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Ch_322b_15.14 - and the reactivity difference between I and...

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Alkenylbenzenes Alkenylbenzenes with a side chain double bond conjugated with the aromatic ring are more stable than nonconjugated systems. C=C-C- conjugated system C-C=C- nonconjugated system more stable less stable In acid-catalyzed dehydration reactions , the more stable, conjugated alkenylbenzene is preferentially formed: CH 2 CHCH 3 OH H + (-H 2 O) CH=CHCH 3 1-phenyl-2-propanol 1-phenyl-1-propene major product
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Additions to Alkenylbenzenes Addition reactions with conjugated alkenylbenzenes proceed through the more stable intermediate . CH=CHCH 3 HBr no peroxides CHCH 2 CH 3 + more stable HBr no peroxides CH 2 CHCH 3 + less stable X Br - CHCHCH 3 Br 1-bromo-1-phenylpropane
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Free Radical Addition of HBr Free radical addition reactions to conjugated alkenylbenzenes proceed by way of the more stable radical intermediate . CH=CHCH 3 HBr peroxides CH 2 CHCH 3 Br 2-bromo-1-phenylpropane CHCHCH 3 more stable via Br . . Br HBr (-Br ) . The regiochemistry of additions to conjugated alkenylbenzenes is explained by preferred formation of the more stable benzyl-type intermediate .
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Quiz 15.014 Explain the products produced in the following addition reactions,
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Unformatted text preview: and the reactivity difference between I and II. CH=CHCH 3 HBr no peroxides CHBrCH 2 CH 3 CH=CHCH 3 HBr no peroxides CHBrCH 2 CH 3 I CH 3 O-CH 3 O-II slower faster solution These are ionic additions where formation of the carbocation intermediate is the rate determining step. The orientation of the addition reaction follows from the greater stability of the benzylic type carbocation compared with a simple secondary carbocation, resulting in a lower transition state energy for formation of the benzylic type carbocation and subsequent formation of the observed products. The faster reaction rate for II reflects the stabilizing influencing of the p-methoxy group on the developing benzylic type carbocation, as shown in the resonance structures below for the intermediate ion. CHCH 2 CH 3 CH 3 O-+ : CHCH 2 CH 3 CH 3 O-+ = CHCH 2 CH 3 CH 3 O-+ = CHCH 2 CH 3 CH 3 O= + =...
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