$R-CH_2-CH_2-X \xrightarrow{alc. \ KOH, \Delta} R-CH=CH_2 + HX$
This is a $\beta$-elimination reaction.
Reactivity of HX: $HI > HBr > HCl$. Reactivity of alkyl groups: tertiary > secondary > primary.

From Vicinal Dihalides (Dehalogenation):

$R-CHBr-CH_2Br + Zn \rightarrow R-CH=CH_2 + ZnBr_2$

From Alcohols (Acidic Dehydration):

$R-CH_2-CH_2-OH \xrightarrow{Conc. \ H_2SO_4, \Delta} R-CH=CH_2 + H_2O$
This is also a $\beta$-elimination reaction.

Properties of Alkenes

Physical:
- $C_2-C_4$ are gases, $C_5-C_{18}$ are liquids, $C_{19+}$ are solids.
- Colourless, odourless (ethene has faint sweet smell), insoluble in water.
- Boiling point increases with molecular mass; branching decreases it.
Chemical: Characterized by addition reactions due to $\pi$ electrons.
1. Addition of Dihydrogen (Hydrogenation): $\xrightarrow{Pt/Pd/Ni}$ Alkanes (Section 9.2.2).
2. Addition of Halogens: $R-CH=CH_2 + Br_2 \rightarrow R-CHBr-CH_2Br$ (vicinal dihalides).
  - Used as a test for unsaturation (decolourises reddish-orange bromine water).
  - Electrophilic addition via cyclic halonium ion.
3. Addition of Hydrogen Halides: $R-CH=CH_2 + HX \rightarrow R-CHX-CH_3$ (alkyl halides).
  - Reactivity: $HI > HBr > HCl$.
  - Markovnikov's Rule: Negative part of addendum adds to the carbon with fewer hydrogen atoms.
  - Anti-Markovnikov Addition (Peroxide Effect/Kharash Effect): In presence of peroxides, HBr adds opposite to Markovnikov's rule (only for HBr).
    - Mechanism: Free radical chain.
4. Addition of Sulphuric Acid: Cold conc. $H_2SO_4$ adds according to Markovnikov's rule to form alkyl hydrogen sulphates.
5. Addition of Water (Hydration): In presence of conc. $H_2SO_4$, forms alcohols according to Markovnikov's rule.
6. Oxidation:
  - Baeyer's reagent (cold, dilute, aqueous $KMnO_4$) forms vicinal diols (glycols).
  - Acidic $KMnO_4$ or dichromate oxidises alkenes to ketones/acids.
7. Ozonolysis: Alkene + $O_3 \rightarrow$ Ozonide $\xrightarrow{Zn/H_2O}$ Aldehydes/ketones.
  - Useful for locating the position of the double bond.
8. Polymerisation: Ethene forms polythene, propene forms polypropene.

Alkynes

Unsaturated hydrocarbons with at least one C≡C triple bond.
General Formula: $C_nH_{2n-2}$
First stable member: Ethyne (acetylene).

Nomenclature and Isomerism

IUPAC: Longest chain with triple bond, suffix '-yne'. Numbering from end nearer to triple bond.
Position Isomers: Differ in the position of the triple bond (e.g., But-1-yne and But-2-yne).
Chain Isomers: Differ in the arrangement of the carbon chain.

Structure of Triple Bond

Each carbon is $sp$ hybridised.
Consists of one $\sigma$ bond (head-on overlap of $sp$ orbitals) and two $\pi$ bonds (lateral overlap of two $2p$ orbitals).
H-C-C bond angle is $180^\circ$ (linear molecule).
C≡C b