Ch_322b_13.08

Ch_322b_13.08 - Chapter 13.8 lecture note

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Electrophilic Attack on Conjugated Dienes: 1,2- and 1,4-Addition Conjugated dienes display a more complicated mode of electrophilic addition than simple alkenes. Typically, more than one product is formed. CH 2 =CH-CH=CH 2 1,3-butadiene HCl 25 o C CH 3 CH-CH=CH 2 + CH 3 -CH=CH-CH 2 Cl Cl 3-chloro-1-butene 1-chloro-2-butene (78%) (22%) The formation of the above two products can be understood within the context of the usual mechanism for electrophilic addition to alkenes, by recognizing the more stable carbocation intermediate . CH 2 =CH-CH=CH 2 1 2 H + C 1 C 2 CH 2 -CH-CH=CH 2 H + H -CH 2 -CH-CH-CH 2 + X less stable primary carbocation more stable allylic-type cation
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Products from 1,2- and 1,4-Additions The products are explained by attack of the nucleophilic Cl - at the two carbons of the allylic-type cation that carry positive charge. H -CH 2 -CH-CH-CH 2 + Cl - CH 3 CH-CH=CH 2 Cl CH 3 -CH=CH-CH 2 Cl The Overall Two-Step Mechanism (1) CH 2 =CH-CH=CH 2 H + slow step electrophilic addition CH 3 CH-CH=CH 2 + CH 3 CH=CH-CH 2 + CH 3 -CH-CH-CH 2 ! + ! + allylic cation
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Ch_322b_13.08 - Chapter 13.8 lecture note

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