Photochlorination

Reaction scheme for the photochlorination of the methyl group of toluene .

The photochlorination is a triggered by light chemical reaction in which a hydrocarbon compound is hydrogen by chlorine is replaced, using as a co-product hydrogen chloride is formed. Alternatively, there is a radical addition of chlorine to aromatic or olefinic hydrocarbons.

In addition to thermal and catalytic chlorination, photochlorination is carried out on an industrial basis, mostly in the liquid phase, sometimes in the presence of inert solvents . The process is exothermic and takes place as a chain reaction that is started by the homolytic splitting of molecular chlorine into chlorine radicals by ultraviolet radiation .

The chlorinated hydrocarbons formed during photochlorination are often only industrial intermediates and react with a large number of basic chemicals to form secondary products such as alcohols , mercaptans , amines and carboxylic acids . The chemical industry uses low molecular weight chlorinated compounds such as carbon tetrachloride as solvents . Higher molecular weight chloroalkanes serve as insecticides , as flame retardants or as plasticizers in plastics and coatings. Chlorinated hydrocarbons are also used as an intermediate in the chemical industry for the production of silicones or detergents .

history

Jean-Baptiste Dumas

In the chlorination is one of the oldest known substitution reactions of chemistry. The French chemist Jean-Baptiste Dumas investigated the substitution of hydrogen by chlorine in the effects of chlorine on candle wax and acetic acid as early as 1830 . He demonstrated that for every mole of chlorine introduced into a hydrocarbon, one mole of hydrogen chloride was formed, and he noticed the photosensitivity of this reaction.

Theodor Grotthuss wrote the first work on the influence of light on the speed of chemical reactions . As early as 1819 Grotthuss published a treatise on the chemical effectiveness of light and formulated the law of photochemical absorption . Consequently, in a physicochemical system, only that fraction of the incident radiation causes an effect that is absorbed by this system ; reflected and transmitted radiation has no effect.

From the work of Max Planck published in 1900 it was known that light consists of discrete quanta . The excitation of a single chemical reaction by a light quantum could be explained by this, but not the quantum yield of reactions like photochlorination. The idea that these reactions could be chain reactions originated from Max Bodenstein in 1913. He assumed that when two molecules react, not only the end product of the reaction can arise, but also unstable, reactive intermediates that be able to continue a chain.

Because of the importance of the reaction to understanding chemistry, substitution patterns, and the resulting derivatives, chemists studied the reaction in depth. However, photochlorination could only be transferred into chemical-industrial practice when, towards the end of the nineteenth century, cheap chlorine was available from chlor-alkali electrolysis .

Chlorinated alkanes were first used in throat sprays. These contained about 1914-1918 chlorinated alkanes in relatively large amounts as a solvent for chloramine-T . Sharpless Solvents Corporation commissioned the first industrial photochlorination plant for chlorinating pentane in 1929 . The commercial production of chlorinated paraffins for use as high pressure additives in lubricants began around 1930. By 1935 the process was technically stable and commercially successful.

But it wasn't until the years after World War II that a major build-up of photochlorination capacity began. In 1950 the United States was producing over 800,000 tons of chlorinated paraffinic hydrocarbons . The main products were ethyl chloride , carbon tetrachloride and methylene chloride . Because of concerns about health and environmental problems such as the ozone depletion behavior of volatile chlorine compounds, the chemical industry developed alternative processes that managed without chlorinated compounds. As a result of the replacement of chlorinated products with non-chlorinated products, the global production volume fell considerably over the years.

background

Visible spectrum of a mercury vapor lamp. The violet line at 405 nm is barely visible to the eye. Particularly strong lines are in the (right) subsequent invisible UV.

Since only absorbed light triggers a primary photochemical reaction, one of the reaction partners has to absorb this light in every photochemical reaction. In the case of photochlorination, the chlorine is the absorbing reactant. Chlorine absorbs light in a wavelength range of around 250 to 450  nanometers , corresponding to absorption in the long-wave ultraviolet and the visible violet spectral range . An energy of 244 kilojoules per mole is required for the homolytic splitting of chlorine  .

According to the photochemical equivalence law , every absorbed photon causes a primary photochemical reaction.

