Hydrogen chloride, displaced

Concentrated sulphuric acid displaces more volatile acids from their salts, for example hydrogen chloride from chlorides (see above) and nitric acid from nitrates. The dilute acid is a good conductor of electricity. It behaves as a strong dibasic acid ... 

Phenazine reacts with benzenesulphinic acid in alcoholic hydrogen chloride to give 2-phenazinyl phenyl sulfone (26 Scheme 4), presumably by an intermediate 5,10-dihy-drophenazine this reaction is evidently a useful method of preparing 2-substituted phenazines, since the sulfone is readily displaced in substitution reactions. 

Reactive halogens in various series have been removed by catalytic hydrogenation with either platinum or palladium catalysts, and other nucleophiles which have been used in chloride displacements include hydroxide ion, alkoxides, hydrosulflde, hydrazine and toluene-p-sulfonylhydrazine, and trimethyl phosphite. 

The use of a flask just large enough to hold the reaction mixture obviates the necessity for an inert atmosphere, for the evolving hydrogen chloride soon displaces the small amount of air over the reaction mixture. 

This enhanced reactivity of fluoromethyl cyanide is undoubtedly due to the inductive effect of the fluorine atom which produces an electron deficit on the carbon atom linked to the nitrogen, and presumably increases still further the polarity of the carbon-nitrogen bond, so that the electron displacements can be pictured as (IX). The increased polarity of the carbon-nitrogen bond will obviously facilitate polar addition of hydrogen chloride and alcohols (or phenols). 

There are several examples of the 1,2-elimination of hydrogen chloride from 9-(2-chloroethyl)carbazoles, using potassium hydroxide in ethanol, generating the 9-vinylcarbazoles. 3-Dimethylamino-9-(2-hydroxyethyl) carbazole comparably lost water on base treatment. Dimethylamine displacement of halogen, then amine N-oxide formation and elimination was utilized to produce 9-alkenylcarbazoles with four, five, and six carbon atoms from the corresponding cu-haloalkyl carbazoles. ... 

In one synthesis of this drug, L-proline (11-2) is acylated with the acid chloride (11-1) obtained from the addition of hydrogen chloride to the double bond in methacrylic acid followed by reaction with thionyl chloride to give amide (11-3) as a mixture of diastereomers. The pure 2S isomer is then isolated from the mixture by fractionation as the dicyclohexylamine salt. Treatment of that compound with ammonium hydrosulfide leads to the displacement of chlorine by a thiol group and the formation of captopril (11-4) [13]. 

This illustrates the general principle that a nonvolatile powerful pro-tonator (H2S04, b.p. 290-317°C) will displace a volatile weaker acid (hydrogen chloride, b.p. -85 °C) on heating. 

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