Strain, torsional

The principles of conformational analysis were established by Barton, D. H. R. Experientia 1950, 6,316. For general references, see reference 2a and Dauben, W. G. Pitzer, K. S. in Newman, M. S., Ed. Steric Effects in Organic Chemistry John Wiley Sons New York, 1956 pp. 1-60. 

Except for explicit references to free energy, in this discussion we will use the word energy to mean enthalpy. 

A very different explanation was put forward by Pophristic and Goodman. They proposed that the rotational barrier in ethane results not from steric destabilization of the eclipsed conformation but, instead, from stabilization of the staggered conformation arising from delocalization of the cr bonding electrons. We will discuss the mechanism of this proposed stabilization in Chapter 4, but there is one point to be made here. When observable physical properties such as torsional energy are attributed to nonobservables—that is, to concepts such as steric effects that are inherently associated with other nonobservables such as molecular orbitals—it is difficult to establish the origin of the physical property 

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Conformations m 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/mol (2 9 kcal/mol) of torsional strain m ethane it is reasonable to assign an energy cost of 4 kJ/mol (1 kcal/mol) to each pair In this chapter we 11 learn of additional sources of strain m molecules which together with torsional strain comprise steric strain... 

At Its most basic level separating the total strain of a structure into its components is a qualita tive exercise For example a computer drawn model of the eclipsed conformation of butane using ideal bond angles and bond distances (Figure 3 8) reveals that two pairs of hydrogens are separated by a distance of only 175 pm a value considerably smaller than the sum of their van der Waals radii (2 X 120 pm = 240 pm) Thus this conformation is destabilized not only by the torsional strain associ ated with its eclipsed bonds but also by van der Waals strain... 

Cyclobutane has less angle strain than cyclopropane and can reduce the torsional strain that goes with a planar geometry by adopting the nonplanar puckered confer matron shown m Figure 3 11... 

Neighboring C—H bonds are eclipsed in any planar cy cloalkane Thus all planar conformations are destabi lized by torsional strain... 

A second but much less stable nonplanar conformation called the boat is shown in Eigure 3 14 Like the chair the boat conformation has bond angles that are approximately tetrahedral and is relatively free of angle strain It is however destabi hzed by the torsional strain associated with eclipsed bonds on four of its carbons The... 

FIGURE 3 14 (a) A ball and spoke model and (b) a space filling model of the boat confor mation of cyclohexane Torsional strain from eclipsed bonds and van der Waals strain involving the flagpole hydrogens (red) make the boat less stable than the chair... 

Torsional strain destabilization that results when adjacent atoms are not staggered... 

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

Neither conformation suffers torsional strain because each has a stag gered arrangement of bonds The gauche conformation is less stable because of van der Waals strain involving the methyl groups... 

Cyclopropane is planar and destabilized by angle strain and torsional strain Cyclobutane is nonplanar and less strained than cyclopropane... 

Section 3 7 Three conformations of cyclohexane have approximately tetrahedral angles at carbon the chair the boat and the skew boat The chair is by far the most stable it is free of torsional strain but the boat and skew boat are not When a cyclohexane ring is present m a compound it almost always adopts a chair conformation... 

Angle strain is the main source of strain in epoxides but torsional strain that re suits from the eclipsing of bonds on adjacent carbons is also present Both kinds of strain are relieved when a ring opening reaction occurs... 

Torsional strain (Section 3 1) Decreased stability of a mole cule associated with eclipsed bonds trans (Section 3 11) Stereochemical prefix indicating that two substituents are on opposite sides of a ring or a double bond (Contrast with the prefix cis )... 

A chiral axis is present in chiral biaryl derivatives. When bulky groups are located at the ortho positions of each aromatic ring in biphenyl, free rotation about the single bond connecting the two rings is inhibited because of torsional strain associated with twisting rotation about the central single bond. Interconversion of enantiomers is prevented (see Fig. 1.16). 

The torsional strain is a sinusoidal function of the torsion angle. Torsional strain results from the barrier to rotation about single bonds as described for ethane on p. 56. For molecules with a threefold barrier such as ethane, the form of the torsional barrier is... 

In cyclic systems such as 1, the dominant conformation is the one with the maximum anomeric effect. In the case of 1, only conformation lA provides the preferred antiperiplanar geometry for both oxygens. Antiperiplanar relationships are indicated by including lone pairs in the oxygen orbitals. Other effects, such as torsional strain and nonbonded repulsion, contribute to the conformational equilibrium, of course. Normally, a value of about 1.5 kcal/mol is assigned to the stabilization due to an optimum anomeric interaction in an acetal. 

Torsional strain refers to the component of total molecular energy that results from nonoptimal arrangement of vicinal bonds, as in the eclipsed conformation of ethane. The origin and stereoelectronic nature of torsional strain were discussed in Section 1.1.1. The... 

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. 

Conversely, processes which convert carbons to sfp- carbons are more favorable for five-membered than for six-membered rings. This can be illustrated by the data for acetolysis of cyclopentyl versus cyclohexyl tosylate. The former proceeds with an enthalpy of activation about 3kcal/mol less than the latter." A molecular mechanics analysis found that the difference was largely accounted for by the relief of torsional strain in the cyclopentyl case." Notice that there is an angle-strain effect which is operating in the opposite direction, since there will be some resistance to the expansion of the bond angle at the reaction center to 120° in the cyclopentyl ring. 

The preference for endo attack in 7,7-dimethylnorbomene is certainly steric in origin, with the 7-methyl substituent shielding the exo direction of approach. The origin of the preferred exo-attack in norbomene is more subject to discussion. A purely steric explanation views the endo hydrogens at C—5 and C—6 as sterically shieldihg the endo approach. There probably is also a major torsional effect Comparison of the exo and endo modes of reproach shows that greater torsional strain develops in the endo mode of... 

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