The One Carbon Ring…. To Rule them All!

1. The One Carbon Ring….To Rule them All! Cyclic and Aromatic Hydrocarbons

2. How do you get from chain to ring? • 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?

3. IUPAC Cyclic Naming • 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 say1-methylcyclohexane.

4. 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 oro-dimethylcyclohexane

5. OMP - OrthoMetaPara • 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…)

6. ORTHO META PARA

7. Priority Groups • 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.

8. And now… Benzene • Has the formula C6H6 • 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.

9. Friedrich Kekule • A man named Friedrich Kekulemade a stunning proposal – whatif it was a ring with alternatingsingle and double bonds? • His proposal fit the bill • For many years this was acceptedas the real structure of benzene,and is often still written this way

10. Kathleen Lonsdale • However, it wasn’t until X-ray crystallography came along that benzene’s true nature was discovered. • Kathleen Lonsdale madethe discovery • She found that all the bondsin benzene are the samelength!

11. The Ruling Ring… The One Ring • 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.

12. Naming Benzene • 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?

13. If benzene is a branch… • 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!