Reaction Review 18 - Elimination Dehydrohalogenation of Alkyl Halides to form Alkenes by E1 and E2 PDF

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Download Reaction Review 18 - Elimination Dehydrohalogenation of Alkyl Halides to form Alkenes by E1 and E2 PDF

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Rxn Review #18 – Beta-Elimination of Alkyl Halides to Form Alkenes The general reaction = converts alkyl halide to alkene by E2.

E2 reactions are 2nd order and concerted. Rate = k[R-X][base]. No carbocation is involved and heat often used to enhance rate of this reaction. What to look for to recognize these reactions? 2o or 3o alkyl halides with strong bases is most common condition. There must be beta protons that can assume anti-coplanar position for reaction to occur. Beta protons removed by base. Rate of reaction by E2 3o alkyl halides > 2o alkyl halides >> 1o alkyl halides Rate of leaving group I > Br > Cl >> F Zaitsev rule favors formation of more substituted alkene if possible by anticoplanar mechanism. See examples. Example of anti-coplanar arrangement necessary for E2 to occur:

Other E2 examples: (can be stereospecific reaction)

E1 Reactions: With weak base solutions and heat, E1 reactions are often observed with alkyl halides to form alkenes. Weak bases are solvents like water and alcohols. E1 follows the same reactivity and Zaitsev rules as E2. No stereospecificity and trans alkenes preferred over cis. We’ll see E1 reactions compete with and/or accompany the SN1 reactions in Ch. 9. Rearrangements are possible. Sample E1 reaction...