anilinium

Anilinium is the positively charged form of aniline, with the chemical formula C6H5NH3+. It is the conjugate acid of aniline (C6H5NH2) and is formed when aniline is protonated by acids. In aqueous solution, anilinium exists in equilibrium with its neutral base: C6H5NH3+ ⇌ C6H5NH2 + H+. The pKa of the anilinium ion is about 4.6, so the protonated form predominates in strongly acidic media, while neutral aniline dominates at higher pH.

Anilinium is typically encountered as salts, such as anilinium chloride (C6H5NH3+ Cl−) or other counterions like sulfate or perchlorate. These salts are common intermediates in organic synthesis and dye chemistry, where the positive charge on nitrogen facilitates various transformations.

A primary application of aniline-derived anilinium salts is in diazotization. In strongly acidic conditions, anilinium is converted to aryldiazonium salts (ArN2+), such as aryldiazonium chloride, by nitrosation with nitrous acid. These diazonium salts are versatile electrophiles used in azo coupling reactions to produce azo dyes and in a range of other synthetic processes.

Safety considerations are important: aniline and its salts are toxic and can be potentially hazardous if inhaled, ingested, or absorbed through the skin. They should be handled with appropriate protective equipment and disposed of following established chemical safety guidelines.