Identify the diazonium group in the diazonium salt ArN₂⁺Br^-.

N₂⁺

The correct answer is option 2. \(N_2^+\).

Let us delve into a detailed explanation:

Aromatic Ring (Ar):

Aromatic compounds are characterized by having a cyclic structure of alternating single and double bonds, known as conjugated pi bonds, and following Hückel's rule (\(4n + 2 \pi \) electrons, where n is an integer). The most common aromatic compound is benzene, which consists of a hexagonal ring of carbon atoms with three alternating double bonds. In organic chemistry notation, "Ar" typically represents an aromatic ring or an aromatic substituent. It is often used as a placeholder to indicate any substituted benzene ring or aromatic system. In the compound \(ArN_2^+Br^-\), "Ar" represents an aromatic ring attached to the rest of the molecule. This aromatic ring could be benzene or any substituted benzene ring.

Diazonium Group \((N_2^+)\):

The diazonium group is a functional group with the chemical formula \(N_2^+\) (diazo functional group) or \(R-N_2^+\) (diazonium cation), where R can be an aromatic or aliphatic group. It consists of two nitrogen atoms (bonded together with a triple bond) and carries a positive charge. The positive charge arises due to the loss of a proton from one of the nitrogen atoms. Diazonium salts are highly reactive and are commonly used in organic synthesis to introduce various functional groups onto aromatic compounds through diazotization reactions.

Bromide Ion \((Br^-)\):

The bromide ion \((Br^-)\) is a negatively charged ion formed by the halogen bromine gaining one electron to achieve a stable electronic configuration. In the compound \(ArN_2^+Br^-\), the bromide ion acts as the counterion to balance the positive charge of the diazonium cation \((N_2^+)\), forming a stable salt. The presence of the bromide ion in the diazonium salt helps enhance its stability and solubility in polar solvents.

Overall Structure of \(ArN_2^+Br^-\): The compound \(ArN_2^+Br^-\) consists of three main components: an aromatic ring \((Ar)\), a diazonium cation \((N_2^+)\), and a bromide ion \((Br^-)\). The diazonium cation  \((N_2^+)\) is attached to the aromatic ring, forming a covalent bond between one of the nitrogen atoms of the diazonium group and one of the carbon atoms of the aromatic ring. The bromide ion \((Br^-)\) is present as a counterion to balance the positive charge of the diazonium cation, forming a salt. Overall, \(ArN_2^+Br^-\) represents a diazonium salt derived from an aromatic compound \((Ar)\) through diazotization, where the diazonium group  \((N_2^+)\) is formed by the reaction of an amine with nitrous acid.

In summary, the compound \(ArN_2^+Br^-\) contains an aromatic ring \((Ar)\) attached to a diazonium cation \((N_2^+)\), with a bromide ion \((Br^-)\) as the counterion. This compound is commonly used in organic synthesis for various functionalization reactions, particularly in the preparation of azo dyes and other aromatic compounds.