ecobeco Posted May 27, 2012 Report Share Posted May 27, 2012 I've read up on how to do this but I still don't understand...for example in toluene, how can I find the number of unique H's and C's? Link to comment Share on other sites More sharing options...
markov79 Posted May 27, 2012 Report Share Posted May 27, 2012 I've read up on how to do this but I still don't understand...for example in toluene, how can I find the number of unique H's and C's? heyo, http://upload.wikimedia.org/wikipedia/commons/2/2e/Toluol.svg i'm working with that, so i hope it's right. i'm fairly certain this is how you do it...: 1) forgive my atrocious numbering. since i forget how to do it, just label the methyl group C1, then the carbon that it's attached to "C2", then go around in a clockwise fashion and label those C3-C7. 2) you know the methyl group is unique because if you look at the others, nothing else has 3 hydrogens sticking off of it. 2) C2 is similarly unique because nothing else has a methyl group on it. 3) now if you look at both C3 and C7, they are the same thing: one H attached to both, as well as the "C2, C1" portion on one side, and then the C4-C6 portion on the other side. as i'll show next, C4 and C6 are the same, and C5 is unique, so the C4-C6 portion is the same wrt both C3 and C7. 4) similarly, C4 and C6 are the same because they have the same things pulling at them: one H, C5 on one side, then the stuff "above" which is the same on both sides (since C3 and C7 are the same, and because from there, the chain is the same from both perspectives). 5) C5 is unique, because there is no other carbon that has exactly symmetrical things pulling at it (except for Cs 1 and 2, but as i said, they're unique... if you compare the surroundings of Cs 1 and 2 to C5, they aren't the same. i think a key thing that you probably would have read about is that toluene is a symmetrical molecule, so you'll likely have a couple of atoms being "equivalent" (C3 and C7; C4 and C6), while a few are different/unique (C1, C2, C5), as well. so: carbons 1, 2 and 5 are unique. carbons 3 and 7 are equivalent, and carbons 4 and 6 are equivalent. carbons 3/7 are NOT equivalent to carbons 4/6. you basically do the same thing with hydrogen. anyone feel free to correct it as i am rusty and just responded because there hadn't been any responses yet. Link to comment Share on other sites More sharing options...
bored Posted May 27, 2012 Report Share Posted May 27, 2012 look for symmetry, and remember that the sp3 hydrogens will all be equal because they are allowed complete rotation. the above ans is pretty good Link to comment Share on other sites More sharing options...
markov79 Posted May 27, 2012 Report Share Posted May 27, 2012 look for symmetry, and remember that the sp3 hydrogens will all be equal because they are allowed complete rotation. the above ans is pretty good score. still got it, to some extent at least. re: hydrogens... yep, forgot about that Link to comment Share on other sites More sharing options...
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