Outline the mechanism of the NaNO2 and HCl reaction and its application in organic synthesis.

The reaction between sodium nitrite (NaNO2) and hydrochloric acid (HCl) is a classic chemical process used in organic synthesis, particularly in the preparation of diazonium salts. The mechanism of this reaction and its application in organic synthesis can be outlined in the following steps:

Step 1: Formation of Nitrous Acid

When sodium nitrite is dissolved in water, it dissociates into sodium ions (Na⁺) and nitrite ions (NO2⁻). The addition of hydrochloric acid, a strong acid, to the aqueous solution of sodium nitrite results in the protonation of nitrite ions to form nitrous acid (HNO2).

$${\text{NaNO}}_2(s)\to {\text{Na}}^{+}(aq)+{\text{NO}}_2^{-}(aq)$$

$$\text{HCl}(aq)+{\text{NO}}_2^{-}(aq)\to {\text{HNO}}_2(aq)+{\text{Cl}}^{-}(aq)$$

Step 2: Formation of Nitrosonium Ion

Nitrous acid is unstable and can decompose to form nitric oxide (NO) and water. However, in the presence of excess hydrochloric acid, nitrous acid is protonated to form the nitrosonium ion (NO⁺), which is a key electrophile in the formation of diazonium salts.

$${\text{HNO}}_2(aq)+{\text{H}}^{+}(aq)\to {\text{NO}}^{+}(aq)+{\text{H}}_2\text{O}(l)$$

Step 3: Reaction with an Aromatic Amine

The nitrosonium ion (NO⁺) is highly reactive and can react with an aromatic amine, such as aniline (C6H5NH2), to form a diazonium salt. This reaction occurs through the attack of the lone pair of electrons on the nitrogen of the amine on the electrophilic nitrosonium ion, followed by the loss of a water molecule.

$${\text{C}}_6{\text{H}}_5{\text{NH}}_2(aq)+{\text{NO}}^{+}(aq)\to {\text{C}}_6{\text{H}}_5{\text{N}}_2^{+}(aq)+{\text{H}}_2\text{O}(l)$$

The product, in this case, is benzene diazonium chloride (C6H5N2⁺Cl⁻), which is a versatile intermediate in organic synthesis.

Step 4: Application in Organic Synthesis

Diazonium salts are used in various reactions in organic synthesis. One of the most common applications is in the Sandmeyer reaction, where the diazonium group is replaced by another substituent, such as a halogen, through the action of copper(I) chloride (CuCl) or copper(I) bromide (CuBr).

For example, the conversion of benzene diazonium chloride to chlorobenzene can be represented as follows:

$${\text{C}}_6{\text{H}}_5{\text{N}}_2^{+}{\text{Cl}}^{-}(aq)+\text{CuCl}(s)\to {\text{C}}_6{\text{H}}_5\text{Cl}(l)+{\text{N}}_2(g)+{\text{Cu}}^{2+}(aq)+{\text{Cl}}^{-}(aq)$$

Another application is the formation of azo dyes through azo coupling reactions, where the diazonium salt reacts with electron-rich aromatic compounds to form colorful azo compounds.

In summary, the reaction between NaNO2 and HCl is a key step in the preparation of diazonium salts, which are important intermediates in organic synthesis for the production of various aromatic compounds, including halogenated aromatics and azo dyes.