Alkanes dehydrochlorination

The chlorination-dehydrochlorination method for producing n-y-olefins from the corresponding rc-alkanes is no longer of industrial importance [30]. 

Typically, the saponification is run with 10% sodium hydroxide solution in a reactor cascade at 95-98°C under stringent pH control. The saponification mixture is separated in a settler. The upper phase consists of alkanes with a small proportion of chloroalkanes, which is removed by oleum refining or dehydrochlorination and high-pressure hydrogenation. The refined alkanes can be recycled to the reactor. In the aqueous lower phase are alkanesulfonates, sodium chloride, and between 4 and 8 wt % hydrotropically dissolved alkanes. An optimal separation can be approached at 95 °C, and residence times of less than 60 min if Fe(III) ions are added and pH values of 3-5 are maintained. 

Only two alkanes have the molecular formula C4H10 butane and isobutane (2-methylpropane)— both of which give two monochlorides on free-radical chlorination. However, dehydrochlorination of one of the monochlorides derived from butane yields a mixture of alkenes. 

Roberts et al. (1993) have provided an in-depth analysis of QSARs for dehydrohalogenation reactions of polychlorinated and polybrominated alkanes. The QSARs were developed based on a dataset of 28 polychlorinated and polybrominated compounds in aqueous solution at 25° C. The QSARs are for the OH- and water-mediated second-order elimination reactions (E2), as Equation (25) shows. The first QSAR is for base-promoted dehydrochlorination and is based on the inductive parameter o, (Equations (26) and (27)) ... 

One of the most interesting recent topics in ruthenium chemistry is undoubted ) C—H bond activation in which the generation of coordinatively unsaturated specie> may play an important role. These species are usually produced by thermal or photo-mediated reductive elimination of dihydrogen, alkanes, alkenes or arenes. Recently, dehydrochlorination from RuHCI(CO) (P BuTMe) is reported to give a 7C-allyl complex via C—H activation of propylene (eq (42)) [144]. 

Annulated pyrroles. l-Dialkylamino-(n+3,n+3)-dichlorobicyclo[n.l.O]alkanes suffer dehydrochlorination, to generate, plausibly, allylic chlorocarbenes for subsequent H-abstraction, cyclization and aromatization. 

The amounts of the various isomers formed on monochlorination of alkanes depends on the relative rates at which the variously bound hydrogen atoms are replaced. Hydrogen atoms in CH3—, —CH2—, and >CH— are replaced at 300° in the proportions 1 3.25 4.43.324f Above 300° the proportion of primary chloride increases the cause of this is that dehydrochlorination, induced by chlorine, occurs preferentially at chlorinated methylene groups.330 Catalysts and irradiation do not affect the above proportions and it follows from them that the amount of terminal chloride formed must decrease continuously with increasing chain length thus, whereas propane gives 48% of 1-chloro compound, only 8.5% is formed from w-dodecane. 

The double elimination of HHal from 1,1- and 1,2-dihalogeno-alkanes to give alkynes (terminal and internal) under very mild conditions is preparatively very simple in petroleum ether, using solid KOBu and catalytic amounts of 18-crown-6 polyether.Different transition-state structures within the E2 mechanism, as well as different initial-state solvation conditions, have been proposed to rationalize the effects of equimolar amounts of crown ether and base on the dehydrochlorination of (p-ClC6H4)2CH2CH(3 x)Ch (x = 1, 2, or 3). ... 

Ring expansion of cycloalkanones. 1-Trimethylsilyloxybicyclo[n.l.0]alkanes (1), prepared by Simmons-Smith reaction with silyl enol ethers of cycloalkanones, react with ferric chloride in DMF containing pyridine to form a 3-chlorocycIo-alkanone (2) in fair to high yield. Dehydrochlorination (sodium acetate) yields a 2-cycloalkenone (3) containing one more carbon atom than the starting cycloalkanone. 

As Hodges and Garnetthad found in the case of arenes, the addition of PtCl " leads to net oxidation to give chloro-arenes or -alkanes, with reduction of Pt(IV) to Pt(II) Methane was converted to MeCH Once again, a 1°>2°>3° selectivity pattern was observed. In CF3CO2H, trifluoroacetate esters were also formed. For cyclohexane, benzene was a major product, perhaps as a result of dehydrochlorination of the chlorocarbon intermediates. 

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