Presentation on theme: "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."— Presentation transcript:
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!