YT2095 Posted March 8, 2008 Share Posted March 8, 2008 what (and why) is the difference between Benzene and 1,3,5 cyclohexatriene? this has been bugging me for ages, and the only Reasonable answer I`v ever had was something to do with the C C bond length being different in them. however there was No explanation as to Why it was different or would be different (if indeed that Is the reason). does any Actually know? feel free to use stick diagrams or whatever diagram you think will show the difference. Link to comment Share on other sites More sharing options...
hermanntrude Posted March 8, 2008 Share Posted March 8, 2008 for a start, under normal conditions, 1,3,5 cyclohexatriene doesn't exist. The difference is that the three double bonds are spread out across the six possible positions, in a phenomenon known as resonance. The reason you got the bond-length explanation before is that if the 3 double bonds were stuck in just one place (like in the imaginary 1,3,5 cyclohexatriene) then 3 bonds would be short and three would be long, but benzene contains six equal length C-C bonds. The true story is a bit more in-depth than that. If you want to know more, first look up resonance and then look up valence bond theory, then look up each of those with reference to benzene. Link to comment Share on other sites More sharing options...
YT2095 Posted March 8, 2008 Author Share Posted March 8, 2008 (like in the imaginary 1,3,5 cyclohexatriene) then 3 bonds would be short and three would be long, but benzene contains six equal length C-C bonds. Whut!? are you MAD? it has 3x c=c and 3x c-c, not 6x c-c (that would be cyclohexane). anyway I think I`v found a reasonable answer since you mentioned resonance here: http://everything2.com/index.pl?node_id=1536635 Link to comment Share on other sites More sharing options...
Karnage Posted March 8, 2008 Share Posted March 8, 2008 Whut!? are you MAD? it has 3x c=c and 3x c-c, not 6x c-c (that would be cyclohexane). anyway I think I`v found a reasonable answer since you mentioned resonance here: http://everything2.com/index.pl?node_id=1536635 No, herman is right. When the people looked at the structure of benzene, they found that all the bonds were of equal length. This was due to resonance. So basically, the electrons are shared between all bonds, not just three bonds as you generally see it. Drawing benzene as 3 double bonds and 3 single bonds isn't a real accurate description because according to Kekule's rule, the double bonds actually resonate. So for benzene you actually have a blend of two possible resonance structures (if you move the double bonds around once you get the other resonance structure). You might notice that the probably best way to represent benzene is the hexagon with the circle in it, to represent the resonance. I think you get the gist. And 1,3,5-cyclohexatriene doesn't exist. It was a structure initially proposed by Kekule and other scientists for benzene, but was later disproven because they found that all the bond lengths were the same and thus resonance was born. Link to comment Share on other sites More sharing options...
Tartaglia Posted March 8, 2008 Share Posted March 8, 2008 It is to do with the symmetry of the p orbitals that form the pi system of the ring. Linear combination of these orbitals lead to three bonding and three antibonding orbitals which are more or less spread over the 6 carbons (I simplify here) and that leads to thermodynamic stabilisation and equivalence of bonds Link to comment Share on other sites More sharing options...
hermanntrude Posted March 9, 2008 Share Posted March 9, 2008 Whut!? are you MAD? it has 3x c=c and 3x c-c, not 6x c-c (that would be cyclohexane). anyway I think I`v found a reasonable answer since you mentioned resonance here: http://everything2.com/index.pl?node_id=1536635 not mad. When I wrote C-C i meant to signify any bond, not specifically single bonds. the bonds (all six of them) are about one-and-a-half bonds in fact. I don't have any fonts with a way of signifying that so i just represented it as a single bond. Sorry to confuse you. 1 Link to comment Share on other sites More sharing options...
YT2095 Posted March 9, 2008 Author Share Posted March 9, 2008 aha, makes sense now, Thanks each although I think someone needs to have a word with Wiki about it though: "IUPAC name Benzene (or 1,3,5-cyclohexatriene)" taken from: http://en.wikipedia.org/wiki/Benzene thanks again each, I`v never covered resonance (with respect to Chemistry) before, Cheerz! Link to comment Share on other sites More sharing options...
hermanntrude Posted March 9, 2008 Share Posted March 9, 2008 have a lok at valence bond theory as well. It explains it a bit further and you'll understand better. If you can stand even MORE, try molecular orbital theory. benzene is a bit of a challenge for all of the bonding theories but the more advanced the theory the better the explanation gets Link to comment Share on other sites More sharing options...
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