Photochlorination
The French chemist Jean-Baptiste Dumas investigated the substitution of hydrogen for chlorine by acetic acid in candle wax as early as 1830.
[3] Photochlorination garnered commercial attention with the availability of cheap chlorine from chloralkali electrolysis.
The Sharpless Solvents Corporation commissioned the first industrial photochloration plant for the chlorination of pentane in 1929.
[5] However, it was only in the years after World War II that a greater build-up of photochloration capacity began.
[7] Because of concerns about health and environmentally relevant problems such as the ozone depletion behavior of light volatile chlorine compounds, the chemical industry developed alternative procedures that did not require chlorinated compounds.
The photochlorination of hydrocarbon is unselective, although the reactivity of the C-H bonds is tertiary>secondary>primary.
The selectivity of photochlorination (with regard to substitution of primary, secondary or tertiary hydrogens) can be controlled by the interaction of the chlorine radical with the solvent, such as benzene, tert-butylbenzene or carbon disulfide.
[14] At higher temperatures, the reaction rates of primary, secondary and tertiary hydrogen atoms equalize.
The liquefied chloromethane (boiling point -24 °C) is mixed with chlorine in the dark and then irradiated with a mercury-vapor lamp.
Due to the high light intensity required, the intermediate products are directly chlorinated, so that mainly tetrachloromethane is formed.
[22][23] The disubstituted benzal chloride is converted to benzaldehyde, a popular flavorant[24] and intermediate for the production of malachite green and other dyes.
Due to chlorine, which is bound directly to the sulfur, the resulting products are highly reactive.
In the presence of oxygen, the hydrogen bromide formed is partly oxidised back to bromine, resulting in an increased yield.
Because of the easier dosage of the elemental bromine and the higher selectivity of the reaction, photobromination is preferred over photochlorination at laboratory scale.
For industrial applications, bromine is usually too expensive (as it is present in sea water in small quantities only and produced from oxidation with chlorine).