${\displaystyle \Delta E_{mol}=N_{\mathrm {A} }h\nu }$

with N _A as Avogadro's constant (N _A = 6.022 10 ²³ mol ⁻¹ ), Planck's quantum of action (h = 6.626 10 ⁻³⁴ Js) and the light frequency with the unit s ⁻¹ . ${\displaystyle \cdot }$ ${\displaystyle \,h}$ ${\displaystyle \cdot }$ ${\displaystyle \,\nu }$

About the relationship:

${\displaystyle \nu =c/\lambda }$

with for the speed of light (c = 299,792,458 ms ⁻¹ ) results ${\displaystyle c}$

${\displaystyle E_{mol}=N_{\mathrm {A} }\cdot hc/\lambda }$

and thus for the wavelength in the unit nm: ${\displaystyle \lambda }$

${\displaystyle \lambda =N_{\mathrm {A} }\cdot hc/E_{mol}}$

Inserting the values ​​results in a value for the wavelength of 491 nanometers, which the incident light may have at most in order to cause chlorine to split. The absorption maximum of chlorine is around 340 nanometers. Light of this wavelength radiates an energy of about 377 kilojoules per mole and is therefore more than sufficient for photolysis of the chlorine.

For photochemical processes it is important how the number of converted molecules relates to the number of absorbed light quanta . This ratio is called the quantum yield (QA) at a certain wavelength of light . The ratio of converted molecules of the light-absorbing substance to the number of absorbed photons is calculated as: ${\displaystyle N_{n}(\lambda )}$ ${\displaystyle N_{\nu }}$

${\displaystyle QA(\lambda )={\frac {\text{umgesetzte Moleküle}}{\text{absorbierte Lichtquanten}}}={\frac {N_{n}(\lambda )}{N_{\nu }(\lambda )}}}$

The quantum yield should not exceed the value 1 in the photochemical primary process, the absorption of the light quantum. In the case of photochlorination, however, this ratio is not equal to or less than 1, but is often considerably higher because of the secondary processes in which the same substances are formed as in the primary photochemical process.

reaction

Substitution reaction

The substitution of hydrogen atoms in a hydrocarbon is purely statistical, with tertiary hydrogen atoms reacting faster than secondary ones and these react faster than primary ones . At a temperature of 30 ° C, the relative reaction rates of primary, secondary and tertiary hydrogen atoms are roughly 1 to 3.25 to 4.43.

When exposed to light, the reaction takes place with the participation of alkyl and chlorine radicals as chain carriers according to the following scheme:

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

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

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

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

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

The chain is terminated by the recombination of chlorine radicals to form molecular chlorine on the vessel wall. Impurities such as oxygen , which is found in electrochemically obtained chlorine, also cause the chain to break.

With photochlorination there is no rearrangement of the carbon chain and all possible monochlorides and multiply chlorinated compounds are formed. However, the target products of photochlorination are mostly the monosubstituted chlorinated hydrocarbons . The formation of multi-substituted products can be reduced within limits by working with a large excess of hydrocarbons or by diluting the chlorine with nitrogen .

The selectivity of the photochlorination with regard to a substitution of primary, secondary or tertiary hydrogen atoms can be controlled by the interaction of the chlorine radical with the solvent, for example benzene , tert- butylbenzene or carbon disulfide . The formation of a complex from benzene and the chlorine radical reduces the reactivity towards a free chlorine radical, which increases the selectivity of the photochlorination. The range of the ratio of substituted primary to secondary hydrogen atoms obtained by the choice of solvent is 1: 3 to 1: 31. At higher temperatures, the reaction rates of primary, secondary and tertiary hydrogen atoms are equal. Therefore, the photochlorination is usually carried out at lower temperatures.

Addition reaction

The addition of chlorine to benzene also takes place as a radical chain reaction:

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

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

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

[...]

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

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

The reaction is carried out at a temperature of 15 to 20 ° C. At a conversion of 12 to 15% the reaction is terminated and the reaction mixture is worked up.

Reaction engineering

Photochemical laboratory reactor with a mercury vapor lamp .

The starting materials (reactants) for photochlorination can be either gaseous or liquid hydrocarbons. In the case of liquid starting materials, chlorine is introduced with stirring. Pure gas reactions, such as the photochlorination of methane, are possible in principle; the temperature rise in a gas phase reaction must largely be absorbed by the specific heat capacities of the gases involved, which limits the conversion. In general, it is necessary to bring the reactants close to the light source in order to obtain the highest possible light yield . For this purpose, the reaction mixture can be irradiated with a suitable light source directly or in a side arm of a reactor through which there is flow . Gaseous hydrocarbons are introduced into an inert solvent and made to react there under irradiation with chlorine.

