Hydrocarbons strained, bonding

The relative reactivities toward diimide cover a range of -10, from 1,2-dimethylcyclohexene to norbornene (ref. 6). Electron attractive substituents increase the reactivity of the double bond towards diimide although the data to place compounds such as maleic acid or acrylonitrile on the scale for Garbisch s hydrocarbons is lacking (ref. 21b). Garbisch et al. found that the main factors that contribute to the observed reactivities in diimide reductions of unsaturated hydrocarbons, eqn. (3), are torsional strain, bond angle... 

One simple difference in the conformations of pure hydrocarbons and heterocyclic rings results from the fact that C-heteroatom bonds are of different lengths than C-C bonds. Bonds to O and N are shorter, often causing increased steric strain. Bonds to S are significantly longer. 

Cycloalkanes C H2 hydrocarbons n = 3, 4 Cyclopropane, cyclobutane (strained bond angles) n = 6 Cyclohexane (most stable, chair conformation) axial (less stable) equatorial (more stable) substituent positions (4-2, 3)... 

Since these double-base proplnts consist essentially of a single phase which bears the total load in any application of force, their mechanical property behavior is significantly different from composite proplnts. In the latter formulations, the hydrocarbon binder comprises only about 14% of the composite structure, the remainder being solid particles. Under stress, the binder of these proplnts bears a proportionately higher load than that in the single phase double-base proplnts. At small strain levels, these proplnts behave in a linear viscoelastic manner where the solids reinforce the binder. As strain increases, the bond between the oxidizer and binder breaks down... 

It might be expected that the strain in cyclopropane, in which the C-C-C bond angles are distorted from 109°28 to 60°, would weaken the bonds and thus lead to an increase in the bond distance. This effect is not observed, however, the carbon-carbon distance in cyclopropane being the same as in the other hydrocarbons to within the accuracy of the investigation. There is even some small indication that the C-C distance in cyclic aliphatic hydrocarbons is slightly smaller (by about 0.01 A.) than the normal distance, the three values reported being 1.53, 1.52, and 1.53 A. 

A crystallographic scale of acidity has been developed. Measuring the mean C—H O distances in crystal structures correlated well with conventional P a(DMSO) values. An ab initio study was able to correlate ring strain in strained hydrocarbons with hydrogen-bond acidity. ... 

The strained hydrocarbon [1,1,1] propellane is of special interest because of the thermodynamic and kinetic ease of addition of free radicals (R ) to it. The resulting R-substituted [ 1.1.1]pent-1-yl radicals (Eq. 3, Scheme 26) have attracted attention because of their highly pyramidal structure and consequent potentially increased reactivity. R-substituted [1.1.1]pent-1-yl radicals have a propensity to bond to three-coordinate phosphorus that is greater than that of a primary alkyl radical and similar to that of phenyl radicals. They can add irreversibly to phosphines or alkylphosphinites to afford new alkylphosphonites or alkylphosphonates via radical chain processes (Scheme 26) [63]. The high propensity of a R-substituted [1.1.1] pent-1-yl radical to react with three-coordinate phosphorus molecules reflects its highly pyramidal structure, which is accompanied by the increased s-character of its SOMO orbital and the strength of the P-C bond in the intermediate phosphoranyl radical. 

In contrast to the series of hydrocarbons, heterocyclic analogues, even of cyclonona-l,4,7-triyne 2 (u = 3), are known. This is because heteroatom linkers more easily adopt smaller bonding angles and thereby provide some relief of overall angle strain. Since heteroatoms have different electronic properties to carbon atoms, the perceived homoconjugative and homoaromatic effects might be expressed more pronouncedly in heterocyclic [n]pericyclines. 

Diamondoids, when in the solid state, melt at much higher temperatures than other hydrocarbon molecules with the same number of carbon atoms in their structures. Since they also possess low strain energy, they are more stable and stiff, resembling diamond in a broad sense. They contain dense, three-dimensional networks of covalent bonds, formed chiefly from first and second row atoms with a valence of three or more. Many of the diamondoids possess structures rich in tetrahedrally coordinated carbon. They are materials with superior strength-to-weight ratio. 

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