Read the following passage and answer questions.
Nucleophiles attack at that part of the substrate molecule which is electron deficient. The reaction in which a nucleophile replaces already existing nucleophile in a molecule is called nucleophilic substitution reaction. Haloalkanes are substrates in these reactions. In this type of reaction, a nucleophile reacts with haloalkane (the substrate) having a partial positive charge on the carbon atom bonded to halogen. A substitution reaction takes place and halogen atom, called leaving group departs as halide ion. Since the substitution reaction is initiated by a nucleophile, it is called nucleophilic substitution reaction.
When NH3 reacts with R-X, what is the major product ?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Primary amine
Ammonia has a lone pair on nitrogen and acts as a nucleophile. A haloalkane heated with an ethanolic solution of ammonia undergoes nucleophilic substitution: the halogen is replaced by –\( \ce{NH2} \). This is ammonolysis, and the first product is a primary amine (as its salt, which a base sets free):
\( \displaystyle \ce{R-X + NH3 -> R-NH3+X^-} \)
\( \displaystyle \ce{R-NH3+X^- + NaOH -> R-NH2 + NaX + H2O} \)
With excess haloalkane the primary amine can react further to give secondary and tertiary amines and the quaternary ammonium salt; ammonia in excess keeps the primary amine as the major product.
Why the other options are incorrect
Option (1) Nitroalkane — Incorrect. It is formed with \( \ce{AgNO2} \).
Option (2) Isonitrile — Incorrect. It is formed with AgCN.
Option (3) Alkyl nitrite — Incorrect. It is formed with \( \ce{KNO2} \).
NCERT (2026-27 edition):
Reactions of Haloalkanes – Table 6.4: Nucleophilic Substitution of Alkyl Halides (R–X)
| Reagent | Substitution product R–Nu | Class of main product |
|---|---|---|
| NaOH (KOH) | ROH | Alcohol |
| H2O | ROH | Alcohol |
| NaOR′ | ROR′ | Ether |
| NaI | R—I | Alkyl iodide |
| NH3 | RNH2 | Primary amine |
| R′NH2 | RNHR′ | Sec. amine |
| R′R″NH | RNR′R″ | Tert. amine |
| KCN | RCN | Nitrile (cyanide) |
| AgCN | RNC | Isonitrile (isocyanide) |
| KNO2 | R—O—N=O | Alkyl nitrite |
| AgNO2 | R—NO2 | Nitroalkane |
| R′COOAg | R′COOR | Ester |
| LiAlH4 | RH | Hydrocarbon |
| R′– M+ | RR′ | Alkane |