Halogenation of alkanes (free radical substitution): Uses homolytic bond fission. Initiated by UV light or heat. Reactivity: $F_{2}$ > $Cl_{2}$ > $Br_{2}$ > $I_{2}$. Selectivity: $Br_{2}$ is more selective (prefers 3° C-H) because bromine radical is less reactive and the transition state is more product-like (Hammond's postulate).

Reaction: C + $Cl_{2}$ --[hν]--> CCl + HCl (chloromethane)

Stability guides synthesis routes: The 3° carbocation intermediate is most stable — synthetic routes that proceed through 3° intermediates (e.g., Markovnikov addition of HBr to propene: CC=C + HBr → CC(Br)C) are kinetically preferred.

Rearrangement reactions in synthesis: Wagner-Meerwein rearrangements occur when a 1° or 2° carbocation can convert to a more stable 3° carbocation via a 1,2-hydride or 1,2-methyl shift. This is critical for predicting actual products in synthetic sequences and is tested in NEET mechanism questions.

Elimination reactions (industrial alkene production): Dehydration of alcohols (acid-catalyzed, or $Al_{2}O_{3}$/$\Delta$) and dehydrohalogenation of alkyl halides (KOH/alcohol, $\Delta$) are the two main industrial routes to alkenes.

Reaction: CCO --[$H_{2}SO_{4}$/$\Delta$]--> C=C + $H_{2}O$ (dehydration of ethanol)

Reaction: CCBr + KOH --[alc., $\Delta$]--> C=C + KBr + $H_{2}O$ (dehydrohalogenation)

Hyperconjugation and alkene stability in polymerization: More substituted alkenes (more hyperconjugative structures) are more stable thermodynamically. This affects the equilibrium in cationic polymerization initiation steps and the regioselectivity of elimination reactions (Zaitsev's rule: the more substituted alkene is the major product).

Electronic effects in pharmaceutical design: -I and -M groups (-$NO_{2}$, -COOH, -CN) on aromatic rings reduce electron density, making the ring less susceptible to electrophilic attack — a principle used in designing selective aromatic sulfonation and nitration sequences for drug intermediates.

IUPAC naming in regulatory submissions: Pharmaceutical regulatory agencies require unambiguous IUPAC names for all active pharmaceutical ingredients. Correct application of the longest-chain rule and lowest-locant rule is essential in medicinal chemistry documentation.

Chiral centers and drug efficacy: The requirement for four different groups at a chiral center (optical isomerism) has profound industrial consequences — one enantiomer of a drug may be therapeutic, and the other toxic (e.g., thalidomide). This makes IUPAC stereodescriptor (R/S) assignment a critical industrial skill rooted in GOC fundamentals.