The reaction between CH3Cl and hydroxide ion to yield methanol and chloride ion follows a second order kinetics, i.e., the rate depends upon the concentration of both the reactants.
-- NCERT Class 12 Chemistry, Ch. 6, p. 171Substitution Mechanisms
8 MCQs9-step worked example
Source: NCERT Organic Compounds Containing HalogensPYQ coverage: NEET 2025Official key: NTA-verifiedLast updated: 24 Sep 2026
Substitution Mechanisms, explained for NEET
The reactivity order reverses between the two mechanisms. That single sentence costs more marks in this unit than any mechanism diagram. In SN2, reactivity runs primary > secondary > tertiary. In SN1, it runs tertiary > secondary > primary. A student who memorises "tertiary is most reactive" and applies it everywhere answers half the substitution questions backwards.
The reversal follows from what each mechanism needs. NCERT Class 12 Chemistry Part 2, Chapter 6, page 173 describes SN2 as a single-step, bimolecular process: the nucleophile attacks the carbon from the side opposite the leaving group while the C–X bond is still breaking. That back-side approach needs space. Three bulky alkyl groups around the carbon block it, so tertiary halides are the slowest. Rate depends on both substrate and nucleophile — second-order kinetics.
SN1, described on page 175, is two-step and unimolecular. The C–X bond ionises first to give a carbocation, then the nucleophile adds. The slow step involves only the substrate, so the rate is first-order and independent of nucleophile concentration. Tertiary carbocations are stabilised by three electron-releasing alkyl groups, so tertiary halides ionise fastest.
Stereochemistry follows from geometry, and NEET tests it as an Assertion–Reason pair. Back-side attack in SN2 flips the configuration: an optically active halide gives a product of inverted configuration, the Walden inversion described on page 179. SN1's planar carbocation intermediate can be attacked from either face with near-equal probability, giving a racemic mixture. Inversion belongs to SN2, racemisation to SN1 — swapping them is the commonest error here.
One more reagent-level distinction. KCN and AgCN both deliver a CN group, but KCN is largely ionic and its nucleophilic carbon gives alkyl cyanides, while AgCN is mainly covalent and attacks through nitrogen to give isocyanides. The reagent decides the product, not the halide.
Watch out: read the kinetic order and substrate class before naming the mechanism. Never assume tertiary means fastest.
Can you answer these Substitution Mechanisms 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
In an SN1 reaction, the rate of reaction depends on the concentration of:
Show answer and why every option is right or wrong
Answer: A. A is correct. SN1 is unimolecular: the rate-determining step is ionisation of the C–X bond, which involves the substrate alone, so the rate is first-order in substrate and independent of nucleophile concentration (NCERT Class 12 Chemistry Part 2, Chapter 6, page 175).
Why B is wrong: B is wrong because the nucleophile enters only in the fast second step, after the rate-determining ionisation, so its concentration does not appear in the rate law.
Why C is wrong: C is wrong because dependence on both concentrations describes SN2, which is bimolecular and second-order overall — this is the reactivity-order reversal trap applied to kinetics.
Why D is wrong: D is wrong because solvent polarity influences the rate by stabilising the carbocation but the solvent is not a kinetic order term in the rate expression.
MCQ 2Easy RecallPractice
The stereochemical outcome when an optically active alkyl halide undergoes substitution by a pure SN2 pathway is:
Show answer and why every option is right or wrong
Answer: C. C is correct. The nucleophile attacks the carbon from the face opposite the leaving group, inverting the spatial arrangement of the remaining three groups — the Walden inversion described in NCERT Class 12 Chemistry Part 2, Chapter 6, page 179.
Why A is wrong: A is wrong because retention would require the nucleophile to approach from the same side as the departing halogen, which the back-side attack geometry of SN2 does not permit.
Why B is wrong: B is wrong because racemisation is the SN1 outcome, arising from a planar carbocation attacked on either face. Assigning racemisation to SN2 is the commonest Assertion–Reason trap in this unit.
Why D is wrong: D is wrong because SN2 is a substitution, not an elimination; the product retains a stereocentre and is optically active, merely inverted.
MCQ 3Easy RecallPractice
Which reagent converts a haloalkane predominantly into an alkyl isocyanide?
Show answer and why every option is right or wrong
Answer: B. B is correct. AgCN is largely covalent, so only the nitrogen lone pair is available for attack, and the product is an alkyl isocyanide (NCERT Class 12 Chemistry Part 2, Chapter 6, page 173).
Why A is wrong: A is wrong because KCN is predominantly ionic; the free cyanide ion attacks through carbon and gives an alkyl cyanide, not an isocyanide.
Why C is wrong: C is wrong because NaCN, like KCN, is ionic and attacks through carbon, giving the alkyl cyanide. The metal ion decides ionic versus covalent character, and both Na and K give the ionic case.
Why D is wrong: D is wrong because HCN is a weak, largely undissociated acid and is not the reagent NCERT specifies for this conversion.
