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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: abcc on November 11, 2006, 04:45:03 PM

Title: bromination
Post by: abcc on November 11, 2006, 04:45:03 PM

i have done an experiment which is the bromination of tran-cinnamic acid .
In theoretically , syn & anti- addition both can occur ;
BUT i only got a anti result , what's the reason????

Title: Re: bromination
Post by: abcc on November 11, 2006, 08:00:47 PM

Brief / revelant explanation is OK :)
THX

Title: Re: bromination
Post by: Dan on November 11, 2006, 08:06:31 PM

Look up the mechanism, it will become clear.

Title: Re: bromination
Post by: abcc on November 12, 2006, 12:00:53 AM

Quote from: Dan on November 11, 2006, 08:06:31 PM

Look up the mechanism, it will become clear.

i know the mechanism ,
there is bromonium ion formed  :)
thus ,anti is prefered=)
but i still have a question,
what's the difference between the bromination of trans-anethole and trans-cinnamic acid ???
why anethole can have syn-addition ???
actually anethole and cinnamic acid both are quite similar.
Is it because the benzene ring of trans-anethole bond to OCH3,
but trans-cinnamic acid doesn't  ???
thx :)

Title: Re: bromination
Post by: Dan on November 12, 2006, 07:28:36 AM

For syn addition to occur, the bromonium ion must be opened (forming a carbocation) so that the bromide can attack the same face as the first bromine. Anethole has a relatively stable carbocation compared to cinnamic acid. Can you see why?

Title: Re: bromination
Post by: abcc on November 12, 2006, 09:26:30 AM

Quote from: Dan on November 12, 2006, 07:28:36 AM

For syn addition to occur, the bromonium ion must be opened (forming a carbocation) so that the bromide can attack the same face as the first bromine. Anethole has a relatively stable carbocation compared to cinnamic acid. Can you see why?

I think the reason for the stable carbocation due to the etherbenzene ring, it has resonnance structures that stabilizes the +ive charge (e- donate), is it ??? 
actually , i am not sure and just guess :)

Title: Re: bromination
Post by: Dan on November 12, 2006, 09:35:53 AM

Quote from: abcc on November 12, 2006, 09:26:30 AM

I think the reason for the stable carbocation due to the etherbenzene ring, it has resonnance structures that stabilizes the +ive charge (e- donate), is it ??? 
actually , i am not sure and just guess :)

It's a good guess! Make sure you can draw the resonance structures. This seems like a good explanation to me anyway.

Title: Re: bromination
Post by: abcc on November 12, 2006, 05:03:32 PM

Quote from: Dan on November 12, 2006, 09:35:53 AM

Quote from: abcc on November 12, 2006, 09:26:30 AM

I think the reason for the stable carbocation due to the etherbenzene ring, it has resonnance structures that stabilizes the +ive charge (e- donate), is it ??? 
actually , i am not sure and just guess :)

It's a good guess! Make sure you can draw the resonance structures. This seems like a good explanation to me anyway.

is my reason possible???
any other reasons are relevant to explain why??
thx

Title: Re: bromination
Post by: Dan on November 12, 2006, 07:55:57 PM

I think it is the reason, again, make sure you can draw the resonance structures.

I can't see any other obvious reasons, but that doesn't mean there aren't any.

Title: Re: bromination
Post by: abcc on November 12, 2006, 08:47:44 PM

Quote from: Dan on November 12, 2006, 07:55:57 PM

I think it is the reason, again, make sure you can draw the resonance structures.

I can't see any other obvious reasons, but that doesn't mean there aren't any.

THE RESONNANC : CH3O=ring=C   
The atom O originally has 2 lone pair e-, thus i think it can donate e- to form double bond and stabilize the +ive charge in the ring
is it the possible structure  ???

anyway , thx your reply !! :)