Propanal conformational stability

The frontier orbital interactions which favored conformation 3 in propanal also stabilize conformation 8. However, 8 is destabilized by dipole-dipole interaction, whereas conformations 9 and 10 are stabilized by other factors. Thus, the CC1 bond is highly polar and moderately strong (78 kcal mol-1), so it has a low-lying C ccl orbital. This permits a stabilizing rcco-0 ccl interaction in conformation 9. 

Combining the results for propanal and chloroethanal suggests that the conformational stability of 2-chloropropanal decreases in the order 13 < 12 < li This order has been confirmed by calculations.5... 

The magnitude of the supermolecule stabilization relative to the isolated reactants was calculated for hydride addition to acetaldehyde, propanal, and chloroacetalde-hyde, maintaining the H, CH3 and Cl substituents, respectively, in the conformation trans to the hydride ion (Scheme 6.15). The increase in the stabilization energy, from 35.5 kcal mol to 37.4 kcal mol to 59kcalmol , was interpreted as a result of the increase in the interactions of the hydride, the n (C=0) orbital, and the antiperi-planar o orbital of the substituent. 

Draw Newman projections of the staggered and eclipsed conformations of propane. Rank them in stability. 

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