Strain, eclipsing

Cyclobutane has less angle strain than cyclopropane (only 19.5°). It is also believed to have some bent-bond character associated with the carbon-carbon bonds. The molecule exists in a nonplanar conformation in order to minimize hydrogen-hydrogen eclipsing strain. 

Eclipsing strain (Section 3.6) The strain energy- in a molecule caused by electron repulsions between eclipsed bonds. Fxlipsing strain is also called torsional strain. 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS Eclipsing strain,... 

If the C s of the cyclobutane ring were coplanar, they would form a rigid square with internal bond angles of 90°. The deviation from 109.5° would not be as great as that for cyclopropane, and there would be less angle strain in cyclopropane. However, this is somewhat offset by the fact that the eclipsing strain involves four pairs of H s, one pair more than in cyclopropane. 

Cyclopentane with a planar ring would have five pairs of eclipsed H s producing considerable eclipsing strain. This strain is reduced, at the expense of some increase in angle strain, when one CHj pushes out of the plane of the ring (Fig. 9-4). The puckering is not fixed but alternates around the ring. 

Calculation of the bond angles and conformation of the cyclic phosphates by minimization of ring and eclipsing strain leads to bond angles.closely in accord... 

Knowing the importance of angle and eclipsing strain in the small-ring cycloalkanes, we should expect that these strains would become still more important in going from cyclobutane to bicyclo[1.1.0]butane or from cyclooctane to pentacyclo[4.2.0.02,5.03 8.04,7]octane (cubane). This expectation is borne out by the data in Table 12-6, which gives the properties of several illustrative smallring polycyclic molecules that have been synthesized only in recent years. 

Theoretical studies have indicated that m-bcnzync is monocyclic with a C(l)-C(3) distance of 2.0 A whereas in tetrafluoro-w-benzyne the increased eclipsing strain between fluorine atoms stabilizes the bicyclo[3.1.0]hexatriene form with a C(l)-C(3) distance of 1.75 A.56 Computational studies coupled with gas-phase experimental studies show that appropriate substituents can be used to tune the reactivity of 1,3-arynes. Thus the presence of NH+ at C(5) makes (13) mildly carbocationic whereas the addition of OH at C(4) in (14) gives a highly reactive (bi)radical.57... 

If cyclopentane were planar, it would have ten hydrogen-hydrogen interactions with a total energy cost of 40 kJ/mol. The measured total strain energy of 26 kJ/mol indicates that 14 kJ/mol of eclipsing strain in cyclopentane (35%) has been relieved by puckering. 

However, not everyone was convinced by the existence of the non-classical carbocation. H. C. Brown 1977 pointed out that the norbornyl compounds are compared with cyclopentyl rather than with cyclohexyl analogues, 2.21 (eclipsing strain), and in such a comparison the endo-isomev is abnormally slow, the exo-isomer being only 14 times faster than cyclopentyl analogues. He also pointed out that the formation of racemic product is due to two rapidly equilibrating classical carbocation species (Scheme 2.17). The interconversion of enantiomeric classical carbocation species must be very rapid on the reaction timescale. 

The BC-3 conformation for cyclooctanone is supported by recent strain energy calculations, which have already been mentioned (Section V). Qualitatively, the BC-3 conformation is also very reasonable, since the non-bonded repulsions between the 3 and 7 methylene groups in the cyclooctane boat-chair conformation are largely removed in the BC-3 form. The 1 position in the boat-chair also has the same kind of advantage that the 3 position has. However, the 3 position is also favored because of the relief of eclipsing strain which occurs in that position, but not in the 1 position (see Table 2 for dihedral angles in the boat-chair). This point will be amplified in the following discussion on methylenecyclooctane. 

Tables 3-1 through 3-3 contain data on which empirical calculations of chemical shifts can be made. The tables represent a fraction of the data available on the fundamental alkane, alkene, and aromatic structures. Moreover, corrections must be applied in order to avoid nonadditivity caused primarily by steric effects. Thus, three groups on a single carbon atom, two large groups cis to each other on a double bond, or any two ortho groups can cause deviations from the parameters listed in the tables. If sufficient model compounds are available, the corrections shown can be applied. Further empirical calculations are possible for any structural entity, so that the eclipsing strain in cyclobutanes, the variety of steric interactions in cyclopentanones, or the variations in angle strain in norbornanes may be taken into account.

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