2. Unsaturated hydrocarbons
In our discussion of the alkanes we mentioned briefly another family of
hydrocarbons, the alkenes, which contain less hydrogen, carbon for carbon, than the
alkanes, and which can be converted into alkanes by addition of hydrogen.
The alkenes were further described as being obtained from alkanes by loss of
hydrogen in the cracking process. Since alkenes evidently contain less than the
maximum quantity of hydrogen, they are referred to as unsaturated hydrocarbons.
This unsaturation can be satisfied by reagents other than hydrogen and gives rise to
the characteristic chemical properties of alkenes.
Structure of ethylene. The carbon-carbon double bond
The simplest member of the alkene family is ethylene, C2H4 . In view of the ready
conversion of ethylene into ethane, we can reasonably expect certain structural
similarities between the two compounds.
The next member of the alkene family is propylene, C3H6 . In view of its great
similarity to ethylene, it seems reasonable to assume that this compound also
contains a carbon-carbon double bond.
Starting with two carbons joined by a double bond, and attaching the other atoms
according to our rule of one bond per hydrogen and four bonds per carbon, we arrive
at the structure
Hybridization and orbital size
The carbon-carbon double bond in alkenes is shorter than the carbon-carbon single
bond in alkanes because four electrons bind more tightly than two. But, in addition,
certain other bonds in alkenes are significantly shorter than their counterparts in
alkanes: for example, the C H distance is 1.103 A in ethylene compared with 1.112 A
in ethane. To account for this and other differences in bond length, we must consider
differences in hybridization of carbon.
4. The butylenes
Going on to the butylenes, C4H8 , we find that there are a number of possible
arrangements. First of all, we may have a straight-chain skeleton as in n-butane, or
a branched-chain structure as in isobutane.
Next, even when we restrict ourselves to the straight-chain skeleton, we find that
there are two possible arrangements that differ in position of the double bond in the
chain. So far, then, we have a total of three structures; as indicated, these are given
the names 1-butene, 2-butene, and isobutylene.
5. To understand the kind of isomerism that gives rise to two 2-butenes, we must
examine more closely the structure of alkenes and the nature of the carbon-carbon
double bond. Ethylene is a flat molecule. We have seen that this flatness is a result
of the geometric arrangement of the bonding orbitals, and in particular the overlap
that gives rise to the π orbital.
For the same reasons, a portion of any alkene must also be flat, the two doubly-
bonded carbons and the four atoms attached to them lying in the same plane. If we
examine the structure of 2-butene more closely, and particularly if we use molecular
models, we find that there are two quite different ways, I and II, in which the atoms
can be arranged (aside from the infinite number of possibilities arising from rotation
about the single bonds). In one of the structures the methyl groups lie on the same
side of the molecule (I), and in the other structure they lie on opposite sides of the
6. As we soon conclude from our examination of these structures, geometric
isomerism cannot exist if either carbon carries two identical groups. Some possible
combinations are shown below:
7. Higher alkenes
As we can see, the butylenes contain one carbon and two hydrogens more than
propylene, which in turn contains one carbon and two hydrogens more than ethylene.
The alkenes, therefore, form another homologous series, the increment being the
same as for the alkanes: CH2 . The general formula for this family is C n H 2n .
Names of alkenes
Common names are seldom used except for three simple alkenes: ethylene,
propylene, and isobutylene. The various alkenes of a given carbon number are,
however, sometimes referred to collectively as the pentylenes (amylenes), hexylenes,
heptylenes, and so on. (One sometimes encounters the naming of alkenes as
derivatives of ethylene: as, for example, tetramethylethylene for (CH3)2C=C(CH3)2.
Most alkenes are named by the IUPAC system.
The rules of the IUPAC system are:
1. Select as the parent structure the longest continuous chain that contains the carbon-
carbon double bond; then consider the compound to have been derived from this
structure by replacement of hydrogen by various alkyl groups. The parent structure is
known as ethane < propene, butene, pentene, and so on, depending upon the number
of carbon atoms; each name is derived by changing the ending -ane of the
corresponding alkane name to -ene:
8. 2. Indicate by a number the position of the double bond in the parent chain.
Although the double bond involves two carbon atoms, designate its position by the
number of the first doubly-bonded carbon encountered when numbering from the
end of the chain nearest the double bond; thus 1-butene and 2-butene.
3. Indicate by numbers the positions of the alkyl groups attached to the parent
H.W / Give the structural formula of: a) 2,3-dimethyl-2-butene; (b) 3-bromo-2-
methylpropene; (c) 2-methyl-3-heptene
Alkenes containing up to five carbon atoms can be obtained in pure from the
petroleum industry. Pure samples of more complicated alkenes must be prepared by
methods like those outlined below. The introduction of a carbon-carbon double
bond into a molecule containing only single bonds must necessarily involve the
elimination of atoms or groups from two adjacent carbons:
1. Dehydrohalogenation of alkyl halides:
11. 2. Dehydration of alcohols:
3. Dehalogenation of vicinal dihalides:
4. Reduction of alkynes:
12. Reactions of the carbon-carbon double bond: addition
Alkene chemistry is the chemistry of the carbon -carbon double bond. What kind
of reaction may we expect of the double bond ? The double bond consists of a
strong or bond and a weak π bond; we might expect, therefore, that reaction would
involve the breaking of this weaker bond. This expectation is correct; the typical
reactions of the double bond are of the sort, where the π bond is broken and two
strong a bonds are formed in its place.
1. Addition of hydrogen. Catalytic hydrogenation.