The order of decreasing basicities of `MeNH_(2),Me_(2)NH` and `Me_(3)N` is benzene is

Options

A. `Me_(3)NgtMe_(2)NHgtMeNH_(2)`
B. `Me_(2)NHgtMeNH_(2)gtMe_(3)N`
C. `Me_(2)NHgtMe_(3)NgtMeNH_(2)`
D. `Me_(2)NgtMeNH_(2)gtMe_(2)NH`

Answer

To determine the order of decreasing basicities of the amines MeNH₂ (methylamine), Me₂NH (dimethylamine), and Me₃N (trimethylamine), we need to analyze the structure of each compound and how the substituents affect the availability of the lone pair of electrons on the nitrogen atom for protonation.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Methylamine (MeNH₂) has one methyl group and one hydrogen atom attached to nitrogen.
- Dimethylamine (Me₂NH) has two methyl groups and one hydrogen atom attached to nitrogen.
- Trimethylamine (Me₃N) has three methyl groups attached to nitrogen.

2. **Consider the Lone Pair Availability**:
- The basicity of amines is largely determined by the availability of the lone pair of electrons on the nitrogen atom to accept a proton (H⁺).
- The more available the lone pair, the stronger the base.

3. **Effect of Alkyl Groups**:
- Alkyl groups (like methyl groups) have a +I (inductive) effect, which increases the electron density on the nitrogen atom.
- More alkyl groups will increase the electron density, making the lone pair more available for protonation.

4. **Analyze Each Compound**:
- **Trimethylamine (Me₃N)**: Has three methyl groups. The +I effect from three methyl groups significantly increases the electron density on nitrogen, making the lone pair highly available for protonation.
- **Dimethylamine (Me₂NH)**: Has two methyl groups. The +I effect is still strong, but less than that of trimethylamine, resulting in lower basicity than Me₃N.
- **Methylamine (MeNH₂)**: Has one methyl group. The +I effect is present, but it is the weakest among the three, resulting in the lowest basicity.

5. **Order of Basicity**:
- Based on the above analysis, the order of decreasing basicity is:
- **Me₃N > Me₂NH > MeNH₂**

### Final Answer:
The order of decreasing basicities of MeNH₂, Me₂NH, and Me₃N is:
**Me₃N > Me₂NH > MeNH₂**