$\text{Cold, dil. KMnO}_4$ (Baeyer's reagent): forms vicinal diols.
Hot, conc. $\text{KMnO}_4$: cleaves double bond, forms ketones/carboxylic acids.
Ozonolysis ($\text{O}_3/\text{Zn/H}_2\text{O}$): cleaves double bond, forms aldehydes/ketones.

Polymerization: E.g., ethene to polythene.

General Formula: $\text{C}_nH_{2n-2}$
Hybridization: $\text{sp}$ (linear geometry)
Acidity of Terminal Alkynes: $\text{R-C}\equiv\text{C-H}$ (acidic due to s-character of sp-hybridized carbon). Reacts with $\text{Na}$, $\text{NaNH}_2$, ammoniacal $\text{AgNO}_3$ (Tollens' reagent), ammoniacal $\text{CuCl}$ to form acetylides.
Preparation:
- Dehydrohalogenation of vicinal/geminal dihalides (with strong base like $\text{NaNH}_2$).
- From tetrahalides.
- From calcium carbide ($\text{CaC}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{C}_2\text{H}_2$).
Reactions (Electrophilic Addition):
- Hydrogenation: $\text{alkyne} \xrightarrow{\text{H}_2/\text{Ni/Pt/Pd}} \text{alkene} \xrightarrow{\text{H}_2/\text{Ni/Pt/Pd}} \text{alkane}$.
- Halogenation: Adds 1 or 2 moles of halogen.
- Hydrohalogenation: Adds 1 or 2 moles of HX (Markovnikov's rule).
- Hydration: $\text{R-C}\equiv\text{CH} + \text{H}_2\text{O} \xrightarrow{\text{HgSO}_4/\text{H}_2\text{SO}_4} \text{ketone}$ (for propyne and higher), $\text{ethyne} \rightarrow \text{acetaldehyde}$.
- Oxidation: Hot, conc. $\text{KMnO}_4$ cleaves triple bond to form carboxylic acids.
- Cyclic Polymerization: $3\text{C}_2\text{H}_2 \xrightarrow{\text{red hot Fe tube}} \text{C}_6\text{H}_6$ (benzene).

Aromaticity (Hückel's Rule):
- Cyclic, planar, fully conjugated.
- $(4n+2)\pi$ electrons ($n=0,1,2,\dots$). E.g., Benzene (6$\pi$), Naphthalene (10$\pi$).
Benzene Structure: Resonant hybrid of Kekulé structures. All C-C bonds are equivalent (139 pm).
Preparation:
- Cyclic polymerization of ethyne ($\text{3C}_2\text{H}_2 \xrightarrow{\text{red hot Fe tube}} \text{C}_6\text{H}_6$).
- Decarboxylation of sodium benzoate.
- Reduction of phenol ($\text{C}_6\text{H}_5\text{OH} + \text{Zn} \rightarrow \text{C}_6\text{H}_6 + \text{ZnO}$).
Reactions (Electrophilic Aromatic Substitution, EAS):
- Nitration: $\text{C}_6\text{H}_6 + \text{HNO}_3 \xrightarrow{\text{conc. H}_2\text{SO}_4} \text{nitrobenzene} + \text{H}_2\text{O}$.
- Halogenation: $\text{C}_6\text{H}_6 + \text{Cl}_2 \xrightarrow{\text{anhyd. AlCl}_3} \text{chlorobenzene} + \text{HCl}$.
- Sulphonation: $\text{C}_6\text{H}_6 + \text{conc. H}_2\text{SO}_4 \xrightarrow{\text{heat}} \text{benzenesulphonic acid} + \text{H}_2\text{O}$.
- Friedel-Crafts Alkylation: $\text{C}_6\text{H}_6 + \text{RCl} \xrightarrow{\text{anhyd. AlCl}_3} \text{alkylbenzene} + \text{HCl}$.
- Friedel-Crafts Acylation: $\text{C}_6\text{H}_6 + \text{RCOCl} \xrightarrow{\text{anhyd. AlCl}_3} \text{acylbenzene} + \text{HCl}$.
Directing Groups:
- Ortho/Para-directing & Activating: Alkyl groups, $-\text{OH}$, $-\text{OR}$, $-\text{NH}_2$, $-\text{NR}_2$.
- Ortho/Para-directing & Deactiva