MCQ 4Direct ApplicationPractice
Three halides are hydrolysed under identical SN2 conditions: 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane. The order of decreasing rate is:
Show answer and why every option is right or wrong
Answer: C. C is correct. SN2 needs an unhindered back-side approach, so reactivity falls as alkyl substitution at the reacting carbon rises: primary > secondary > tertiary (NCERT Class 12 Chemistry Part 2, Chapter 6, page 173).
Why A is wrong: A is wrong because it applies the SN1 carbocation-stability order to an SN2 reaction. This reversal is the highest-cost error in this topic.
Why B is wrong: B is wrong because it places the secondary halide above the primary one; steric crowding increases monotonically from primary to tertiary, so the primary halide is fastest.
Why D is wrong: D is wrong because the rate-controlling factor in SN2 is steric access to the reacting carbon, not leaving-group identity, which is common to all three here.
MCQ 5Direct ApplicationPractice
A haloalkane reacts with aqueous hydroxide and the measured rate is found to double when the hydroxide concentration is doubled at fixed substrate concentration. The mechanism is:
Show answer and why every option is right or wrong
Answer: D. D is correct. A rate that responds to nucleophile concentration means the nucleophile is present in the rate-determining step, which is the defining kinetic signature of the single-step bimolecular SN2 mechanism (NCERT Class 12 Chemistry Part 2, Chapter 6, page 173).
Why A is wrong: A is wrong because in SN1 the slow ionisation step does not involve the nucleophile, so changing hydroxide concentration would leave the rate unaltered.
Why B is wrong: B is wrong because elimination is a competing pathway with its own kinetics; the data given describe the substitution rate law and identify it unambiguously.
Why C is wrong: C is wrong because nucleophile strength affects how fast the SN2 step is, but it does not change SN1 kinetics; the observed concentration dependence, not the nucleophile's strength, assigns the mechanism.
MCQ 6Direct ApplicationPractice
A single enantiomer of 3-bromo-3-methylhexane is warmed with aqueous ethanol containing no added strong nucleophile. The expected substitution product is:
Show answer and why every option is right or wrong
Answer: C. C is correct. C3 carries four different groups — Br, CH₃, C₂H₅ and C₃H₇ — so the starting halide is chiral and the question about configuration is a real one. A tertiary halide in a polar protic solvent with no strong nucleophile follows SN1, and the planar carbocation intermediate is attacked from either face with near-equal probability, so the single starting enantiomer gives a roughly 50:50 mixture of both product enantiomers (NCERT Class 12 Chemistry Part 2, Chapter 6, page 179).
Why A is wrong: A is wrong because inversion is the SN2 signature, and back-side attack is exactly what a tertiary carbon blocks.
Why B is wrong: B is wrong because retention would require the intermediate to hold its original geometry, but the carbocation is planar and has no memory of which face the halogen left from.
Why D is wrong: D is wrong because tertiary halides are the most reactive class under SN1 conditions, not inert; only their SN2 reactivity is negligible.
MCQ 7CalculationPractice
An optically active secondary bromide is treated first with a high concentration of a strong nucleophile in a polar aprotic solvent, and in a separate run with a dilute weak nucleophile in aqueous ethanol. The pair of stereochemical outcomes, in that order, is:
Show answer and why every option is right or wrong
Answer: B. B is correct. High nucleophile concentration in a polar aprotic solvent favours the bimolecular SN2 route, which inverts configuration; a dilute weak nucleophile in a polar protic solvent favours ionisation to a carbocation and gives substantial racemisation (NCERT Class 12 Chemistry Part 2, Chapter 6, pages 173 and 179).
Why A is wrong: A is wrong because it pairs each set of conditions with the other mechanism's stereochemistry — the inversion-versus-racemisation swap this topic is built to catch.
Why C is wrong: C is wrong because it ignores the effect of the second set of conditions; a weak dilute nucleophile in a protic solvent cannot sustain a bimolecular pathway.
Why D is wrong: D is wrong because a strong nucleophile at high concentration in an aprotic solvent drives the bimolecular route, whose stereochemical outcome is inversion, not racemisation.
MCQ 8Concept TrapPractice
Two statements are made about nucleophilic substitution. Statement I: a tertiary halide reacts faster than a primary halide under SN1 conditions. Statement II: a tertiary halide reacts faster than a primary halide under SN2 conditions. Which is correct?
Show answer and why every option is right or wrong
Answer: A. A is correct. Statement I holds because tertiary carbocations are the most stabilised, so ionisation is fastest. Statement II fails because SN2 requires back-side access, which three alkyl groups obstruct, making tertiary halides the slowest (NCERT Class 12 Chemistry Part 2, Chapter 6, pages 173 and 175).
Why B is wrong: B is wrong because it asserts one reactivity order for both mechanisms, which is precisely the reversal error; the orders run in opposite directions.
Why C is wrong: C is wrong because it inverts both statements: SN1 genuinely does favour tertiary substrates, so Statement I is correct as written.
Why D is wrong: D is wrong because Statement I is a correct description of SN1 reactivity; only Statement II is false.
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How do you solve a Substitution Mechanisms question? A worked example
- 1
Given.
