Elimination Reactions of Alcohols

In alkyl halide reactions  2º and 3º-alkyl halides experienced rapid E2 elimination when treated with strong bases, such as hydroxide and alkoxides. But-

*Alcohols do not undergo such base-induced elimination reactions and are, in fact, often used as solvents for such reactions. This is a example of how leaving group stability often influences the rate of a reaction.

*When an alcohol is treated with sodium hydroxide, the following acid-base equilibrium occurs. Most alcohols are slightly weaker acids than water (why?) so the left side is favored.

R–O–H   +   Na⁽⁺⁾ OH^(–)   <=====>  R–O^(–) Na⁽⁺⁾   +   H–OH

*This type of elimination in which water released from an alcohol is called dehydration. 

*It is important to note that-

(a) "Water is a much better leaving group than hydroxide ion, it is sensible to use acid-catalysis rather than base-catalysis to achieve such reactions".

(b) "Hydrohalic acids (HX) are not normally used as catalysts because their conjugate bases are good            nucleophiles and may give substitution products"

(c) "The conjugate bases of sulfuric and phosphoric acids are not good nucleophiles and do not give substitution  under the usual conditions of their use.

(d) "The acid-catalyzed dehydrations are the reverse of the acid-catalyzed hydration reactions of alkenes.

(e) "The laws of thermodynamics require that the mechanism in both directions proceed by the same reaction path. This is known as the principle of microscopic reversibility.  

Reactions:

The first two examples (top row) are typical, and the more facile elimination of the 3º-alcohol suggests predominant E1 character for the reaction. This agrees with the tendency of branched 1º and 2º-alcohols to give rearrangement products, as shown in the last example. The last two reactions also demonstrate that the Zaitsev Rule applies to alcohol dehydrations as well as alkyl halide eliminations. Thus the more highly-substituted double bond isomer is favored among the products.

E1 - Mechanism:

To illustrate, the following diagram lists the three steps in each transformation.

The dehydration reaction is shown by the blue arrows; the hydration reaction by magenta arrows. The intermediates in these reactions are common to both, and common transition states are involved.