Amines are basic in nature due to the presence of lone pair of electrons on N atom of `-NH_(2)` group. The basic strength of amines can be expressed by their dissociation constant, `K_(b)` or `pK_(b)`.
`RNH_(2) + H_(2) to R NH_(3)^(+) + OH^(-)`
`K_(b) = ([RHH_(3)^(+)][OH^(-)])/([RNH_(2)])` and `pK_(b) = -log K_(b)`
Greater the `K_(b)` value or smaller the `pK_(b)` value, more is the basic strength of amine. Aliphatic amines are stronger bases than ammonia due to the electron releasing effect of alkyl groups. The basic strength among amines decreases as :
`2^(@) gt 1^(@) gt 3^(@)`
Aryl amines such as aniline are less basic than aliphatic amines due to the involvement of lone pair of electrons on N atom with the resonance in benzene. In derivatives of aniline, the electron releasing groups increase the basic strength while electron withdrawing groups decrease the basic strength. The base weakening effect of electron withdrawing group and base strengthening effect of electron releasing group is more marked at p-position than at m-position. Every o- substituted aniline is less basic than aniline due to ortho effect.
Which of the following group does not decrease the basic strength of aniline ?

Options

A. `-OCH_(3)`
B. `-NO_(2)`
C. `-CN`
D. `-"halogen"`

Answer

To determine which group does not decrease the basic strength of aniline, we need to analyze the effects of various substituents on the aniline molecule. Aniline (C6H5NH2) has a basic nitrogen atom that can accept protons (H+). The basic strength of aniline can be influenced by the presence of different substituent groups, which can either donate or withdraw electron density from the nitrogen atom.

### Step-by-Step Solution:

1. **Understand Basic Strength**: Basic strength arises from the ability of a compound to accept protons (H+). This ability is enhanced by the availability of a lone pair of electrons on the nitrogen atom.

2. **Identify Electron Donating and Withdrawing Groups**:
- **Electron Donating Groups (EDGs)**: These groups increase electron density on the nitrogen atom, enhancing basic strength. Examples include alkyl groups and groups with lone pairs such as -OCH3 (methoxy).
- **Electron Withdrawing Groups (EWGs)**: These groups decrease electron density on the nitrogen atom, reducing basic strength. Examples include -NO2 (nitro), -CN (cyano), and halogens (like -Cl, -Br).

3. **Analyze the Given Groups**:
- **OCH3 (Methoxy Group)**: This group has a lone pair of electrons and exhibits a +M (mesomeric) effect, which increases electron density on the nitrogen atom, thus increasing basic strength.
- **NO2 (Nitro Group)**: This group is a strong EWG and decreases electron density, thus decreasing basic strength.
- **CN (Cyano Group)**: This group is also an EWG and decreases electron density, thus decreasing basic strength.
- **Halogens (like Cl)**: They generally exhibit a -I (inductive) effect, which withdraws electron density and decreases basic strength.

4. **Conclusion**: Among the groups analyzed, the methoxy group (-OCH3) is the only group that does not decrease the basic strength of aniline; instead, it increases it due to its electron-donating effect.

### Final Answer:
The group that does not decrease the basic strength of aniline is **OCH3 (Methoxy group)**.