Q.• What is the relationship between two compounds having the same molecular formula?
• What is meant by stereoisomerism ?

A carbon atom bonded to four different groups is called a chiral carbon (marked with an asterisk); a molecule containing one such centre -- for example C-2 of 2-chlorobutane, CH3-*CHCl-CH2CH3, bearing H/Cl/CH3/C2H5 -- cannot be superimposed on its own mirror image, and is called a chiral molecule. This handedness (chirality, from the Greek for 'hand') means a chiral molecule and its mirror image are non-identical stereoisomers of each other, called enantiomers (or enantiomorphs/optical antipodes). To detect this experimentally, plane polarised light (ordinary light passed through a Nicol prism so only oscillations in one plane survive) is passed through the compound: an optically active compound rotates the plane of this light, either dextrorotatory (right, d-/+) or laevorotatory (left, l-/-), by a measurable angle called the optical rotation -- optical activity is the experimentally observed consequence of the underlying structural fact of chirality. Enantiomers share every physical property except the sign of this rotation, and every chemical property except reactivity towards another optically active reagent; an equimolar mixture of both enantiomers, called a racemic modification/mixture ((dl)- or (+/-)-), is optically inactive overall because the …