A Guidebook to Mechanism in Organic Chemistry

Bonding in Carbon Compounds

Conformations are denned as different arrangements of the same
group of atoms that can be converted into one another without the
breaking any bonds.

H

% Eclipsed Staggered

The staggered conformation is likely to be the more stable of

the two for the hydrogen atoms are as far apart as they can get and

any interaction is thus at a minimum, whereas in the eclipsed con­

formation they are suffering the maximum of crowding. The long

cherished principle of free rotation about a carbon-carbon single

ttBfici is not contravened, howeve'r, as it has been shown that the

eclipsed and staggered conformations differ by only « 3 kcal/mole in

energy content and this is'srffell enough to allow their ready inter-

conversion through the agency of ordinary thermal motions at room

temperature. That such crowding can lead to a real restriction of

rotation about a carbon-carbon single bond has been confirmeTP by

the isolation of two forrik of CHBra-CHBrj, though admittedly only

a^low temperatures wnere collisions between molecules do not

provide enough energy to effect the interconversion.

(ii) Carbon-carbon double bonds

In ethylene each carbon atom is bonded to only three other atoms, two

hydrogens and one carbon. Strong a bonds are formed with these

three atoms by the use of three hybrid orbitals derived by hybridising

the Is and, this time, two only of the carbon atom's 2p atomic orbitals—

an atom will normally only mobilise as many hybrid orbitals as it has

atoms or groups to form strong a bonds with. The resultant sp^2 hybrid

orbitals all lie in the same plane and are inclined at 120° to each other

(plane trigonal orbitals). In forming the molecule of ethylene, two of the

sp^2 orbitals of each carbon atom are seen as overlapping with the Is

orbitals of two hydrogen atoms to form two strong a C—H bonds,

while the third sp^2 orbital of each carbon atom is used to form a strong

a C—C bond between them.