elimination mechanisms Flashcards
what is the difference between the way the attacking ion behaves in substitution vs elimination reactions?
sub (Sn1 and Sn2) : Y- acts as a Nucleophile
elim (E1 and E2): Y- acts as a BASE and attacks at an H
rates of reaction / energy profiles of E1 and E2 mechs
exactly the same as Sn1 and Sn2
E1: RDS only dependent on LG
rate = k1[RL]
E2: different transition state formed, but exactly the same energy profile
rate = k2[RL][Nu]
best orientation of orbitals for E1 reaction following formation of carbocation
C-H sigma bond of the H being removed must be PARALLEL to the empty p orbital for BEST OVERLAP
(in order for the alkene-forming step to proceed)
what are the two HOMO-LUMO interactions occurring in an E1 reaction?
- LP on Base (HOMO) + sigma* C-H (LUMO)
–> break C-H bond - C-H sigma (HOMO) + empty p-orbital (LUMO)
–> form double bond
best orbital orientation for E2 reactions
concerted process, so C-H sigma and C-LG sigma* must lie ANTIPERIPLANAR to one another
what are the two HOMO-LUMO interactions occurring in an E2 reaction?
LP on Base (HOMO) + sigma* C-H (LUMO)
C-H sigma (HOMO) + C-LG sigma* (LUMO)
concerted process
expected stereochemistry of the product of an E1 reaction
E or trans isomer expected (major)
minimisation of steric clash
possible bc rotation about C-C bond possible at carbocation intermediate
stereochemical outcome of E2 reactions
dependent on orbital overlap
as rotation is not possible in transition state
halide leaving group ability for E1 and E2 mechanisms
Br- / I- are both good for E1 and E2
-OH leaving group ability for E1 and E2 mechanisms
NEVER happens
(OH2)+ leaving group ability for E1 and E2 mechanisms
OH under acidic conditions.
Good for E1
NEVER happens for E2, which is always carried out under basic conditions
how to perform E2 mech for an alcohol
ie - how to get -OH to leave
cannot protonate to OH2+ bc E2 only happens in basic conditions
instead convert to TsO-
quaternised amine NR3+ leaving group ability for E1 and E2 mechanisms
good for both E1 and E2
synthesis of a quaternary ammonium salt
amine N lp attack alkyl halide (halide good lg) in Sn2 reaction
four factors to consider for deciding slim or sub occurring
substrate structure
basicity of Nu/Base
Size of Nu/Base
Temperature
how does substrate structure affect whether elim or sub occurs
typical Sn1 substrates also work well in E1 (and often E2!)
those that work well in Sn2 often good in E2 (sometimes E1)
how does basicity of Nu/B affect whether elim or sub occurs
strong bases give mainly elimination, weak bases mainly sub
higher pKa = stronger base
how does size of Nu/B affect whether elim or sub occurs
elim more likely for BULKY nucleophiles (esp if also basic), even for attacking primary halides
common base choice when elim reaction is desired
potassium tert-butoxide – basic, bulky Nu
how does temperature effect elim / sub??
HIGHER temp favours ELIM - entropy: 2 molecules become 3 after elim, vs 2 molecules stay two in sub
what does increasing branching in the substrate favour?
favours elimination over substitution
due to increased carbocation stability for E1 and increased number of protons available for elimination by either mechanism
E2 only occurs with what type of nucleophile?
STRONGLY BASIC
either hindered or unhindered, but most favoured w bulky bases eg. BuO-
good weakly basic nucleophiles tend to react by…
by Sn2, except for tertiary substrates where it reacts by Sn1 or E1.
nucleophiles include I- and RS-
how to influence competition between E1 and Sn1 reactions
high temp favours E1
