is no positive inductive effect to stabilize the conjugate acid.

Therefore, in the gas phase, the basicity increases with the number of alkyl groups due to the increasing stabilization of the conjugate acid by the positive inductive effect.

The order of basicity in the gas phase is:

Triethylamine > Diethylamine > Ethylamine > Ammonia

Thus, among the given options, Triethylamine is the strongest base in the gas phase.