Aliphatic Hydrocarbons

Aliphatic hydrocarbons—encompassing alkanes, alkenes, and alkynes—form the backbone of organic chemistry in the Class 12 BISE curriculum. Alkanes, or paraffins, are saturated hydrocarbons characterized by strong $sp^3$ hybridized $\sigma$-bonds. Because of this, they are generally unreactive, though they readily undergo free radical substitution reactions such as halogenation in the presence of sunlight. A key board exam focus here is the stepwise mechanism of alkane halogenation (initiation, propagation, and termination steps), as well as methods of preparation like the Wurtz reaction and catalytic hydrogenation.

Unsaturated hydrocarbons introduce greater chemical reactivity due to the presence of $\pi$-bonds. Alkenes (olefins) contain a double bond ($sp^2$ hybridization) and are famous for electrophilic addition reactions, including the addition of halogens, hydrogen halides (governed strictly by Markovnikov's rule), and sulfuric acid. Alkynes contain a triple bond ($sp$ hybridization) and exhibit characteristic acidic properties due to the high $s$-character of terminal alkynes, allowing them to form metallic acetylides. Examiners frequently test your understanding of geometrical (cis-trans) isomerism in alkenes and the comparison of acidity between alkanes, alkenes, and alkynes.

A major portion of BISE board questions revolves around interconversions and naming reactions. You must thoroughly memorize preparation methods such as the dehydration of alcohols and dehydrohalogenation of alkyl halides to form alkenes, and dehalogenation of vicinal dihalides or dehydrohalogenation of tetrahalides to form alkynes. Additionally, mastering oxidation reactions—such as the hydroxylation of alkenes with cold alkaline $KMnO_4$ (Baeyer's test for unsaturation) and ozonolysis for locating double bonds—is essential for securing top marks in both subjective and conversion-based questions.