Dehydration of Alcohols

Dehydration of Alcohols  

In the dehydration of alcohols, the H and OH are lost from adjacent carbons.

We have seen that treatment of alcohols with mineral acid at elevated temperatures results in alkene formation by loss of water, a process called dehydration, which proceeds by E1 or E2 pathways.

Dehydration of Alcohols

The ease of elimination of water from alcohols increases with increasing substitution of the hydroxy-bearing carbon. For alcohols to undergo substitution or elimination reactions, the OH must first be converted into a better leaving group. 

The simplest way of turning the hydroxy substituent in alcohols into a good leaving group is to protonate the oxygen to form an alkyloxonium ion.The positive charge therefore resides on the oxygen atom. Protonation changes OH from a bad leaving group into neutral water, a good leaving group. This reaction is reversible and, under normal conditions, the equilibrium lies on the side of unprotonated alcohol. However, this is immaterial if a nucleophile is present in the mixture that is capable of trapping the oxonium species.

Alkyloxonium ions derived from primary alcohols are subject to such nucleophilic attack. Thus, the butyloxonium ion resulting from the treatment of 1-butanol with concentrated HBr undergoes displacement by bromide to form 1-bromobutane. 

Alkyloxonium ions derived from secondary and tertiary alcohols, in contrast with their primary counterparts, lose water with increasing ease to give the corresponding carbocations.

The reason for this difference in behavior is the difference in carbocation stability.Primary carbocations are too high in energy to be accessible under ordinary laboratory conditions, whereas secondary and tertiary carbocations are generated with increasing ease. Thus, primary alkyloxonium ions undergo only SN2 reactions, whereas their secondary and tertiary relatives enter into SN1 and E1 processes. When good nucleophiles are present, we observe SN1 products.