Which of the following is an antiaromatic compound ?

Options

A.
B.
C.
D.

Answer

To determine which of the given compounds is antiaromatic, we will apply Huckel's rule. Here’s a step-by-step solution:

### Step 1: Understand Huckel's Rule
Huckel's rule states that for a compound to be aromatic, it must satisfy two conditions:
1. The compound must be cyclic and planar.
2. It must have a certain number of π (pi) electrons, specifically following the formula \( 4n + 2 \) where \( n \) is a non-negative integer (0, 1, 2, ...).

If a compound has \( 4n \) π electrons (where \( n \) is a positive integer), it is classified as antiaromatic.

### Step 2: Analyze Each Option
We will analyze each option provided (A, B, C, D) to check if they meet the criteria for being antiaromatic.

#### Option A:
- **Structure**: A cyclic ketone with a double bond.
- **Hybridization**: All atoms are sp² hybridized.
- **π Electrons**: There are 2 π electrons from the double bond.
- **Conclusion**: Since \( 2 \) does not fit the \( 4n \) formula (where \( n \) is a positive integer), this compound is not antiaromatic.

#### Option B:
- **Structure**: A compound with a boron atom.
- **Hybridization**: Boron has a vacant p orbital contributing to the cyclic cloud.
- **π Electrons**: There are 4 π electrons in total.
- **Conclusion**: Since \( 4 \) fits the \( 4n \) formula (with \( n = 1 \)), this compound is antiaromatic.

#### Option C:
- **Structure**: A compound with a nitro group.
- **Hybridization**: All atoms are sp² hybridized.
- **π Electrons**: There are 6 π electrons due to conjugation.
- **Conclusion**: Since \( 6 \) fits the \( 4n + 2 \) formula (with \( n = 1 \)), this compound is aromatic.

#### Option D:
- **Structure**: Another cyclic compound.
- **Hybridization**: All atoms are sp² hybridized.
- **π Electrons**: There are 6 π electrons.
- **Conclusion**: Since \( 6 \) fits the \( 4n + 2 \) formula (with \( n = 1 \)), this compound is aromatic.

### Step 3: Final Conclusion
After analyzing all options, the compound that is antiaromatic is **Option B**.