Spectroscopy
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In infra-red spectroscopy, what do 2/3/4/5 adjacent hydrogen atoms in an arene mean exactly? (from the Nuffield Book of Data). I thought it meant how many hydrogen atoms are around the ring but in one question an arene with five hydrogen atoms around its ring has '2 adjacent hydrogen atoms' according to the answer.
Igloo writes ...
The adjacent hydrogen atoms refer to the ‘spare’ hydrogen atoms left on the ring after substitution has taken place. The best way to make things clear is to give you a few examples:
A monosubstituted arene, e.g. phenol, C6H5OH, or chlorobenzene, C6H5Cl
There are 5 adjacent hydrogen around the ring, so, according to the Data Book, carbon-hydrogen bending absorptions would be expected at 750 cm-1 and 700 cm-1
A disubstituted arene, e.g. benzene-1,2-diol, C6H4(OH)2
The hydroxyl groups are next to each other, so there are 4 adjacent hydrogen atoms around the ring, and carbon-hydrogen bending absorption would be expected at 750 cm-1 only.
Another disubstituted arene, e.g. 1,3-dichlorobenzene, C6H4Cl2
Here the hydroxyl groups are next door-but-one to each other, so there is an isolated hydrogen atom giving rise to a carbon-hydrogen bending vibration at 880 cm-1 and also another 3 adjacent hydrogen atoms, giving an absorption at 780 cm-1.
These examples should help to make things clear. I do not know the question to which you are referring, but judging from your question, I think that you already understood correctly what it was all about. It may well be that the answer to this question is wrong.
Risk assessment
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updated: 15 December 2006
