Types of Organic Reactions
The four pillars of organic mechanisms: Substitution, Addition, Elimination, and Rearrangement. Learn how to identify them instantly based on substrates and reagents.
Module Focus: The 4 Core Pathways
Despite the millions of organic compounds, their reactions generally fall into four fundamental categories: Substitution (swapping parts), Addition (breaking $\pi$ bonds to add groups), Elimination (removing groups to form $\pi$ bonds), and Rearrangement (reorganizing the skeleton). Identifying the reaction type is step one in solving any organic mechanism problem in NEET.
1. Substitution Reactions
A reaction in which an atom or a group of atoms in a molecule is directly replaced (substituted) by another atom or group. The overall number of bonds remains the same.
An electron-rich nucleophile replaces a leaving group.
Alkyl Halides + aq. KOH $\rightarrow$ Alcohols
Key Mechanisms: $S_N1$ (Carbocation) and $S_N2$ (Inversion).
An electron-deficient electrophile replaces an atom (usually H).
Benzene + $Cl_2/FeCl_3$ $\rightarrow$ Chlorobenzene
Characteristic of Aromatic rings (EAS).
Reaction driven by odd-electron species (radicals).
$CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl$
Characteristic of Alkanes and allylic/benzylic positions.
2. Addition Reactions
Reactions where two molecules combine to form a single product. This typically involves the breaking of a $\pi$ (pi) bond to form two new $\sigma$ (sigma) bonds. Unsaturated compounds become saturated.
Electrophilic Addition
Characteristic of Alkenes and Alkynes. The electron-rich $\pi$ cloud attacks an electrophile first.
Follows Markovnikov's Rule.
$CH_3-CH=CH_2 + HBr \rightarrow CH_3-CH(Br)-CH_3$
Nucleophilic Addition
Characteristic of Aldehydes and Ketones. A nucleophile attacks the electron-deficient carbonyl carbon.
Ketone + $HCN \rightarrow$ Cyanohydrin
Markovnikov's Rule states the negative part of the addendum (e.g., $Br^-$) goes to the carbon with fewer hydrogens (proceeds via most stable carbocation).
Kharasch Effect (Peroxide Effect) yields the Anti-Markovnikov product (proceeds via free radical mechanism).
3. Elimination Reactions
The exact opposite of addition. Two atoms or groups are removed from a molecule, resulting in the formation of a $\pi$ (pi) bond. Saturated compounds become unsaturated.
When elimination can produce two different alkenes, which one is the major product?
- Zaitsev (Saytzeff) Rule: The more substituted (more stable) alkene is the major product. This is the default pathway (e.g., using alc. KOH).
- Hofmann Rule: The less substituted (less stable) alkene is the major product. This happens if the base is exceptionally bulky (like tert-butoxide), or if the leaving group is very poor (like $-N(CH_3)_3^+$ or $-F$).
4. Rearrangement Reactions
Reactions involving the migration of an atom or group from one atom to another within the same molecule, creating a new structural skeleton.
In basic GOC, this is almost exclusively seen with Carbocations. A less stable carbocation ($1^\circ$ or $2^\circ$) will undergo a 1,2-hydride shift or 1,2-alkyl shift to become a more stable carbocation ($3^\circ$ or resonance-stabilized).
Alkyl Halide + alc. KOH $\rightarrow$ Elimination
NEET Grand Test: Reaction Types
15 High-Order Thinking Questions testing mechanism identification, stereochemistry, and regioselectivity.
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