Eliminations Flashcards
(14 cards)
What is involved in elimination reactions?
A base (either negatively charged or have a lone pair)
Forms an alkene
What is an E2 reaction?
Bimolecular elimination reaction with second order rate kinetics
What is an E1 reaction?
Unimolecular elimination reaction with first order rate kinetics
Favours tertiary compounds
Equation for E2 reactions
Rate of reaction depends on the concentration of both the substrate and the base
Rate = k [base] [R-X]
Where R-X is the alkylhalide (substrate)
What does the E2 mechanism involve?
Removal of a proton from the alkylhalide
The proton being removed is in plane and opposite the leaving group
Importance of product structures in E2 mechanism
The reactive conformation is the antiperiplaner one where the proton is opposite the leaving group
The unreactive conformation would be when the leaving group and proton are next to each other
E2 vs SN2 reactions
An SN2 reaction will be inhibited when there is a strong base so instead, elimination (E2) will occur
When a small nucleophile is used, it will be SN2
Equation for E1 reaction
Rate = k [R-X]
Where X is the leaving group
What does an E1 reaction involve?
Loss of leaving group —> slow step
Proton abstraction and double bond formation on a carbocation intermediate
E1 vs SN1 reactions
SN1 reactions are favoured by small, reactive nucleophiles = Nucleophilic attack
E1 reactions are favoured by bulky bases and high temperatures = elimination
Biosynthesis of terpene
GPP with a diphosphate leaving group undergoes heterolysis and cyclisation to form an alpha-terpinyl cation which can react in different ways depending on the conditions
E1 or SN1 in terpene biosynthesis
SN1 reaction has GPP reacting with water and forms alpha-terpinol = found in pine trees
E1 reaction removes a proton from GPP and forms limonene = found in citrus fruits
E1 reaction energy profile
Energy profile for E2 reactions
