Organic reactions can be classified into the following categories: (i) Substitution reactions (ii) Addition reactions (iii) Elimination reactions (iv) Rearrangement reactions You will be studying these reactions in Unit 9 and later in class XII.
-- NCERT Class 11 Chemistry, Ch. 8, p. 278Common Reactions Organic
8 MCQs9-step worked example
Source: NCERT Organic Chemistry — Some Basic PrinciplesOfficial key: NTA-verifiedLast updated: 8 Oct 2026
Try this first
Which list gives the four categories of organic reactions named in NCERT?
- A.Substitution, addition, oxidation, reduction
- B.Combustion, addition, elimination, hydrolysis
- C.Substitution, polymerisation, elimination, oxidation
- D.Substitution, addition, elimination, rearrangement
Tap to see the answer
Answer: D. NCERT classifies organic reactions into substitution, addition, elimination and rearrangement reactions (NCERT Class 11 Chemistry, Chapter 8, page 278).
A is wrong: A is wrong because oxidation and reduction are not among the four categories NCERT lists here (NCERT Class 11 Chemistry, Chapter 8, page 278).
B is wrong: B is wrong because combustion and hydrolysis are not among the four categories NCERT lists (NCERT Class 11 Chemistry, Chapter 8, page 278).
C is wrong: C is wrong because polymerisation and oxidation are not among the four categories NCERT lists (NCERT Class 11 Chemistry, Chapter 8, page 278).
Common Reactions Organic, explained for NEET
In one flask a haloalkane loses HBr and becomes an alkene. In another, an alkene takes up bromine. Both are called organic reactions, but they are not the same kind. What four families do chemists sort organic reactions into, and how do you tell which family a given equation belongs to?
NCERT classifies organic reactions into four categories: substitution, addition, elimination and rearrangement reactions. It adds that these are studied in Unit 9 and later in Class XII, and the chapter summary repeats the same four types (NCERT Class 11 Chemistry, Chapter 8, page 278; NCERT Class 11 Chemistry, Chapter 8, page 291).
Substitution: one or more hydrogen atoms of alkanes can be replaced by halogens, a nitro group or a sulphonic acid group; the reactions in which hydrogen atoms of alkanes are substituted are known as substitution reactions (NCERT Class 11 Chemistry, Chapter 9, page 302).
Addition: alkenes and alkynes undergo addition reactions, which are mainly electrophilic additions, and the addition of halogens to alkenes is an example of electrophilic addition (NCERT Class 11 Chemistry, Chapter 9, page 311; NCERT Class 11 Chemistry, Chapter 9, page 326).
Elimination: dehydrohalogenation is the removal of halogen acid from a haloalkane to give an alkene; it is a β-elimination reaction, because the hydrogen atom is eliminated from the carbon atom next to the one to which the halogen is attached (NCERT Class 11 Chemistry, Chapter 9, page 310).
Rearrangement is named in NCERT as the fourth category, and the Class 11 pages name it without working an example, so for NEET you need to recognise the word and the other three families (NCERT Class 11 Chemistry, Chapter 8, page 278).
The quick test: count what happens to the substrate. A hydrogen replaced by another atom is substitution; two atoms added across a multiple bond is addition; a small molecule such as HX removed is elimination.
Watch out: the same reagent can do different jobs. Chlorine with an alkane in light replaces hydrogen; chlorine with an alkene adds across the double bond.
How do you solve a Common Reactions Organic question? A worked example
- 1
Given
Three changes are written: (A) CH₄ + Cl₂ gives CH₃Cl + HCl in light; (B) CH₂=CH₂ + Br₂ gives BrCH₂CH₂Br; (C) CH₃CH₂Br gives CH₂=CH₂ + HBr.
- 2
Required
Classify each change as substitution, addition or elimination.
- 3
Concept
Substitution replaces hydrogen atoms of alkanes by another atom or group (NCERT Class 11 Chemistry, Chapter 9, page 302). Addition of halogens to alkenes is electrophilic addition (NCERT Class 11 Chemistry, Chapter 9, page 311). Removal of halogen acid is dehydrohalogenation, a β-elimination (NCERT Class 11 Chemistry, Chapter 9, page 310).
- 4
Formula
No formula is needed; compare the substrate before and after: swap, add, or remove.
- 5
Substitution
A: an H on carbon is swapped for Cl, and HCl leaves. B: Br₂ is taken up and the C=C becomes C–C. C: HBr is lost and a C=C forms.
- 6
Calculation
A: C₁H₄ → C₁H₃Cl, same carbon count, one H replaced. B: C₂H₄ → C₂H₄Br₂, two atoms added. C: C₂H₅Br → C₂H₄, HBr removed.
- 7
Final answer
A is substitution. B is addition (electrophilic addition of a halogen). C is elimination (dehydrohalogenation, β-elimination).
