Dehydrocondensation

Pyrolysis. Benzene undergoes thermal dehydrocondensation at high temperatures to produce small amounts of biphenyls and terphenyls (see Biphenyl AND terphenyls). Before the 1970s most commercial biphenyl was produced from benzene pyrolysis. In a typical procedure benzene vapors are passed through a reactor, usually at temperatures above 650°C. The decomposition of benzene iato carbon and hydrogen is a competing reaction at temperatures of about 750°C. Biphenyls are also formed when benzene and ethylene are heated to 130—160°C ia the presence of alkaH metals on activated AI2O3 (33). 

Biphenyl has been produced commercially in the United States since 1926, mainly by The Dow Chemical Co., Monsanto Co., and Sun Oil Co. Currently, Dow, Monsanto, and Koch Chemical Co. are the principal biphenyl producers, with lesser amounts coming from Sybron Corp. and Chemol, Inc. With the exception of Monsanto, the above suppHers recover biphenyl from high boiler fractions that accompany the hydrodealkylation of toluene [108-88-3] to benzene (6). Hydrodealkylation of alkylbenzenes, usually toluene, C Hg, is an important source of benzene, C H, in the United States. Numerous hydrodealkylation (HDA) processes have been developed. Most have the common feature that toluene or other alkylbenzene plus hydrogen is passed under pressure through a tubular reactor at high temperature (34). Methane and benzene are the principal products formed. Dealkylation conditions are sufficiently severe to cause some dehydrocondensation of benzene and toluene molecules. 

Since the thermal dehydrocondensation proceeds by a free-radical mechanism (37), various radical-forrning promoters like acetone, ethanol, or methanol have been found useful in improving conversion of ben2ene to condensed polyphenyls. In the commercial dehydrocondensation process, ben2ene and some biphenyl are separated by distillation and recycled back to the dehydrocondensation step. Pure biphenyl is then collected leaving a polyphenyl residue consisting of approximately 4% o-terphenyl, 44% y -terphenyl, 25% -terphenyl, 1.5% triphenylene, and 22—27% higher polyphenyl and tars. Distillation of this residue at reduced pressure affords the mixed terphenyl isomers accompanied by a portion of the quaterphenyls present. 

Pure (9-terphenyl can be obtained by fractional distillation. To obtain high purity m- or -terphenyl, the appropriate distillation fraction has to be further purified by recrysta11i2ing, 2one refining, or other refining techniques. Currently, litde demand exists for pure isomers, and only a mixture is routinely produced. Small amounts of acetone, ethanol, or methanol are used to promote dehydrocondensation, and as a result, minor amounts of methyl- or methylene-substituted polyphenyls accompany the biphenyl and terphenyls produced. For most purposes, the level of such products (<1%) is so small that their presence can be ignored. For appHcations requiring removal of these alkyl-polyphenyl impurities, an efficient process for their oxidative destmction has been described (38). 

Rehable estimates of annual production of biphenyl in the United States are difficult to obtain. The 1990 figure is probably on the order of 16 million kg/yr of which about half is derived from hydrodealkylation sources. About 10% of the biphenyl derived from HD A sources is consumed, as 93—95% grade, in textile dye carrier appHcations. The remainder is used for alkylation or upgraded to >99.9% grades for heat-transfer purposes. Essentially all of the high purity biphenyl produced by dehydrocondensation of ben2ene is used as alkylation feedstock or is utili2ed directly in heat-transfer appHcations. 

Moreover, it has been established that dehydrocondensation can also be applied to 3,5-diethynyl-l-methylpyrazole, which makes it possible to produce polymer (88%) with an extended system of conjugate bonds possessing semiconductor properties (2001UP2). 

Takakura, K., Toyota, T, and Sugawara, T. (2003). A novel system of self-reproducing giant vesicles. J. Am. Chem. Soc., 125, 8134-8140. See also Takakura, K Toyota, T, Yamada, K., et al. (2002). Morphological change of giant vesicles triggered by dehydrocondensation reaction. ChemLett., 31,404-5. 

Intramolecular dehydrocondensation of the /3 -germylethanethiol (65) gives the 2-germathietane, which decomposes by /3-elimination. The germathione intermediate then ring inserts into the germathietane (Scheme 98) (80MI12000). 

High purity biphenyl is currently produced by Monsanto in the United States and United Kingdom by direct dehydrocondensation of benzene. Terphenyls are also obtained from the higher boiling polyphenyl byproducts that accompany the biphenyl. 

Reaction with Further Electrophiles of Group IVA (Sl,Ge,Sn). IV-Silylated aziridines can be prepared from ethyleneimine by amination of chlorosilanes in the presence of an HC1 acceptor, by dehydrocondensation with an organosilicon hydride or by cleavage of a silicon—carbon bond in 2-furyl-, 2-thienyl-, benzyl-, or allylsilanes in the presence of an alkali metal catalyst (262—266). N-Silylated aziridines can react with carboxylic anhydrides to give acylated aziridines, eg, A/-acetylaziridine [460-07-1] in high yields (267). At high temperatures, A/-silylaziridines can be dimerized to piperazines (268). Aldehydes can be inserted... 

Hydrosilatrane (49) reacts readily with alcohols and phenols in boiling xylene (equation 62). The process is catalyzed by sodium alkoxides or phenoxides304. As the acidity of the phenols decreases, the dehydrocondensation rate increases. An opposite tendency is observed for nucleophilic substitution by alkoxide ions. In this case the steric effect of the bulky alcohol plays a more important role than the electronic effect in governing the reaction rate. 

The dehydrocondensation of hydridsilatrane with alcohols, phenols or organic acids can yield corresponding alkoxy-, aroxy- or acyloxysilatranes (in the presence of alkali or ZnCI2) ... 

In 1957, Anderson found that perfluoroalkanoic acids dehydrocondense with Et3GeH without catalyst to give Et3GeOOCR (R = CF3, C2F5, C3F7), whereas the reaction did... 

In 1962, Lesbre and Satge360 discovered that the dehydrocondensation reaction of trialkylgermane and carboxylic acids could be catalyzed by copper powder. For instance, the reaction of Bu3GeH and MeCOOH gave B GeOOCMe in 60% yield. 

In 1961, Satge276 carried out the dehydrocondensation of Et3GeH and PI1SO3H, which resulted in Et3Ge0S02Ph. 

By dehydrocondensation of B(OH)3 with Et3GeH in 1962 Lesbre and Satge360 obtained tris(triethylgermyl)borate (Et3GeO)3B. 

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