e2 reaction mechanism

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E2 REACTION MECHANISM Immanuelle Orchidea By PresenterMedia.com

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Page 1: E2 reaction mechanism

E2 REACTION MECHANISMImmanuelle Orchidea

By PresenterMedia.com

Page 2: E2 reaction mechanism

Keywords :• bimolecular • concerted • dehydrohalogenatio

n • kinetics • mechanism

• periplanar ("syn" and "anti")

• regioisomers • second-order

reaction • staggered

conformation • stereoisomers

Page 3: E2 reaction mechanism

Overview

The term E2 stands for "elimination bimolecular.“The product of an E2 reaction has one more degree of unsaturation than the starting materials did.

Page 4: E2 reaction mechanism

E2 Reactionconversion of tert- butyl bromide to isobutylene

the base-induced elimination of "HX" (dehydrohalogenation) of an alkyl halide gives rise to an alkene

Page 5: E2 reaction mechanism

Mechanism

Base (B: ) attacks a neighboring C—H bond and begins to remove the H at the same time as the alkene double bond starts to form and the X group starts to leave

Transition StateNeutral alkene is produced when C—H bond is fully broken and the X group has departed with the C –X bond electron pair

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Stereoselectivity in E2

Anti-periplanar Syn-periplanar

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Stereoselectivity

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Stereoselectivity

In most simple acyclic cases, ANTI elimination is found to be very much preferred

The degree of stereoselectivity may be influenced to some extent by :-Polarity-Ion solvating ability (solvent)

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Stereoselectivity In cyclic compounds the conformation from which elimination can take place may to a considerable extent be enforced by the relative rigidity of the ring structure.

The following degrees of stereoselectivity were observed for HY elimination from the cyclic compounds (CH2)nCHY:

Ring Size %SYN elimination

Cyclobutyl 90

Cyclopentyl 46

Cyclohexyl 4

Cycloheptyl 37

Page 10: E2 reaction mechanism

Orientation in E2

In substrates which have alternative b-Hydrogen atoms available, it is possible to obtain more than one alkene on elimination.

How do we forecast which alkene is the more likely to be produced?

Hofmann rulesSaytzev rules

Saytzev vs Hofmann

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Orientation in E2

Hofmann : that alkene will predominate which has LEAST alkyl substituents on the double bond carbons (karbo kation yang diserang yang kurang stabil)

Saytzev : that alkene will predominate which has MOST alkyl substituents on the double bond carbons (karbo kation yang diserang yang lebih stabil, yang lebih banyak mengandung alkil substituen)

Saytzev vs Hofmann

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That increase in the size of Y and more particularly branching in it, leads to increasing proportion of HOFMANN elimination with the same alkyl group.

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