Cyclic Compounds
Cycloalkanes
Cycloalkanes again only contain carbon-hydrogen bonds and carbon-carbon single bonds, but this time the carbon atoms are joined up in a ring. The smallest cycloalkane is cyclopropane.
If you count the carbons and hydrogens, you will see that they no longer fit the general formula CnH2n+2. By joining the carbon atoms in a ring, you have had to lose two hydrogen atoms.
You are unlikely to ever need it, but the general formula for a cycloalkane is CnH2n.
Don't imagine that these are all flat molecules. All the cycloalkanes from cyclopentane upwards exist as "puckered rings".
Cyclohexane, for example, has a ring structure which looks like this:
This is known as the "chair" form of cyclohexane - from its shape which vaguely resembles a chair.
Reactivity
Cycloalkanes are very similar to the alkanes in reactivity, except for the very small ones - especially cyclopropane. Cyclopropane is much more reactive than you would expect.
The reason has to do with the bond angles in the ring. Normally, when carbon forms four single bonds, the bond angles are about 109.5°. In cyclopropane, they are 60°.
With the electron pairs this close together, there is a lot of repulsion between the bonding pairs joining the carbon atoms. That makes the bonds easier to break.
The effect of this is explored on the page about reactions of these compounds with halogens which you can access from the alkanes menu below.
Examples- to see the names highlight name to left of structure
| Name | Structure |
|
3,3-dimethylcyclopropene |
 |
| cyclopentene |
|
| 1,1-dichlorocyclopentane |  |
| 1,1-Dichloro-2,2-dimethylcyclopropane |  |
| 1,1-difluorocyclopentane |  |
| cyclopentanone |  |
| bromocyclopentane |  |
| chlorocyclopentane |  |
| iodocyclopentane |  |
