For simple amines in aqueous solution, which of the following gives the correct general order of basic strength from strongest base to weakest base?

Introduction / Context:
Amines are organic derivatives of ammonia and act as bases because the nitrogen atom has a lone pair of electrons that can accept a proton. The basic strength of amines in aqueous solution depends on several factors, including the inductive effect of alkyl groups and solvation of the resulting ammonium ions. Exam questions often ask students to compare the basicity of primary, secondary and tertiary amines and ammonia. This question focuses on the general order of basic strength in water for simple aliphatic amines.

Given Data / Assumptions:
- We consider simple aliphatic amines without special steric or resonance effects.
- Basic strength is measured in aqueous solution, not in the gas phase.
- Alkyl groups donate electron density to nitrogen through the inductive effect, tending to increase basicity compared with ammonia.
- Solvation of the protonated amine by water molecules also influences the observed basic strength.

Concept / Approach:
In aqueous solution, secondary amines are generally the strongest bases among simple aliphatic amines. This is because they have two alkyl groups that push electron density toward nitrogen, increasing its ability to accept a proton, while still allowing good solvation of the resulting ammonium ion. Primary amines are usually slightly less basic than secondary amines, and tertiary amines may be less basic than both in water because bulky alkyl groups hinder solvation of the protonated species. Ammonia, which has no alkyl groups, is the weakest base in this series. Therefore, the general basicity order in water is secondary amines greater than primary amines greater than tertiary amines greater than ammonia.

Step-by-Step Solution:
Step 1: Recall that alkyl groups exert a positive inductive effect, pushing electron density toward nitrogen and increasing its tendency to accept a proton. Step 2: Recognise that secondary amines have two alkyl groups and therefore receive a stronger inductive effect than primary amines, which have only one. Step 3: Understand that tertiary amines have three alkyl groups, but in water their protonated forms can be less well solvated because of steric hindrance, which can reduce their effective basicity compared with secondary amines. Step 4: Note that ammonia has no alkyl groups and therefore lacks the inductive electron donation that makes alkyl amines stronger bases, so it is the weakest base in the group. Step 5: Combine these points to obtain the basicity order: secondary amines (R2NH) > primary amines (RNH2) > tertiary amines (R3N) > ammonia (NH3).

Verification / Alternative check:
Experimental pKb or pKa values for protonated amines in water generally confirm this trend for simple aliphatic amines. Secondary amines often show the smallest pKb values, indicating the greatest basic strength, followed by primary and then tertiary amines, with ammonia being the weakest. Textbook discussions of amine basicity frequently show this order and explain it using a combination of inductive and solvation arguments. Although there can be exceptions for specific structures, the pattern given in this question is a well established general rule for basicity in aqueous solution.

Why Other Options Are Wrong:
Option A suggests that tertiary amines are always the strongest bases, which might be closer to the trend in the gas phase but does not hold in water due to solvation effects. Option C gives a basicity order that does not match common experimental data and undervalues the effect of having two alkyl groups in secondary amines. Option D places ammonia as the strongest base, which contradicts the strong electron donating influence of alkyl groups. Option E claims that all amines and ammonia have exactly the same basic strength, which is not supported by measured equilibrium constants. Only option B expresses the standard aqueous basicity order for simple aliphatic amines.

Common Pitfalls:
Students may focus solely on the inductive effect and conclude that tertiary amines must always be the strongest bases because they have the most alkyl groups. This reasoning neglects the role of solvation, which is very important in water. Another pitfall is to memorise a trend without remembering the conditions, leading to confusion between gas phase and solution phase basicity. By keeping both inductive and solvation effects in mind and remembering that the order given applies specifically to aqueous solution, learners can answer such questions correctly and understand the underlying chemistry rather than relying on incomplete rules.

Final Answer:
In aqueous solution, the general order of basic strength for simple amines is Secondary amines (R2NH) > primary amines (RNH2) > tertiary amines (R3N) > ammonia (NH3).