Carbon-hydrogen fission

Olah, continuing his studies on electrophilic reactions at sigma bonds, has described the chlorination (carbon-hydrogen fission) and chlorolysis (carbon-carbon fission) of alkanes. Successive reaction of a thiol with chlorocarbonyl sulphenyl chloride and triphenylphosphine effects conversion into the corresponding alkyl chloride (Scheme 136). 

All the oxidants convert primary and secondary alcohols to aldehydes and ketones respectively, albeit with a great range of velocities. Co(III) attacks even tertiary alcohols readily but the other oxidants generally require the presence of a hydrogen atom on the hydroxylated carbon atom. Spectroscopic evidence indicates the formation of complexes between oxidant and substrate in some instances and this is supported by the frequence occurrence of Michaelis-Menten kinetics. Carbon-carbon bond fission occurs in certain cases. 

Further studies on 1,3-dipolar addition reactions of diazophosphonates have been recorded,122 and work on 2-diazo-l-hydroxyalkylphosphonates also continues.123 The ester (155 R = H) reacts with esters of acetylenedicarboxylic acid without liberation of nitrogen to give stereoisomeric C-phosphorylated pyrazolines, which can be decomposed with both phosphorus-carbon and carbon-carbon bond fission, affording mixtures containing dimethyl acetylphosphonate, dimethyl hydrogen phosphonate, and tri(alkoxycarbonyl)pyrazolines. In the reaction between the same diazophosphonate and diazomethane, the latter conceivably acts as a basic catalyst for proton transfer in a series of steps which includes phosphonate-phosphate isomerization. The importance of a labile proton is demonstrated by the fact that the ester (155 R = Me) does not react in the manner described above. 

Next to the isotope effects of hydrogen the most studied element has been carbon where the effects range as high as 15% with " C measurements of radioactivity are inherently of lower precision than mass spectrometric measurements making C the preferable isotope. Results from isotope effects on carbon can be most informative as the majority of organic reactions involve carbon bond fission. 

Kossiakov and Bice 2) have shown that the energy of activation for the fission of a carbon-hydrogen bond is least at a tertiary carbon atom, the energy for this case being 2 kcaJ. less than for a secondary carbon atom and 4 kcal. less than for a primary carbon atom. It would therefore seem that, for work of this type, hydrocarbons containing one or more tertiary carbon atoms would be most suitable for study. Davies and Elton (S) showed that optically active 2-phenylbutane imdergoes racemization when adsorbed on charcoal, homolysis occurring at the tertiary carbon atom. 

Although the C—C—C bond angles in the C5 ring are close to that for tetrahedral carbon, the fission of a C—C bond by hydrogenolysis occurs more readily than that of linear alkanes. Reaction takes place at lower temperatures and with lower activation energies, and the order of reaction in hydrogen (for methylcyclopentane) is positive, where for an acyclic alkane it would be negative. This is... 

Aquation of [Co(NCO)(NH3)5] + is also acid catalysed, indeed rates are proportional to hydrogen ion concentration. The products are [Co(NH3)6] + and carbon dioxide, so here the mechanism involves not cobalt-nitrogen but rather intraligand nitrogen-carbon bond fission. ... 

The use of larger particles in the cyclotron, for example carbon, nitrogen or oxygen ions, enabled elements of several units of atomic number beyond uranium to be synthesised. Einsteinium and fermium were obtained by this method and separated by ion-exchange. and indeed first identified by the appearance of their concentration peaks on the elution graph at the places expected for atomic numbers 99 and 100. The concentrations available when this was done were measured not in gcm but in atoms cm. The same elements became available in greater quantity when the first hydrogen bomb was exploded, when they were found in the fission products. Element 101, mendelevium, was made by a-particle bombardment of einsteinium, and nobelium (102) by fusion of curium and the carbon-13 isotope. 

In mechanism (8.43) the bridgehead hydrogens of barrelene should be found at the a positions of semibullvalene (2a, 0/3,0y). Mechanism (8.44) can give three different hydrogen-label distributions. If the final bond formation is concerted with bond fission, and bond fission and formation take place at the same carbon atom [mechanism (8.44A)], the label distribution should be (la, 0/3, ly). If bond formation is concerted with bond fission but with a preference for bond formation at the carbon allylic to bond fission [mechanism (8.44B)], the label distribution should be (2a, 0/3, Oy). If there is a symmetric allylic biradical which has a finite existence [mechanism (8.44AB)], then the hydrogen-label distribution should be (1.5a, 0/3,0.5y). 

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