Steric interference

Trivalent ( classical carbenium ions contain an sp -hybridized electron-deficient carbon atom, which tends to be planar in the absence of constraining skeletal rigidity or steric interference. The carbenium carbon contains six valence electrons thus it is highly electron deficient. The structure of trivalent carbocations can always be adequately described by using only two-electron two-center bonds (Lewis valence bond structures). CH3 is the parent for trivalent ions. 

In the foregoing discussions of theoretical models and experimental results, we have focused on linear polymers. We have seen the effect of chain substituents on viscosity. All other things being equal, bulky substituents tend to decrease f and thereby lower 17. The effect is primarily due to the opening up of the liquid because of the steric interference with efficient packing arising from the substituents. With side chains of truly polymeric character, the picture is quite different. 

If the a position of an enamine is carbon substituted, providing the possibility of an isomeric enamine, and if the amine group and other substituent groups are sufficiently removed from the sites of electrophilic attack as to not cause any steric interference, then simple alkylation of an enamine by an acrylate ester can be followed with a second electrophilic... 

The Diels-Alder reactions of the methyl or ethyl ester of benzenesulfonylindole-2-acrylic acid with several l-alkoxycarbonyl-l,2-dihydropyridines are reported and only a single stereoisomer was obtained, as in the case of l-methoxy(ethoxy)-carbonyl-1,2-dihydropyridines. However, when the Diels-Alder reaction of 17 was carried out with 8g[R = (CHsjsC], a mixture of two stereoisomers 18gand25were obtained in a 1 1 ratio (65% total yield). The bulky rerr-butyl group creates sufficient steric interference with the indole ring to cause the loss of stereochemistry ... 

Cis alkenes are less stable than their trans isomers because of steric strain between the two larger substituents on the same side of the double bond. This is the same kind of steric interference that we saw previously in the axial conformation of methylcyclohexane (Section 4.7). 

The transfer of chirality from sulfur to carbon as well as the high stereoselectivity were explained by a preference of rearrangement through transition state 11a over lib due to steric interference between the p-tolyl and a-alkyl group. 

Not only are steric interferences intolerably severe in the fully extended configuration XI it is usually impossible to eliminate them altogether through bond rotation, as may be seen from models. Even when the substituents are no larger than methyl, as in polyisobutylene (X = Y=CH3), steric interferences are so great as to preclude construction of a scale model for any configuration of the chain if the normal C—C and C—H distances and the usually accepted van der Waals radii are used. If Stuart models are used, in which the van der Waals... 

The continuity of the polymer chain brings about a crowding of successive substituents of head-to-tail vinyl polymers which is not apparent in formulas as ordinarily written but is clearly shown by scale models. Steric interferences between substituents in polymers made up of disub-... 

Steric effects play a dominant role with more highly substituted dienes. Hexachloro-cyclopentadiene, for example, shows a higher endo preference than cyclopentadiene because the 5-chlorine causes steric interference with exo substituents.13... 

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