Torsion strain

Torsional energy and torsional strain Torsional energy is the energy required for rotating about the C-C a bond. In ethane, this is very low (only 3 kcal). Torsional strain is the strain observed when a conformer rotates away from the most stable conformation (i.e. the staggered form). Torsional strain is due to the slight repulsion between electron clouds in the C-H bonds as they pass close by each other in the echpsed conformer. In ethane, this is also low. 

It must be emphasized diat angle strain, torsional strain, and transannular strain are simply terms invented by chemists to categorize structural features in rings which lead to increased energy (strain). The molecules diemselves adopt structures which are the most stable and have die greatest amount of bonding energy. Nevertheless, strain is an important feature in molecules to keep in mind since it is manifested by altered chemical reactivity. 

Internal strain in cyclic systems 93,95) incorporates angle strain (Baeyer strain), torsional strain (Pitzer strain) and transannular strain. In the case of nitrogen inversion, large effects are expected to arise from angle strainb>, the nature of the cyclic system imposing a value of the angle 0... 

Spin-spin coupling in nmr, 242 Spontaneous reaction, 36 Stability, heat of combustion, 90 of radicals, 61 resonance, 24 Staggered conformation, 51 Stereoisomers, 69 Stereomers, conformational, 80 Stereoselective reactions, 91 Steric strain, 54, 126 Stilbene, 218 Strain, torsional, 54, 167 Structure of benzene, 189 cyclooctatetraene, 195 Substitution, ally lie, 105 in arenes, 205 in naphthalene, 213 nucleophilic aromatic, 215 ... 

The destabihzation that comes from eclipsed bonds on adjacent atoms is called torsional strain. Torsional strain is one of several structural features resulting from its three-dimensional makeup that destabilize a molecule. The total strain of all of the spatially dependent features is often called steric strain. Because three pairs of eclipsed bonds produce 12 kJ/mol (2.9 kcal/mol) of torsional strain in ethane, it is reasonable to assign an energy cost of 4 kJ/mol (1 kcal/mol) to each pair of eclipsed bonds. 

Eclipsed, Eclipsing Two ligands on adjacent atoms are eclipsed if their torsion angle is near 0° i.e., synperiplanar). See also bisecting conformation, eclipsing strain, torsion angle. 

The electrons in a C—H bond will repel the electrons in another C—H bond if the bonds get too close to each other. The staggered conformation, therefore, is the most stable conformation of ethane because the C—H bonds are as far away from each other as possible. The eclipsed conformation is the least stable conformation because in no other conformation are the C—H bonds as close to one another. The extra energy of the eclipsed conformation is called torsional strain. Torsional strain is the name given to the repulsion felt by the bonding electrons of one substituent as they pass close to the bonding electrons of another substituent. The investigation of the various conformations of a compound and their relative stabilities is called conformational analysis. 

There are a number of different modes of stress-strain that must be taken into account by the designer. They include tensile stress-strain, flexural stress-strain, compression stress-strain, torsional stress-strain, and/or shear stress-strain (Table 7.9). 

Torque (rotational shear stress) Torsional strain Torsional modulus... 

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