Ch 5: Dehydrohalogenation

Chapter 5:
Structure and Preparation of Alkenes.
Elimination Reactions

Dehydrohalogenation of Alkyl Halides

Reaction type: 1,2- or β-Elimination

Summary

When heated with strong bases, alkyl halides typically undergo a 1,2-elimination reactions to generate alkenes.
Typical bases are NaOH or KOH or NaOR or KOR (alkoxide) especially NaOEt or KOtBu in the alcohol as solvent.
Nucleophilic substitution is a potential side reaction typically leading to alcohols or ethers. (summary)
Alkyl halide relative reactivity order : 3^o > 2^o > 1^o
Alkyl halide relative reactivity order : I > Br > Cl > F (n.b. elimination of F is rarely used).
Product distribution:

Regioselectivity is usually controlled by the relative stability of the product alkenes (Zaitsev's rule).
However bulky bases such as t-butoxide (tBuO-) often lead to the anti-Zaitsev product.
Similarly, subsituted cyclic halides may also give the anti-Zaitsev product.
Stereoselectivity due to the preferred antiperiplanar (180^o) arrangment of -H and -X in the transition state.