A disadvantage of photochemical processes is the low efficiency of converting electrical energy into radiant energy of the required wavelength. In addition to radiation, light sources generate a lot of heat, which in turn requires cooling . In addition, most light sources emit polychromatic light, although only monochromatic light is required. However, a high quantum yield offsets these disadvantages. The quantum yield for the photochlorination of the n -heptane is, for example, about 7000. In technical systems for photochlorination, the quantum yield is about 100. In contrast to thermal chlorination, which can use the heat of reaction , the photochemical method of operation requires the energy to maintain the reaction are constantly being replenished.

Scheme of a bubble column reactor for photochlorination.

The presence of inhibitors such as oxygen or nitrogen oxides must be avoided. Excessive chlorine concentrations lead to excessive absorption in the vicinity of the light source and have a negative effect. Working at low temperatures is advantageous since side reactions are avoided, since the selectivity is increased and since gaseous reactants are less expelled from a solvent, which increases the yield . Before the reaction, some of the starting materials can be cooled to such an extent that the heat of reaction is absorbed without further cooling of the mixture. In the case of gaseous or low-boiling starting materials, work under pressure is necessary. Because of the large number of possible raw materials, a large number of processes have been described. The photochlorination is usually carried out in a stirred tank reactor , a bubble column reactor or a tubular reactor, which, depending on the target product, are followed by further processing stages. In the case of the stirred tank reactor, the lamp, which is usually shaped as an elongated cylinder, is provided with a cooling jacket and immersed in the reaction solution. Tube reactors are quartz or glass tubes that are irradiated from the outside. The stirred tank variant has the advantage that no light is lost to the environment. However, the light intensity drops rapidly with distance from the light source due to adsorption by the reactants.

The influence of radiation on the reaction speed can often be represented by a power law based on the quantum current density , i.e. the moles of light quanta (previously measured in Einstein units ) per area and time. One goal when designing reactors is therefore to determine the most economically advantageous dimensioning in terms of optimizing the quantum current density.

Products

Chlorinated products can be converted into further intermediate and end products via a variety of reactions , for example by hydrolysis into alcohols or by reaction with alkali cyanides into nitriles , which can be hydrolyzed with water to carboxylic acids or reduced with hydrogen to amines . By conversion with metallic magnesium in Grignard reactions , carbon skeletons can be built up via the intermediate stage of alkyl magnesium halides . In Friedel-Crafts alkylations , chloroalkanes are used to prepare alkyl aromatics.

Chlorinated paraffins

2,3,4,5,6,8-hexachlorodecane, a short-chain chlorinated paraffin

Chlorinated paraffins can be prepared from alkanes by photochlorination. Compared to thermal chlorination, the risk of the formation of secondary products through thermolysis , for example through elimination of hydrogen chloride, is very low. Due to the radical reaction, the selectivity is low and mixtures of several chlorinated paraffins with a complex composition are formed. The degree of chlorination varies, and the exact composition of the resulting product mixtures is often not known. The world annual production in 1985 was 300,000 tons; since then, production volumes in Europe and North America have declined. In China, on the other hand, production rose sharply. In 2007, China produced over 600,000 tons of chlorinated paraffins, in 2004 the amount was still below 100,000 tons.

The chlorinated paraffins have the general formula C _x H _{(2 x - y +2)} Cl _y and are divided into three groups. The low molecular weight chloroparaffins are the short- chain chloroparaffins (SCCP) with 10 to 13 carbon atoms , followed by the medium-chain chloroparaffins (MCCP) with carbon chain lengths of 14 to 17 carbon atoms and the long-chain chloroparaffins ( Long Chain Chloroparaffins ( LCCP)), where the carbon chain length is greater than 17 carbon atoms. About 70% of the chlorinated paraffins produced are MCCPs with a degree of chlorination of 45 to 52%. The remaining 30% are divided equally between SCCP and LCCP.

The short chain chloroparaffins are highly toxic and easily accumulate in the environment. The European Union has classified the SCCP as a category III carcinogen and has restricted its use.

Benzyl chloride, benzal chloride and benzotrichloride

See also : SSS rule

The mono- to trichlorinated products, the most important representative of which is benzyl chloride , can be produced by photochlorination of the side chain of toluene . Converted into benzyl alcohol , it serves as an intermediate product in the manufacture of plasticizers. By converting it into the benzyl cyanide with subsequent hydrolysis, phenylacetic acid is finally obtained.

