Dehydrohalogenation; mechanism and kinetics of E1 [partly illegible]

Dehydrohalogenation

An elimination reaction is a reaction in which two atoms or groups are removed from adjacent carbon atoms of a molecule, forming a double (or triple) bond, without being replaced by other groups. Dehydrohalogenation of an alkyl halide is the elimination of H and X (as HX) from adjacent carbons, usually by heating with a base such as alcoholic KOH, to form an alkene.

E1 mechanism

Mechanism: E1 (two steps).

Step 1 (slow): ionisation to carbocation
 (CH₃)₃C—Br  ---slow--->  (CH₃)₃C⁺ + Br⁻

Step 2 (fast): base removes a β-hydrogen
 OH⁻ + H—CH₂—C⁺(CH₃)₂ ---fast---> H₂O + CH₂=C(CH₃)₂

Overall: (CH₃)₃C–Br + KOH(alc) → (CH₃)₂C=CH₂ + KBr + H₂O (2-methylpropene)

• The carbocation is an intermediate, so rearrangement is possible, and E1 competes with SN1.
• E1 is favoured by 3° halides, a weak base (or low base concentration) and a polar protic solvent.

Kinetics

Only the alkyl halide takes part in the slow, rate-determining step (carbocation formation), so:

Rate = k [R–X]

The reaction is first order overall (unimolecular) and its rate does not depend on the concentration of the base. The rate increases with carbocation stability: 3° > 2° > 1°.