Sec-Butyl radical

The resulting free radicals react with chlorine to give the corresponding alkyl chlorides Butyl radical gives only 1 chlorobutane sec butyl radical gives only 2 chlorobutane... 

These results are in accordance with the findings of Boothe and coworkers26, who found that the reactions of four diastereomeric 2-bromo-3-phenylsulfinylbutanes with tributyl-tin radicals generate /3-phenylsulfinyl sec-butyl radicals (8) which eliminate PhSO radicals to form the 2-butenes in a stereoselective manner. The stereoselectivities observed in this free radical elimination must result from the fact that the rate constant for elimination is greater than that for rotation about the C—C bond. Furthermore, a neighboring phenyl group on the radical center seems to stabilize the radical enough so that the internal rotation can compete with the -elimination reaction. It is also noteworthy that the small... 

Talbutal differs from butabarbital in that a sec-butyl radical is used in talbutal, whereas in butabarbital, an isobutyl radical can be used as a substituent on C,. Talbutal is used as a sedative, soporific drug for the same indications as butabital. Synonyms for this drug are profundol, lotusate, and others. 

An unambiguous proof that decarbonylation of the acyl radical takes place at least in part in the solvent cage has been obtained by the photolysis of the 3,5-di-f-butylphenyl ester of (5,)-( + )-2-methylbutanoic acid (125) in diox-ane.62 3,5-Di-/-butylphenyl-sec-butyl ether (127), isolated in 3% yield, was shown to be completely racemic. This means that the sec-butyl radical had to exist as an independent, free-rotating radical intermediate in the solvent cage. The relatively high yield of ether, the assumed lifetime of radicals 65-68, and radical concentration at the usual solution concentrations (10-3 to 10 1... 

If every collision of a chlorine atom with a butane molecule resulted in hydrogen abstraction, the -butyl/sec-butyl radical ratio and, therefore, the 1 -chloro/2-chlorobutane ratio, would be given by the relative numbers of hydrogens in the two equivalent methyl groups of CH3CH2CH2CH3 (six) compared with those in the two equivalent methylene groups (four). The product distribution expected on a statistical basis would be 60% 1-chloro-butane and 40% 2-chlorobutane. The experimentally observed product distribution, however, is 28% 1-chlorobutane and 72% 2-chlorobutane. sec-Butyl radical is therefore formed in greater amounts, and n-butyl radical in lesser amounts, than expected statistically. 

Rabinovitch et al. (85) studied the reaction of H atoms with trans-ethylene-d2 as a function of ethylene pressure in the temperature range — 78 to 160°C. They were able to account for all secondary reactions of the hot ethyl radicals and to determine the rates of their decomposition (relative to stablization). Simultaneously they calculated the theoretical rates on the basis of the Rice-Ramsperger-Kassel theory of uni-molecular reactions, using expressions derived by Marcus (71), and found a reasonable agreement with the experimental values. Similar satisfactory agreements had been found previously by Rabinovitch and Die-sen (84) for hot sec-butyl radicals. Extensive studies of hot radicals produced by H or D atom additions to various olefins have been carried... 

The Percentage of sec-Butyl Radicals Initially Formed in the Reaction of Hg 6(3Pi)... 

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 ... 

To provide a less ambiguous method for measuring an olefin scavenger/unimolecular decomposition method was devised (143). When a 1 10 mixture of H C0/cis-2-C4H8 is photolyzed at wavelengths near 300 nm, the photolytic H atoms are scavenged by cis-2-butenes to form vibrationally hot sec-butyl radicals. 

The highly excited sec-butyl radical decomposes unimolecularly with a lifetime of 1 x 10 s to give a CH3 radical and a C3H6 molecule, unless stabilized by collision with a bath molecule (M). The sequence of the relevant processes is summarized as follows ... 

The other chain is produced by the decomposition of the sec-butyl radicals (reaction 6 ) to give methyl radicals + propylene. The chain cycle consists of reactions 6 and 8 and would be represented by the over-all stoichiometry... 

We would expect from independent considerations that the bond strengths of secondary H are less than primary II in hydrocarbons, and this is in accord with the greater yields of sec-butyl radicals which would be required to give the major yields of CH4 and CsHe which are observed. 

In Sec. 7.3, we said that attachment of chlorine to either face of the sec butyl radical is equally likely. This is in effect true, but deserves closer examination. Consider any conformation of the free radical 1, for example. It is clear that attack by chlorine from the top of I and attack from the bottom are not... 

The disproportionation-combination ratio is temperature dependent. For sec-butyl radicals, this ratio is 2.3 at 375°K. (11) and 11 at 90°K. (10). Adopting the same values for pentyl radicals we calculate a ratio of 7.5 at 143°K. (melting point of n-pentane) at which recombination occurs. 

Finally, we should also note an experiment by Rabinovitch and his students [71.0] in which the thermal decomposition of the sec-butyl radical (formed in a chemical activation process) showed no variation in rate from 0.01 to 200 atmospheres of added hydrogen. ... 

One would further predict that the D atom would be transferred to one of the two CH2 groups in the cyclopropane ring to form a sec-butyl radical, rather than to the carbon atom carrying the methyl group, which would result in the formation of the thermodynamically less stable primary butyl radical. If the reaction were to begin as described here, the transfer of a second particle could then only result in the formation of CH2DCH2CH2CH3, which is the observed product. 

In the case of the sec-butyl radical, the carbon bearing the unpaired electron is sp hybridized and the unpaired electron lies in the unhybridized 2p orbital. Reaction... 

Finally, there is a chemical activation gas bulb experiment from which it has been deduced that complete energy redistribution is very rapid in sec-butyl radicals excited above the unimolecular dissociation threshold-In these experiments H atoms add to cis-2-butene to produce vibrationally excited sec-butyl radicals... 

It is the authors feeling that the above experimental result does not necessarily mean that complete intramolecular vibrational energy redistribution will occur within lO" s in an isolated sec-butyl radical. In the measurements of D/S versus pressure hydrogen is used as the quenching gas M. It is a notoriously inefficient deactivator and may promote intramolecular vibrational energy redistribution by a combination of small impact parameter inefficient collisions and large impact parameter weak collisions. 

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