NCERT · Class 12 · Chemistry · Alcohols, Phenols and EthersExplain the mechanism of acid-catalyzed dehydration of alcohols to alkenes with a detailed step-by-step mechanism. Also, explain why tertiary alcohols undergo dehydration much faster than primary alcohols under similar conditions.

Acid-Catalyzed Dehydration of Alcohols to Alkenes

\nThe acid-catalyzed dehydration of alcohols involves the elimination of a water molecule from an alcohol in the presence of an acid catalyst (such as concentrated $H_2SO_4$ or $H_3PO_4$) to form an alkene. This reaction is an example of an elimination reaction, specifically an $E1$ elimination pathway for secondary and tertiary alcohols.

Step-by-Step Mechanism

1. Protonation of Alcohol:

   • The oxygen atom of the alcohol molecule has lone pairs of electrons, making it basic. In the presence of an acid catalyst, it accepts a proton ($H^+$) to form an oxonium ion (protonated alcohol).
   • Equation: $R-CH_2-OH + H^+ \rightleftharpoons R-CH_2-O^+H_2$
   • This step is extremely fast and reversible.

2. Formation of Carbocation (Rate-Determining Step):

   • The protonated alcohol loses a water molecule ($H_2O$) to form a carbocation. Because water is a very good leaving group, this step proceeds with the cleavage of the C-O bond.
   • Equation: $R-CH_2-O^+H_2 \rightarrow R-CH_2^+ + H_2O$
   • This is the slowest step of the mechanism and hence is the rate-determining step of the overall reaction.

3. Formation of Alkene by Deprotonation:

   • The carbocation loses a proton ($H^+$) from an adjacent carbon atom (alpha-carbon) to form a neutral alkene molecule. The proton is accepted by a base (like water or hydrogensulfate ion), regenerating the acid catalyst.
   • Equation: $R-CH-CH_2^+ + B \rightarrow R-CH=CH_2 + BH^+$

Reactivity Order of Alcohols

• The order of reactivity of alcohols towards acid-catalyzed dehydration is: $$\text{Tertiary (3°)} > \text{Secondary (2°)} > \text{Primary (1°)}$$
• Reason: The rate-determining step (Step 2) involves the formation of a carbocation intermediate. Since the stability of carbocations follows the order $3^\circ > 2^\circ > 1^\circ$, tertiary alcohols readily form stable tertiary carbocations and therefore undergo dehydration easily and rapidly. Primary alcohols form less stable primary carbocations, requiring harsher conditions (higher temperature and concentrated acid) for dehydration.