C=C Formation (Elimination)

Question 1

substrate structure and E vs. Sn

Answer 1

typical Sn1/Sn2 substrates:

• work well for E1/E2 respectively
• often/sometimes work well for E2/E1 respectively
• flexibility in elimination pathway, not inextricably linked to Sn1/2

steric factors disfavouring Sn2 at hindered centres don’t affect elimination i.e. α-proton to LG being removed is much less affected by steric bulk

Question 2

E vs. Sn

Answer 2

normally both occur there will just be majority of product as one

Question 3

nucleophile/base basicity and E vs. Sn

Answer 3

STRONG BASES MAINLY GIVE ELIMINATION
(so drives reaction by deprotonation)

WEAK BASES GIVE NUCLEOPHILIC SUBSTITUTION

use pKaH

Question 4

size of nucleophile/base and E vs. Sn

Answer 4

Sn2 DISFAVOURED ON HINDERED ALKYL HALIDES
In Sn2, Nu attacks C (squeezes though substrate substituents to acmes σ*)

accessing more exposed α hydrogen in E is far easier

E PREFERRED OVER SN WHEN USING BULKY BASIC NUCLEOPHILES

Question 5

reaction temperature and E vs. Sn

Answer 5

HIGH TEMPS FAVOUR ELIMINATION

ENTROPY: 2 MOLECULES BECOME 3 IN ELIMINATION
2 MOLECULES BECOME 2 IN SUBSTITUTION

ENTROPY CHANGE GREATER IN ELIMINATION
ΔG=ΔH-TΔS
@ high temperatures higher enthalpy change becomes more important

want to make ΔG more negative

Elimination normally carried out @ room temperature or above

Question 6

Common base choice when elimination required

Answer 6

tBuOK - hindered base/nucleophile
(CH3)3C-O-
substitution = slow