We know from our discussion of hybridisation, that the overlap of two sp2 orbitals and the additional overlap of two p orbitals forms a double bond.
A double bond can be considered as a functional group, just as a single bond can. Molecules containing a C-C double bond are known as alkenes. However, unlike alkanes, which were very unreactive, alkenes can react with many species. The naming of alkenes follows the same system as that of alkanes, but the ending is changed from “-ane” to “-ene” i.e. propane and propene:
Unlike a C-C single bond, the double bond does not permit rotation.
The reason for this is that the rotation about a double bond would cause the p orbitals not to overlap momentarily, and this is equivalent to breaking the pi bond. Hence there is an energy barrier to rotation that is much greater than in the case of a single bond.
This lack of rotation means that there are no interchanging conformations as there are for alkanes, but instead, there can be (cis and trans) isomers:
|cis (same side)|
|trans (opposite sides)|
Clearly, the two molecules above (both 2-butene) are different if they cannot rotate about the double bond (which they can’t). Form the perspective of the double bond, one has the two methyl groups on the same side, and the other has them on opposite sides. These are denoted cis and trans respectively.
Although the two molecules will not interconvert on their own, they can be encouraged to do so by use of a catalyst. e.g. a strong acid:
When this is done, and the system is allowed to reach equilibrium, it is found that there are unequal quantities of the two isomers. The proportions are roughly 3:1 of trans : cis. The reason for this is that the isomers have different stabilities.
The cis isomer, being less abundant at equilibrium, is obviously the less stable form, and this is because of steric interactions between the two large groups on the same side. i.e. there isn’t enough room for the two methyl groups to be held together in the same line, so they bash into each other, and cause strain for the molecule. The trans isomer has a methyl on the same side as a hydrogen atom, which isn’t large enough to seriously interfere with the methyl group, and so is of lower energy.
With more complicated alkenes (i.e. tri and tetrasubstituted alkenes – those with, respectively, three or four non hydrogen groups attached to the double bond) a naming system has to be introduced to allow systematic definition of what is cis and what is trans, or more properly, what is E (from German “entgegen” meaning “opposite”) and Z (from German “zusammen” meaning “together”).
The rules for this are also used later in determining the stereochemistry of molecules. They are known as the Cahn-Ingold-Prelog rules:
1. Rank the atoms attached directly to the double bond in order of decreasing atomic mass. i.e. an hydrogen atom is the lowest priority possible. Where there are two identical atoms, their immediate neighbours are compared until a difference is found. e.g:
Here, the methyl is of lower priority than the acid group, because the methyl carbon is only attached to H atoms, which are of lower priority than the O atoms that the acid carbon is attached to. Clearly, the alcohol group takes highest priority as oxygen has a mass number of 16, compared to carbon’s 12. Hydrogen is obviously of lowest priority.
2. If the two highest priority groups (1 and 2) are on the same side (and hence consequently the two lowest priority groups (3 and 4) are also on the same side), then the compound is designated Z. If the highest are not on the same side, as in the example above, then the compound is designated E.
Unlike alkanes, alkenes can perform a variety of reaction, some of which we will look at next.