Question

Which statement is not correct? (A) Pure Alkyl bromides are colourless (B) Alkyl bromides and iodides develop colour when exposed to light (C) \(\mathrm{R}-\mathrm{CN}\) is obtained as a major product on reaction between \(\mathrm{R}-\mathrm{X}\) and \(\mathrm{AgCN}\) (D) \(\mathrm{R}-\mathrm{NO}_{2}\) is obtained as a major product on reaction between \(\mathrm{R}-\mathrm{X}\) and \(\mathrm{AgNO}_{2}\)

Short answer

The incorrect statement is (D) \(\mathrm{R}-\mathrm{NO}_{2}\) is obtained as a major product on reaction between \(\mathrm{R}-\mathrm{X}\) and \(\mathrm{AgNO}_{2}\). Instead, the major product will be \(\mathrm{R}-\mathrm{ONO}\) (alkyl nitrite).

Step-by-step solution

Statement (A) Pure Alkyl bromides are colourless

Alkyl bromides (R-Br) are organic compounds that contain a bromine atom bonded to a carbon atom. In general, they are colourless when pure. So, statement A is correct.

Statement (B) Alkyl bromides and iodides develop colour when exposed to light

Alkyl bromides and iodides (R-X where X is either Br or I) can react with light to form radicals, which may result in colour development. Therefore, statement B is also correct.

Statement (C) \(\mathrm{R}-\mathrm{CN}\) is obtained as a major product on reaction between \(\mathrm{R}-\mathrm{X}\) and \(\mathrm{AgCN}\)

In a reaction between an alkyl halide (R-X) and silver cyanide (AgCN), the major product will be \(\mathrm{R}-\mathrm{CN}\) along with the formation of silver halide (AgX). This is due to the nucleophilic substitution reaction, where the halogen (X) is replaced by the cyanide (CN) group. So, statement C is correct.

Statement (D) \(\mathrm{R}-\mathrm{NO}_{2}\) is obtained as a major product on reaction between \(\mathrm{R}-\mathrm{X}\) and \(\mathrm{AgNO}_{2}\)

In a reaction between an alkyl halide (R-X) and silver nitrite (AgNO₂), the major product is not \(\mathrm{R}-\mathrm{NO}_{2}\). Instead, the major product will be \(\mathrm{R}-\mathrm{ONO}\), which is the alkyl nitrite. This is because nitrite ion (NO₂⁻) acts as a nucleophile, attacking the electrophilic carbon center of the alkyl halide. The bond between the nitrogen atom and the oxygen atom with the unshared electron pair is formed, leading to the formation of alkyl nitrite as the major product. So, statement D is incorrect. Based on our analysis, the answer is (D) \(\mathrm{R}-\mathrm{NO}_{2}\) is obtained as a major product on reaction between \(\mathrm{R}-\mathrm{X}\) and \(\mathrm{AgNO}_{2}\).

Key concepts

Alkyl Halides

Alkyl halides, also known as haloalkanes, are a class of chemical compounds where one or more hydrogen atoms in an alkane are replaced by halogen atoms, such as fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). These compounds form the basis of many synthetic organic reactions, such as nucleophilic substitution, due to the reactivity of the carbon-halogen bond.

For instance, when discussing alkyl bromides, these are colorless liquids or gases at room temperature, and their physical properties like boiling point and density generally increase with heavier halogens. However, under the influence of light or heat, alkyl halides can undergo decomposition or radical formation, leading to color changes. This attributes to their photosensitivity, especially noticed in compounds containing bromine or iodine.

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions are fundamental to organic chemistry. They occur when a nucleophile, which is a species with a pair of electrons to donate, replaces a leaving group in a molecule. This reaction is commonly seen with alkyl halides, where the halogen acts as the leaving group.

There are two main types of nucleophilic substitution reactions: SN1 and SN2. SN1 reactions involve the formation of a carbocation intermediate and occur in two steps, typically with tertiary alkyl halides. On the other hand, SN2 reactions occur in a single step, where the nucleophile attacks the carbon atom at the same time the leaving group departs, and they are commonly associated with primary alkyl halides. The outcome of these reactions is influenced by factors like the nature of the nucleophile, the solvent, and the structure of the alkyl halide.

Chemical Properties of Alkyl Halides

The chemical properties of alkyl halides are defined by the reactivity of the carbon-halogen bond. They typically undergo two types of reactions: nucleophilic substitution, where the halide atom is replaced by another nucleophile, and elimination, where the halide atom is removed with the formation of a double bond.

In reactions where alkyl halides are paired with silver compounds such as silver cyanide (AgCN) or silver nitrite (AgNO2), the type of nucleophile determines the major product. With AgCN, the cyanide ion (CN-) is the nucleophile, and it replaces the halogen to form an alkyl cyanide (R-CN), as in statement (C) of the exercise. However, with AgNO2, the product formed is an alkyl nitrite (R-ONO), as the nitrite ion acts as the nucleophile and not the nitro group (NO2-), which is demonstrated in statement (D) of the exercise. This distinction is critical for students to understand the premise of nucleophilicity and the potential outcomes of reactions involving alkyl halides.