Q.Amongst the given set of reactants, the most appropriate for preparing 2° amine is ____.
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Nucleophilic Addition – From Intuition to Precision
Imagine you have a molecule with a carbon–oxygen double bond — a carbonyl group (C=O). That oxygen is greedy for electrons; it pulls them away from carbon, leaving the carbon slightly positive (δ+) and the oxygen slightly negative (δ−). Now, if you bring a species that is rich in electrons (a nucleophile, meaning "nucleus-loving"), it will naturally be attracted to that electron-deficient carbon. The nucleophile attacks the carbon, the π bond breaks, and the oxygen picks up a proton (or some other electrophile) to become stable. That, in a nutshell, is nucleophilic addition.
The key idea: a nucleophile adds across a polar multiple bond (usually C=O or C≡N), breaking the π bond and forming two new sigma bonds.
The Precise Statement
Nucleophilic addition is a reaction in which a nucleophile (an electron-rich species) forms a sigma bond with an electrophilic carbon atom of a polar multiple bond (typically a carbonyl group, C=O, or a nitrile, C≡N), while the π bond breaks. The resulting intermediate then captures a proton (or another electrophile) to give a neutral product.
In general form:
RX2C=O+NuX−HX+RX2C(OH)Nu
The nucleophile (NuX−) attacks the carbonyl carbon; the oxygen becomes negatively charged; then a proton (HX+) from the medium attaches to the oxygen, yielding an alcohol.
Why It Happens – The Driving Force
The carbonyl carbon is electrophilic because:
- Oxygen is more electronegative than carbon, so the C=O bond is polarised: CXδ+=OXδ−.
- The π bond is weaker than a σ bond, so it can break relatively easily.
A nucleophile (like OHX−, CNX−, or NHX3) has a lone pair or a negative charge. It seeks positive centres. The attack forms a new σ bond, and the π electrons move entirely to oxygen, creating an alkoxide ion (RX2C−OX−). This intermediate is then quenched by a proton.
A common mistake: thinking the nucleophile attacks the oxygen. No — oxygen is already electron-rich; the nucleophile goes to the carbon because it is electron-deficient.
A Concrete Example – Addition of HCN to a Ketone
Take acetone (CHX3COCHX3) and hydrogen cyanide (HCN). In the presence of a base, CNX− (the nucleophile) attacks the carbonyl carbon:
CHX3COCHX3+CNX−CHX3C(OX−)(CN)CHX3
The alkoxide intermediate then picks up a proton from HCN (or from water) to give a cyanohydrin:
CHX3C(OX−)(CN)CHX3+HX+CHX3C(OH)(CN)CHX3
The product is acetone cyanohydrin. Notice: two new sigma bonds formed (C−CN and O−H), and the π bond is gone.
What Makes a Good Nucleophile?
Strong nucleophiles are usually negatively charged or have lone pairs:
- OHX−, CNX−, NHX2X−, CHX3OX−, HX− (from hydride reagents like NaBHX4 or LiAlHX4)
- Neutral but polarisable: NHX3, HX2O (weaker, but can add under acidic conditions) …
Why this formula?
Nucleophilic Substitution Reactions: Why the Key Formulas Hold
Nucleophilic substitution reactions are a cornerstone of organic chemistry. Instead of just memorizing the rate laws, let's understand why they arise from the molecular events.
1. The Two Main Mechanisms: A Tale of Timing
The key formulas (rate laws) for nucleophilic substitution come directly from how many molecules are involved in the rate-determining step (RDS) — the slowest step that controls the overall reaction speed.
SN1: Unimolecular — The Leaving Group Goes First
The Rate Law:
Rate=k[RX]
Why?
The reaction happens in two steps:
- Slow step (RDS): The C–X bond breaks spontaneously, forming a carbocation intermediate. Only the substrate (RX) is involved.
RXslowR++X−
- Fast step: The nucleophile (Nu−) attacks the carbocation.
R++Nu−fastRNu
Since the slow step depends only on the concentration of RX, the rate law has no dependence on [Nu−]. The nucleophile arrives after the carbocation is formed — it cannot affect the speed of the first step.
Key insight: The rate is determined by how easily the leaving group leaves, not by how fast the nucleophile attacks.
SN2: Bimolecular — Simultaneous Attack and Departure
The Rate Law:
Rate=k[RX][Nu−]
Why?
