Consider the following statements about the nucleophilic substitution reactions of haloalkanes.
(A) The nucleophile attacks the carbon atom bonded to the halogen, because this carbon atom bears a partial positive charge.
(B) The halogen atom departs as a halide ion and is called the leaving group.
(C) Groups such as cyanide and nitrite, which possess two nucleophilic centres, are called bidentate nucleophiles.
(D) In these reactions, the haloalkane is called the nucleophile.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (C) and (D) only
In a nucleophilic substitution reaction, an electron rich species (the nucleophile) replaces the halogen of the haloalkane. The haloalkane is the substrate, the molecule that is attacked.
(A) The nucleophile attacks the carbon atom bonded to the halogen, because this carbon atom bears a partial positive charge. — Correct statement. Halogen is more electronegative than carbon, so the C–X bond is polarised and the carbon is electron deficient.
(B) The halogen atom departs as a halide ion and is called the leaving group. — Correct statement. The halogen leaves with the bonding electron pair, as X−.
(C) Groups such as cyanide and nitrite, which possess two nucleophilic centres, are called bidentate nucleophiles. — Incorrect statement. Such groups are called ambident nucleophiles. Cyanide can link through C or N, and nitrite through O or N. (“Bidentate” is a term for ligands that bind a metal ion through two donor atoms at the same time.)
(D) In these reactions, the haloalkane is called the nucleophile. — Incorrect statement. The haloalkane is the substrate. The nucleophile is the attacking species, such as OH−, CN− or NH3.
\( \displaystyle \ce{Nu^- + R-X -> R-Nu + X^-} \)
NCERT (2026-27 edition):
6.7.1 Reactions of Haloalkanes – “…nucleophiles are electron rich species. Therefore, they attack at that part of the substrate molecule which is electron deficient. … Haloalkanes are substrate 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.”
“Groups like cyanides and nitrites possess two nucleophilic centres and are called ambident nucleophiles.”