Isobutane, reaction with methylene

Propane and cyclopentane give isopropyl chloride and cyclopentyl chloride, respectively, whereas isobutane is transformed to ferf-butyl chloride under the same reaction conditions (yields are 69%, 74%, and 76%, respectively). Neopentane undergoes isomerization to yield 2-chloro-2-butane (88%). When saturated, hydrocarbons were allowed to react with methylene bromide and SbF5 bromoalkanes were obtained in comparable yields (64-75%). Formation of the halogenated product can be best explained by the mechanistic pathway (I) depicted in Scheme 5.55. Since SbF5 always contains some HF, mechanism (II) may also contribute to product formation (Scheme 5.55). 

CARBENE. The name quite generally used for the methylene radical, CH,. It is formed during a number of reactions. Thus the flash photochemical decomposition of ketene (CH2=C=0) has been shown to proceed in two stages. The first yields carbon monoxide and CHj. the latter then reacting with more ketene to form ethylene and carbon monoxide. Carbcne reacts by insertion into a C- H bond to form a C-CH, bond. Thus carbene generated from ketene reacts with propane to form, i-butane and isobutane. Carbene generated by pyrolysis uf diazomethane reacts with diethyl ether to form ethylpropyl ether and ethylisopropyl ether. 

It will now be instructive to examine the n-butane reaction (76). In this case the reaction follows almost exclusively a single path leading to the formation of sec-butyl radicals. The percentage of the quenching done by the two methylene groups is very nearly the same as that for the tertiary C-H bond in isobutane (i.e. >90%). However, the primary yield of w-butyl radicals ( 2%) from w-butane is decidedly less than that for isobutyl radicals ( " 14%) from isobutane. This behavior can be readily interpreted on the basis of a cyclic transition-state structure, but not with an open-chain transition state. For the two reaction sequences, we may write ... 

Using a similar technique Priola and coworkers (S) studied the reaction between t-butyl chloride and EtjAl, Et2AlCl, EtAlClj and AICI3 using methylene chloride and methyl chloride solvents at —78° C, for 2 h. The results of this study can be summarized as follows 1. The major reaction products are isobutane and 2,2-dimethylbutane in reactions where EtsAl is used. 2. Other products such as 23-dimethyl-butane, isooctane and l-chloro-33 -dimethylbutane are also formed in amounts strictly dependent on the molar ratios of t-BuCl/alkylaluminum or the chlorine content of the alkylaluminum. 3. When EtjAlO or EtAlCl2 react with f-BuCl, the product consists of branched Q, Cg, Cg hydrocarbons and a higher alkyl chloride. 4. Interestingly, AlClj/does not react with t-BuCl, however, it yields a crystalline complex at —78° in the absence of an added olefln. 

An insertion-type mechanism for the S( Z)) atom reaction is further supported by the indiscriminative nature of the attack on the various paraffinic C—H bonds. For the propane and isobutane molecules it was found that the two possible isomeric mercaptans were formed in a statistical ratio, i.e., n-PrSH/iso-PrSH 5 3.0, and iso-BuSH/f-BuSH iii 9.0. From methylene chemistry it is well known that singlet CH2 in its insertion reactions may be completely indiscriminative, but the abstractive attack of free radicals upon C—H bonds is known to be very sensitive to bond order and strongly increases in the sequence 0° < 1° < 2° < 3°. Insertive interactions with C—H bonds have also been postulated for singlet and triplet carbon atoms, NH, C2O radicals, and recently for the reactions of 0( Z)) atoms with alkanes. ... 

A similar increase in reactivities in the methyl-methylene-methine series is found in the free-radical oxidations of lower alkanes with oxygen in the presence of hydrogen bromide as an initiator of the reaction. Ethane gives a 64% yield of acetic acid at 220 °C, propane gives a 72% yield of acetone at 189 °C, and isobutane gives a 69.5% yield of terf-butyl hydroperoxide, a 10% yield of fm-butyl alcohol, and a 6% yield of di-rm-butyl peroxide at 163 °C [54],... 

These results are at some variance with those of MacKay and Wolfgang (1962) who see no phase effect in acetylene yield and only a small effect on ethylene in comparing gaseous and liquid isobutane. They do, however, note that stabilization of the Cg build-up products may be going on since they observe an increase in both isopentane and 2-methyl-3-butene. The isopentane yield is probably enhanced because of stabilization of the methylene-insertion products. They also found a 25% decrease in acetylene from ethylene oxide on going from the gas to the solid which they ascribed to reaction of the intermediate with the cage. Ethylene in the gas, liquid, and solid phases was also studied. 

---