With butane things start to get slightly more complicated. Now we have effectively replaced two hydrogen atoms in ethane by larger methyl groups. These are large enough to get in the way of each other, and steric repulsion becomes a significant contribution to the rotational energy barriers. However, the main complication is that, as we rotate about the central C–C bond, not all the staggered conformations are the same, and neither are all the eclipsed con formations. The six conformations that butane can adopt as the central C–C bond is rotated in 60° intervals are shown below. The green Me group and the brown hydrogens are rotating while the substituents on the other carbon atom remain still.

Look closely at these different conformations. The conformations with dihedral angles 60° and 300° are actually mirror images of each other, as are the conformations with angles 120° and 240°. This means that we really only have four different maxima or minima in energy as we rotate about the central C–C bond: two types of eclipsed conformation, which will repre sent maxima in the energy–rotation graph, and two types of staggered conformation, which will represent minima. These four different conformations have names, shown in the bottom row of the diagram. In the syn-periplanar and anti-periplanar conformations the two C–Me bonds lie in the same plane; in the synclinal (or gauche) and anticlinal conformations they slope towards (syn) or away from (anti) one another.

Before we draw the energy–rotation graph, let’s just stop and think what it might look like. Each of the eclipsed conformations will be energy maxima but the syn-periplanar conformation (θ = 0°) will be higher in energy than the two anticlinal conformations (θ = 120° and 240°): in the syn-periplanar conformation two methyl groups are eclipsing each other whereas in the anticlinal conformations each methyl group is eclipsing only a hydrogen atom. The staggered conformations will be energy minima but the two methyl groups are furthest from each other in the anti-periplanar conformation so this will be a slightly lower minimum than the two synclinal (gauche) conformations.

As in ethane, the eclipsed conformations are not stable since any rotation leads to a more stable conformation. The staggered conformations are stable since they each lie in a potential energy well. The anti-periplanar conformation, with the two methyl groups opposite each other, is the most stable of all. We can therefore think of a butane molecule as rapidly inter converting between synclinal and anti-periplanar conformations, passing quickly through the eclipsed conformations on the way. The eclipsed conformations are energy maxima, and therefore represent the transition states for interconversion between conformations. If we managed to slow down the rapid interconversions in butane (by cooling to very low temperature, for example), we would be able to isolate the three stable conformations—the anti-periplanar and the two synclinal conformations. These different stable conformations of butane are some sort of isomers. They are called conformational isomers or conformers for short.

● Conformations and conformers Butane can exist in an infinite number of conformations (we have chosen to show only the six most significant) but has only three conformers (potential energy minima)—the two synclinal (gauche) conformations and the anti-periplanar conformation.

You now have a more thorough explanation of the zig-zag arrangement of carbon chains, when we showed you how to draw molecules realistically. This is the shape you get if you allow all the C–C bonds to take up the anti-periplanar conformation, and it will be the most stable conformation for any linear alkane.

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