Interactions eclipsing

KutMtocul barrier increases with ihe number of CH3/H eclipsing interactions. 

Changing the atom bound to a methyl group from carbon to nitrogen to oxygen, as in going from ethane to methylamine to methanol, produces a decrease in the rotational barrier from 2.88 to 1.98 to 1.07kcal/mol. This closely approximates the 3 2 1 ratio of the number of H—H eclipsing interactions in these three molecules. 

Rotational bairier decreases with the number of H/H eclipsing interactions. 

For example, cyclohexanone is reduced by sodium borohydride 23 times faster than cyclopentanone." The explanation for this difference lies in the relative torsional strain in the two systems. Converting an sp atom in a five-membered ring to sp increases the torsional strain because of the increase in the number of eclipsing interactions in the alcohol. A similar change in a six-membered ring leads to a completely staggered (chair) arrangement and reduces torsional strain. 

Using the data incorporated in Fig. 3.3 and assuming the additivity of gauche and eclipsing interactions of similar type, sketch the rotational energy profile you would expect for 2,3-dimethylbutane. 

Apply the bent-bond model to the preferred conformations of acetaldehyde and propene. Do bent-bonds maintain or remove eclipsing interactions in the equilibrium structures of the two molecules Formulate a simple rule based on the bent-bond model for predicting conformational preferences in systems containing trigonal atoms. 

Each H <— H eclipsing interaction in ethane costs about 4.0 kj/mol. Wow many such interactions are present in cyclopropane What fraction of the overall 115 kj/mol (27.5 kcal/mol) strain energy of cyclopropane is due to torsional strain ... 

Problem 4,10 How many H eclipsing interactions would be present if cyclopentane were planar Assuming an energy cost of 4.0 kj/mol for each eclipsing interaction, how much torsional strain would planar cyclopentane have Since the measured total Strain of cyclopentane is 26 kj/mol, how much of the torsional strain is relieved by puckering ... 

Cyclopropane (115 kj/mol strain) and cyclobutane (110.4 kj/mol strain) have both angle strain and torsional strain. Cyclopentane is free of angle strain but has a substantial torsional strain due to its large number of eclipsing interactions. Both cyclobutane and cyclopentane pucker slightly away from planarity to relieve torsional strain. 

Ab initio calculations on these conformers indicate that the anti geometry is 6.04 kcal/mol more stable than the syn291 Some of this energy difference is undoubtedly due to the eclipsing interaction of the hydrogens but this can not account for the total rotational barrier. Thus, the a—p+ interaction effect must be responsible for some major component of the rotational barrier. 

Amine oxide pyrolysis occurs at temperatures of 100-150° C. The reaction can proceed at room temperature in DMSO.229 230 231 232 If more than one type of [> hydrogen can attain the eclipsed conformation of the cyclic transition state, a mixture of alkenes will be formed. The product ratio parallels the relative stability of the competing transition states. Usually, more of the E-alkene is formed because of the additional eclipsed interactions present in the transition state leading to the Z-alkene. The selectivity is usually not high,... 

The spherical shape causes a pyramidalization of the C atoms and, therefore, a large amount of strain energy. Addition chemistry is driven by the strain reKef introduced by the formation of almost strain-free sp -C atoms. At a certain degree of addition this strain relief mechanism has to compete with new strain-introducing processes such as the increasing introduction of eclipsing interactions and the formation of planar cyclohexane substructures. 

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