The structure of benzene is best described in terms of the molecular orbital theory. All the six carbon atoms of benzene are sp² hybridized. Six sp² hybrid orbitals of carbon linearly overlap with six one is orbitals of hydrogen atoms to form six C - H sigma bonds. Overlap between the remaining Sp² hybrid orbitals of carbon forms six C-C sigma bonds

All the bonds in benzene lie in one plane with bond angle 120°. Each carbon atom in benzene possess an un hybridized p-orbital containing one electron. The lateral overlap of their p-orbital produces 3 \(\pi\)-bond The six electrons of the p-orbitals cover all the six carbon atoms and are said to be delocalised. Due to delocalization, strong -bond is formed which makes the molecule stable. Hence unlike alkenes and alkynes benzene undergoes substitution reactions rather addition reactions under normal conditions.

Among anthracene and cyclopentadiene - anthracene is aromatic in nature.

Anthracene		Cyclopentadiene
1.	Planar structure	Planar structure
2.	Contains 14 delocalised \(\pi\) electrons (4n + 2 = 14\(\pi\)e-)	Contains 4 non-delocalised \(\pi\) electrons
3.	Hence aromatic	Hence non-aromatic

Huckel proposed that aromaticity is a function of electronic structure. A compound may be aromatic, if it obeys the following rules
(i) The molecule must be co-planar
(ii) Complete delocalization of \(\pi\) electron in the ring
(iii) Presence of (4n+2) \(\pi\) electrons in the ring where n is an integer (n=0,1,2 .... ) is known as Huckle's rule.

1.	Cyclo penta diene	(i) It has a planar Structure (ii) It has four \(\pi\) electron but the elecron are not delocalised and hence it is not an aromatic compound.
2.	Cyclooctatetraene	Molecule is non-planar and hence it is not an aromatic compound.
3.	Cyclopropenylcation	(i) Cyclopropenyl cation has planar structure. (ii) It has 2 delocalised \(\pi\) electron. (iii) 4n + 2 = 2 4n = 0 n = 0 (an interger) and hence it is aromatic compount.

Alkyne undergoes two types of polymerisation reaction
(i) Linear Polymerisation :
Ethyne forms linear polymer, when passed into a solution of cuprous chloride and ammonium chloride.
\(2CH\equiv CH\overset { { CI }_{ 2 }/{ NE }_{ 4 }CI }{ \longrightarrow } { CH }_{ 2 }=\underset { \overset { | }{ C\equiv CH } }{ CH } \)
                                                   Vinyl ethylene
(ii) Cyclic polymerisation:
Ethyne undergoes cyclic polymerization on passing through red hot iron tube. Three molecules of ethyne polymerises to benzene.

Polymerisatlon:
A polymer is a large molecule formed by the combination of larger number of small molecules. The process in known as polymerisation. Alkenes undergo polyrnerisation at high temperature and pressure, in the presence of a catalyst.