elimination Flashcards
what is the process of E1
unimolecular
Halogen removed first forming carbocation intermediate (RDS),
LP on-base bonds on hydrogen from sp3 carbon which is removed and alkene bond is formed, leaving X- and BH+
what is the rate law, order of reactivity and stereochemical outcome of E1
rate = K[R2CHCR2X]
order: T > S > P > Me
Most substituted alkene is formed
what is the process of E2 elimination1
bimolecular
Leaving group and proton must be anti-periplanar for good orbital overlap.
the one-step process to TS where B- bonds to H which forms alkene bond and X leaves in same step.
alkene + X- + BH formed
what is the rate law, order of reactivity and stereochemical outcome of E2
rate = K[R2CHCR2X][B-]
order: T > S > P
most substituted alkene is formed
explain trends in reactivity of E2
more bulk around group faster ROR as more probability of attack when more beta-H
more substituted alkene is more stable and formed more rapidly through a stabilised TS. (hammonds postulate)
how is orbital overlap important in E2
H and leaving group must be anti periplanar for good overlap of sigma and sigma* and electron transfer from sigma to leaving group form p orbital, the two p orbitals align.
what is saytzev’s rule
in dehydrohalogenation the preserved product is the alkene that has the greatest number of alkyl groups attached to the doubly bonded carbon atom.
how does Regioselectivity in E2 occur
more substituted alkene more stable, E2 transition state has partial double bond character, same factors that stabilise alkene, stabilise TS
how does regioselectivity occur in E1
TS for 2nd step of E1 has partial double bond character, same preferences for most substituted alkene.
define stereospecific
reaction in which stereoisomeric starting materials afford stereoisomerically different product under same reaction conditions
what are the conditions of E2 for cyclohexanes
both C-H and C-X must be axial for anti-periplanar layout
how can E1 eliminations be stereoselective
drawing newman projection of carbocation p orbital should be co-planar with proton
how can E2 be forced
use a concentrated solution of a strong base due to rate equation
how do hard and soft nucleophiles compare for elimination and substitution
strong, hard base = elimination E2
soft base = substitution Sn2
hard BULKY base favours the elimination and less substituted alkene for steric reasons. removed less hindered protons
it is too slow for Sn2 for steric reasons. can force elimination E2
what are the effects of temperature on the favouring of substitution and elimination
increasing temperature favours elimination
deltaG = deltaH - TdeltaS
in substitution there are 2 reactants and 2 products so deltaS = 0
elimination 2 reactants and 3 products deltaS > 0
