Why is benzene not an alkene?

[Halcyon32]

How is benzene not an alkene if it contains double bonds. Also, doesn't the suffix -ene mean a compound is an alkene? On top of that why are benzenes bonds longerthan alkene bonds?

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[keowens91]

First of all there's the equation CnH2n for alkenes and you can see that benzene does not follow this rule. It is an aromatic compound with the formula C6H6. Due to the resonance the bonds are constantly "flipping", so they are not considered true double bonds. Pi bonds are stronger so the internuclear distance of Carbons is shorter, thus a single bond is a length of around 148 ppm, and a double bond around 133 ppm. All benzene double bonds are identical having a bond length somewhere inbetween around 139 ppm.

 

[Halcyon32]

So, since it exhibits double bond character but doesn't have isolated double bonds thats what makes it longer than isolated double bonds and shorter than single bonds?

 

[keowens91]

If by double bond character you mean pi electrons yes, but I wouldn't use the word isolated...it just is not a true double bond alternating with a single bond. Hence why benzene should really be drawn as a hexagon with an inner circle (forget chemdraw). The pi electrons are delocalized, and aromatic compounds have a very different orbital structure than alkenes (remember how much fun MO diagrams are). I suggest just reading a quick chapter or wiki on resonance, bond length, etc.

 

[Halcyon32]

by double bond character I meant that since the electrons are delocalized they move throughout the molecule and give some bonds a strength closer to but not quite as strong as double bonds. And i thought isolated was just another way of saying true double bonds but I guess not? Is my unnderstanding of the terminology correct? I don't even know what MO diagrams are yet because I literally just read the first chapter of orgo in my EK prep book so I suppose I'll get to those later and have a deeper understanding of the concept as a whole.

 

[keowens91]

What year are you/what chemistry classes have you taken?

 

[keowens91]

I just meant that you can have alkenes with conjugated double bonds, that are considered alkenes and not aromatic compounds. So what distinguishes benzene is more the fact of it's orbital structure and chemical properties. But yes you are correct in the way you're thinking of the electrons and bond length.

 

[Halcyon32]

What year are you/what chemistry classes have you taken?

I'm a sophomore and have only taken gen chem. No orgo. and thanks a lot for your help!

 

[keowens91]

Oh OK, no problem. Fortunately the new MCAT is a lot of gen chem and very little orgo. You'll have to bust your butt for the class, but then just know a few basic reactions for MCAT. EK should be sufficient for that, Good luck!!

 

[Halcyon32]

Oh OK, no problem. Fortunately the new MCAT is a lot of gen chem and very little orgo. You'll have to bust your butt for the class, but then just know a few basic reactions for MCAT. EK should be sufficient for that, Good luck!!

Oh wow. I dunno how I feel about that. haha here's hoping gen chem turns out to be much easier. Thanks a lot!