In each of the following pair of compounds, which is more basic in aqueous solution? Give an explanation for your choice:
`C_(6)H_(5)N(CH_(3))_(2)" or " 2,6` - dimethyl-N-N-dimethylaniline

Answer

To determine which compound is more basic in aqueous solution between `C6H5N(CH3)2` (N,N-dimethylaniline) and `2,6-dimethyl-N,N-dimethylaniline`, we need to analyze the structures and the effects of substituents on the nitrogen atom's lone pair.

### Step-by-Step Solution:

1. **Identify the Structures**:
- **N,N-dimethylaniline**: This compound has a phenyl group (C6H5) attached to a nitrogen atom that has two methyl groups (CH3) attached to it.
- **2,6-dimethyl-N,N-dimethylaniline**: This compound has the same N,N-dimethylaniline structure, but it also has two additional methyl groups attached to the 2nd and 6th positions of the benzene ring.

2. **Analyze Basicity**:
- Basicity in organic compounds is often related to the availability of the nitrogen atom's lone pair for protonation. The more available the lone pair, the more basic the compound.
- In N,N-dimethylaniline, the lone pair on nitrogen can participate in resonance with the aromatic ring. This delocalization reduces the availability of the lone pair for bonding with protons, thus decreasing its basicity.

3. **Consider the Effect of Methyl Groups**:
- In the case of 2,6-dimethyl-N,N-dimethylaniline, the additional methyl groups at the ortho positions can have a significant effect. These methyl groups can create steric hindrance and electronic effects.
- The ortho effect refers to the influence of substituents on the aromatic ring that can affect the lone pair's availability. The methyl groups can cause repulsion, pushing the lone pair out of the plane of the aromatic system.

4. **Conclusion on Basicity**:
- In 2,6-dimethyl-N,N-dimethylaniline, the repulsion from the methyl groups causes the nitrogen's lone pair to be less involved in resonance with the benzene ring. This means that the lone pair is more available for protonation, making this compound more basic than N,N-dimethylaniline.

### Final Answer:
**2,6-dimethyl-N,N-dimethylaniline is more basic in aqueous solution than N,N-dimethylaniline due to the ortho effect of the methyl groups, which makes the nitrogen's lone pair more available for protonation.**