C=C + excess Br(aq) Watch

TheDoublethinker
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Hi there,

CIE Chemistry A-Level, Paper 42 Nov 17:

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Why doesn't any Br (or Br + OH) add to both Carbons in that double bond? Any suggestions? Do you think that would be an alternative correct answer?

Thank you
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alstudent_2019
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(Original post by TheDoublethinker)
Hi there,

CIE Chemistry A-Level, Paper 42 Nov 17:

Name:  2.gif
Views: 13
Size:  35.5 KB

Why doesn't any Br (or Br + OH) add to both Carbons in that double bond? Any suggestions? Do you think that would be an alternative correct answer?

Thank you
it says electrophillic substitution (assuming thats part of the question) in such case then the mark scheme answer is correct because whats the benzene ring undergoes is electrophillic substitution (-H replaced by :Br-) however the bit u have circled is not electrophillic substition but electrophillic addition.
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TheDoublethinker
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It is the mark scheme that says Electrophilic substitution. Sorry I think I haven't given enough information on my post.

The original question only says what happens after adding Serotonin + excess Br(aq). No mention to temperature or presence of light. I understand the electrophilic substitution bit. I just don't understand why no Br (or OH since it's aq) is added to the double C=C bond. I'm sure I have seen cyclic chains of carbons undergoing electrophilic addition.

Do you think that would be an alternative correct answer or can you think of any reason why that would NOT be a correct answer?

Thanks again
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charco
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(Original post by TheDoublethinker)
It is the mark scheme that says Electrophilic substitution. Sorry I think I haven't given enough information on my post.

The original question only says what happens after adding Serotonin + excess Br(aq). No mention to temperature or presence of light. I understand the electrophilic substitution bit. I just don't understand why no Br (or OH since it's aq) is added to the double C=C bond. I'm sure I have seen cyclic chains of carbons undergoing electrophilic addition.

Do you think that would be an alternative correct answer or can you think of any reason why that would NOT be a correct answer?

Thanks again
The issue here is that the pyrrhole ring is aromatic, just like benzene and will not undergo electrophilic addition.

But how you are supposed to know that at A' level is beyond me.

The rules for aromaticity:
The molecule is cyclic (a ring of atoms)
The molecule is planar (all atoms in the molecule lie in the same plane)
The molecule is fully conjugated (p orbitals at every atom in the ring)
The molecule has 4n+2 π electrons (n=0 or any positive integer)
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TheDoublethinker
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(Original post by charco)
The issue here is that the pyrrhole ring is aromatic, just like benzene and will not undergo electrophilic addition.

But how you are supposed to know that at A' level is beyond me.

The rules for aromaticity:
The molecule is cyclic (a ring of atoms)
The molecule is planar (all atoms in the molecule lie in the same plane)
The molecule is fully conjugated (p orbitals at every atom in the ring)
The molecule has 4n+2 π electrons (n=0 or any positive integer)
Thank you very much. And no, I hadn't seen those rules ever before. Not mentioned on my books or syllabus. Just another typical CIE question.
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