Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: MHatsune197 on April 25, 2017, 02:09:23 PM
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Hello everyone,
I am writing a lab report for the bromination of (E)-stilbene in which I obtained a melting point range that supported the meso-stilbene dibromide. However I am stuck cause I was given a question that asks how does this supports the hypothesis that we would obtain meso as the major product. Does it has to do with Markovnikov's rule or is it the anti-Markovnikov? And why does it not go through the carbocation, does it had to do with the stability? or I was thinking it was because it was better to go through Sn2...help me please ???
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Hi,
No it doesn't go through the formation of a carbocation, nor is it SN2. Think about the size of the bromine atom relative to the two carbon atoms. The intermediate formed is a cyclic bromonium ion. Again, due to the large size of the bromine atom, the second bromine attacks from the opposite side, mainly for steric reasons. Hopefully this helps some bit.
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The bromonium ion is correct, but your use of sterics as a rationale for the stereochemistry of the ring-opening isn't. This second step is an SN2 reaction, and as such it is stereospecific and goes with inversion of stereochemistry. In other words, the second bromide can only attack in the way you've drawn, regardless of any steric factor.
Since stilbene is symmetrical Markovnikov's rule has no consequence.
Hope this makes it a bit more clear.
Hi,
No it doesn't go through the formation of a carbocation, nor is it SN2. Think about the size of the bromine atom relative to the two carbon atoms. The intermediate formed is a cyclic bromonium ion. Again, due to the large size of the bromine atom, the second bromine attacks from the opposite side, mainly for steric reasons. Hopefully this helps some bit.