Strain in molecules

Conformations in which the torsion angles between adjacent bonds are other than 60° are said to have torsional strain. Eclipsed bonds produce the most torsional strain staggered bonds none. Because three pairs of eclipsed bonds are responsible for 12 kJ/rnol (2.9 kcal/rnol) of torsional strain in ethane, it is reasonable to assign an energy cost of 4 kJ/rnol (1 kcal/rnol) to each pair. In this chapter we ll learn of additional sources of strain in molecules, which together with torsional strain comprise steric strain. 

In particular, they seem to indicate that the strain in molecules 20-29 is not large enough to preclude their synthesis. The synthesis of [l.l.ljgeminane (21) and that of molecules 22, 23, and tru s-28(I) would allow one to study the properties of a bond formed by inverted carbon atoms different from the central bond in small-ring propellanes while that of tricycloheptane 20 and trans-28(II) and trans-29(I) would provide the opportunity to analyze the bond between the carbon atoms with inverted and tetrahedral configuration of substituents, respectively. 

An example of the application of molecular mechanics in the investigation of chemical reactions is a study of the correlation between steric strain in a molecule and the ease of rupture of carbon-carbon bonds. For a series of hexasubstituted ethanes, it was found that there is a good correlation between the strain calculated by the molecular mechanics method and the rate of thermolysis. Some of the data are shown in Table 3.3. 

Conformational analysis is far- simpler in cyclopropane than in any other cycloalkane. Cyclopropane s three carbon atoms are, of geometric necessity, coplanar-, and rotation about its carbon-carbon bonds is impossible. You saw in Section 3.4 how angle strain in cyclopropane leads to an abnormally large heat of combustion. Let s now look at cyclopropane in more detail to see how our orbital hybridization bonding model may be adapted to molecules of unusual geometry. 

In this chapter we explored the three-dimensional shapes of alkanes and cycloalkanes. The most important point to be taken from the chapter is that a molecule adopts the shape that minimizes its total strain. The sources of strain in alkanes and cycloalkanes are ... 

The difference in energy between the staggered and eclipsed forms is due almost entirely to the torsional strain in the eclipsed conformation. At any instant, almost all the molecules of ethane reside in the staggered conformation. 

What are the facts To measure the amount of strain in a compound, we have to measure the total energy of the compound and then subtract the energy of a strain-free reference compound. The difference between the two values should represent the amount of extra energy in the molecule due to strain. The simplest way to do this for a cycloalkane is to measure its heat of combustion, the amount of heat released when the compound burns completely with oxygen. The more energy (strain) the compound contains, the more energy (heat) is released on combustion. 

Draw- the most stable chair conformation of the following molecules, and estimate the amount of strain in each ... 

Torsional strain (Section 3.6) The strain in a molecule caused by electron repulsion between eclipsed bonds. Torsional strain is also called eclipsing strain. 

The Ag-C bonds tend to be asymmetric study of silver cycloalkene complexes shows their stability to decrease in the order C5 > C6 > C7 > C8, corresponding to relief of strain in the cyclic molecules consequent upon the lengthening of the double bond on coordination. 

An interesting anomaly is a//-rra j-l,2,3,4,5,6-hexaisopropylcyclohexane, in which the six isopropyl groups prefer the axial position, although the six ethyl groups of the corresponding hexaethyl compound prefer the equatorial position. The alkyl groups of these compounds can of course only be all axial or all equatorial, and it is likely that the molecule prefers the all-axial conformation because of unavoidable strain in the other conformation. 

Most of the organic pollutants described in the present text act at relatively low concentrations because they, or their active metabolites, have high affinity for their sites of action. If there is interaction with more than a critical proportion of active sites, disturbances will be caused to cellular processes, which will eventually be manifest as overt toxic symptoms in the animal or plant. Differences between species or strains in the affinity of a toxic molecule for the site of action are a common reason for selective toxicity. 

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