Chem--benzene and cyclohexane

1) cyclohexane exist in the form of "chair" or "boat" conformation. Each of the carbon atom in cyclohexane form 2 bonds with hydrogen and 2 with the adjacent carbon atom. This causes a sp3 hybrid to form and each carbon centre would appear like a "tetrahedral". Technically, we can force it to be planar but it exists naturally as a chair or a boat conformation. you can refer to this for more information:

http://en.wikipedia.org/wiki/Cyclohexane_conformation

如果係中文解既話, 就可以話因為佢係sp3 嘛, 所以每個carbon 都係form 4個bond, 你試下將你自己放係個carbon centre 上面, 你會見到, 每個carbon 都係in forms of tetrahedral 架. 而benzene 就係planar, 因為benzene 入面有C=C,個carbon centre 而家變左做sp2, 所以會係planar structure 黎既.

2) (b) Explain the difference in the C-C-C bond angles of the two compounds.
(Given: benzene: C-C-C bond angle = 120 cyclohexane: C-C-C bond angle = 109.5)

Again, of you have considered the structure both compounds, you will realise that benzene is planar where cyclohexane is no planar. The carbon centre in benzene is sp2, which means it will form bonds with three other atoms. You can visualise the carbon centre as a trigonal planar form, which means the angle is 120o. From part 1) as the carbon centre is sp3, the carbon centre will be able to form 4 bonds with neighbouring atoms. This results in tetrahedral structure with the angle of 109.5o.

3) (c) Explain the difference in the C-C bond lengths of the two compounds.(Given: benzene: C-C bond length = 0.139nm cyclohexane: C-C bond length = 0.154nm)

>> Consider the structure of the compound, benzene contains C=C bonds where cyclohexane does not. The bond strength of C=C is stronger than C-C. As such, the bond length in benzene is shorter relative to the bond length in cyclohexane. Notice that benzene is an aromatic compound with pi- electron cloud.This would contribute to its bond length as well :)

hope that helps :)

b.
think about the hybridization of orbitals of C atom.

c.
think about the bonding nature. resonance, sideway-overlapping of p-orbitals..