The three carbon atoms in cyclopropane must lie in a plane since it is always possible to draw a plane through any three points. All the C–C bond lengths are the same, which means that the three carbon atoms are at the corners of an equilateral triangle. we know that there is considerable strain in this molecule. Most of this is due to the bond angles deviating so greatly from the ideal tetrahedral value of 109.5°. Most—but not all. If we view along one of the C–C bonds we can see a further cause of strain—all the C–H bonds are eclipsed.

The eclipsed conformation of ethane is an energy maximum and any rotation leads to a more stable conformation. In cyclopropane it is not possible to rotate any of the C–C bonds and so all the C–H bonds are forced to eclipse their neighbours. In fact, in any planar conformation all the C–H bonds will be eclipsed with their neighbours. In cyclobutane, the ring distorts from a planar conformation in order to reduce the eclipsing interactions, even though this reduces the bond angles further and so increases the bond angle strain. Cyclobutane adopts a puckered or ‘wing-shaped’ conformation.

This explains why cyclopentane is not entirely strain-free even though in a planar conformation the C–C–C bond angles are close to 109.5°. The heat of combustion data give us an indication of the total strain in the molecule, not just the contribution of angle strain. There is strain in planar cyclopentane caused by the eclipsing of adjacent C–H bonds. As in cyclo butane, the ring distorts to reduce the eclipsing interactions but this increases the angle strain. Whatever happens, there is always going to be some strain in the system. The mini mum energy conformation adopted is a balance of the two opposing effects. Cyclopentane adopts a shape approximating to an ‘open envelope’, with four C atoms in a plane and one above or below it. The atoms in the ring rapidly take turns not to be in the plane, and cyclo pentanes have much less well-defined conformational properties than cyclohexanes, to which we shall now return.

مواضيع ذات صلة

E1 reactions can be stereoselective

The role of the leaving group

Substrate structure may allow E1

E1 and E2 mechanisms

Size

Elimination happens when the nucleophile attacks hydrogen instead of carbon

Axially and equatorially substituted rings react differently

Steroid

Decalins

What happens with more than one substituent on the ring

Substituted cyclohexanes

The ring inversion (flipping) of cyclohexane