Record the following sets of compounds according to increasing `pK_(a) ( = - log Ka)`
Hexylamine, aniline, methylamine

Answer

To determine the order of the compounds hexylamine, aniline, and methylamine according to increasing \( pK_a \) (which is inversely related to basicity), we can follow these steps:

### Step 1: Understand the Compounds
- **Hexylamine**: A straight-chain primary amine with a six-carbon alkyl group.
- **Methylamine**: A primary amine with a one-carbon alkyl group (methyl group).
- **Aniline**: An aromatic amine where the amino group is attached to a benzene ring.

### Step 2: Analyze Basicity
- Basicity of amines is influenced by the availability of the lone pair of electrons on the nitrogen atom for protonation.
- In **aniline**, the lone pair on the nitrogen is involved in resonance with the benzene ring, making it less available for protonation. This results in lower basicity.
- In **methylamine**, the lone pair is more available because there is no resonance effect, making it more basic than aniline.
- **Hexylamine** has an even larger alkyl group compared to methylamine, which can enhance the electron-donating effect (inductive effect) due to the larger number of carbon atoms. This makes hexylamine more basic than methylamine.

### Step 3: Order of Basicity
From the analysis:
- Aniline is the least basic due to resonance.
- Methylamine is more basic than aniline but less basic than hexylamine.
- Hexylamine is the most basic due to the strong inductive effect from the larger alkyl group.

Thus, the order of basicity is:
1. Aniline (least basic)
2. Methylamine
3. Hexylamine (most basic)

### Step 4: Relate Basicity to \( pK_a \)
Since \( pK_a \) is inversely related to basicity:
- Lower basicity corresponds to higher \( pK_a \).
- Higher basicity corresponds to lower \( pK_a \).

### Final Order of Increasing \( pK_a \)
Thus, the order of increasing \( pK_a \) is:
1. Hexylamine (lowest \( pK_a \))
2. Methylamine
3. Aniline (highest \( pK_a \))

### Summary
The final order according to increasing \( pK_a \) is:
**Hexylamine < Methylamine < Aniline**