Which of the following C-H bond has the lowest bond dissociation energy

Options

A. Primary `(1^(@))C-H` bond
B. Secondary `(2^(@))C-H` bond
C. Tertiary `(3^(@)) C-H` bond
D. All of these

Answer

To determine which C-H bond has the lowest bond dissociation energy, we need to analyze the stability of the radicals formed when these bonds are broken. Here's a step-by-step solution:

### Step 1: Identify the Types of C-H Bonds
We are comparing three types of C-H bonds:
1. Primary C-H bond (1°) - Example: CH3-CH2- (ethyl group)
2. Secondary C-H bond (2°) - Example: CH3-CH- (isopropyl group)
3. Tertiary C-H bond (3°) - Example: CH3-C(CH3)- (tert-butyl group)

### Step 2: Understand Bond Dissociation Energy (BDE)
Bond dissociation energy is the energy required to break a bond homolytically, resulting in the formation of radicals. The stability of the resulting radicals is crucial in determining the BDE.

### Step 3: Analyze the Stability of Radicals
When a C-H bond is broken, a radical is formed:
- Breaking a primary C-H bond forms a primary radical (R•).
- Breaking a secondary C-H bond forms a secondary radical (R•).
- Breaking a tertiary C-H bond forms a tertiary radical (R•).

The stability of these radicals follows this order:
- Tertiary radical > Secondary radical > Primary radical

### Step 4: Relate Radical Stability to Bond Dissociation Energy
The more stable the radical, the lower the bond dissociation energy of the C-H bond:
- Tertiary radicals are the most stable, leading to the lowest bond dissociation energy for tertiary C-H bonds.
- Primary radicals are the least stable, leading to the highest bond dissociation energy for primary C-H bonds.

### Step 5: Conclusion
Based on the stability of the radicals formed, we conclude that the tertiary C-H bond has the lowest bond dissociation energy.

### Final Answer
The C-H bond with the lowest bond dissociation energy is the **tertiary C-H bond**.

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