Chemistry · General Principles Of Organic Chemistry · NEET
A substitution reaction is one in which an atom or a group of atoms in a molecule is replaced by another atom or group. The carbon skeleton stays the same size, so the number of carbons and the degree of unsaturation do not change. Memory hook: SWAP, not GROW and not SHRINK. Substitution swaps one group for another, addition makes the molecule grow, and elimination makes it lose atoms.
In substitution, a leaving group (Br) is replaced by an incoming group (OH) at the same carbon, so the molecule keeps its size and degree of unsaturation. The three NEET types differ by the attacking species: nucleophile, electrophile, or free radical.
Your doubts, answered
What exactly is a substitution reaction and how do I spot one?
In a substitution reaction, one atom or group on the substrate is replaced by another atom or group. The carbon framework does not gain or lose carbons, and the degree of unsaturation (number of double bonds or rings) stays the same. Quick check: if a group leaves AND a new group takes its place at the same carbon, it is substitution. Example: CH3-Br + OH- gives CH3-OH + Br-. Here -Br is swapped for -OH, so it is substitution.
How is substitution different from addition and elimination?
Substitution swaps one group for another, so the molecular size stays similar. Addition adds atoms across a double or triple bond, so a pi bond is used up and the molecule gains atoms (for example Br2 adding across C=C). Elimination removes atoms (usually H and a halogen) and creates a new double bond, so the molecule loses atoms. Trick: substitution = SWAP, addition = GROW (pi bond breaks), elimination = SHRINK (pi bond forms).
What are the three types of substitution reactions?
They are named after the attacking species. (1) Nucleophilic substitution: a nucleophile (electron-rich, like OH-, CN-) replaces a leaving group, common at sp3 carbon in haloalkanes. (2) Electrophilic substitution: an electrophile (electron-poor, like Cl+, NO2+) replaces a hydrogen, typical of benzene and aromatic rings. (3) Free radical substitution: a radical replaces H, seen in the halogenation of alkanes under UV light.
Is the chlorination of methane substitution or addition?
It is a free radical substitution. Methane (CH4) has no double bond, so nothing can add. Instead, under UV light Cl2 splits into Cl radicals, one H of methane is replaced by Cl to give CH3Cl plus HCl. A group (H) is swapped for another (Cl), which is the definition of substitution.
Why does benzene react with Cl2 by substitution but an alkene by addition?
Benzene has a stable aromatic ring (6 delocalised pi electrons). Adding across it would destroy that stability, so instead one ring H is replaced by Cl using a catalyst like AlCl3: this is electrophilic substitution. An alkene has a reactive, localised C=C, so Cl2 simply adds across it to give a dihalide (addition). This is a classic NEET distinction.
⚠️ The NEET trap ✗ Benzene + Cl2 with AlCl3 is an addition reaction because Cl2 reacts with the ring. ✓ Benzene + Cl2 with AlCl3 is electrophilic SUBSTITUTION: one ring hydrogen is replaced by Cl to give chlorobenzene + HCl. Only under UV light (free radical) does Cl2 ADD to benzene giving C6H6Cl6. 🧠 Same reactants, different conditions give different reaction types. AlCl3 catalyst means substitution; UV light means addition. Read the condition above the arrow before choosing.
Real NEET questions
2019
Among the following, the reaction that proceeds through an electrophilic substitution is:
A · C6H5-N2+Cl- + Cu2Cl2 -> C6H5-Cl + N2
B · C6H6 + Cl2 (AlCl3) -> C6H5-Cl + HCl ✓
C · C6H6 + Cl2 (UV light) -> C6H6Cl6
D · C6H5-CH2-OH + HCl (heat) -> C6H5-CH2-Cl + H2O
Solution: With AlCl3, Cl2 generates the electrophile Cl+, which replaces a ring hydrogen of benzene. This is electrophilic aromatic substitution. Option A is a Sandmeyer-type reaction, option C is free radical addition (giving BHC), and option D is nucleophilic substitution at an sp3 carbon. So only B is electrophilic substitution.
2025
How many products (including stereoisomers) are expected from the monochlorination of 2-methylbutane, (CH3)2CH-CH2-CH3?
A · 5
B · 6 ✓
C · 2
D · 3
Solution: Free radical monochlorination replaces one H at a time. 2-Methylbutane has four kinds of replaceable hydrogen, giving four constitutional products: 1-chloro-2-methylbutane, 2-chloro-2-methylbutane, 2-chloro-3-methylbutane and 1-chloro-3-methylbutane. Two of these (1-chloro-2-methylbutane and 2-chloro-3-methylbutane) have a chiral carbon, so each exists as a pair of enantiomers. Counting enantiomers separately gives 6 products.
2016
For the reactions (a) CH3CH2CH2Br + KOH -> CH3CH=CH2 + KBr + H2O, (b) bromo-dimethylcyclohexane + KOH gives the hydroxy-dimethylcyclohexane (Br replaced by OH), (c) cyclohexene + Br2 -> 1,2-dibromocyclohexane, which statement is correct?
A · (a) and (b) are elimination and (c) is addition
B · (a) is elimination, (b) is substitution and (c) is addition ✓
C · (a) is elimination, (b) and (c) are substitution
D · (a) is substitution, (b) and (c) are addition
Solution: Reaction (a) loses HBr to form an alkene, so it is elimination. Reaction (b) simply replaces -Br by -OH with no change to the skeleton, so it is nucleophilic substitution. Reaction (c) adds Br2 across the C=C with no atom lost, so it is addition. Hence (a) elimination, (b) substitution, (c) addition.
Solved General Principles Of Organic Chemistry NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is a reaction where one atom or group in a molecule is replaced by a different atom or group, while the rest of the carbon skeleton stays the same. Example: -Br is swapped for -OH.
What are the main types of substitution reactions for NEET?
Three types: nucleophilic substitution (nucleophile replaces a leaving group, common in haloalkanes), electrophilic substitution (electrophile replaces H, common in benzene), and free radical substitution (radical replaces H, as in alkane halogenation).
How do I quickly tell substitution from addition and elimination?
Substitution swaps a group and keeps the size and unsaturation the same. Addition breaks a pi bond and adds atoms (molecule grows). Elimination removes atoms and forms a new pi bond (molecule shrinks). Remember: SWAP, GROW, SHRINK.
Is halogenation of an alkane substitution?
Yes. Alkanes have no double bond, so halogens cannot add. Under UV light a halogen radical replaces a hydrogen, which is free radical substitution, for example CH4 to CH3Cl.
Why is aromatic substitution important for NEET?
Benzene and its derivatives almost always react by electrophilic substitution (nitration, halogenation, sulphonation, Friedel-Crafts). NEET frequently asks you to classify these and to distinguish them from addition, so it is high-yield.
