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General Aspects

Photohalogenation are in common cases addition or substitution reactions. Photochlorination and photobromination are the most relevant members of this reaction type. The reactions are radical chain reactions.


The reactions start by absorption of light, which provides enough energy for the dissociation of the halogen-halogen bond. In pure solvents, the quantum yield can be considerably higher than (104) for the photochlorination. The thermal dissociation of the chlorine-chlorine bond requires temperatures larger than 200C and dissociation of bromine occurs between 250C to 400C. The minimum energy, which is required to break the halogen-halogen bond for chlorine amounts to 240 kJ/mol and for bromine 190 kJ/mol, which correspond to a radiation wavelength 500 nm and 630 nm.


Photochlorinations give access to chlorinated products which are used as synthetic intermediates or solvents. Typical products are benzyl chloride, dichlormethane and tetrachlormethane. Introduction of chlorine into polymers is also possible. This reaction type represents the most commonly used photochemical reaction on industrial scale. The reaction proceeds a radical chain mechanism. Under suited reaction conditions, namely in the absence of impurities, very large quantum yields can be reached. This is quite attractive for industrial application, because the required light intensity to achieve a certain production volume is lower. The following equation illustrates the general reaction steps when chlorine is added to olefins. haleq2

Beside addition reactions, substitution can occur for photochlorinations. The most common case is the substitution of a hydrogen atom, as generally shown in Figure 1. This reaction path can beneficially be used for the chlorination of the side chains of alkylbenzenes. Although addition to the aromatic ring remains possible, the reaction rates of the substitution are faster. This enables selective introduction of chlorine into the side chain.


Figure 1: Photochlorinatoin of toluene at the side chain and aromatic ring.

Chlorination of the side chain be further enhanced by use of moderately high reaction temperatures, because for such conditions, the addition to the aromatic ring becomes reversible. As a consequence, side chain chlorination becomes the dominant reaction. Ring chlorinations become relevant, when the ring is electrophilic. This can be provoked by addition of lewis acid catalysts such as FeCl3 or AlCl3 and favored in polar solvents. Hence, metallic impurities must be removed from the reaction solution to maintain high selectivities. This is especially important on an industrial scale, because high purities of the solvents and reactants might not be possible due to technical or economic reasons. In this case, complexing agents can be used to suppress ring chlorination and the use of metals for construction should be avoided. Substitution on the aromatic ring is favored for low reaction temperatures and high chlorine concentrations.

Side chain chlorination of toluene and xylenes are of interest for chemical industries for the production of intermediates for plasticiers, phtalic acid derivates and various other organic products with a world production of several tens of thousand tons. The chlorination at the side chain becomes dominant when the temperature is moderately high and chlorine concentration low. The reaction mechanism is similar to the radical substitution of saturated aliphatic hydrocarbons. The reaction steps can be controlled by variation of operating conditions. Depending on the chlorination degree, the target product can be isolated. The obtained product distribution of the photchlorination of toluene is shown in Figure 2.[1][25]]


Figure 2: Product distribution of the photochlorination of toluene at 100 °C as a function ofthe degree of chlorination.[1]



The addition reaction of bromine to olefins are generally reversible and radical chain mechanism is accepted. But, to explain the stereoselectivity of addition reaction formation of bridged bromoalkyl radikal proposed. The proposed mechanism is shown in the following equations:



 The rotation around of C-C bond is prevented by formation of bridged radical and hence for steric reasons the trans-product must be formed. The stereoselectivity can be explained including reaction rate of bromalkyl radical with bromine. Depending on rotation rate around C-C bond and reaction rate bromalky radical with bromine can cis- or trans-product be formed. The stereoselectivity of product (cis or trans) depends on stability of the different rotational isomers of the radical as well as the steric hindrance of substituents.




  1. BRAUN, A. M.; MAURETTE, M.-T.; OLIVEROS, E.: Photochemical technology, 1991, Wiley, ISBN 0471926523, DOI: 10.1002/ange.19921041147.
  2. BECKER, H.; BÖTTCHER, H.; DIETZ, F.; REHOREK, D.; ROEWER, G.; SCHILLER, K.; TIMPE, H.-J.: Einführung in die Photochemie, 3. bearb. aufl. edn., 1991, Deutscher Verlag der Wissenschaften GmbH, ISBN 978–3–3260–0604–8.
  3. MATTAY, J.; GRIESBECK, A. G.: Photochemical Key Steps in Organic Synthesis. An Experimental Course Book, 1994, Wiley-VCH, ISBN 978–3–5272–9214–1.
  4. HOFFMANN, N.: Photochemical Reactions as Key Steps in Organic Synthesis, Chem. Rev., 2008, 108 (3), 1052–1103, DOI: 10.1021/cr0680336.
  5. ALBINI, A.; FAGNONI, M. (Eds.): Handbook of Synthetic Photochemistry, 1. edn., 2009, Wiley-VCH Verlag GmbH & Co. KGaA, ISBN 978–3–5273–2391–3.


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