Which alcohol is most reactive for dehydrogenation ?

Options

A. `CH_3-overset(OH)overset(|)(CH)-CH_2-NO_2`
B. `CH_3-overset(OH)overset(|)(CH)-CH_2-CHO`
C. `CH_3-overset(OH)overset(|)(CH)-CH_2-CN`
D. `CH_3-overset(OH)overset(|)(CH)-CH_2-COCH_3`

Answer

To determine which alcohol is most reactive for dehydrogenation, we need to analyze the stability of the carbocations formed during the elimination reaction. The reactivity of alcohols in dehydrogenation is directly related to the stability of the resulting carbocation after the hydroxyl group (OH) is removed.

### Step-by-Step Solution:

1. **Understanding Dehydrogenation**:
Dehydrogenation of alcohols involves the removal of hydrogen from the alcohol group, leading to the formation of a carbocation. The stability of this carbocation is crucial for the reactivity of the alcohol.

**Hint**: Remember that the more stable the carbocation, the more reactive the alcohol will be.

2. **Formation of Carbocations**:
When an alcohol undergoes dehydrogenation, it loses an OH group and forms a carbocation. For example, if we consider a generic alcohol (R-OH), the carbocation formed will be R+.

**Hint**: Visualize the structure of the alcohol and how the carbocation forms after the elimination of OH.

3. **Evaluating Carbocation Stability**:
The stability of carbocations can be influenced by the substituents attached to the carbon bearing the positive charge. Electron-donating groups stabilize carbocations, while electron-withdrawing groups destabilize them.

**Hint**: Identify the substituents on the carbocation and determine whether they are electron-donating or electron-withdrawing.

4. **Order of Electron-Withdrawing Effects**:
The order of electron-withdrawing effects (−I effect) of different groups is important. For example, groups like NO2, CN, CHO, and COCH3 exhibit varying degrees of −I effects. The stronger the −I effect, the less stable the carbocation.

**Hint**: List the groups and their −I effects to compare their influence on carbocation stability.

5. **Determining Carbocation Stability**:
Based on the −I effects, we can determine the stability of the carbocations formed from different alcohols. The order of stability will be inversely related to the strength of the −I effect of the substituents.

**Hint**: Use the order of −I effects to rank the stability of the carbocations.

6. **Conclusion**:
After analyzing the stability of the carbocations formed from the different alcohols, we can conclude that the alcohol that leads to the most stable carbocation will be the most reactive for dehydrogenation.

**Hint**: Summarize your findings to identify which alcohol is the most reactive based on the stability of the carbocation.

### Final Answer:
The alcohol that forms the most stable carbocation (with the least electron-withdrawing effect) will be the most reactive for dehydrogenation. Based on the analysis, the order of reactivity for the alcohols can be established, with the one leading to the most stable carbocation being the most reactive.