Solutions_Manual_for_Organic_Chemistry_6th_Ed 329

Solutions_Manual_for_Organic_Chemistry_6th_Ed 329 - 1 5-7...

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Unformatted text preview: 1 5-7 continued ( e) H 3C - ?� A ?? :; + - == CH2 H H _ g � (? ? + same carbocation as in (d) == CH2 H 3C - a llylic -H 3C - t wo c arbons are e lectron defic ient so nucleophile can attack either H2 R H H ??f == H H + H2 I + + , H2 } R OH H 3C - C - C == CH2 H I OH I H I + H 3C - C == C - CH2 H I I H I 1 5 -8 ( a) Br H 2C - C - C == CH2 I Br I Br A H I H I + H 2C - C == C - CH2 I Br I ( b) � n / B r - Br H H ..... B H I H I Br I + 20 H H .. :Br : Br - -- allylic •• - :Br: •• H2C - C - C == CH2 I Br I Br A H I H I + H 2C - C == C - CH2 I Br I two carbons are electron deficient s o nucleophi le can attack either B H I H I ( c) The resonance form A + , w hich e ventuall y leads to product A , h as positi ve charge on a 2° c arbon and is a m ore significant resonance contributor than structure B + . W ith greater positive charge on the 2° c arbon than on the 1 ° c arbon , we would expect bromide ion attack on the 2 ° c arbon to have lower activation energy. Therefore, A m ust be the k inetic p ro d uc t . A t higher temperature , however, the last step becomes reversible, and the stabi lity of the products becomes the dominant factor in determining product ratios. As B h as a disubstituted alkene whereas A i s only monosubstituted, it is reasonable that B is the major, thermodynamic p roduct at 60° C . 3 22 ...
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