What is the correct order of reactivity of alcohols toward reactions involving cleavage of C-O bond?

3°>2°>1°

The correct answer is option 2. \(3°>2°>1°\).

To explain why the correct order of reactivity of alcohols toward reactions involving cleavage of the \(C-O\) bond is \( \text{3°} > \text{2°} > \text{1°} \), we need to delve into the mechanisms of these reactions and the stability of the intermediates formed.

Mechanism of C-O Bond Cleavage in Alcohols

Reactions involving the cleavage of the \(C-O\) bond in alcohols often proceed through a mechanism that involves the formation of a carbocation intermediate. This is particularly true for reactions such as:

Dehydration (elimination of water to form alkenes)

Substitution reactions with acids like \(HCl\), \(HBr\), or \(HI\)

Carbocation Stability

The reactivity of alcohols in these reactions is largely determined by the stability of the carbocation that is formed when the \(C-O\) bond is cleaved. The stability of carbocations follows the order:

\(\text{3° (tertiary)} > \text{2° (secondary)} > \text{1° (primary)}\)

Why are Tertiary Carbocations Most Stable

Hyperconjugation: Tertiary carbocations benefit from hyperconjugation, where adjacent \(C-H\) bonds can donate electron density to the empty p-orbital of the carbocation, stabilizing it.

Inductive Effect: Alkyl groups are electron-releasing through the inductive effect, which helps to disperse the positive charge on the carbocation. Tertiary carbocations have more alkyl groups than secondary and primary carbocations.

Resonance: In some cases, tertiary carbocations can also be stabilized by resonance if there are adjacent double bonds or aromatic rings.

Reactivity of Alcohols

Tertiary Alcohols (3°): These can form stable tertiary carbocations. The stability of the tertiary carbocation makes the \(C-O\) bond cleavage easier, resulting in higher reactivity.

Secondary Alcohols (2°): These form less stable secondary carbocations compared to tertiary, but still more stable than primary. Thus, they are moderately reactive.

Primary Alcohols (1°): These form the least stable primary carbocations. The instability of primary carbocations makes the \(C-O\) bond cleavage more difficult, resulting in lower reactivity.

Examples of Reactions

1. Dehydration to form alkenes:

Tertiary alcohols dehydrate easily to form alkenes due to the stability of the tertiary carbocation intermediate. Secondary alcohols also undergo dehydration but less readily than tertiary alcohols. Primary alcohols are the least reactive in dehydration reactions because the primary carbocation is highly unstable.

2. Substitution with HX (e.g., \(HCl, HBr, HI\)):

Tertiary alcohols react rapidly with HX to form alkyl halides due to the stable tertiary carbocation. Secondary alcohols react at a moderate rate. Primary alcohols react slowly because the primary carbocation is unstable.

Conclusion

The correct order of reactivity of alcohols toward reactions involving cleavage of the \(C-O\) bond is determined by the stability of the carbocation intermediate formed during the reaction. Therefore, the reactivity order is:

\(\text{3°} > \text{2°} > \text{1°}\)

This order reflects the increasing stability of the carbocations formed from tertiary, secondary, and primary alcohols, respectively.