Hexane reactions with hydrogen

On catalytic hydrogenation over a rhodium catalyst the compound shown gave a mixture containing as 1 ten butyl 4 methylcyclohexane (88%) and trans 1 ten butyl 4 methylcyclo hexane (12%) With this stereochemical result in mind consider the reactions in (a) and (b)... 

Reaction of various aldehydes with hydrogen sulfide leads to substituted thiophenes, dihydrothiophenes, dithiolanes and trithiolane, as well as to six-membered ring thiopyran derivatives and dithiins. Ledl (33) obtained 2,4-dimethylthiophene (1, R Me) as a product of the reaction of propionaldehyde with hydrogen sulfide in the presence of ammonia. Sultan (29) reported the formation of 2,4-diethylthiophene (1, R - Et), 2,4-dibutyl-thiophene (1, R - Bu), and their dehydro derivatives from the reaction of ammonium sulfide with butyraldehyde and caproaldehyde (hexanal), respectively. The mechanism suggested for their formation is depicted in Scheme 1. Space limitations do not allow us to discuss the mechanism here in detail (for additional information, see ref. 29). 

A 5000-ml pressure-resistant reactor was charged with the step 3 product (0.036 mol) dissolved in 200 ml THF and 10% palladium on carbon (2.8 g). The reactor was then pressurized to 0.4 MPa with hydrogen and the reaction performed at 60°C for 3 hours. The mixture was then filtered with Celite , concentrated, and the cis and trans isomers isolated in 95%. The isomers were then recrystallized in 100 ml hexane and the trans isomer isolated. The filtrate was concentrated, treated with t-butoxy potassium (0.25 mol), and 300 ml A. A -dimcthylformamidc and then refluxed at 100°C for 6 hours to convert the cis isomer to the trans isomer. The reaction was stopped by the dropwise addition of 500 ml water and the material purified according to the step 1 procedure. After recrystallization 11.51 g of product was isolated. 

In the reactions with aliphatic hydrogen-containing compounds such as hexane, and halogen compounds such as carbon tetrachloride, hydrogen and halogen respectively are abstracted from these compounds these are reactions characteristic of active free radicals. 

To confirm that structure I was in fact the cause of the off-odor, compound I was produced synthetically by a reaction of hydrogen sulfide with mesityloxide. Also the off-odor from the problem ham was first extracted with methanol and hexane and then concentrated. Subsequent study with GC showed the synthesized compound I (Fig. 13-4), whose structure was confirmed by MS, had the same FID retention time as the substance producing the off-odor at the end of the GC column identified by sniffing the ham extract. With this result the identity of the cat-like off-odor was explained, but not the mechanism of formation. 

Xu and co-workers examined 3,3, 4,4, 5-pentachlorobiphenyl [59] and eight di- thru hexachlorobiphenyls [61] in hexane. Masazaki et al. studied all 209 PCB congeners in decane [60], a real tour de force, while Wang and co-workers examined 22 congeners in hexane [62,63]. In all cases the photoreactions were pseudo first order, the dominant reaction was reductive dechlorination in which chlorine is replaced with hydrogen (the fate of the chlorine is unknown) ortho chlorines (2,2, 6,6 ) were removed faster than meta (3,3/,5,5 ) and para (4,4 ) chlorines, probably because the homolysis of an ortho C - Cl bond is exothermic while that at the meta and para sites is endothermic, and in a few instances, migration of a chlorine atom from one site to another on the biphenyl was observed. 

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