Types of Organic Reactions: Substitution, Addition, Elimination, Rearrangement

NCERT lists exactly four: (i) substitution, (ii) addition, (iii) elimination, and (iv) rearrangement. Learn these four names first — almost every NEET GOC reaction fits into one of them. Substitution swaps a group, addition puts two groups onto a multiple bond, elimination removes a small molecule to form a multiple bond, and rearrangement moves atoms within the same molecule.

In a substitution reaction one atom or group replaces another, so the total number of groups on the carbon stays the same (example: R-Br + OH- gives R-OH + Br-). In an addition reaction two new groups add across a C=C or C=C double/triple bond and nothing leaves, so the molecule gets bigger and loses one degree of unsaturation (example: alkene + Br2 gives a dibromide). Quick test: if the double bond disappears and nothing leaves, it is addition.

They are opposites. Addition destroys a multiple bond (two groups add on). Elimination creates a multiple bond (a small molecule like HBr or H2O is removed from two neighbouring carbons). If a C=C forms and a small molecule leaves, it is elimination; if a C=C is used up, it is addition.

It is elimination. Dehydrohalogenation means removing H and a halogen (like HBr) from adjacent carbons of an alkyl halide, using alcoholic KOH, to form an alkene. Because a small molecule leaves and a new double bond forms, it is an elimination reaction, not substitution. This is a very common NEET trap.

In a beta-elimination, the leaving group (like Br) goes from one carbon and a hydrogen goes from the neighbouring (beta) carbon, forming a double bond between them. Zaitsev's rule says the more substituted alkene (the one with more carbon groups on the C=C) is the major product because it is more stable. Example: 2-bromopentane with base gives mainly pent-2-ene, not pent-1-ene.

In a rearrangement the atoms within a single molecule shift their positions to give a structural isomer — the molecular formula stays the same but the arrangement changes. A common example is a carbocation rearranging by a 1,2-hydride or 1,2-methyl shift to become more stable. Keto-enol tautomerism is also a type of intramolecular rearrangement.

Both are substitutions, but the attacking species differs. In nucleophilic substitution an electron-rich nucleophile attacks an electron-poor carbon (common in alkyl halides, sp3 carbon). In electrophilic substitution an electron-poor electrophile attacks an electron-rich site (common in benzene, aromatic rings). Example: benzene + Cl2 with AlCl3 is electrophilic aromatic substitution.

Count atoms and watch the multiple bonds. If one group is swapped for another and the count stays same, it is substitution. If two groups add onto a C=C or triple bond (bond disappears), it is addition. If a small molecule leaves and a new double bond forms, it is elimination. If atoms just move inside the same molecule to form an isomer, it is rearrangement.

Yes. Elimination removes a small molecule to create a multiple bond, while addition consumes a multiple bond by adding groups across it. They are opposite processes, and many reactions can go either way depending on conditions.

Alcoholic KOH provides ethoxide/alkoxide, a strong base that pulls off a beta-hydrogen and favours elimination to form an alkene. Aqueous KOH provides OH-, a good nucleophile that attacks the carbon and favours substitution, replacing the halogen with -OH.

Classifying a given reaction as substitution, addition or elimination is a repeat favourite (2016, 2020). Electrophilic vs nucleophilic substitution (2019) is also common. Practising atom-counting on real reactions is the fastest way to score these marks.