Answer to Question #86431 in Organic Chemistry for Gayatri

Question

a) Explain isocyanide reaction and its importance.
b) Give examples of the following reactions:
i) Gattermann reaction
ii) Gomberg – Bachmann reaction
iii) Schiemann reaction

Answer 1

a) The carbylamine reaction (also known as the Hofmann isocyanide synthesis) is the synthesis of an isocyanide by the reaction of a primary amine, chloroform, and base. The conversion involves the intermediacy of dichlorocarbene.

Illustrative is the synthesis of tert-butyl isocyanide from tert-butylamine in the presence of catalytic amount of the phase transfer catalyst benzyltriethylammonium chloride.

Me₃CNH₂ + CHCl₃ + 3 NaOH → Me₃CNC + 3 NaCl + 3 H₂O

Similar reactions have been reported for aniline.

The mechanism involves the addition of amine to dichlorocarbene, a reactive intermediate generated by the dehydrohalogenation of chloroform. Two successive base-mediated dehydrochlorination steps result in formation of the isocyanide.

Isocyanide is a useful synthetic building block. Being stable carbenes, isonitriles are highly reactive compounds that can react with almost any type of reagents (electrophiles, nucleophiles and even radicals). Today isocyanide chemistry is a broad and important part of organic chemistry, inorganic, coordination, polymeric, combinatorial and medicinal chemistry.

b) Examples:

i) The Gattermann reaction, (also known as the Gattermann formylation and the Gattermann salicylaldehyde synthesis) is a chemical reaction in which aromatic compounds are formylated by a mixture of hydrogen cyanide (HCN) and hydrogen chloride (HCl) in the presence of a Lewis acid catalyst such as AlCl₃. It is named for the German chemist Ludwig Gattermann and is similar to the Friedel–Crafts reaction.

ii) The Gomberg–Bachmann reaction, named for the Ukrainian-American chemist Moses Gomberg and the American chemist Werner Emmanuel Bachmann, is an aryl-aryl coupling reaction via a diazonium salt.

The arene compound 1 (here benzene) is coupled with base with the diazonium salt 2 to the biaryl 3 through an intermediate aryl radical. For example, p-bromobiphenyl may be prepared from 4-bromoaniline and benzene:

BrC₆H₄NH₂ + C₆H₆ → BrC₆H₄−C₆H₅

The reaction offers a wide scope for both diazonium component and arene component but yields are generally low following the original procedure (less than 40%), given the many side-reactions of diazonium salts. Several improvements have been suggested. One possibility is to employ diazonium tetrafluoroborates in arene solvent together with a phase-transfer catalyst another is to use 1-aryl-3,3-dialkyltriazenes.

iii) The Schiemann reaction (also called the Balz–Schiemann reaction) is a chemical reaction in which a primary aromatic amine is transformed to an aryl fluoride via a diazoniumtetrafluorofluoroborate intermediate. Both thermal and photolytic decomposition of the diazonium intermediate proceed through an arene cation, as evidenced by the equal product ratios in both cases. Named after the German chemists Günther Schiemann and Günther Balz, this reaction is the preferred route to fluorobenzene and some related derivatives, including 4-fluorobenzoic acid.

The conversion of aryl amines to aryl fluorides via diazotisation and subsequent thermal decomposition of the derived tetrafluoroborates or hexafluorophosphates. The decomposition may also be induced photochemically.