The disubstituted benzal chloride is the raw material for the production of benzaldehyde . Benzaldehyde is used as a pure substance to give foods an almond odor . As an intermediate product, it is used in the production of malachite green and other dyes. The trisubstituted benzotrichloride is hydrolyzed to synthesize benzoyl chloride :

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

Benzoyl chloride can be converted into the corresponding esters by reaction with alcohols. With sodium peroxide it reacts to form benzoyl peroxide , a radical initiator for polymerizations . However, the atom economy of these syntheses is low, since stoichiometric amounts of salts are obtained.

Chloromethane

By-products of the photochlorination of methane (schematic representation without taking stoichiometry into account ).

An example of photochlorination at low temperatures and under normal pressure is the chlorination of methyl chloride to methylene chloride . The liquefied methyl chloride, which boils at -24 ° C, is mixed with chlorine in the dark and then irradiated with a mercury vapor lamp. The resulting methylene chloride has a boiling point of 41 ° C and is later separated from the methyl chloride by distillation.

The photochlorination of methane has a lower quantum yield than the chlorination of methylene chloride. The resulting high use of light results in direct further chlorination of the intermediate products, so that mainly carbon tetrachloride is formed.

Monochlorononane and dodecane

General structural formula of the nonylphenol ethoxylates.

Monochlorononane reacts in a Friedel-Crafts alkylation with phenol to form nonylphenol and can be further converted to nonylphenol ethoxylates by ethoxylation . These nonionic surfactants are used as emulsifiers and as detergents and cleaning agents. Due to their xenoestrogenic properties, the use of nonylphenol ethoxylates and nonylphenols has been severely restricted in the EU.

Another target product is monochlorododecane , which also reacts with benzene in a Friedel-Crafts alkylation to form a detergent raw material, linear alkylbenzene , which is further processed into sodium dodecylbenzenesulfonate . In the photochlorination of dodecane , the formation of undesired 1-isomers can be suppressed by choosing a suitable solvent such as benzene. In the meantime, the production of alkylbenzene mostly takes place via the hydrogen fluoride -catalyzed reaction of 1-dodecene with benzene.

Chlorinated polymers

Polyethylene , dissolved in carbon tetrachloride, can be converted photochemically in a polymer-like reaction into a chlorinated polyolefin, which is used as an impact modifier to improve the notched impact strength of polyvinyl chloride . Polyvinyl chloride films can be further chlorinated at room temperature by photochlorination. The non-chlorinated methylene groups of the polymer are chlorinated. The quantum yield for reactions between polymers in the solid phase and chlorine is typically in the region of 1, since a chain reaction is not possible or only possible to a limited extent in this case.

Polyolefin membranes made of polyethylene, polypropylene and polystyrene films can be chlorinated in the solid phase by photochlorination to form membranes with a chlorine content of up to 12%. This improves the gas permeation , the wettability and the water permeability.

Hexachlorocyclohexane

Structural formula of lindane .

In the photochlorination of benzene, of the eight possible isomers, the four α-, β-, γ-, δ- hexachlorocyclohexane isomers are formed on a larger scale. These are all in a chair conformation and differ in the occupation of axial and equatorial positions in the molecule. Only the γ-isomer, which contains three chlorine atoms in the axial and three in the equatorial position and is produced in concentrations of 10 to 18%, has an insecticidal effect. It is separated from the other isomers by extraction processes, which are further processed to trichlorobenzene by dehydrochlorination . The technically pure γ-isomer is still used as an insecticide outside the EU under trade names such as lindane and gammexane.

Other products

Vinylene carbonate can be obtained by photochlorination of ethylene carbonate via the stage of monochloroethylene carbonate with subsequent dehydrochlorination , for example with triethylamine (NEt ₃ ). Vinylene carbonate is a reactive monomer for homopolymerization and copolymerization, for example with isobutyl vinyl ether .

Small traces of trichlorosilane in ultra-pure tetrachlorosilane can be removed by photochlorination.

Utilization of hydrogen chloride

Hydrogen chloride can be used for further chlorination, for example by the addition reaction of hydrogen chloride onto an olefinic double bond, by esterification of alcohols or by oxychlorination of alkanes and olefins . The hydrogen chloride formed during the photochlorination of methane can be selectively converted to methyl chloride, for example by esterification with methanol . At elevated temperatures, methylene chloride is also formed under catalysis.

