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Elimination Reactions: Leaving Groups, Mechanisms, and Selectivity, Study notes of Stereochemistry

University of Indianapolis (UIndy)Stereochemistry

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

  • What are the differences between E1 and E2 reactions in terms of substrate dependence and stereochemistry?
  • What are the mechanisms of E1, E2, SN1, and SN2 reactions?
  • What are elimination reactions and how do they differ from substitution reactions?
  • What are the key factors that influence the selectivity of elimination reactions?
  • How does the choice of base/nucleophile and leaving group affect the outcome of elimination reactions?

Typology: Study notes

2021/2022

Uploaded on 09/27/2022

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Common Leaving Groups
RH2C N N
Leaving group
NC
CH2R N N
NC:
RH2C N N
RH2C S C4F9
O
O
RH2C S CH3
O
O
RH2C I
RH2C Br
RH2COH
H
RH2COH
CH3
RH2C Cl
RH2CNCH3
CH3
CH3
Diazonium salt
Class of compound Leaving group
Nonaflate C4F9SO3-
Mesylate
Iodides
Bromides
Protonated alcohols
Protonated ethers
Chlorides
Quaternary Ammonium Salts
Excellent leaving groups Good leaving groups
N2
CH3SO3-
I-
Br-
H2O
CH3OH
Cl-
N(CH3)3
pf3
pf4
pf5
pf8
pf9
pfa
pfd
pfe
pff
pf12
pf13
pf14
pf15
pf16

Partial preview of the text

Download Elimination Reactions: Leaving Groups, Mechanisms, and Selectivity and more Study notes Stereochemistry in PDF only on Docsity!

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:

  • A substrate bearing a good leaving group attached to a tetrahedral carbon atom.
  • A substrate that can form a relatively stable carbocation.

The difference between E1 and SN1 reactions is in the type species which reacts with the substrate. E1 reactions are favoured with:

  • Bases that are poor nucleophiles (good nucleophiles will favour substitution reactions).
  • Remember: Substitution and Elimination reactions are always competing (whenever possible).

Elimination Reactions – The E1 Mechanism

A different elimination product is possible for every unique type of H beta (β) to the carbocation carbon.

E1 Reactions – Stereochemistry and Regiochemistry

70% 30%

H H

H

H 2 SO 4

OH

α β β

γ

δ

Alkene Stability

C atoms with more s character tend to form stronger bonds with other carbons.

R

R R

R R

R H

R R

R H

H H

R H

R H

R H

H

> >^ = >

H

R H

R (^) H

R R

H

trans > cis

Conjugate > skipped

Endocyclic > exocyclic

Elimination Reactions - Kinetic vs. Thermodynamic Products

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

C

H 3 C

H 3 C

H 3 C

HO (^) Br C

H 3 C

H 3 C CH 2

  • H 2 O + Br

C

H 3 C

H 2 C

H 3 C

Br

HO

C

CH 3

CH 3

H 2 C

H

H

O H

Br

The mechanism:

Why?

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".

E2 Reactions – Stereochemistry and Regiochemistry

H

C C

Ph

Ph

H

H 3 C

Br

Base C C Ph Ph

H H

3 C^ = C C

Ph Ph

H 3 C H

H

C C

Ph

H

Ph

H 3 C

Br

Base C C Ph H

H Ph 3 C^ = (^) C C

Ph H

H 3 C Ph

H

C C

Ph

Ph

H

H 3 C

Br

H

C C

Ph

Ph

H

H 3 C

Stereochemical Consequences

E2:

E1:

E2 Reactions – Elimination of Primary Alcohols

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?

E2 Reactions – E2 vs. SN 2

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

H 3 CH 2 C O

H 3 CH 2 C O CH 2 CH 3 99%

Because many good nucleophiles are also good bases, SN2 often competes with E for those substrates that are good for SN 2

E2 Reactions – Preparation of Alkynes

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

E1 vs. E2 vs. SN1 vs. SN2 - Summary

  • As a general rule, elimination reactions can always compete with substitution reactions. We can, however, alter the reaction conditions to favour one process over another.
  • To favour E1 over SN1 for alcohols, use an acid with a non-nucleophilic conjugate base (H 2 SO 4 , H 3 PO 4 ). To favour SN1 over E1, use a good nucleophile.
  • To favour E2 over SN2, use a strong, bulky non-nucleophilic base. To favour SN 2 over E2, use good nucleophiles that are relatively weak bases.
  • It is important to keep in mind that although you might choose reaction conditions that will favour one reaction over another, more often than not you will still see traces of the competing reaction.
  • Before considering the possibility of an elimination reaction, make sure there are β-hydrogen atoms available to eliminate!