Summary

Alkanes ($C_nH_{2n+2}$): Saturated hydrocarbons with only single bonds. Key reactions: free-radical halogenation (e.g., $CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl$) and combustion ($C_nH_{2n+2} + \frac{3n+1}{2}O_2 \rightarrow nCO_2 + (n+1)H_2O$). Conformation of ethane: staggered (more stable) vs. eclipsed (less stable).

Alkenes ($C_nH_{2n}$): Unsaturated with one $C=C$ double bond. Show geometrical isomerism (cis/trans) due to restricted rotation. Key reactions: electrophilic addition (e.g., $CH_2=CH_2 + Br_2 \rightarrow CH_2BrCH_2Br$), Markovnikov's rule (H adds to the carbon with more H atoms), and ozonolysis (cleavage of $C=C$ to give carbonyl compounds).

Alkynes ($C_nH_{2n-2}$): Unsaturated with one $C\equiv C$ triple bond. Terminal alkynes are acidic ($RC\equiv C-H$ can be deprotonated by strong bases like $NaNH_2$). Key reactions: electrophilic addition (e.g., $HC\equiv CH + 2Br_2 \rightarrow CHBr_2CHBr_2$), hydration via enol (Keto-enol tautomerism), and polymerisation to benzene (cyclic trimerisation).

Aromatic hydrocarbons (e.g., benzene $C_6H_6$): Planar, cyclic, with delocalised $\pi$-electrons (Hückel's rule: $4n+2$ $\pi$-electrons). Undergo electrophilic substitution (nitration, halogenation, sulphonation, Friedel-Crafts alkylation/acylation) rather than addition. The ring is stabilised by resonance. …