Diazonium salts, with nitrite anion, aromatic

The first widely used intermediates for nucleophilic aromatic substitution were the aryl diazonium salts. Aryl diazonium ions are usually prepared by reaction of an aniline with nitrous acid, which is generated in situ from a nitrite salt.81 Unlike aliphatic diazonium ions, which decompose very rapidly to molecular nitrogen and a carbocation (see Part A, Section 4.1.5), aryl diazonium ions are stable enough to exist in solution at room temperature and below. They can also be isolated as salts with nonnucleophilic anions, such as tetrafluoroborate or trifluoroacetate.82 Salts prepared with 0-benzenedisulfonimidate also appear to have potential for synthetic application.83... 

They are prepared by the action of HNO2 on aromatic amines. The amine is dissolved in excess of mineral acid and sodium nitrite is added slowly until a slight excess of HNO2 is present. The reaction is usually carried out in ice-cold solution. The solution then contains the diazonium salt of the mineral acid used, anhydrous diazonium salts of unpredictable stability may be precipitated with complex anions like PF , SnCl6 BF4 . 

Before we talk about this group of aromatic Sfjl reactions in more detail, let s consider how to make the diazonium salt. The reagent we need is the reactive nitrogen electrophile NO. You met NO in Chapter 20, but to remind you, it forms when the nitrite anion (usually sodium nitrite) is treated with acid at around 0 °C. Protonation of nitrite gives nitrous acid, HONO protonation again gives a cation, which can lose water to form NO. Butyl nitrite (or other alkyl nitrites) can also be used as a source of NO. ... 

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