1. In the case of malonic acid CH2(COOH)2, propane CH2Me2, or any other molecule of the form CH2Y2, if we replace either of the CH2 hydrogens by a group Z, the identical compound results. The two hydrogens
are thus equivalent. Equivalent atoms and groups need not, of course, be located on the same carbon atom. For example, all the chlorine atoms of hexachlorobenzene are equivalent as are the two bromine atoms of 1,3-
dibromopropane.
2. In the case of ethanol CH2MeOH, if we replace one of the CH2 hydrogens by a group Z, we get one enantiomer of the compound ZCHMeOH (91), while replacement of the other hydrogen gives the other enantiomer (92). Since the two compounds that result upon replacement of H by Z (91 and 92) are not