The reaction occurs in one concerted step:
- The nucleophile attacks the carbon from one side at the same time as the leaving group departs from the opposite side.
- Both RX and Nu− must collide with the correct orientation and sufficient energy.
The rate depends on the frequency of productive collisions between the two molecules. This is directly proportional to the product of their concentrations:
Rate∝[RX]×[Nu−]
Key insight: Both partners are involved in the transition state simultaneously — if either is missing, the reaction cannot proceed.
2. The Transition State: Why the Formulas Are Not Just "Given"
For SN2, the transition state has a pentavalent carbon (five bonds partially formed/broken). The energy barrier depends on steric hindrance — bulkier groups around carbon make it harder for the nucleophile to approach, which is why SN2 is favored at primary carbons. …
The key idea is that direct alkylation of ammonia (option A) gives a mixture of primary, secondary, and tertiary amines, so it is not suitable for preparing a pure secondary amine. Option B yields a primary amine after reduction of the nitrile. Option D is the Gabriel phthalimide synthesis, which gives a pure primary amine, not secondary. …
The key idea is that reductive amination of an aldehyde with a primary amine selectively gives a secondary amine without over-alkylation. The correct option is (C).
To prepare a secondary amine cleanly, you need a method that avoids mixtures of primary, secondary, and tertiary products. Let's see why each option works or fails.
Nucleophilic Substitution Reactions — the core concept here — involve a nucleophile attacking an electrophilic carbon. When you use an alkyl halide with ammonia, the product (a primary amine) is itself a better nucleophile than ammonia. So it immediately attacks another alkyl halide molecule, giving a cascade of over-alkylation. That's the fundamental problem with direct alkylation.
Let's examine each option:
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Option (A): 2° R–Br + NH₃
Ammonia attacks the secondary alkyl bromide to give a primary amine salt. But the free primary amine formed is more nucleophilic than ammonia, so it reacts with another molecule of R–Br to give a secondary amine, then a tertiary amine, and finally a quaternary ammonium salt. You end up with a mixture — not a clean route to a secondary amine.
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Option (B): 2° R–Br + NaCN followed by H₂/Pt
This gives a nitrile (R–CN) which on reduction yields a primary amine (R–CH₂–NH₂). The carbon skeleton gains one extra carbon, and the product is a primary amine, not secondary. So this is outright wrong for preparing a secondary amine.
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Option (C): 1° R–NH₂ + RCHO followed by H₂/Pt
This is reductive amination. The aldehyde and primary amine first form an imine (Schiff base) by condensation. Catalytic hydrogenation then reduces the C=N bond to a C–N single bond, giving a secondary amine.
The beauty: no free alkyl halide is present to cause over-alkylation. The imine intermediate is less nucleophilic than the starting amine, so further reaction is suppressed. This is the most controlled, high-yield method.
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Option (D): 1° R–Br (2 mol) + potassium phthalimide followed by H₃O⁺/heat …
Concept: Preparation of Secondary Amines via Reductive Amination
The most appropriate method for preparing a secondary (2°) amine is option (C).
Method Name: Reductive Amination
Steps:
- Condensation reaction A primary amine (1°R−NH2) reacts with an aldehyde (RCHO) to form an imine (Schiff base):
R−NH2+RCHO→R−N=CH−R+H2O
- Catalytic hydrogenation The imine is reduced using H2 in the presence of a Pt catalyst:
R−N=CH−R+H2PtR−NH−CH2−R
Final product: A secondary amine (R−NH−CH2−R).
Why other options fail:
- (A) 2°R−Br+NH3 → gives a mixture of primary, secondary, and tertiary amines (not selective). …
Here are the common mistakes students make on this question, along with how to avoid each.
Mistake 1: Confusing the product of the Hoffmann Ammonolysis (Option A)
- The Mistake: Students see 2∘R−Br+NH3 and think it directly gives a pure 2∘ amine. They forget that NH3 is a nucleophile and the product (1∘ amine) is also a nucleophile.
- Why it's wrong: The reaction doesn't stop. The 1∘ amine product reacts with more alkyl halide to give a 2∘ amine, then a 3∘ amine, and finally a quaternary ammonium salt. You get a mixture.
- How to avoid: Remember the Hoffmann Ammonolysis rule: Alkylation of ammonia always gives a mixture of 1∘, 2∘, 3∘ amines, and quaternary salt. It is never a clean method for preparing a specific 2∘ amine.
