(i) How does Huckel rule help to decide the aromatic character of a compound?
(ii) Draw cis-trans isomers for the following compounds
(a) 2-chloro-2-butene (b) `CH_(3)-C CI=CH-CH_(2)CH_(3)`

Explanation

(i) A compound is said to be aromatic, if it obeys the following rules:
1. The molecule must be cyclic.
2. The molecule must be co-planar.
3. Complete delocalisation of r-electrons in the ring.
4. Presence of `(4n+2)pi` electrons in the ring where n is an integer (n-0,1,2 ...)
This is known as Huckel.s rule.

  (i) It is cyclic one.
(ii) It is a co-planar molecule.
(iii) It has six delocalised 7 electrons.
(iv) 4n+ 2 = 6
4n = 6-2
4n = 4
implies n=1
It obey Huckel.s rule, with n = 1, hence benzene is aromatic in nature.
(ii) (a) 2-Chloro-2-butene: