Organic Chemistry 353-Chapter 8 - WhenY=HandZ= Alkenes...

Elimination Reactions of Alkyl Halides
Alkyl halides can also undergo elimination
U d t th i lk Used to synthesise alkenes
When Y=H and Z=Halogen Dehydrohalogenation

Alkenes

Elimination Mechanisms : E2 Reaction
Again kinetic studies are useful to probe mechanism
Transition state must involve the alkyl halide and alkoxide

E2 and S
N
2

The Zaitsev (Saytzeff) Rule
In elimination reactions, the most stable alkene usually predominates

E1 Reaction
An E1 reaction exhibits first ‐ order kinetics: An E1 reaction exhibits first order kinetics:
Mechanisms of Elimination E1 Rate=k[(CH
3
)
3
CCI]

E1 Reaction

Competition with S
N
1
E1 and S
N
1 occur together, t ‐ butyl bromide in ethanol

Rearrangements

Stereochemistry of the E2 Reaction
Anticoplanar TS important in cyclic systems

The Stereochemistry of E2 Reactions
Antiperiplanar arrangement is required
Allows for best overlap of orbitals in the developing π ‐ bond
and staggered conformation is more stable
Cyclohexane molecules provide experimental evidence

The Stereochemistry of E2 Reactions
Two axial cyclohexane β ‐ Hydrogens
Zaitsev’s rule is satisified Zaitsev s rule is satisified