- 8
Common trap
Calling A an addition because chlorine "adds" to the molecule. In A a hydrogen leaves as HCl, so one atom has taken the place of another.
- 9
Similar NEET-style question
Propene reacts with bromine to give 1,2-dibromopropane. Classify the reaction. (Answer: addition, specifically electrophilic addition, because bromine adds across the C=C and nothing leaves.)
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Can you answer these Common Reactions Organic MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Which list gives the four categories of organic reactions named in NCERT?
Show answer and why every option is right or wrong
Answer: D. NCERT classifies organic reactions into substitution, addition, elimination and rearrangement reactions (NCERT Class 11 Chemistry, Chapter 8, page 278).
Why A is wrong: A is wrong because oxidation and reduction are not among the four categories NCERT lists here (NCERT Class 11 Chemistry, Chapter 8, page 278).
Why B is wrong: B is wrong because combustion and hydrolysis are not among the four categories NCERT lists (NCERT Class 11 Chemistry, Chapter 8, page 278).
Why C is wrong: C is wrong because polymerisation and oxidation are not among the four categories NCERT lists (NCERT Class 11 Chemistry, Chapter 8, page 278).
MCQ 2Easy RecallPractice
In a substitution reaction of alkanes, what is replaced?
Show answer and why every option is right or wrong
Answer: B. NCERT states that one or more hydrogen atoms of alkanes can be replaced by halogens, a nitro group or a sulphonic acid group; these are substitution reactions (NCERT Class 11 Chemistry, Chapter 9, page 302).
Why A is wrong: A is wrong because it is the hydrogen atoms that are replaced, not the carbon atoms (NCERT Class 11 Chemistry, Chapter 9, page 302).
Why C is wrong: C is wrong because alkanes have only single bonds, and converting a double bond to a single bond is an addition (NCERT Class 11 Chemistry, Chapter 9, page 311).
Why D is wrong: D is wrong because alkanes have no hydroxyl group to replace (NCERT Class 11 Chemistry, Chapter 9, page 302).
MCQ 3Easy RecallPractice
Dehydrohalogenation of a haloalkane means
Show answer and why every option is right or wrong
Answer: C. NCERT defines dehydrohalogenation as the removal of halogen acid, and it is an example of a β-elimination reaction (NCERT Class 11 Chemistry, Chapter 9, page 310).
Why A is wrong: A is wrong because adding halogen acid across a double bond is the reverse of this and is an addition reaction (NCERT Class 11 Chemistry, Chapter 9, page 311).
Why B is wrong: B is wrong because dehydrohalogenation removes both a hydrogen and a halogen as halogen acid, not one for the other (NCERT Class 11 Chemistry, Chapter 9, page 310).
Why D is wrong: D is wrong because replacing a halogen by a nitro group would be a substitution (NCERT Class 11 Chemistry, Chapter 9, page 302).
MCQ 4Direct ApplicationPractice
Methane reacts with chlorine in the presence of sunlight to give chloromethane and hydrogen chloride. This reaction is classified as
Show answer and why every option is right or wrong
Answer: D. A hydrogen atom of the alkane is replaced by a halogen, which is a substitution reaction; NCERT notes that halogenation takes place at higher temperature or in diffused sunlight or ultraviolet light (NCERT Class 11 Chemistry, Chapter 9, page 302).
Why A is wrong: A is wrong because nothing adds across a multiple bond; methane has only single bonds and a hydrogen leaves (NCERT Class 11 Chemistry, Chapter 9, page 302).
Why B is wrong: B is wrong because the reaction does not remove a small molecule to make a double bond; the hydrogen is replaced by chlorine (NCERT Class 11 Chemistry, Chapter 9, page 302).
Why C is wrong: C is wrong because the carbon skeleton is unchanged; atoms are exchanged, not rearranged (trap: reading any change in a molecule as rearrangement).
MCQ 5Direct ApplicationPractice
Ethene reacts with bromine to give 1,2-dibromoethane. This is an example of
Show answer and why every option is right or wrong
Answer: A. Addition of halogens to alkenes is an example of electrophilic addition, and alkenes mainly undergo electrophilic additions (NCERT Class 11 Chemistry, Chapter 9, page 311; NCERT Class 11 Chemistry, Chapter 9, page 326).
Why B is wrong: B is wrong because free radical substitution is the main reaction type of alkanes, not of alkenes (NCERT Class 11 Chemistry, Chapter 9, page 326).
Why C is wrong: C is wrong because elimination removes atoms such as HX from the molecule; here atoms are taken up (NCERT Class 11 Chemistry, Chapter 9, page 310).
Why D is wrong: D is wrong because the bromine atoms are added rather than the skeleton being reorganised (trap: calling every change of the substrate a rearrangement).
