elimination mechanisms

Question 1

what is the difference between the way the attacking ion behaves in substitution vs elimination reactions?

Answer 1

sub (Sn1 and Sn2) : Y- acts as a Nucleophile
elim (E1 and E2): Y- acts as a BASE and attacks at an H

Question 2

rates of reaction / energy profiles of E1 and E2 mechs

Answer 2

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]

Question 3

best orientation of orbitals for E1 reaction following formation of carbocation

Answer 3

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)

Question 4

what are the two HOMO-LUMO interactions occurring in an E1 reaction?

Answer 4

1. LP on Base (HOMO) + sigma* C-H (LUMO)
   –> break C-H bond
2. C-H sigma (HOMO) + empty p-orbital (LUMO)
   –> form double bond

Question 5

best orbital orientation for E2 reactions

Answer 5

concerted process, so C-H sigma and C-LG sigma* must lie ANTIPERIPLANAR to one another

Question 6

what are the two HOMO-LUMO interactions occurring in an E2 reaction?

Answer 6

LP on Base (HOMO) + sigma* C-H (LUMO)

C-H sigma (HOMO) + C-LG sigma* (LUMO)

concerted process

Question 7

expected stereochemistry of the product of an E1 reaction

Answer 7

E or trans isomer expected (major)
minimisation of steric clash
possible bc rotation about C-C bond possible at carbocation intermediate

Question 8

stereochemical outcome of E2 reactions

Answer 8

dependent on orbital overlap
as rotation is not possible in transition state

Question 9

halide leaving group ability for E1 and E2 mechanisms

Answer 9

Br- / I- are both good for E1 and E2

Question 10

-OH leaving group ability for E1 and E2 mechanisms

Answer 10

NEVER happens

Question 11

(OH2)+ leaving group ability for E1 and E2 mechanisms

Answer 11

OH under acidic conditions.

Good for E1
NEVER happens for E2, which is always carried out under basic conditions

Question 12

how to perform E2 mech for an alcohol

Answer 12

ie - how to get -OH to leave
cannot protonate to OH2+ bc E2 only happens in basic conditions
instead convert to TsO-

Question 13

quaternised amine NR3+ leaving group ability for E1 and E2 mechanisms

Answer 13

good for both E1 and E2

Question 14

synthesis of a quaternary ammonium salt

Answer 14

amine N lp attack alkyl halide (halide good lg) in Sn2 reaction

Question 15

four factors to consider for deciding slim or sub occurring

Answer 15

substrate structure
basicity of Nu/Base
Size of Nu/Base
Temperature

Question 16

how does substrate structure affect whether elim or sub occurs

Answer 16

typical Sn1 substrates also work well in E1 (and often E2!)
those that work well in Sn2 often good in E2 (sometimes E1)

Question 17

how does basicity of Nu/B affect whether elim or sub occurs

Answer 17

strong bases give mainly elimination, weak bases mainly sub
higher pKa = stronger base

Question 18

how does size of Nu/B affect whether elim or sub occurs

Answer 18

elim more likely for BULKY nucleophiles (esp if also basic), even for attacking primary halides

Question 19

common base choice when elim reaction is desired

Answer 19

potassium tert-butoxide – basic, bulky Nu

Question 20

how does temperature effect elim / sub??

Answer 20

HIGHER temp favours ELIM - entropy: 2 molecules become 3 after elim, vs 2 molecules stay two in sub

Question 21

what does increasing branching in the substrate favour?

Answer 21

favours elimination over substitution

due to increased carbocation stability for E1 and increased number of protons available for elimination by either mechanism

Question 22

E2 only occurs with what type of nucleophile?

Answer 22

STRONGLY BASIC

either hindered or unhindered, but most favoured w bulky bases eg. BuO-

Question 23

good weakly basic nucleophiles tend to react by…

Answer 23

by Sn2, except for tertiary substrates where it reacts by Sn1 or E1.

nucleophiles include I- and RS-

Question 24

how to influence competition between E1 and Sn1 reactions

Answer 24

high temp favours E1