An optically active haloalkane, 2-bromobutane, is hydrolysed in aqueous sodium hydroxide. Experiment shows that halving the NaOH concentration halves the observed rate. Substrate class: secondary. Nucleophile: hydroxide, strong.
- 2
Required.
Name the mechanism and predict the stereochemistry of the product.
- 3
Concept.
Two mechanisms are available to a secondary halide. The kinetic order fixes which one operates: an SN2 rate law contains both substrate and nucleophile concentrations, an SN1 rate law contains only the substrate. Stereochemistry then follows from the mechanism's geometry.
- 4
Formula.
For SN2, rate = k[RX][Nu⁻]. For SN1, rate = k[RX].
- 5
Substitution.
Halving [OH⁻] halves the rate, so the rate is first-order in hydroxide. Hydroxide concentration therefore appears in the rate law, which matches rate = k[RX][OH⁻] and excludes rate = k[RX].
- 6
Calculation.
Overall order = 1 (substrate) + 1 (nucleophile) = 2. The kinetic orders here are counting integers from the rate law, not measured quantities, so they carry no significant-figure implications; the "halving" is an exact experimental ratio of 2 (exact), not a measurement.
- 7
Final answer.
The reaction proceeds by SN2. The hydroxide attacks the C2 carbon from the side opposite bromine, so the product is butan-2-ol with inverted configuration — a single enantiomer, not a racemate.
- 8
Common trap.
The trap is to see "secondary halide" and assume a carbocation forms, then answer racemisation. Secondary halides can follow either route; only the kinetic data decide. The second trap is reaching for the tertiary-is-fastest reflex, which belongs to SN1 alone.
- 9
Similar NEET-style question.
An optically active halide gives a product that is optically inactive and shows no net rotation, while the rate is unchanged when the nucleophile concentration is tripled. Identify the mechanism, the substrate class most consistent with it, and the stereochemical term for the product.
What to remember before solving Substitution Mechanisms questions
SN1 reactions are generally carried out in polar protic solvents (like water, alcohol, acetic acid, etc.). The reaction between tert-butyl bromide and hydroxide ion yields tert-butyl alcohol and follows the first order kinetics, i.e., the rate of reaction depends upon the concentration of only one reactant, which is tert-butyl bromide. It occurs in two steps. In step I, the polarised C-Br bond undergoes slow cleavage to produce a carbocation and a bromide ion. The carbocation thus formed is then attacked by nucleophile in step II to complete the substitution reaction.
-- NCERT Class 12 Chemistry, Ch. 6, p. 173Key Fact
Reactivity order for SN2
Thus the order of reactivity followed is: Primary halide > Secondary halide > Tertiary halide.
-- NCERT Class 12 Chemistry, Ch. 6, p. 173In case of optically active alkyl halides, the product formed as a result of SN2 mechanism has the inverted configuration as compared to the reactant. This is because the nucleophile attaches itself on the side opposite to the one where the halogen atom is present. When (-)-2-bromooctane is allowed to react with sodium hydroxide, (+)-octan-2-ol is formed with the -OH group occupying the position opposite to what bromide had occupied. Thus, SN2 reactions of optically active halides are accompanied by inversion of configuration. In case of optically active alkyl halides, SN1 reactions are accompanied by racemisation.
-- NCERT Class 12 Chemistry, Ch. 6, p. 179Definition
Racemisation
If a 50:50 mixture of A and B is obtained then the process is called racemisation and the product is optically inactive, as one isomer will rotate the plane polarised light in the direction opposite to another.
-- NCERT Class 12 Chemistry, Ch. 6, p. 179Definition
Optical activity and the d/l notation
Plane of plane polarised light produced by passing ordinary light through Nicol prism is rotated when it is passed through the solutions of certain compounds. Such compounds are called optically active compounds. The angle by which the plane polarised light is rotated is measured by an instrument called polarimeter. If the compound rotates the plane of plane polarised light to the right, i.e., clockwise direction, it is called dextrorotatory or the d-form and is indicated by placing a positive (+) sign before the degree of rotation.
-- NCERT Class 12 Chemistry, Ch. 6, p. 175Haloalkanes react with KCN to form alkyl cyanides as main product while AgCN forms isocyanides as the chief product. KCN is predominantly ionic and provides cyanide ions in solution. Although both carbon and nitrogen atoms are in a position to donate electron pairs, the attack takes place mainly through carbon atom and not through nitrogen atom since C-C bond is more stable than C-N bond. However, AgCN is mainly covalent in nature and nitrogen is free to donate electron pair resulting in the formation of isocyanide as the main product.
-- NCERT Class 12 Chemistry, Ch. 6, p. 171Substitution Mechanisms questions from past NEET papers
1 question from NEET 2025. Answers verified against NTA official keys.
NEET 2025
1A is false but R is true
2Both A and R are true and R is the correct explanation of A
3Both A and R are true but R is not the correct explanation of A
4A is true but R is false
NTA Answer: Option 2(final)
All 7 past-paper questions from Organic Compounds Containing Halogens →
Sources
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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