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Photosulfochlorination

Introducing sulfon groups into alkanes or alkyl aromates is of particular interest for the production of anionic tensides. Alkane sulfonic acids/sulfonates show advantages when compared to the often used alkylbenzoic sulfonic acids/sulfonates, because they possess better biological degradation properties.[1] Despite this, it is problematic to produce these compounds on a technical scale. Alkane sulfonates with a backbone of 12 to 18 carbon-atoms are especially relevant for chemical industries.

One possible synthesis route for alkane sulfonic acids utilizes photosulfochlorinations. These reactions are initiated by breaking the Cl–Cl-bond through irradiation by light with a wavelength of λ < 500 nm. The single reaction steps from initiation to propagation and termination are summarized in Figure 1. When termination steps can be suppressed sufficiently, what can be realized by use of reactants of high purity, quantum yields of φ > 4·104 can be reached. Due to impurities present in technical grade chemicals, the quantum yield is typically lower. Photosulfochlorinations are associated with stoichiometrical formation of HCl.

Another way to generate sulfonic acid moieties are photosulfoxidations. These reactions are initiated through exiting SO2 with a wavelength of 180 nm to 390 nm. The proposed mechanism is shown in Figure 1. Excited SO2 transfers the energy to the alkane, resulting in abstraction of a proton.[2] Compared to photosulfochlorinations, an important difference is the formation of persulfonic acids (RSO2O2H) which decomposes into two radicals. Hence, the propagation is maintained autocatalytically. Under ideal conditions, the reaction proceeds until full conversion is reached with only a single, short irradiation. This reaction does not show any stoichiometric byproducts. On an industrial scale, photosulfoxidations are conducted in the presence of water, which acts as a radical scavenger. The light-water-process, implemented by Hoechst on a technical scale, uses water to suppress the formation and deposition of tarry substances at the reactor wall. This avoids a decrease of the photon flux reaching the reaction solution. With this, the performance of the process is maintained over time. Additionally, the alkane persulfonic acids are decomposed to the corresponding alkane sulfonic acids and H2SO4.[3] In this case, the reaction is not autocatalytic anymore and a permanent regeneration of radicals through irradiation is required. The quantum yield in this case is φ ≈ 10. To increase the reaction rate, initiators can be used.

sulfonation1

Figure 1: General reaction schemes of the photosulfochlorination and the photosulfoxidation.

 

  1. BRAUN, A. M.; MAURETTE, M.-T.; OLIVEROS, E.: Photochemical technology, 1991, Wiley, ISBN 0471926523, DOI: 1002/ange.19921041147.
  2. KOSSWIG, K.: Surfactants, Ullmann’s Encyclopedia of Industrial Chemistry, 2012-June, DOI: 1002/14356007.a25_747.
  3. SCHIMMELSCHMIDT, K.: Verfahren zur Herstellung von Sulfonsaeuren, Tech. Rep. DE910165, Hoechst AG, 1964-April.

 
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