Factors Affecting E2 Flashcards
Can primary, secondary or tertiary react in E2?
Tertiary can but not in SN2
Secondary substrates produce half E2 and half SN2
Primary favours SN2 reactions and only forms a small amount of E2 product
Describe the effect of base in E2 reactions
Strong bases and increasing concentration favours E2- part of RDS
What do large bulky bases favour?
They favour E2 reactions as primary halogenoalkanes react in E2
There would be too much steric hindrance to react with tertiary in E1
What solvents prefer E2?
Aprotic solvents favour E2 reactions as they do not solvate and hinder the base
How do leaving groups affect the rate?
Good leaving groups increase the rate of E2
This is because the rate determining step involves the cleavage of HX
The better the leaving group, the faster the cleavage and so the faster the rate
Why is the E2 reaction stereospecific?
It is stereospecific as it has no choice and only one product is formed
Only the E isomer is formed due to steric strain
Proceeds via an antiperiplanar conformation
What is saytzeffs rule?
The most substituted alkene is formed as it is more stable
What is the product if the base is large?
Hoffmann rule applies
This states at the least substituted alkene is formed as the primary carbon hydrogen is more accessible
What arrangement is necessary for elimination reaction?
Diaxial arrangement is necessary for elimination reaction
What is the curtin Hammett principle?
This states that the ratio of activation energy dictates the product ratio- kinetic control
What is Bredts rule?
Double bonds do not form at bridgeheads
What conditions are required for the E1cb mechanism ( conjugate base)
Required conditions:
- protons easy to remove
Resultant anion- stabilised
The leaving group must be hesitant to leave
