Redistribution (chemistry)

In chemistry, redistribution usually refers to the exchange of anionic ligands bonded to metal and metalloid centers. The conversion does not involve redox, in contrast to disproportionation reactions. Some useful redistribution reactions are conducted at higher temperatures; upon cooling the mixture, the product mixture is kinetically frozen and the individual products can be separated. In cases where redistribution is rapid at mild temperatures, the reaction is less useful synthetically but still important mechanistically.

Examples

Rapid redistribution reactions are exhibited by methylboranes. Thus monomethyldiborane rapidly converts at room temperature to diborane and trimethylborane:[1]

6 MeB2H5 → 5 B2H6 + 2 Me3B

Useful redistribution reactions are found in organoaluminium, organoboron, and organosilicon chemistry.[2][3]

BCl3 + 2 B(C2H5)3 → 3 BCl(C2H5)2

In another example, tetramethylsilane is an undesirable product of the industrially important direct process, but it can be converted (recycled) into more useful products by redistribution with silicon tetrachloride:

SiMe4 + SiCl4 → 2 SiMe2Cl2

In organotin chemistry, the mixed alkyl tin chlorides are produced by redistribution, a reaction called the Kocheshkov comproportionation:[4]

3 SnBu4 + SnCl4 → 4 SnBu3Cl

Many metal halides undergo redistribution reactions, usually to afford nearly statistical mixtures of products. For example, titanium tetrachloride and titanium tetrabromide redistribute their halide ligands, one of many reactions in this conversion is shown:[5]

TiCl4 + TiBr4 → 2 TiBr2Cl2

References
  1. Bell, R. P.; Emeléus, H. J. (1948). "The Boron Hydrides and Related Compounds". Quarterly Reviews, Chemical Society. 2 (2): 132. doi:10.1039/QR9480200132.. The authors refer to redistributions as "disproportionations".
  2. Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
  3. Many mixed organo-chloro derivatives of many metalloids are produced in this manner. In one example, Köster, R.; Binger, P. (2007). Chlorodiethylborane and Chlorodiphenylborane" 2007;. Inorganic Syntheses. Vol. 15. pp. 149–153. doi:10.1002/9780470132463.ch33. ISBN 9780470132463.
  4. G. G. Graf (2005). "Tin, Tin Alloys, and Tin Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a27_049. ISBN 978-3527306732.
  5. S. P. Webb and M. S. Gordon (1999). "Intermolecular Self-Interactions of the Titanium Tetrahalides TiX4 (X = F, Cl, Br)". J. Am. Chem. Soc. 121 (11): 2552–2560. doi:10.1021/ja983339i.
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