Question

Gauche effect full explanation with example ​

Answer 1

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The term "gauche" refers to conformational isomers (conformers) where two vicinal groups are separated by a torsion angle of 60°. IUPAC defines groups as gauche if they have a "synclinal alignment of groups attached to adjacent atoms".

In stereochemistry, gauche interactions hinder bond rotation. For example, sighting along the C2–C3 bond in staggered butane, there are two possible relative potential energies. The two methyl groups can be in an anti-bonding relationship, or offset at sixty degree dihedral angles. In the latter configuration, the two methyls are said to be in a gauche relationship, and the relative potential energy of each methyl-methyl gauche interaction is 0.9 kilocalories per mole (4 kJ/mol). In general a gauche rotamer is less stable than an anti-rotamer.

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Answer 2

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Substituted alkanes

In compounds of the generic structure, X-C-C-Y, where X and Y are both electronegative groups, the balance of conformational preference shifts from app to gauche

This is now due to the orbital overlap (effect 1 in theory) starting to dominate over the other two controlling factors. The relative energies of the σ-orbitals and σ*-orbitals of C-X or C-Y bonds have the following order:

⇐ small σ-σ* gap | large σ-σ* gap ⇒

⇐ Large E(2) | Small E(2) ⇒

low energy acceptor: σ*C-F < σ*C-Cl < σ*C-O ≈ σ*C-+NMe3 < σ*C-C≡N < σ*C-N < σ*C-C < σ*C-H < σ*C-Si high energy acceptor:

high energy donor: σC-Sn > σC-Si > σC-H > σC-C > σC-N > σC-O > σC-Cl > σC-F low energy donor

Thus vertical E(2) values on the left hand side of this list, for which the σ-σ* energy gap is small, are large whilst those on the rhs are much weaker:

Examples: 1,2-Difluoroethane

gauche anti-periplanar (app)

two σC-H/σ*C-F interactions, E(2) = 4.9

two σC-H/σ*C-H, E(2) = 2.6

two σC-F/σ*C-H E(2)= 0.8

=16.6 kcal/mol in total

four σC-H/σ*C-H, E(2) = 2.5

two σC-F/σ*C-F, E(2) = 1.8

= 13.6 kcal/mol in total

The app is thus stabilized by ~3.0 kcal/mol less than the gauche.

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