e2 reaction mechanism
TRANSCRIPT
E2 REACTION MECHANISMImmanuelle Orchidea
By PresenterMedia.com
Keywords :• bimolecular • concerted • dehydrohalogenatio
n • kinetics • mechanism
• periplanar ("syn" and "anti")
• regioisomers • second-order
reaction • staggered
conformation • stereoisomers
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.
E2 Reactionconversion of tert- butyl bromide to isobutylene
the base-induced elimination of "HX" (dehydrohalogenation) of an alkyl halide gives rise to an alkene
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
Stereoselectivity in E2
Anti-periplanar Syn-periplanar
Stereoselectivity
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)
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
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
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
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.