Strain in small rings

In both the above examples the conversion takes place easily because of the release of strain in small rings. 

Because cyclopentane and cyclobutane (Sections 12-3E and 12-3F) also have nonplanar carbon rings, it is clear that the Baeyer postulate of planar rings is not correct. Nonetheless, the idea of angle strain in small rings is important. There is much evidence to show that such strain produces thermodynamic instability and usually, but not always, enhanced chemical reactivity. 

The C—C=C angle in alkenes normally is about 122°, which is 10° larger than the normal C—C—C angle in cycloalkanes. This means that we would expect about 20° more angle strain in small-ring cycloalkenes than in the cycloalkanes with the same numbers of carbons in the ring. Comparison of the data for cycloalkenes in Table 12-5 and for cycloalkanes in Table 12-3 reveals that this expectation is realized for cyclopropene, but is less conspicuous for cyclobutene and cyclopentene. The reason for this is not clear, but may be connected in part with the C-H bond strengths (see Section 12-4B). 

Bond strain in small-ring molecules generally shows itself in a displacement of deformation density peaks outside of the ring. As a consequence of the curvature of the C-C bond paths in strained molecules, the charge density is not distributed so as to maximize the force of attraction between the nuclei. As a result of this effect, the bonds are weakened in spite of the fact that in general they exhibit intranuclear distances shorter than normal bonds. 

Fragmentation reactions are particularly common in small rings. Relief of strain and the gain in stability in forming certain common fragments (such as N2, CO2), as felt in the... 

In the main, the physical and chemical properties of saturated and partially unsaturated alicyclic compounds closely resemble those of the analogous acyclic compounds formally derived by cleavage of the carbon ring at a point remote from any functionality. Relatively small, but often significant, differences in properties arise from conformational effects, and from strain effects in small rings, and these differences can be striking in properties which are particularly sensitive to molecular shape. 

Angle strain (Section 4.3) The strain introduced into a molecule when a bond angle is deformed from its ideal value. Angle strain is particularly important in small-ring cycloalkanes, where it results from compression of bond angles to less than their ideal tetrahedral values. 

Bent bonds (Section 4.4) The bonds in small rings such as cyclopropane that bend away from the internuclear line and overlap at a slight angle, rather than head-on. Bent bonds are highly strained and highly reactive. 

Strain of small ring molecules cannot be fully understood in terms of the deviation of the bond angles from the ideal ones of the hybrid orbitals. The mechaitism of the relaxation of the strain has been proposed. Here, we briefly review a relaxation by the a—kj interactions between the geminal a bonds in the rings and % relaxation by the Ji—kj interaction between the endocyclic n bond with the vicinal a bonds on the ring atoms. A more detailed review is made by Naruse and Inagaki elsewhere in this volume. 

Ring strain in small cyclic alkanes and silanes... 

Other factors which affect the case of electrocyclic ring opening include the nature of substituents which can stabilize or destabilize the development of possible charge and the release of strain in small cyclic systems. Thus different stereochemistries have been observed in the ring opening of cyclopropyl derivatives. All cis derivatives generate an all-cis allyl cation but the anti derivatives will form the all trans cation. 

The complexity of the parameter-fitting procedure in the MINDO models can only be appreciated by a detailed study of the inherent assumptions. It is perhaps indicative to say the only molecular integral that is calculated exactly from the basis atomic orbitals is the overlap integral, all others being approximated or given empirical values. The repulsion potential of the atomic cores is, for example, one of the critical functions in the theory. There has to the present time been four distinct versions of the MINDO parameterization, and the latest (MINDO/3) (102) is said to remove certain deficiencies in the earlier versions (such as the prediction that HjO was linear and the underestimation of the strain energies in small ring hydrocarbons). 

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