Ortho/Para-directing & Deactivating: Halogens (weakly deactivating).

Meta-directing & Deactivating: $-NO_2, -CN, -CHO, -COOH, -SO_3H, -COR$.

Characteristic of Alkenes and Alkynes.
Mechanism:
1. Electrophile ($E^+$) attacks $\pi$-bond, forming carbocation.
2. Nucleophile ($Nu^-$) attacks carbocation.
Markovnikov's Rule: In addition of $HX$ to unsymmetrical alkene, $H$ adds to carbon with more $H$ atoms, and $X$ adds to carbon with fewer $H$ atoms. (Forms more stable carbocation).
Anti-Markovnikov's Rule (Peroxide Effect): In addition of $HBr$ to unsymmetrical alkene in presence of peroxides, $H$ adds to carbon with fewer $H$ atoms. (Free radical mechanism).
Examples: Addition of $H_2$ (Hydrogenation), $X_2$ (Halogenation), $HX$ (Hydrohalogenation), $H_2O$ (Hydration).

Characteristic of Aldehydes and Ketones (due to polar $C=O$ bond).
Mechanism:
1. Nucleophile attacks electrophilic carbon of carbonyl group.
2. Protonation of oxygen.
Reactivity: Aldehydes $>$ Ketones (due to steric hindrance and +I effect of alkyl groups in ketones).
Examples: Addition of $HCN$, $NaHSO_3$, Grignard Reagents, Alcohols.

Example: Anti-Markovnikov addition of $HBr$ to alkenes in presence of peroxides.
Mechanism:
1. Initiation: Peroxide breaks into free radicals.
2. Propagation: Radical reacts with $HBr$, then alkyl radical reacts with alkene.
3. Termination: Radicals combine.

Two steps: Carbocation formation (slow, RDS) then loss of $H^+$ from adjacent carbon.
Rate = $k[\text{Alkyl Halide}]$.
Favored by: $3^\circ$ alkyl halides, polar protic solvents, weak bases, high temperature.
Competition with $S_N1$.

One step: Base removes $H^+$, leaving group departs, double bond forms (concerted).
Rate = $k[\text{Alkyl Halide}][\text{Base}]$.
Favored by: $1^\circ$ or $2^\circ$ alkyl halides, strong bases, high temperature.
Competition with $S_N2$.

In elimination reactions, the major product is the more substituted (more stable) alkene.
$CH_3-CH_2-CH(Br)-CH_3 \xrightarrow{Alc. KOH} CH_3-CH=CH-CH_3 \text{ (Major)} + CH_3-CH_2-CH=CH_2 \text{ (Minor)}$

When the leaving group is bulky (e.g., quaternary ammonium salts) or a bulky base is used, the least substituted alkene is the major product.

Oxidation: Increase in oxygen atoms, decrease in hydrogen atoms.
- Alcohols to Aldehydes/Ketones/Carboxylic Acids ($K_2Cr_2O_7/H^+$, $KMnO_4$).
- Alkenes to Diols ($Baeyer's \text{ reagent}$, cold dil. $KMnO_4$).
- Ozonolysis of Alkenes/Alkynes.
Reduction: Decrease in oxygen atoms, increase in hydrogen atoms.
- Aldehydes/Ketones to Alcohols ($LiAlH_4$, $NaBH_4$).
- Carboxylic Acids to Alcohols ($LiAlH_4$).
- Alkenes/Alkynes to Alkanes ($H_2/Ni, Pd, Pt$).
- Nitro compounds to Amines ($Fe/HCl$, $Sn/HCl$).