Aromaticity makes benzene very stable, and specialized reactions are needed to modify aromatic compounds. Know what makes the structure of benzene unique, including resonance stabilization and aromaticity. Electron withdrawing groups such as nitro, nitrile, and carbonyl direct new groups to the meta-position and slow the reaction down. KMnO4 doesn't touch the benzene system, but it will chew-up alkyl groups attached to a benzene converting them to benzoic acid. Achiral molecules have internal symmetry, there is a plane of symmetry or a point of symmetry.
Chiral molecules have a point of asymmetry - a chiral center.
Typically a carbon with four different groups attached to it is a chiral center. Know the rules for determining R or S configuration. Diastereomers have more than one chiral center per molecule - and some of these chiral centers are the same, others are different. Meso compounds have more than one chiral center, but the molecule as a whole is NOT chiral because the molecule itself is symmetrical - it has a point or plane of symmetry. Optical activity is a property of chiral compounds.