Bromine, acetylene action

Action of bromine on acetylene, action of nitric acid on aromatic hydrocarbons. The accidents involving both reactions are very specific.They are the result of application of an unsuitable temperature. If it is too low, the reaction is too slow and causes an accumulation of reagent that is not converted and whose concentration increases, causing an acceleration of the reaction, i.e. a rise in temperature. The temperature then becomes too high and causes a more or less violent speeding-up of the reaction. 

Bromine. Slip the glass cover of a jar momentarily aside, add 2-3 ml. of bromine water, replace the cover and shake the contents of the jar vigorously. Note that the bromine is absorbed only very slowly, in marked contrast to the rapid absorption by ethylene. This slow reaction with bromine water is also in marked contrast to the action of chlorine water, which unites with acetylene with explosive violence. (Therefore do not attempt this test with chlorine or chlorine water.)... 

Two examples have been given for the temperature/concentration action in the earlier reference to thermal effects (actions of bromine on acetylene and nitric acid on aromatic hydrocarbons). 

C) Action of Acetylene on Bromine. Pass acetylene through 5 ml of bromine water. Note the changes. Aside from the color change, what other evidence is there for the reaction ... 

Addition methods were used for the determination of m-saturated compounds and olefins. The methods are based on the additions of bromine to unsaturated bonds, and the waves for the brominated compounds corresponding to the reduction of o, j8-dibromides (involving elimination) are measured. Their heights are proportional to the concentration of the unsaturated compound. Thus vinylchloride and 1,2-dichloroethylene were transformed into l-chloro-l,2-dibromoethane and l,2-dichloro-l,2-dibromoethane, by the action of a 3 M solution of bromine in methanol saturated with sodium bromide. The excess of bromine was removed with ammonia and the polarographic analysis was performed with sodium sulphite or lithium chloride as a supporting electrolyte. On the other hand, acetylene, vinyl-chloride, 1,2-dichloroethylene and 1,1,2-trichloroethylene were determined ) after a 24 hr reaction with bromine in glacial acetic acid (1 1). The excess bromine was removed with a stream of nitrogen or carbon dioxide. An aliquot portion is diluted (1 10) with a 3 M solution of sodium acetate in 80 per cent acetic acid and after deaeration the curve is recorded. 

Formation of allene (263) by the action of hydrobromic acid on acetylene (264) in the presence of cuprous bromide has been found to follow first-order kinetics and an S i process is believed to be involved in the rate-determining step. The reaction is stereoselective since the optically active acetylenic alcohol (265) gives optically active allene (266) and thus the reaction proceeds with entry of the bromine from the same side as the OH group. ... 

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