Dehydration of Alcohols

Summary:

• When heated with strong acids catalysts (most commonly H₂SO₄, H₃PO₄), alcohols typically undergo a 1,2-elimination reactions to generate an alkene and water.
• Also known as dehydration since it involves the removal of a molecule of water.
• Alcohol relative reactivity order : 3^o > 2^o > 1^o
• Regioselectivity : major product is usually the more highly substituted alkene (alkene stability) Zaitsev's Rule.
• Stereoselectivity : trans- > cis- again controlled by stability
• Reaction of secondary or tertiary alcoholsusually proceeds via an E1 mechanism which proceeds via a carbocation intermediate, that can often undergo rearrangement.
• Primary alcohols will proceed via an E2 mechanism since the primary carbocation is highly unfavorable.
• Other common strong acids such as HCl, HBr or HI are less suitable catalysts as nucleophilic substitution reactions will probably interfere.

Dehydrohalogenation of Alkyl Halides

Summary:

• When heated with strong bases, alkyl halides typically undergo a 1,2-elimination reactions to generate alkenes.
• Typical bases are sodium or potassium hydroxides or alkoxides in the alcohol as solvent.
• Alkyl halide relative reactivity order :  I > Br > Cl > F   (nb. elimination of F is rarely used).
• Product distribution:
• Regioselectivity is usually controlled by the relative stability of the product alkenes (Zaitsev's rule).
• Stereoselectivity due to the preferred antiperiplanar (180^o) arrangment of -H and -X in the transition state.