Some Results with Unsaturated Molecules

In this section we shall consider a few results of work on the exchange of unsaturated molecules under conditions where the rate of addition of hydrogen or deuterium is negligible compared with the rate of exchange i.e., the molecules are behaving in a way similar to the behavior of saturated molecules. There is not suflBcient space to discuss the important and extensive work involving the simultaneous exchange and deuteration of un-saturated molecules, but this subject has been reviewed by Taylor (16). 

Flanagan and Rabinovitch were able to establish another point of general interest. They deduced from the relative rates of exchange and isomerization of rans-d2-ethylene that there is an isotope effect in the rupture of the carbon- hydrogen bond when adsorbed ethyl radicals dissociate to form adsorbed ethylene molecules. The ratio of the rupture probabilities of C—H and C—D decreased from 15.9 at —78° to 1.4 at 429°. More evidence of this kind would obviously be valuable because it suggests that some revision may be necessary of the theory for calculating initial distributions of 

---

The Michael addition mechanism, whereby sulfur nucleophiles react with organic molecules containing activated unsaturated bonds, is probably a major pathway for organosulfur formation in marine sediments. In reducing sediments, where environmental factors can result in incomplete oxidation of sulfide (e.g. intertidal sediments), bisulfide (HS ) as well as polysulfide ions (S 2 ) are probably the major sulnir nucleophiles. Kinetic studies of reactions of these nucleophiles with simple molecules containing activated unsaturated bonds (acrylic acid, acrylonitrile) indicate that polysulfide ions are more reactive than bisulfide. These results are in agreement with some previous studies (30) as well as frontier molecular orbital considerations. Studies on pH variation indicate that the speciation of reactants influences reaction rates. In seawater medium, which resembles pore water constitution, acrylic acid reacts with HS at a lower rate relative to acrylonitrile because of the reduced electrophilicity of the acrylate ion at seawater pH. 

Nitrogen dioxide, NO2. This is also an odd-electron molecule, and some of its reactions resemble those of a free radical, for example, its dimerization, its power of removing hydrogen from saturated hydrocarbons, and its addition reactions with unsaturated and aromatic hydrocarbons. A microwave study gives N—O, 1-197 A and the 0—N—O angle, 134° 15, in agreement with the results of earlier electron diffraction and infrared studies. 

The reactions can either be centered at any of the atoms of the CNO moiety (Scheme 3), where the substituent groups are omitted for clarity), leading to oxime (or oximate), imine or other types of complexes (in some cases with metallacycles and in others with formation of unsaturated organic N,0-products) or occur at another part of the oxime molecule. In fact, coordination does not always lead to the activation of the oxime functional group, but instead a protecting effect can result as observed (see Section 1.29.3.6) in the chlorination of some salicylaldoxime Pt11 complexes which occurs exclusively at the metal and at the benzene ring (ortho and para positions) of the oxime in contrast with the known reactivity of free oximes. 

Recent advances in computational chemistry have made it possible to calculate enthalpies of formation from quantum mechanical first principles for rather large unsaturated molecules, some of which are outside the practical range of combustion thermochemistry. Quantum mechanical calculations of molecular thermochemical properties are, of necessity, approximate. Composite quantum mechanical procedures may employ approximations at each of several computational steps and may have an empirical factor to correct for the cumulative error. Approximate methods are useful only insofar as the error due to the various approximations is known within narrow limits. Error due to approximation is determined by comparison with a known value, but the question of the accuracy of the known value immediately arises because the uncertainty of the comparison is determined by the combined uncertainty of the approximate quantum mechanical result and the standard to which it is compared. 

Cross-metathesis of two different alkenes to give an acyclic alkene is complicated by the possible formation of not only the desired cross-metathesis product, but also self-metathesis products, each as a mixture of alkene isomers. However, some alkenes are amenable to efficient cross-metathesis to give the desired substituted alkene. This is particularly the case with alkenes that are slow to homod-imerize, such as a, -unsaturated carbonyl compounds or alkenes bearing bulky substituents. Hence, cross-metathesis of methyl acrylate with an alkene proceeds efficiently (2.116). The ruthenium catalyst reacts preferentially with the more electron-rich alkene 98, which then undergoes cross-metathesis with the acrylate or self-metathesis with another molecule of the alkene 98. The latter reaction is reversible and hence a high yield of the desired substituted acrylate results over time. The use of 1,1-disubstituted alkenes as partners in cross-metathesis provides a route to trisubstituted alkenes. This chemistry is therefore a useful alternative to conventional syntheses of alkenes, such as by the Wittig reaction. 

---