CH 3 CHCH 3
Br
C 2 H 5 Ο−Na
O C 2 H 5
+ + C 2 H 5 OH
(−^55 NaBr)oC CH^3 CHCH^3
+ H 2 CCHCH 3
SN 2 (21%) E2 (79%)
- Tertiary halides: no SN2 reaction, elimination reaction is highly favored
CH 3 CCH 3
Br
C 2 H 5 Ο−Na
O C 2 H 5
+ + C^2 H^5 OH
25 oC
(−NaBr)
+ H 2 CCHCH 3
CH 3
SN 2 (9%) E2 (91%)
CH 3 CCH 3
CH 3
H 2 C CCH 3
CH 3
C 2 H 5 Ο−Na + C 2 H 5 OH
E2 + E1
(100%)
C 2 H 5 OH
55 oC
(−NaBr)
CH 3 CCH 3
Br
+ +
CH 3
1) Elimination is favored when the reaction is carried out at higher temperature.
i) Eliminations have higher free energies of activation than substitutions because
eliminations have a greater change in bonding (more bonds are broken and
formed).
ii) Eliminations have higher entropies than substitutions because eliminations
have a greater number of products formed than that of starting compounds).
2) Any substitution that occurs must take place through an SN1 mechanism.
6.19B TERTIARY HALIDES: SN 1 VERSUS E1
- E1 reactions are favored:
- with substrates that can form stable carbocations.
- by the use of poor nucleophiles (weak bases).
- by the use of polar solvents (high dielectric constant).
- It is usually difficult to influence the relative position between SN1 and E1
products.
