Decreasing order of -I effect of the triad `[-NO_(2),overset(o+)(N)H_(3),-CN]` is

Options

A. `-overset(o+)(N)H_(3)gt -NO_(2) tgt -CN`
B. `-overset(o+)(N)H_(3) gt -CN gt -NO_(2)`
C. `-CN gt - NO_(2) gt overset(o+)(N)H_(3)`
D. `-NO_(2) gt -CNgt overset(o+)(N)H_(3)`

Answer

To determine the decreasing order of the -I (negative inductive) effect of the triad [-NO₂, -NH₃⁺, -CN], we need to analyze the -I effect of each group based on their electronegativity and the presence of any charges.

### Step-by-Step Solution:

1. **Understanding the -I Effect**:
The -I effect refers to the ability of a substituent to withdraw electron density from the rest of the molecule. This effect is influenced by the electronegativity of the atoms involved and the presence of any charges.

2. **Analyzing Each Group**:
- **-NH₃⁺ (Ammonium Ion)**:
- The nitrogen in the ammonium ion (NH₃⁺) carries a positive charge.
- Nitrogen is electronegative, and the positive charge increases its tendency to attract electrons from neighboring atoms.
- Therefore, -NH₃⁺ exhibits a strong -I effect due to the combination of electronegativity and the positive charge.

- **-NO₂ (Nitro Group)**:
- The nitro group (-NO₂) contains nitrogen bonded to two oxygen atoms, which are highly electronegative.
- The electronegativity difference between nitrogen and oxygen allows the nitro group to withdraw electron density from adjacent atoms.
- Thus, -NO₂ exhibits a moderate -I effect, less than that of -NH₃⁺ but stronger than -CN.

- **-CN (Cyano Group)**:
- The cyano group (-CN) consists of a carbon atom triple-bonded to a nitrogen atom.
- While nitrogen is electronegative, the carbon atom is less electronegative than nitrogen, leading to a weaker -I effect compared to the other groups.
- Therefore, -CN exhibits the weakest -I effect among the three groups.

3. **Arranging in Decreasing Order**:
Based on the analysis:
- **-NH₃⁺** has the strongest -I effect.
- **-NO₂** has a moderate -I effect.
- **-CN** has the weakest -I effect.

Thus, the decreasing order of -I effect is:
**-NH₃⁺ > -NO₂ > -CN**

### Final Answer:
**The decreasing order of -I effect is: -NH₃⁺ > -NO₂ > -CN.**