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Stephens reaction and carbylamine test
Answers
Stephen aldehyde synthesis, a named reaction in chemistry, was invented by Henry Stephen. This reaction involves the preparation of aldehydes from nitriles using tin(II) chloride, hydrochloric acid and quenching the resulting iminium salt with water. During the synthesis, ammonium chloride is also produced.
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.[1]
Me3CNH2 + CHCl3 + 3 NaOH → Me3CNC + 3 NaCl + 3 H2O
Similar reactions have been reported for aniline.
As it is only effective for primary amines, the carbylamine reaction can be used as a chemical test for their presence. In this context, the reaction is also known as Hofmann's isocyanide test.[2] In this reaction, the analyte is heated with alcoholic potassium hydroxide and chloroform. If a primary amine is present, the isocyanide (carbylamine) is formed, as indicated by a foul odour. The carbylamine test does not give a positive reaction with secondary and tertiary amines.
Mechanism
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.Stephen aldehyde synthesis, a named reaction in chemistry, was invented by Henry Stephen (OBE/MBE). This reaction involves the preparation of aldehydes (R-CHO) from nitriles (R-CN) using tin(II) chloride (SnCl2), hydrochloric acid (HCl) and quenching the resulting iminium salt ([R-CH=NH2]+Cl−) with water (H2O).[1][2] During the synthesis, ammonium chloride is also produced.
Stephen aldehyde synthesis
Stephen aldehyde synthesis: Reaction mechanism
By addition of hydrogen chloride the used nitrile (1) reacts to its corresponding salt (2). It is believed that this salt is reduced by a single electron transfer by the tin(II) chloride (3a and 3b).[3] The resulting salt (4) precipitates after some time as aldimine tin chloride (5). Hydrolysis of 5 produces a amide (6) from which an aldehyde (7) is formed.
Substitutes that increase the electron density promote the formation of the aldimin-tin chloride adduct. By electron withdrawing substituents, the formation of amide chloride is facilitated.[4] In the past, the reaction was carried out by precipitating the aldimine-tin chloride, washing it with ether and then hydrolyzing it. However, it has been found that this step is unnecessary and the aldimine tin chloride can be hydrolysed directly in the solution.[5]
This reaction is more efficient when aromatic nitriles are used instead of aliphatic ones. However, even for some aromatic nitriles (e. g. 2-formylbenzoic acid ethyl ester) the yield can be low.[5]