Which of the following molecules has longest C=C strength?

Options

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

Explanation

To determine which of the given molecules has the longest C=C bond length, we need to analyze the effect of hyperconjugation and the number of alpha hydrogens present in each molecule. Here’s a step-by-step solution:

### Step 1: Understand the concept of C=C bond strength and length
The strength of a carbon-carbon double bond (C=C) is influenced by the degree of hyperconjugation. Hyperconjugation occurs when alkyl groups donate electron density to the π-bond of the double bond, effectively increasing the single bond character of the double bond. This results in a longer bond length.

### Step 2: Identify the alpha hydrogens in each molecule
Alpha hydrogens are the hydrogens attached to the carbon atoms adjacent to the carbon-carbon double bond. The more alpha hydrogens present, the greater the hyperconjugation effect, which leads to a longer C=C bond.

### Step 3: Analyze each option
Let's analyze the four options provided:

1. **Option 1**: Identify the number of alpha hydrogens adjacent to the C=C bond.
- Count the hydrogens on the carbons adjacent to the double bond.

2. **Option 2**: Repeat the process for the second molecule.
- Count the alpha hydrogens.

3. **Option 3**: Analyze the third molecule.
- Count the alpha hydrogens.

4. **Option 4**: Finally, analyze the fourth molecule.
- Count the alpha hydrogens.

### Step 4: Compare the number of alpha hydrogens
After counting the alpha hydrogens for each molecule:
- **Option 1**: 2 alpha hydrogens
- **Option 2**: 3 alpha hydrogens
- **Option 3**: 0 alpha hydrogens
- **Option 4**: 6 alpha hydrogens (3 from each adjacent carbon)

### Step 5: Determine the longest C=C bond
The molecule with the highest number of alpha hydrogens will exhibit the most significant hyperconjugation effect, leading to the longest C=C bond. In this case, **Option 4** has the most alpha hydrogens (6), indicating that it will have the longest C=C bond.

### Conclusion
Thus, the molecule with the longest C=C bond strength is **Option 4**.

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