Which of the following species is/are planar ?

Options

A. tert-Butyl free radical
B. tert-Butyl carbocation
C. tert-Butyl carbanion
D. Allyl carbanion

Explanation

To determine which of the given species are planar, we will analyze each species based on the steric number of the central carbon atom. The steric number is calculated by adding the number of bond pairs and lone pairs around the central atom. A steric number of 3 indicates a planar geometry.

### Step-by-Step Solution:

1. **Identify the Species**: The species to analyze are:
- Tert-butyl free radical
- Tert-butyl carbocation
- Tert-butyl carbanion
- Allyl carbanion

2. **Tert-butyl Free Radical**:
- The central carbon atom is bonded to 3 methyl groups (C).
- Lone pairs = 0, Bond pairs = 3.
- Steric number = 0 + 3 = 3.
- **Conclusion**: This species is planar.

3. **Tert-butyl Carbocation**:
- The central carbon atom is also bonded to 3 methyl groups (C).
- Lone pairs = 0, Bond pairs = 3.
- Steric number = 0 + 3 = 3.
- **Conclusion**: This species is planar.

4. **Tert-butyl Carbanion**:
- The central carbon atom is bonded to 3 methyl groups (C) and has a negative charge.
- Lone pairs = 1 (due to the negative charge), Bond pairs = 3.
- Steric number = 1 + 3 = 4.
- **Conclusion**: This species is not planar (it has a tetrahedral geometry).

5. **Allyl Carbanion**:
- The central carbon atom is bonded to 2 hydrogen atoms and has a negative charge.
- Lone pairs = 1 (due to the negative charge), Bond pairs = 2.
- Steric number = 1 + 2 = 3.
- **Conclusion**: This species is planar due to resonance.

### Final Answer:
The planar species are:
- Tert-butyl free radical (A)
- Tert-butyl carbocation (B)
- Allyl carbanion (D)

Thus, the correct options are A, B, and D.

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