Mistake 2: Misidentifying the product of the Nitrile Route (Option B)
- The Mistake: Students think 2∘R−Br+NaCN followed by reduction gives a 2∘ amine.
- Why it's wrong: The 2∘ alkyl halide undergoes SN2 with CN− to give a nitrile (R−CN). Reduction of a nitrile (H2/Pt) gives a 1∘ amine (R−CH2−NH2). The carbon skeleton increases by one, but the amine is still primary.
- How to avoid: Trace the carbon chain. The CN− adds a carbon, but the NH2 group ends up on that new carbon. The product is always R−CH2−NH2 — a 1∘ amine, regardless of the starting halide's degree.
Mistake 3: Forgetting the Reductive Amination Mechanism (Option C)
- The Mistake: Students see 1∘R−NH2+RCHO and think it forms a 2∘ amine directly, or they confuse it with simple imine formation.
- Why it's correct (and why students miss it): The 1∘ amine reacts with the aldehyde to form an imine (R−N=CH−R′). The H2/Pt then reduces the C=N bond to a C−N bond, giving a 2∘ amine (R−NH−CH2−R′). This is reductive amination — the most reliable method.
- How to avoid: Memorize the two-step logic:
- Condensation: 1∘ amine + aldehyde/ketone → imine.
- Reduction: Imine + H2/catalyst → 2∘ amine. This avoids over-alkylation (unlike option A).
Mistake 4: Misapplying the Gabriel Phthalimide Synthesis (Option D)
- The Mistake: Students think using 2 moles of 1∘R−Br with potassium phthalimide gives a 2∘ amine. …
- TG EAPCET 2026Set eng-2026-05-10-FN1 markMCQQ.The reagent which is used to distinguish C6H5N(CH3)2 and (C2H5)2NH is (anhy = anhydrous, conc = concentrated, alc = alcoholic) (A) C6H5SO2Cl (B) Anhy. ZnCl2 ∣ conc. HCl (C) CrO3 ∣ H2SO4 (D) CHCl3 ∣ alc. KOH
›Reveal solutionSolution
The pair is a tertiary amine and a secondary amine. Only Hinsberg's reagent, C6H5SO2Cl (option A), separates them: the secondary amine gives an alkali-insoluble sulphonamide, the tertiary amine does not react.
The concept first
Before choosing a reagent, classify the two compounds:
- C6H5N(CH3)2 (N,N-dimethylaniline): the nitrogen carries three carbon groups → tertiary amine, no N–H.
- (C2H5)2NH (diethylamine): the nitrogen carries two carbon groups and one H → secondary amine, one N–H.
So you need a test that distinguishes 2∘ from 3∘ — i.e. a test that keys off the presence of an N–H bond, not off basicity.
Hinsberg's test does exactly that. Benzenesulphonyl chloride acylates the nitrogen, but only if there is an N–H available:
- 1∘ amine → C6H5SO2NHR. The N–H left over is made acidic by the two electron-withdrawing S=O groups → the sulphonamide dissolves in KOH/NaOH.
- 2∘ amine → C6H5SO2NR2. No N–H remains, so the sulphonamide is a solid that is insoluble in alkali.
- 3∘ amine → no reaction (no N–H to substitute); the amine remains as an oily layer, dissolving only on adding acid.
Step-by-step
- Test option (D), CHCl3 + alc. KOH (carbylamine). It gives the foul-smelling isocyanide only with primary amines. Both compounds here are non-primary → both give a negative test → cannot distinguish. ✗
- Test option (B), anhy. ZnCl2 + conc. HCl (Lucas reagent). This is a test for alcohols (distinguishing 1∘/2∘/3∘ ROH by turbidity). Amines are not alcohols. ✗ …
- TG EAPCET 2024Set ap-2024-05-08-FN1 markMCQQ.An organic compound C7H9N on reduction with reagent X gave Y. Reaction of Y with p-toluene sulphonyl chloride gave Z which is insoluble in alkali. X and Y respectively are (A) LiAlH4 , C6H5−CH2−NH2 (benzylamine) (B) NaBH4 , C6H5−NHCH3 (N-methylaniline) (C) H2∣Ni , C6H5−CH2−NH2 (benzylamine) (D) H2∣Ni , C6H5−NHCH3 (N-methylaniline)
›Reveal solutionSolution
The alkali-insoluble benzenesulphonamide tells us Y must be a secondary amine, which fixes Y=C6H5NHCH3; a secondary amine of this shape arises from catalytic reduction of an isocyanide, so X=H2/Ni. Option (D).
