ACCESS. This view of a model of tertbutyl disulfide, (CH»)8C—SS—C(CH,),f shows the relatively free access to the backside of the sulfur atoms. Attack by phenyl radicals on this hindered sulfide is about equal on sulfur and on hydrogen. Phenylazotriphenylmethane was used as a phenyl radical source in this study
RESEARCH
Radicals May Cause Displacement Reactions by Walden Inversion Mechanism Rate pattern followed by radicals in S H 2 reactions similar to that followed by anions in S N 2 reactions Evidence suggesting that radicals cause displacement reactions by the Walden inversion mechanism has been obtained by Dr. William A. Pryor of Louisiana State University, Baton Rouge, and Harold Guard of Purdue University, Lafayette, Ind. They find that the rate pattern followed by radicals in homolytic displacement reactions (SH2 reactions) is similar to the pattern followed by anions in nucleophilic displacement reactions (SN2 reactions). The SN2 reaction occurs by Walden inversion, Dr. Pryor says. The mechanism was discovered by Paul Walden, a Russian chemist, during the 1890's. In this mechanism, the entering anion attacks a molecule from the backside (the side opposite to that from which the displaced group leaves). This gives a transition state 38
C & E N MAR. 3 0,
1964
in which the attacking anion, the central carbon atom, and the leaving group lie in a straight line. The central carbon is inverted in this process. If the original molecule is optically active, this inversion changes the molecule from the D to the L configuration. However, the SN2 reaction is generally thought to involve the Walden inversion regardless of whether the starting material is optically active, according to Dr. Pryor. This mechanism leads to characteristic rate profiles for reactions of similarly substituted compounds. The rate profiles can be used to identify the Walden inversion mechanism in compounds that are not optically active. Although the SN2 reaction has been known for many years to be a backside displacement, Dr. Pryor
points out that there has not been an unambiguous demonstration that radicals also cause displacements by this mechanism. Theoretical calculations made about 30 years ago by Dr. Henry Eyring (while at Princeton University and now at the University of Utah) and others suggested that the SN2 and the SH2 reactions involve similar transition states. Attempts to demonstrate this have failed, though, because competing hydrogen or halogen abstractions by radicals are faster than is attack on carbon atoms, he notes. Previous work by Dr. Richard Noyes of the University of Oregon on alkyl iodides indicated that SH2 reactions might occur by a backside displacement mechanism. However, his research and that done by Dr. Sidney W. Benson of Stanford Research Institute show that alkyl iodides react with radicals largely by iodine abstraction, rather than by an SH2 process at a carbon atom. S—S Bond. The very high reactivity of the sulfur-sulfur bond toward free radicals suggested to Dr. Pryor that a study of the SH2 reaction with disulfides rather than alkyl iod'des might be possible. He planned therefore, to try to relate the rate profile of the SH2 reaction on disulfides to the SN2 reaction on the same compounds (since the SN2 reaction on disulfides occurs by a Walden inversion mechanism). Dr. Pryor uses the reactions of phenyl radicals (generated from phenylazotriphenylmethane) with aliphatic disulfides for his studies [/. Am. Chem. Soc, 86, 1150 (1964)]. Two reactions occur. In one, the phenyl radical attacks a hydrogen of the disulfide to form benzene: C6H5 + RSSR -> C e H 5 -H + RSSR In the other reaction, the phenyl radical attacks a sulfur atom to give a phenylalkyl sulfide and a thiyl radical: CeH5· + RSSR -> C e H 5 - SR + RS This reaction is an SH2 reaction on
sulfur. It is one of the few displace ment reactions by radicals that takes place at an atom having a valence larger than one, Dr. Pryor says. These reaction rates were obtained by studying them in the presence of carbon tetrachloride. The phenyl radicals then also react by abstract ing a chlorine atom from carbon tetra chloride to form chlorobenzene: C e H 5 + CC14 -» C 6 H 5 - C 1 + CC13 The ratios of the products of these three reactions can be measured and their relative rates determined by gas chromatography. A graph of the logarithm of the rel ative rate of attack by phenyl radicals on the sulfur atom of disulfides cor relates closely with the logarithm of the rate of attack by nucleophiles on the same disulfides. Since the loga rithm of a rate constant is proportional to energy, Dr. Pryor concludes that the energetics of both the S H 2 and the S N 2 reactions on these disulfides are similar. These results strongly indi cate that both reactions involve a backside transition state (three atoms in a line). In addition, the geometry of disul fides is similar to that of alkyl halides. The two pairs of unshared elec trons on sulfur occupy positions simi lar to those occupied by two of the four groups attached to carbon, and the remaining bond angles are similar. The profile for attack by phenyl radi cals on sulfur atoms of these disul fides correlates well with the general ized pattern for the S N 2 reaction on carbon. Labile Hydrogens. Dr. Pryor also finds that the disulfide hydrogens are very labile, even though disulfides are attacked by phenyl radicals more on sulfur than on hydrogen. The alpha-
The transition states for S H 2 and S N 2 reactiens on disulfides and the S N 2 reaction on alkyl halides have similar configurations Κ. J
, Λ Λ ^
^ I N
sHZ(sumir)orhomolytic homolytic displacement
S N 2 (sulfur)-
Y.Û'S4R Ί~...:Ζ£%
Nucleophilic displacement
S N 2 (carbon)—
Nucleophilic displacement
hydrogens of disulfides are compar able in activity to benzylic hydrogens; the beta-hydrogens are much less la bile than are the alpha-hydrogens. Comparing primary hydrogens, alphahydrogens are 6.8 times more reactive than are beta-hydrogens. The reactivity of primary, second ary, and tertiary alpha-hydrogens is 1:3.2:9.1, Dr. Pryor calculates. By comparison, reactivity of benzylic al pha-hydrogens is 1:4.6:9.7, as calcu lated by Dr. G. A. Russell and R. F. Bridger of Iowa State University. Thus, hydrogen atoms located alpha to a sulfur atom are similar to benzylic hydrogens in reactivity and sensitivity to substituent effects. Rare. Quantitative parallels of steric effects are much rarer than are parallels of polar factors, Dr. Pryor says. Thus, the parallelism in the rates of the S H 2 and S N 2 reactions on disulfides has greater significance, suggesting that the two reactions have similar mechanisms. Continuing work in this area, Dr. Pryor and his group at LSU are meas uring the rate profiles for the reac tions of radicals with aliphatic perox ides. It's already clear, however, that the S—S bond has special reac tivity toward free radicals, he says. The oxygen-oxygen bond of peroxides doesn't seem to have this high reac tivity.
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BRIEFS o-Nitrophenylacetyl chloride readily explodes in air, according to Dr. Shin Hayao of the chemical therapeutics research laboratory, Miles Labora tories, Elkhart, Ind. Two explosions with the compound took place at Miles. In both instances, o-nitrophenylacetic acid was suspended in chloroform, and thionyl chloride added. The mixture was refluxed for two to three hours, and solvent re moved under vacuum (water pump). In one case, the solvent-free residue decomposed violently, Dr. Hayao says, generating a lot of black smoke and a red flame. In the other case, the residue exploded as soon as the sol vent was evaporated; the glass equip ment was destroyed. The finding is similar to the attempted distillation of o-nitrobenzoyl chloride [/. Am. Chem. Soc, 68, 344 (1946)] by Dr. W. A. Bonner and Dr. C. D. Hurd of North western University. The compound detonated violently.
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