e2 reaction mechanism

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  • 1.E2 REACTION MECHANISMImmanuelle Orchidea By PresenterMedia.com

2. Keywords : bimolecular concerted dehydrohalogenation kinetics mechanism periplanar ("syn" and "anti") regioisomers second-order reaction staggered conformation stereoisomers 3. 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. 4. conversion of tert- butyl bromide to isobutylenethe base-induced elimination of "HX" (dehydrohalogenation) of an alkyl halide gives rise to an alkene 5. Base (B: ) attacks a neighboring CH 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 leaveTransition StateNeutral alkene is produced when CH bond is fully broken and the X group has departed with the C X bond electron pair 6. Stereoselectivity in E2 7. Stereoselectivity 8. 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) 9. 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 eliminationCyclobutyl90Cyclopentyl46Cyclohexyl4Cycloheptyl37 10. Orientation in E2 In substrates which have alternative bHydrogen 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? Hofmannrules Saytzev rules 11. Orientation in E2 Saytzev vs HofmannHofmann : 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) 12. 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.