Which of the following compounds will have the longest C = C bonds?

Options

A. `CH_(3) - CH = CH_(2)`
B. `CH_(3)-underset(CH_3)underset(|)overset(CH_3)overset(|)(C )-CH=CH_(2)`
C. `CH_(2) = C = CH_(2)`
D. `CH_(3)-underset(CH_3)underset(|)(C )=CH_(2)`

Answer

To determine which of the given compounds has the longest C=C bond, we need to analyze the influence of the +I (inductive) effect from substituents on the carbon atoms involved in the double bond. The +I effect refers to the electron-donating ability of substituents, which can lead to increased electron density and, consequently, increased repulsion between the electrons in the bond, resulting in longer bond lengths.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Compound 1: CH3CH=CH2 (Propene)
- Compound 2: CH3C(CH3)=CH2 (Isobutylene)
- Compound 3: CH2=CH2 (Ethylene)
- Compound 4: CH3C=CH2CH3 (Butene)

2. **Analyze the +I Effect**:
- The +I effect is stronger with more alkyl groups attached to the carbon atoms involved in the double bond. Alkyl groups donate electron density, which can lead to increased bond length due to electron repulsion.

3. **Assess Each Compound**:
- **Compound 1 (Propene)**: Has one methyl group attached to the double bond, leading to a moderate +I effect.
- **Compound 2 (Isobutylene)**: Has two methyl groups attached to the double bond (one on each carbon), leading to a stronger +I effect compared to Compound 1.
- **Compound 3 (Ethylene)**: Has no alkyl substituents, so it has the least +I effect.
- **Compound 4 (Butene)**: Has one methyl group on one carbon of the double bond, leading to a moderate +I effect, but less than Compound 2.

4. **Compare the +I Effects**:
- Compound 2 has the highest +I effect due to the presence of two methyl groups, which increases the electron density and repulsion at the double bond, resulting in a longer C=C bond.

5. **Conclusion**:
- Based on the analysis, Compound 2 (CH3C(CH3)=CH2) will have the longest C=C bond due to the strongest +I effect from the two methyl groups.

### Final Answer:
The compound with the longest C=C bond is **Compound 2: CH3C(CH3)=CH2**.