Alkane substrates

The photochlorination of hydrocarbon is unselective, although the reactivity of the C-H bonds is tertiary>secondary>primary. At 30 °C the relative reaction rates of primary, secondary and tertiary hydrogen atoms are in a relative ratio of approximately 1 to 3.25 to 4.43. The C-C bonds remain unaffected.^{[9] [10]}

Upon radiation the reaction involves alkyl and chlorine radicals following a chain reaction according to the given scheme:

${\displaystyle \mathrm {Cl_{2}\ \xrightarrow {h\nu } \ Cl{\cdot }+{\cdot }Cl\quad (chain\ start)} }$

${\displaystyle \mathrm {Cl{\cdot }+RH\longrightarrow {\cdot }R+HCl\quad (chain\ propagation)} }$

${\displaystyle \mathrm {R{\cdot }+Cl_{2}\longrightarrow {\cdot }Cl+RCl\quad (chain\ propagation)} }$

${\displaystyle \mathrm {Cl{\cdot }+{\cdot }R\longrightarrow RCl\quad (chain\ termination)} }$

${\displaystyle \mathrm {Cl{\cdot }+{\cdot }Cl\longrightarrow Cl_{2}\quad (chain\ termination)} }$

Chain termination occurs by recombination of chlorine atoms.^[11] Impurities such as oxygen (present in electrochemically obtained chlorine) also cause chain termination.

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.^[12] Selectivity increases in aromatic solvents.^[13] By varying the solvent the ratio of primary to secondary hydrogens can be tailored to ratios between 1: 3 to 1: 31.^[14] At higher temperatures, the reaction rates of primary, secondary and tertiary hydrogen atoms equalize. Therefore, photochlorination is usually carried out at lower temperatures.^[9]

Aromatic substrates

The photochlorination of benzene proceeds also via a radical chain reaction:^[15]

${\displaystyle \mathrm {Cl_{2}\ \xrightarrow {h\nu } \ Cl{\cdot }+{\cdot }Cl\quad (chain\ start)} }$

${\displaystyle \mathrm {Cl{\cdot }+C_{6}H_{6}\longrightarrow {\cdot }C_{6}H_{6}Cl\quad (chain\ propagation)} }$

${\displaystyle \mathrm {{\cdot }C_{6}H_{6}Cl+Cl_{2}\longrightarrow {\cdot }Cl+C_{6}H_{6}Cl_{2}\quad (chain\ propagation)} }$

[…]

${\displaystyle \mathrm {{\cdot }C_{6}H_{6}Cl_{5}+Cl_{2}\longrightarrow {\cdot }Cl+C_{6}H_{6}Cl_{6}\quad (chain\ propagation)} }$

${\displaystyle \mathrm {Cl{\cdot }+{\cdot }Cl\longrightarrow Cl_{2}\quad (chain\ termination)} }$

In some applications, the reaction is carried out at 15 to 20 °C. At a conversion of 12 to 15% the reaction is stopped and the reaction mixture is worked up.^[15]

Chloromethanes

Subsequent products of photochlorination of methane (schematic representation without consideration of stoichiometry).

An example of photochlorination at low temperatures and under ambient pressure is the chlorination of chloromethane to dichloromethane. The liquefied chloromethane (boiling point -24 °C) is mixed with chlorine in the dark and then irradiated with a mercury-vapor lamp. The resulting dichloromethane has a boiling point of 41 °C and is later separated by distillation from methyl chloride.^[16]

The photochlorination of methane has a lower quantum yield than the chlorination of dichloromethane. Due to the high light intensity required, the intermediate products are directly chlorinated, so that mainly tetrachloromethane is formed.^[16]

Chlorinated paraffins

A major application of photochlorination is the production of chloroparaffins. Mixtures of complex composition consisting of several chlorinated paraffins are formed. Chlorinated paraffins have the general sum formula C_xH_(2x−y+2)Cl_y and are categorized into three groups: Low molecular weight chlorinated paraffins are short chain chloroparaffins (SCCP) with 10 to 13 carbon atoms, followed by medium chain chloroparaffins (MCCP) with carbon chain lengths of 14 to 17 carbon atoms and long chain chlorinated paraffins (LCCP), owing a carbon chainwith more than 17 carbon atoms. Approximately 70% of the chloroparaffins produced are MCCPs with a degree of chlorination from 45 to 52%. The remaining 30% are divided equally between SCCP and LCCP.^[6] Short chain chloroparaffins have high toxicity and easily accumulate in the environment. The European Union has classified SCCP as a category III carcinogen and restricted its use.^[17]

In 1985 the world production was 300,000 tonnes; since then the production volumes are falling in Europe and North America.^[18] In China, on the other hand, production rose sharply. China produced more than 600,000 tonnes of chlorinated paraffins in 2007, while in 2004 it was less than 100,000 tonnes.^[19]

The quantum yield for the photochlorination of n-heptane is about 7000, for example.^[20] In photochlorination plants, the quantum yield is about 100. In contrast to the thermal chlorination, which can utilize the formed reaction energy, the energy required to maintain the photochemical reaction must be constantly delivered.^[21]

The presence of inhibitors, such as oxygen or nitrogen oxides, must be avoided. Too high chlorine concentrations lead to high absorption near the light source and have a disadvantageous effect.^[14]

Benzyl chloride, benzal chloride and benzotrichloride

The photochlorination of toluene is selective for the methyl group. Mono- to trichlorinated products are obtained. The most important of which is the mono-substituted benzyl chloride, which is hydrolyzed to benzyl alcohol. Benzyl chloride can also be converted via benzyl cyanide with subsequent hydrolysis into phenylacetic acid.^[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.^[25] The trisubstituted benzotrichloride is used for the hydrolysis of the synthesis of benzoyl chloride:^[26]

${\displaystyle \mathrm {Ph{-}CCl_{3}+H_{2}O\longrightarrow } }$ ${\displaystyle \mathrm {Ph-COCl+2\ HCl\ } }$

By reaction with alcohols, benzoyl chloride can be converted into the corresponding esters. With sodium peroxide it turns into dibenzoyl peroxide, a radical initiator for polymerizations. However, the atom economy of these syntheses is poor, since stoichiometric amounts of salts are obtained.

Sulfochlorination

The sulfochlorination first described by Cortes F. Reed in 1936 proceeds under almost identical conditions as the conventional photochlorination.^[27] In addition to chlorine, sulfur dioxide is also introduced into the reaction mixture. The products formed are alkylsulfonyl chlorides, which are further processed into surfactants.^[28]

Hydrochloric acid is formed as a coupling product, as is the case with photochlorination. Since direct sulfonation of the alkanes is hardly possible, this reaction has proven to be useful. Due to chlorine, which is bound directly to the sulfur, the resulting products are highly reactive. As secondary products there are alkyl chlorides formed by pure photochlorination, as well as several sulfochlorinated products in the reaction mixture.^[29]

Photobromination

Reaction scheme of the photobromination of the methyl group of toluene

Photobromination with elemental bromine proceeds analogous to photochlorination also via a radical mechanism. 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).^[30][31] Instead of elemental bromine, N-bromosuccinimide is also suitable as a brominating agent.^[32] The quantum yield of photobromination is usually much lower than that of photochlorination.