Cyclic and Aromatic Hydrocarbons

 An alkane (or alkene) can become a ring if its two end carbons react to form a bond.  When you have a ring, it is known as a cyclic hydrocarbon  There are a few special rules for naming a cyclic molecule, but first, there’s a new prefix: cyclo-.  A six-carbon ring with all single bonds is called “cyclohexane” – only a bit different, right?

 If there are no groups, you just call it cyclo-  If there are groups, how do you start numbering? Are there “ends”?  Assign #1 to the carbon with the highest priority group. If there’s only one branch (or multiple bond), you don’t need to say 1-methylcyclohexane.

 If you’ve got more than one group, ALWAYS NUMBER THE RING. Start with #1 at the highest priority group, and number in the direction that gives the smallest numbers to all the branches.  Here, neither methyl is more important, but no matter which you pick, you’ll end up with #2 at the other methyl… so  1,2-dimethylcyclohexane or o- dimethylcyclohexane

 When you are working with six-carbon rings, there’s a trick – three actually. They’re called:  Ortho – two similar groups are on carbons side-by-side ( 1,2-dimethyl… )  Meta – two similar groups are on carbons that are separated by a carbon ( 1,3-dimethyl …)  Para – two similar groups are on carbons opposite each other ( 1,4-dimethyl…)

ORTHO META PARA

 Highest priority at the top  Triple Bond  Double Bond  Large Alkyl Branch  Small Alkyl Branch  So, if there’s a triple bond, it gets numbered as #1.  After that, look for the smallest next number, regardless of priority of the groups.

 Has the formula C 6 H 6  Early chemists couldn’t figure out a structure – they proposed molecules with two double bonds & a triple bond, or two triple bonds. However, these models didn’t explain the ridiculous stability of benzene.

 A man named Friedrich Kekule made a stunning proposal – what if it was a ring with alternating single and double bonds?  His proposal fit the bill  For many years this was accepted as the real structure of benzene, and is often still written this way

 However, it wasn’t until X-ray crystallography came along that benzene’s true nature was discovered.  Kathleen Lonsdale made the discovery  She found that all the bonds in benzene are the same length!

 How does that work? If it was all single bonds, it would be cyclohexane! If it was all double bonds, there would be no room for hydrogens!  The truth lies halfway between – each carbon makes approximately 1.5 bonds with each neighboring carbon. The electrons end up in delocalized bonds (remember metallic bonding?)  That’s why benzene is now written as shown here – the circle indicates all the carbon bonds are equal, but more than a single bond.

 First, benzene is aromatic – it is a ring with alternating single and double bonds. All molecules containing the benzene ring are aromatic.  Naming it is pretty simple – benzene is its parent chain name, and branches are named as normal.  What is the name here?

 It doesn’t happen often, but if benzene is on a molecule with a double or triple bond, the benzene ring is now a branch.  If this is the case, its simply called ‘n-phenyl’ (n is the carbon number it is attached to, and it’s phenyl because benzene is sometimes called “phenol”.)  That’s it!