The concept: why alkali-solubility identifies the class of amine
When an amine reacts with a sulphonyl chloride (Hinsberg's reagent, here p-toluenesulphonyl chloride):
- 1° amine RNH2 gives RNH−SO2Ar. The N–H left on nitrogen sits between an electron-withdrawing SO2 group and the ring, so it is acidic — the sulphonamide dissolves in NaOH forming a salt.
- 2° amine R2NH gives R2N−SO2Ar. Nitrogen now has no hydrogen at all, nothing to ionise, so the product is insoluble in alkali.
- 3° amine does not react at all.
So "Z is insoluble in alkali" is a direct statement that Y is a secondary amine.
Step-by-step
- Use the Hinsberg clue. Z insoluble in alkali ⇒ Y has no N–H after sulphonylation ⇒ Y is 2°.
- Screen the options. C6H5CH2NH2 (benzylamine) is a primary amine — its tosylamide C6H5CH2NH−SO2C6H4CH3 still has an acidic N–H and would dissolve in KOH. So options (A) and (C) are out. Y must be C6H5NHCH3.
- Now identify the reduction. A secondary amine bearing an N−CH3 group is the hallmark product of reducing an isocyanide (carbylamine): C6H5−NC+4[H] H2/Ni C6H5−NH−CH3 …
- TG EAPCET 2024Set eng-2024-05-10-AN1 markMCQQ.Identify the set, in which X and Y are correctly matched (A) NH2OH, Hydrazone (B) NH2NH2, Semicarbazone (C) C6H5NH2, Schiff base (D) RNH2, Oxime
›Reveal solutionSolution
The question asks which pair of reagent (X) and product (Y) is correctly matched. The correct match is aniline (C6H5NH2) with Schiff base, so option (C) is correct.
Concept & Intuition
This is a classic organic chemistry matching problem about carbonyl derivatives. Each reagent (X) reacts with a carbonyl compound (aldehyde or ketone) to give a specific nitrogen-containing derivative. The key is to recall the functional group of the product formed:
- Oximes come from hydroxylamine (NH2OH).
- Hydrazones come from hydrazine (NH2NH2).
- Semicarbazones come from semicarbazide (NH2NHCONH2).
- Schiff bases (imines) come from primary amines (RNH2), especially aromatic ones like aniline.
Let’s check each option step by step.
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Option (A): NH2OH → Hydrazone
Hydroxylamine (NH2OH) reacts with a carbonyl to form an oxime (with a C=NOH group), not a hydrazone. Hydrazones come from hydrazine. So this is incorrect.
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Option (B): NH2NH2 → Semicarbazone
Hydrazine (NH2NH2) gives a hydrazone (C=NNH2). Semicarbazones require semicarbazide (NH2NHCONH2). So this is incorrect.
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Option (C): C6H5NH2 → Schiff base …
- TG EAPCET 2023Set eng-2023-05-12-AN1 markMCQQ.Identify what is Y in the following reaction sequence?
[!FORMULA] CH3−CO−NH2Br2NaOH (aq)X(i) NaNO2+HCl(ii) H2OY
(A) CH3NH2 (B) CH3CONHBr (C) CH3OH (D) BrCH2CONH2›Reveal solutionSolution
Acetamide undergoes Hofmann bromamide degradation to methylamine (X); nitrous acid converts this primary aliphatic amine into an unstable diazonium ion that immediately loses NX2 and picks up water, giving methanol — option (C).
The concept first: why aliphatic diazonium salts die instantly
Diazotisation (NaNOX2+HCl) makes the same −NX2X+ group from any primary amine. The difference is stability. In an aryl diazonium salt the −NX2X+ is conjugated with the ring, so at 273–278 K it survives long enough to be used in coupling/Sandmeyer reactions. In an alkyl diazonium ion there is no such delocalisation, and NX2 is an outstanding leaving group — so the ion falls apart the moment it forms, giving a carbocation that the solvent (water) captures. That is why primary aliphatic amines simply effervesce and give alcohols with nitrous acid.
