Types of Organic Reactions

A comprehensive classification of organic reactions into four fundamental types.

By chemca Team • Updated Jan 2026

Organic reactions involve the breaking of old bonds and the making of new ones. Based on the nature of bond cleavage and formation, they are broadly classified into four categories: Substitution, Addition, Elimination, and Rearrangement.

1. Substitution Reactions

Definition: A reaction in which an atom or group of atoms in a molecule is replaced by another atom or group without changing the degree of saturation.

$R-X + Y \rightarrow R-Y + X$

Types of Substitution:

Nucleophilic Substitution ($S_N$): A nucleophile ($Nu^-$) attacks a positively charged center (usually carbon in alkyl halides).
SN1 Unimolecular, two-step, carbocation intermediate, racemization.
SN2 Bimolecular, one-step, transition state, inversion of configuration (Walden Inversion).
Electrophilic Substitution ($S_E$): An electrophile ($E^+$) replaces another electrophile (usually $H^+$). Characteristic of Aromatic Compounds.
Examples: Nitration, Sulfonation, Friedel-Crafts Alkylation of Benzene.
Free Radical Substitution: Occurs via free radicals in the presence of UV light/heat.
Example: Chlorination of Methane ($CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl$).

2. Addition Reactions

Definition: Two molecules combine to form a single product. This typically involves the breaking of a $\pi$-bond (double/triple) and formation of two $\sigma$-bonds.

$>C=C< + A-B \rightarrow A-C-C-B$

Types of Addition:

Electrophilic Addition: Initiated by an electrophile. Characteristic of Alkenes and Alkynes.
Example: Addition of HBr to Propene (Follows Markovnikov's Rule).
Nucleophilic Addition: Initiated by a nucleophile. Characteristic of Aldehydes and Ketones (Polar Carbonyl group $>C=O$).
Example: Addition of HCN to Acetaldehyde.
Free Radical Addition: Initiated by radicals.
Example: Addition of HBr to Alkenes in presence of Peroxide (Anti-Markovnikov/Kharasch Effect).

3. Elimination Reactions

Definition: Two atoms or groups are removed from a molecule (usually from adjacent carbons) to form a multiple bond (double/triple). It is the reverse of addition.

$H-C-C-X \xrightarrow{\text{Base}} >C=C< + HX$

Types of Elimination:

$\beta$-Elimination (Dehydrohalogenation): Hydrogen is removed from the $\beta$-carbon and Halogen from the $\alpha$-carbon.
E1 Unimolecular, two-step, carbocation intermediate.
E2 Bimolecular, one-step, transition state.
Dehydration: Removal of water from alcohols to form alkenes (using conc. $H_2SO_4$).
$\alpha$-Elimination: Groups removed from the same carbon (forms Carbenes).

4. Rearrangement Reactions

Definition: Involves the migration of an atom or group from one site to another within the same molecule to form a structural isomer (usually more stable).

Common Examples:

1,2-Hydride/Methyl Shift: Occurs in Carbocations to gain stability (e.g., $1^\circ \rightarrow 2^\circ \rightarrow 3^\circ$).
Isomerization of Alkanes: $n$-Butane $\xrightarrow{AlCl_3/HCl}$ Isobutane.

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