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An in-depth analysis of elimination reactions, focusing on the role of leaving groups, mechanisms, and selectivity. It covers various types of elimination reactions, including E1, E2, SN1, and SN2, and discusses their differences in terms of substrate dependence, stereochemistry, and importance of base/nucleophile and leaving group. It also highlights the competition between elimination and substitution reactions.
What you will learn
Typology: Study notes
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Common Leaving Groups RH 2 C N N
Leaving group
NC CH 2 R N N
NC:
RH 2 C N N
RH 2 C S C 4 F 9
O
O RH 2 C S CH 3
O
O RH 2 C I RH 2 C Br
RH 2 C O
H
H
RH 2 C O
H
CH 3
RH 2 C Cl
RH 2 C N CH 3
CH 3
CH 3
Diazonium salt
Class of compound Leaving group
Nonaf late (^) C 4 F 9 SO 3 -
Mesylate
Iodides Bromides
Protonated alcohols
Protonated ethers
Chlorides
Quaternary Ammonium Salts
Excellent leaving groups
Good leaving groups
N 2
CH 3 SO 3 -
I- Br-
H 2 O
CH 3 OH
Cl-
N(CH 3 ) 3
RH 2 C F
O
C CH 3
O RH 2 C
RH 2 C H
RH 2 C OH
RH 2 C NH 2 RH 2 C CH 3
Fluorides
Acetates (^) Acetate anion, CH 3 CO 2 -
Alcohols (^) Hydroxide, HO- Hydrides (^) Hydride, H- Amines Amide, NH 2
Alkanes
Very poor leaving groups F-
CH 3 -
Poor Leaving Groups
The substrates that favour E1 reactions are the same that favour SN1 reactions:
The difference between E1 and SN1 reactions is in the type species which reacts with the substrate. E1 reactions are favoured with:
A different elimination product is possible for every unique type of H beta (β) to the carbocation carbon.
70% 30%
H H
H
H 2 SO 4
OH
α β β
γ
δ
H
R H
R (^) H
R R
H
trans > cis
Conjugate > skipped
Endocyclic > exocyclic
Alkene stability is determined by heats of hydrogenation.
C
H 3 C
H 3 C
H 3 C
Br C
H 3 C
H 3 C
H 3 C
OCH 3
If you want SN1, what nucleophile is best?
The E2 Reaction
HO (^) Br C
Br
Br
The mechanism:
H C C
H
CH 3
CH 3
H (^) Br
H O
H C^ C CH 3
H^ CH^3
Br
H O H
The β-proton pulled off by the base must be anti-periplanar to the leaving group. This reaction is referred to as a "beta-elimination".
Ph
Ph
Br
Base C C Ph Ph
Ph Ph
Ph
Ph
Br
Base C C Ph H
H Ph 3 C^ = (^) C C
Ph H
H 3 C Ph
Ph
Ph
Br
Ph
Ph
Stereochemical Consequences
E2:
E1:
It is possible to convert 1°alcohols to alkenes:
H C C
H
H
H
R (^) OH 2
SO 3 H
O
H C^ C H
R H
H
O
H
H C C H
H
H
R (^) OH
H 2 SO 4
What kind of problems could we expect with the above reaction?
H 3 C C CH 3
H 3 C (^) Br H 3 CH 2 C O H 3 C C CH 2
CH 3 100%
H 3 C C CH 3
H (^) Br H 3 CH 2 C O
H 3 C C CH 3
H (^) OCH 2 CH 3
H 3 C C CH 2
H
21%
79%
CH 3 CH 2 Br
Because many good nucleophiles are also good bases, SN2 often competes with E for those substrates that are good for SN 2
Ph
H H
Ph Br 2
Ph
Ph
H 2 N
Ph H
Br Ph
H 2 N
Ph Ph
H
Br
BrH
Elimination reactions can be used to prepare alkynes:
Br
H H 2 N