Which of the following is the most reactive isomer ?

Options

A. `CH_(3)CH_(2)CH_(2)CH_(2)-overset(O) overset("||")C-H`
B. `CH_(3)CH_(2)CH_(2)-overset(O)overset("||")C-CH_(3)`
C. `CH_(3)CH_(2)-overset(O)overset("||")C-CH_(2)CH_(3)`
D. `CH_(3)-overset(O)overset("||")C-underset(CH_(3))underset(|)(CH)-CH_(3)`

Answer

To determine the most reactive isomer among the given options, we need to analyze the reactivity in terms of nucleophilic addition reactions. The reactivity is influenced by two main factors: the electron deficiency (denoted as δ+) of the sp² carbon and the steric hindrance (crowding) around that carbon.

### Step-by-Step Solution:

1. **Identify the Structure of Each Isomer**:
- Let's denote the four isomers as A, B, C, and D based on their structural formulas.

2. **Evaluate Electron Deficiency (δ+)**:
- The δ+ on the sp² carbon indicates how electron-deficient that carbon is. A higher δ+ means that the carbon is more susceptible to nucleophilic attack.
- In each isomer, determine which groups are attached to the sp² carbon. Electron-withdrawing groups will increase δ+, while electron-donating groups will decrease it.

3. **Assess Steric Hindrance**:
- Steric hindrance refers to the crowding around the sp² carbon. More bulky groups around the carbon will hinder the approach of nucleophiles, decreasing reactivity.
- Identify the size and number of substituents around the sp² carbon in each isomer.

4. **Compare the Isomers**:
- For each isomer, evaluate the balance between δ+ and steric hindrance.
- Isomer A: Less crowding, higher δ+ due to electron-withdrawing groups.
- Isomer B: More crowding, lower δ+ because of electron-donating groups.
- Isomer C: Similar to B, with significant crowding and lower δ+.
- Isomer D: Highest crowding and lowest δ+.

5. **Determine the Most Reactive Isomer**:
- Based on the analysis, Isomer A has the least steric hindrance and the highest δ+, making it the most reactive towards nucleophilic addition.

### Conclusion:
The most reactive isomer is **Isomer A** due to its favorable balance of high electron deficiency and low steric hindrance.