Process variants

Radiation chemical chlorination

PC chlorination in the Bitterfeld Chemical Combine (1960).

Instead of ultraviolet light, gamma radiation is also used to start the radical chain in chlorination. The chemical combine Bitterfeld carried out the dry post-chlorination of PVC in a fluidized bed , the so-called PC chlorination, using radiation chemistry . The long-lived isotope Cobalt-60 was used as a gamma radiation source for this.

Sulfochlorination

The sulfochlorination first described by Cortes F. Reed in 1936 takes place under almost identical conditions and the same reaction procedure as conventional photochlorination . In addition to chlorine, sulfur dioxide is also introduced into the reaction mixture. The products are alkylsulfonyl chlorides that are processed into surfactants .

As with photochlorination, hydrogen chloride is produced as a by-product. Since direct sulfonation of the alkanes is hardly possible, this reaction has proven useful. Due to the chlorine bound directly to the sulfur , the resulting products are extremely reactive. The by-products in the reaction mixture are alkyl chlorides, which are formed by pure photochlorination, as well as multiple sulfochlorinated products.

Photobromination

Reaction scheme for the photobromination of the methyl group of toluene.

The photobromination with elemental bromine also proceeds according to a radical mechanism analogous to photochlorination. In the presence of oxygen, some of the hydrogen bromide formed is oxidized to bromine, which leads to an increased yield. Because the elementary bromine is easier to dose and the higher selectivity of the reaction, photobromination is preferred to photochlorination for work on a laboratory scale. For industrial applications, however, bromine, which is only contained in small quantities in seawater and is released from it through oxidation with chlorine, is usually too expensive. Instead of elemental bromine, N- bromo succinimide is also suitable as a brominating agent . The quantum yield of photobromination is usually much lower than that of photochlorination.

literature

• Dieter Wöhrle , Michael Tausch , Wolf-Dieter Stohrer : Photochemistry: Concepts, Methods, Experiments . Wiley & Sons, 1998, ISBN 3-527-29545-3 .
• Theodor Weyl (original), Josef Houben (ed.), Eugen Müller (ed.): Methods of organic chemistry. IV / 5a photochemistry . Thieme Verlag, Stuttgart 1975, ISBN 3-13-201904-6 .
• Mario Schiavello (Ed.): Photoelectrochemistry, Photocatalysis and Photoreactors Fundamentals and Developments . Springer Netherlands, 2009, ISBN 978-90-481-8414-9 .

Web links

Commons : Photochlorination  - collection of images, videos and audio files

Wiktionary: Photochlorination  - explanations of meanings, word origins, synonyms, translations

Individual evidence

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4. ↑ Max Planck: On the theory of the law of energy distribution in the normal spectrum. In: Negotiations of the German physical society. 2, No. 17, 1900, p. 245, Berlin (presented on December 14, 1900); (PDF) ( Memento of the original from August 7, 2015 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. 
5. ↑ Max Bodenstein: Photochemical kinetics of the chlorine explosion gas. In: Journal of Electrochemistry and Applied Physical Chemistry. 19, 1913, pp. 836-856, doi: 10.1002 / bbpc.19130192104 .
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21. ↑ Mario Schiavello (Ed.): Photoelectrochemistry, Photocatalysis and Photoreactors Fundamentals and Developments . Springer Netherlands, 2009, ISBN 978-90-481-8414-9 , p. 564.
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40. ↑ Directive 2003/53 / EC of the European Parliament and of the Council of June 18, 2003 on the 26th amendment to Directive 76/769 / EEC on restrictions on the placing on the market and use of certain dangerous substances and preparations (nonylphenol, nonylphenol ethoxylate and cement) , accessed on 25 March 2016 .
41. ↑ Marion L. Sharrah, Geo. C. Feighner: Synthesis of Dodecylbenzene - Synthetic Detergent Intermediate. In: Industrial & Engineering Chemistry. 46, 1954, pp. 248-254, doi: 10.1021 / ie50530a020 .
42. ↑ C. Decker, M. Balandier, J. Faure: Photochlorination of Poly (vinyl Chloride). I. Kinetics and Quantum Yield. In: Journal of Macromolecular Science: Part A - Chemistry. 16, 2006, pp. 1463-1472, doi: 10.1080 / 00222338108063248 .
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