查看完整版本 : organic chemistry question urgent

銀月遊俠 2012-12-14 05:11 PM

organic chemistry question urgent

I noticed that there is different way to open a unsymmetrical three member ring I want to ask which direction is correct for each three member rings(a or b).
[img=143,253]http://i1189.photobucket.com/albums/z428/AnthonyCChem/Captu_zps66543b80.png[/img]
the opening of brominum ion and the second the carbocation from alkyne( by the way is there a name for this ion?)
[img=142,232]http://i1189.photobucket.com/albums/z428/AnthonyCChem/Cap_zps5a81a928.png[/img]
the first one is acid-catalysed ring opening of epoxides and the second one is base-catalysed

jmlo 2012-12-14 11:15 PM

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[quote] I noticed that there is different way to open a unsymmetrical three member ring I want to ask which direction is correct for each three member rings(a or b). [/quote]
Path a is more favorable. The partial positive charge built at the transition state is more stabilized when it resides on the more substituted carbon. The regioselectivity is predicted by the Markovnikov's rule.

[quote] the opening of brominum ion and the second the carbocation from alkyne( by the way is there a name for this ion?) [/quote]
Very often we just call it bromonium ion intermediate.

[quote] the first one is acid-catalysed ring opening of epoxides and the second one is base-catalysed [/quote]
The first case requires an acid as catalyst as H2O is a weak nucleophile. On the other hand, the second case does not need any catalyst since OH- is already a strong nucleophile.

The regioselectivity of these two cases are different. For acid-catalyzed epoxide ring opening, the Markovnikov's rule is followed. The nucleophile attacks the more substituted carbon. That is, path a is more favorable. On the contrary, for the ring opening by a strong nucleophile, the attack occurs on the less substituted carbon. In other words, path b is more favorable.
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