Benzenediazonium Ion Synthesis Essay

Benzenediazonium Tetrafluoroborate1

[369-57-3]  · C6H5BF4N2  · Benzenediazonium Tetrafluoroborate  · (MW 191.92)

(much higher stability than the corresponding chlorides;2 shock-insensitive; often used when pure, isolated arenediazonium salts are needed;1 reagent for introducing aryl, arylazo, arylhydrazono, or amino groups; forms fluoroarenes upon heating;3 building block for heterocycles)

Alternate Name: phenyldiazonium tetrafluoroborate.

Physical Data: colorless solid after recrystallization; turns pink at 80 °C; decomposes at 114-116 °C.

Solubility: fairly sol polar solvents such as acetonitrile, acetone, pyridine, DMF, DMSO, and HMPA with decomposition; slightly sol water; insol hydrocarbons and Et2O; solubilized in nonpolar media by crown ethers.

Form Supplied in: 4-nitrobenzenediazonium tetrafluoroborate is widely available (nearly colorless solid). A large variety of arenediazonium BF4 salts are readily prepared from aromatic amines (see below).

Preparative Methods: 1a,1d,3,4 The most commonly used procedures consist in diazotization of an aromatic amine (ArNH2) with NaNO2 in aqueous HCl or H2SO4 followed by precipitation of the salt with added NaBF4 or fluoroboric acid.5 Alternatively, the diazotization can be carried out directly in 40-50% aqueous HBF4.2c Aromatic amines that do not dissolve in aqueous mineral acids can be reacted with NO+BF4- in an anhydrous organic solvent or in liquid SO2.6 This method can also be applied to N-(trimethylsilyl)anilines7 and for the preparation of those salts that are not easily isolated from water.8 An improved, high-yielding one-pot procedure9 employs ArNH2, t-BuONO, and BF3.Et2O in an anhydrous organic

General Reactivity Patterns.

Similar to the chloride salts, arenediazonium tetrafluoroborates can be transformed into covalent azo compounds by addition of a nucleophile and into various functionalized arenes by displacement of the N2+ group. While the chloride salts are typically prepared in situ in an acidic aqueous or ethanolic solution, the tetrafluoroborates are usually isolated and can be employed as pure compounds in the solvent of choice or in suspension. Their low solubility in nonpolar organic solvents may be a problem, but it can be overcome by phase-transfer techniques.1c,11

Addition to Unsaturated Compounds with Retention of N2.

Diarylazo compounds (Ar1-N=N-Ar2) are obtained by coupling of arenediazonium salts with sufficiently electron-rich (hetero)aromatic substrates. The vast majority of these important azo coupling reactions have been carried out in the reaction media obtained by diazotization of aromatic amines with NaNO2 in aqueous HCl or H2SO4 (see Benzenediazonium Chloride). Comparisons of reactivity between in situ generated arenediazonium chlorides and tetrafluoroborates (used as isolated salts) are rare.12 Thus 4-nitrobenzenediazonium chloride reacts only slowly with indole and its 1-, 2-, and 3-Me derivatives in aqueous neutral solution, whereas the corresponding BF4 salt in H2O-ethanol undergoes azo coupling rapidly and almost quantitatively (eq 1).13a Similarly,


Abstract

InChI = 1S/C6H5N2.BF4/c7-8-6-4-2-1-3-5-6;2-1(3,4)5/h1-5H;/q+1;-1

InChIKey = JNCLVGMBRSKDED-UHFFFAOYSA-N

(much higher stability than the corresponding chlorides;2 shock-insensitive; often used when pure, isolated arenediazonium salts are needed;1 reagent for introducing aryl, arylazo, arylhydrazono, or amino groups; forms fluoroarenes upon heating;3 building block for heterocycles)

Alternate Names: phenyldiazonium tetrafluoroborate.

Physical Data: colorless solid after recrystallization; turns pink at 80 °C; decomposes at 114–116 °C.

Solubility: fairly sol polar solvents such as acetonitrile, acetone, pyridine, DMF, DMSO, and HMPA with decomposition; slightly sol water; insol hydrocarbons and Et2O; solubilized in nonpolar media by crown ethers.

Form Supplied in: 4-nitrobenzenediazonium tetrafluoroborate is widely available (nearly colorless solid). A large variety of arenediazonium BF4 salts are readily prepared from aromatic amines (see below).

Preparative Method:1a,1d,3, 4 The most commonly used procedures consist in diazotization of an aromatic amine (ArNH2) with NaNO2 in aqueous HCl or H2SO4 followed by precipitation of the salt with added NaBF4 or fluoroboric acid.5 Alternatively, the diazotization can be carried out directly in 40–50% aqueous HBF4.2c Aromatic amines that do not dissolve in aqueous mineral acids can be reacted with NO+BF4 in an anhydrous organic solvent or in liquid SO2.6 This method can also be applied to N-(trimethylsilyl)anilines7 and for the preparation of those salts that are not easily isolated from water.8 An improved, high-yielding one-pot procedure9 employs ArNH2, t-BuONO, and BF3·Et2O in an anhydrous organic solvent, typically CH2Cl2.

Handling, Storage, and Precautions: the dry parent salt can be stored for more than a month at rt or for a few years at −20 °C under N2 in the dark, but decomposes when exposed to direct sunlight. Rapid recrystallization from warm water2b or from acetonitrile–Et2O is possible without decomposition. Although arenediazonium tetrafluoroborates, in contrast to the chloride salts, are renowned in general for their enhanced thermal stability and shock-insensitivity, some care should nevertheless be taken. Some salts are known to decompose while drying, e.g. 3-methoxybenzenediazonium, 2-methylbenzenediazonium, and certain heteroarenediazonium tetrafluoroborates.3, 10 Avoid contact with metals.

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