MCQ 6Direct ApplicationPractice
A haloalkane loses one molecule of HBr and forms an alkene. The hydrogen that leaves is on the carbon next to the carbon bearing bromine. This is classified as
Show answer and why every option is right or wrong
Answer: C. Removal of halogen acid, with the hydrogen lost from the carbon next to the one carrying the halogen, is the β-elimination of dehydrohalogenation (NCERT Class 11 Chemistry, Chapter 9, page 310).
Why A is wrong: A is wrong because nothing takes the place of the leaving atoms; HBr is removed and a double bond forms (NCERT Class 11 Chemistry, Chapter 9, page 310).
Why B is wrong: B is wrong because addition combines molecules across a multiple bond, which is the reverse direction (NCERT Class 11 Chemistry, Chapter 9, page 311).
Why D is wrong: D is wrong because electrophilic addition is the characteristic reaction of alkenes, which here is the product, not the reactant (NCERT Class 11 Chemistry, Chapter 9, page 326).
MCQ 7CalculationPractice
Ethane reacts with chlorine in sunlight to give chloroethane (C₂H₅Cl) and HCl. Ethene reacts with chlorine to give 1,2-dichloroethane (C₂H₄Cl₂) alone. Which pair of classifications is correct, in this order?
Show answer and why every option is right or wrong
Answer: C. In ethane a hydrogen is replaced by chlorine and HCl is a by-product, so it is substitution (NCERT Class 11 Chemistry, Chapter 9, page 302). In ethene the halogen adds across the double bond and nothing leaves, so it is electrophilic addition (NCERT Class 11 Chemistry, Chapter 9, page 311).
Why A is wrong: A is wrong because it reverses the two classifications: the alkane substitutes and the alkene adds (NCERT Class 11 Chemistry, Chapter 9, page 326).
Why B is wrong: B is wrong because ethane loses a hydrogen but gains chlorine, so it is not an elimination; nothing is removed to form a multiple bond (NCERT Class 11 Chemistry, Chapter 9, page 310).
Why D is wrong: D is wrong because the ethene reaction adds chlorine and removes nothing, so it is addition, not elimination (NCERT Class 11 Chemistry, Chapter 9, page 311).
MCQ 8CalculationPractice
A bromoalkane of molecular formula C₄H₉Br undergoes dehydrohalogenation, losing one molecule of HBr. What is the molecular formula of the alkene formed?
Show answer and why every option is right or wrong
Answer: D. Dehydrohalogenation removes one molecule of halogen acid, here HBr (NCERT Class 11 Chemistry, Chapter 9, page 310). C₄H₉Br − HBr = C₄H₈, which is an alkene.
Why A is wrong: A is wrong because C₄H₁₀ is an alkane and would need hydrogen added, not HBr removed (NCERT Class 11 Chemistry, Chapter 9, page 310).
Why B is wrong: B is wrong because C₄H₉ is only the bromoalkane minus Br; HBr, not Br alone, is removed (trap: removing only the halogen).
Why C is wrong: C is wrong because C₄H₆ would result from removing two molecules of HBr or an extra H₂ (trap: removing two hydrogens).
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What to remember before solving Common Reactions Organic questions
6 NCERT lines
Summary
Chapter summary: organic reactions are broadly substitution, addition, elimination and rearrangement
Organic reactions can be broadly classified into following types; substitution, addition, elimination and rearrangement reactions.
-- NCERT Class 11 Chemistry, Ch. 8, p. 291One or more hydrogen atoms of alkanes can be replaced by halogens, nitro group and sulphonic acid group.
-- NCERT Class 11 Chemistry, Ch. 9, p. 302This reaction is known as dehydrohalogenation i.e., removal of halogen acid. This is example of E-elimination reaction, since hydrogen atom is eliminated from the E carbon atom (carbon atom next to the carbon to which halogen is attached).
-- NCERT Class 11 Chemistry, Ch. 9, p. 310Addition of halogens to alkenes is an example of electrophilic addition reaction involving cyclic halonium ion formation which you will study in higher classes.
-- NCERT Class 11 Chemistry, Ch. 9, p. 311The important reactions of alkanes are free radical substitution, combustion, oxidation and aromatization. Alkenes and alkynes undergo addition reactions, which are mainly electrophilic additions.
-- NCERT Class 11 Chemistry, Ch. 9, p. 326Common Reactions Organic: NEET previous year questions (PYQs) with answers
12 questions in Organic Chemistry — Some Basic Principles
No question in our NEET 2020–2025 set targets this topic directly.
All 12 past-paper questions from Organic Chemistry — Some Basic Principles →
More in Organic Chemistry — Some Basic Principles: 9 exam traps and mistakes · 1 formula · 1 question pattern from its other lessons.
Sources
NCERT refs: Class 11 Chemistry Chapter 8, p.278 | Class 11 Chemistry Chapter 8, p.291 | Class 11 Chemistry Chapter 9, p.302 | Class 11 Chemistry Chapter 9, p.311 | Class 11 Chemistry Chapter 9, p.326 | Class 11 Chemistry Chapter 9, p.310
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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