Strain, angular interaction

The increased stabilization upon thienoannulation as compared to benzo-annulation may be caused by the more effective stabilization of the positive charge, by diminished angular ring strain, and by limited peri interactions between hydrogen atoms (due to the heteroatoms) and hence by nearly perfect planarity. 

The following conformations, free from angular strain, can be postulated for bicyclo[3.3.1]nonane itself chair-chair, CC (2a), chair-boat, CB (2b) boat-boat, BB (2c). Although conformations 2a-2c are free from angular strain, none is free from strong destabilizing interactions between nonbonded atoms. In order to discuss the experimental and computational data more systematically, it is necessary to consider the factors that stabilize and destabilize conformation 2a-2c. 

For the unsubstituted diazabicyclo[3.3.1]nonane carbohydrate backbone there are three stable strain energy minimized conformations cc, cb, and bb (see Chart 9). While aU three conformations are free from angular strain, each is to some extent destabilized by nonbonded interactions. 

Any ring tends to adopt a conformation in which the sum of all possible sources of strain is minimized. Thus, 5-membered rings, as shown above, adopt the envelope (Cs) or twisted conformation (C2). This increases the angular strain in comparison with a planar conformation, but decreases interaction between adjacent hydrogen atoms (eclipsed in the planar conformation). 

The interaction and angular strain energies considered so far have been derived from heats of combustion, i.e. heats of oxidation to carbon dioxide and water. Such data are not always available for okfinic compounds, but a considerable amount of work has been carried out on the heats of Iwdrogenation of these unsaturated compounds. The products of such reactions, the hydrogenated compounds, generally differ in each case. Hero the heat of hydrogenation is a measure of the difference between the heats of formation of the saturated and unsaturated compounds, as shown in Fig. 11. 

These examples show how heats of combustion and of hydrogenation of isomeric compounds, or of a homologous series, can be used to determine the relative strain energies which exist in the compounds. Strain energies can be divided, somewhat artificially, but usefully into three different types angular strain, due to compression of the angle normal to the particular type of carbon hybridization interaction strain, due to repulsion between non-bonded atoms and, finally, torsional strain of double bonds out of the plane where the greatest orbital overlap can occur. 

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