Hydrocarbons

Hydrocarbons are compounds of carbon and hydrogen and nothing else, and they are where organic chemistry stops being about rules and starts being about reactions. The chapter takes the four families in turn. Alkanes are the unreactive ones, so their chemistry is substitution by free radicals, and they bring with them conformations, the study of what rotation about a single bond costs. Alkenes and alkynes have loosely held pi electrons, so their chemistry is electrophilic addition, and this is where Markovnikov's rule, the peroxide effect, ozonolysis and the tests for unsaturation live. Alkynes add a twist: a terminal triple bond is acidic enough to lose a proton to sodium, which no other hydrocarbon will do. Then comes benzene, which by every rule so far should behave like a triene and does not, and the delocalisation that explains why, along with aromaticity, electrophilic substitution and the directive influence that decides where the next substituent goes. Everything in Class 12 organic chemistry starts from a hydrocarbon, and the reactions and mechanisms here reappear in every one of those chapters. It is also one of the heaviest scoring chapters in both JEE and NEET, because most of what is asked is reagent-to-product recall backed by a mechanism you can reconstruct.