What is the correct order of basicity of amines in the gaseous state compared with ammonia?

Introduction / Context:
Amines are derivatives of ammonia where one or more hydrogen atoms are replaced by alkyl or aryl groups. Their basicity depends on the availability of the lone pair on nitrogen for protonation. In different phases, such as gas or aqueous solution, the order of basic strength can change because solvation effects become important in solution. This question specifically asks for the order of basicity in the gaseous state, where intrinsic electron donating effects dominate.

Given Data / Assumptions:
- We compare primary, secondary and tertiary aliphatic amines with ammonia (NH3).
- The phase is gaseous, so solvent effects like hydrogen bonding with water are absent.
- Basicity is related to the tendency to donate the nitrogen lone pair to a proton (H+).
- Only inductive and electronic effects of alkyl groups are considered in this context.

Concept / Approach:
In the gaseous state, the electron donating inductive effect of alkyl groups increases the electron density on the nitrogen atom, enhancing its basicity. Tertiary amines have three alkyl groups, secondary have two and primary have one. Ammonia has no alkyl groups. Therefore, in the absence of solvation, the order of basicity is primarily determined by how many electron releasing groups are attached to nitrogen. This leads to tertiary amine being the most basic, followed by secondary, then primary, and finally ammonia as the least basic among them in the gas phase.

Step-by-Step Solution:
Step 1: Recall that alkyl groups push electron density towards nitrogen through the +I inductive effect, increasing basicity. Step 2: Note that tertiary amines have three alkyl groups, so they experience the strongest electron donating effect on nitrogen. Step 3: Recognise that secondary amines, with two alkyl groups, are next, followed by primary amines with one alkyl group. Step 4: Remember that ammonia, with no alkyl groups, has the lowest electron density on nitrogen among the species considered. Step 5: Conclude that in the gaseous state, the basicity order is tertiary amine greater than secondary amine greater than primary amine greater than ammonia.

Verification / Alternative check:
In contrast, in aqueous solution, the order of basicity can differ because solvation stabilises certain conjugate acids more effectively. There, secondary amines can sometimes be more basic than tertiary amines due to better solvation, but this does not apply in the gas phase. Many textbooks explicitly state that without solvent, the pure inductive effect dominates and leads to tertiary amines being the strongest bases. Comparing these two scenarios helps verify that the question specifically about the gaseous state must have tertiary amines at the top of the order.

Why Other Options Are Wrong:
Option A, secondary amine greater than primary greater than tertiary greater than ammonia, might be confused with some solution phase discussions but does not reflect gas phase behaviour. Option C, primary greater than secondary greater than tertiary greater than ammonia, reverses the expected effect of increasing alkyl substitution. Option D, secondary greater than tertiary greater than primary greater than ammonia, again misplaces tertiary amines. In all these incorrect orders, the role of the inductive effect in the gas phase is not applied correctly.

Common Pitfalls:
Students often memorise only one order of basicity for amines without noticing whether it refers to aqueous solution or the gaseous state. This leads to confusion when questions change the phase. Another pitfall is ignoring the effect of solvent entirely and assuming that the same reasoning always applies. To avoid these mistakes, learners should clearly associate the inductive effect dominated order with the gas phase and remember that solvation can modify the order in solution.

Final Answer:
In the gaseous state, the correct order of basicity of amines compared with ammonia is Tertiary amine > Secondary amine > Primary amine > NH3.