Chirality depends on whether a molecule is different from its mirror image or not. A molecule is said to be chiral if it is isomeric with its own mirror image and a molecule is said to be achiral if it is identical with its mirror image.
An atom which is attached to 4 different atoms or groups is called a chiral centre. It is always sp3 hybridized. Sp2 and sp hybridized atoms are always achiral.
Superimpossibilty of mirror image is the original test to check for chirality. Molecular chirality is caused due to dis-symmetry or lack of symmetry. Any object which has a plane or point of symmetry will be achiral whereas object having no plane or point of symmetry will be chiral, i.e., different from its mirror image.
Presence of plane of symmetry or point of symmetry is enough of an evidence for superimpossibilty to assume mirror images of an molecule to be identical, irrespective of projections we are presented with.
To check the chirality in a closed chain system, first of all we need to compare atoms or groups outside the closed chain and then compare the connectivity in clockwise and anticlockwise direction. If groups and connectivity are different, then the centre is chiral. If groups are different , no need to check for connectivity, the molecule is chiral. But if groups are same, check for connectivity. If connectivity is different, the centre is chiral, else if connectivity is same , the centre is achiral.
Numbers of chiral centre are-
1. Ethane – 0
2. Butane – 0
3. 2-methyl butane – 0
4. 2-chloro butane – 1
5. 1,2-dichloro cyclohexane – 2
6. 1,3-dichloro cyclohexane – 2
7. 1,4-dicholoro cyclohexane – 0