Step-by-step
Step 1 — Identify X. Acetamide with bromine in aqueous alkali is the textbook Hofmann bromamide degradation:
CHX3CONHX2+BrX2+4NaOHCHX3NHX2+NaX2COX3+2NaBr+2HX2O …
- TG EAPCET 2023Set eng-2023-05-13-AN1 markMCQQ.An amine (X) reacts with p-toluene sulphonyl chloride to give the product Y, which is insoluble in alkali. The product of X with benzoyl chloride is (A) CH3CH2CH(NH2)−COC6H5 (B) CH3CH2CH2N(CH3)COCH2C6H5 (C) CH3CH2NHCOC6H5 (D) CH3CH2N(CH3)COC6H5
›Reveal solutionSolution
A sulphonamide that is insoluble in alkali can only come from a secondary amine, so X is CHX3CHX2NHCHX3; benzoyl chloride then acylates its nitrogen to give CHX3CHX2N(CHX3)COCX6HX5 — option (D).
The concept first: why alkali-solubility fingerprints the amine class
p-Toluenesulphonyl chloride (a Hinsberg-type reagent) reacts with the amine's lone pair, replacing an N−H hydrogen with the bulky −SOX2Ar group:
- Primary amine R−NHX2 → R−NH−SOX2Ar. One N−H remains. That hydrogen is acidic, because the resulting anion is stabilised by the strongly electron-withdrawing sulphonyl group. Hence the product dissolves in KOH/NaOH.
- Secondary amine RX2NH → RX2N−SOX2Ar. No N−H is left. There is no acidic proton to remove, so the product is insoluble in alkali. ✓
- Tertiary amine RX3N → no reaction (there is no N−H to substitute in the first place), so no product Y at all.
The stem says a product Y forms and is insoluble in alkali. Both facts together force X to be secondary.
Step-by-step
Step 1 — Classify X. Product formed ⇒ not tertiary. Product alkali-insoluble ⇒ not primary. Therefore X is a 2∘ amine.
Step 2 — Benzoylation (Schotten–Baumann). Benzoyl chloride CX6HX5COCl acylates the amine nitrogen:
RX2NH+CX6HX5COClRX2N−CO−CX6HX5+HCl
The amide nitrogen ends up carrying both original alkyl groups plus the benzoyl group — i.e. it is a tertiary (N,N-disubstituted) amide with no N−H.
Step 3 — Test each option against "benzoyl on a 2∘ nitrogen".
- (A) CHX3CHX2CH(NHX2)COCX6HX5 — the benzoyl is on carbon, and a free −NHX2 survives. Wrong on both counts. …
- TG EAPCET 2022Set eng-2022-07-18-FN1 markMCQQ.Lithium nitrate on heating gives (A) Li2O+NO2 (B) Li2O+NO2+O2 (C) LiNO2+O2 (D) Li2O2+NO2+O2
›Reveal solutionSolution
Lithium nitrate decomposes differently from other alkali metal nitrates because of the small size and high polarising power of Li⁺; it gives Li2O, NO2, and O2, so the correct option is (B).
Concept & Intuition
Most alkali metal nitrates (like NaNO₃, KNO₃) decompose on strong heating to give the nitrite and oxygen:
2MNO3→2MNO2+O2.
But lithium is an exception. Because Li⁺ is very small, it has a high charge density and strongly polarises the nitrate ion. This destabilises the nitrate, causing it to break down further — lithium nitrite (LiNO₂) itself is unstable at high temperature and decomposes to lithium oxide, nitrogen dioxide, and oxygen. So the final products are not simply nitrite + O₂, but oxide + NO₂ + O₂.
Step-by-step reasoning
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General trend for alkali metal nitrates
For Na, K, Rb, Cs:
2MNO3Δ2MNO2+O2
The nitrite is stable at the decomposition temperature.
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Lithium’s anomaly
Li⁺ is much smaller than other alkali ions. Its high polarising power weakens the N–O bonds in the nitrate ion, so decomposition occurs at a lower temperature and proceeds further.
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First step – formation of nitrite
Initially, LiNO₃ does form LiNO₂ and O₂:
2LiNO3→2LiNO2+O2
But LiNO₂ is not stable at the temperature needed for decomposition.
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Second step – further decomposition of LiNO₂
Lithium nitrite decomposes to lithium oxide, nitrogen dioxide, and oxygen:
2LiNO2→Li2O+NO2+NO
However, NO reacts immediately with O₂ to give NO₂: …
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