Reactions of Hydrocarbons
A double bond is formed when two carbon atoms share two pairs of electrons. Since a carbon-carbon single bond is very strong and unreactive, it might seem logical that a carbon-carbon double bond would be even stronger and more unreactive than a single bond. It is true that it is more difficult to break both bonds of a double bond than to break a single bond. However, it is actually easier to break one of the bonds in a double bond than it is to break a single bond.
The unreactivity of the carbon-carbon single bond is one of the real strengths of organic compounds. It is this quality that allows nature as well as the chemist to construct such large and elaborate molecules.
Compounds with carbon-carbon multiple bonds such as the carbon double bond and the carbon triple bond are said to be unsaturated. A compound with a double bond has two fewer hydrogen atoms than the corresponding alkane with the same number of carbons. A compound with a triple bond has four fewer hydrogens. More hydrogen or other atoms such as the halogens—fluorine (F2), chlorine (Cl2), bromine (Br2), and iodine (I2)—can be added to these compounds.
The single-bonded hydrocarbons such as methane, ethane, and propane are not able to accommodate any more atoms and are said to be saturated. That is, under normal conditions no more atoms can be added to them. For example, ethane, a saturated hydrocarbon, does not react with chlorine under ordinary conditions—room temperature, atmospheric pressure, and no catalyst. But if a mixture of bromine and ethane is exposed to a bright light or sunlight a reaction occurs. Light acts as an energy source, causing the mixture to react. This is an example of a substitution reaction.
In a substitution reaction one atom in a molecule is replaced by another atom. In this example a chlorine atom is substituted for a hydrogen atom in ethane. Note that in a substitution reaction there are two products.
Let’s look at another type of reaction. The following equations illustrate the addition of hydrogen atoms to the unsaturated hydrocarbon ethylene to form the saturated hydrocarbon ethane. Notice in the second equation what happens when hydrogen is introduced to the saturated hydrocarbon ethane.
C2H4 + H2 → C2H6
The first equation in this section illustrates a saturated hydrocarbon, methane, reacting with a halogen. The second illustrates an unsaturated hydrocarbon reacting with additional hydrogen.
Unsaturated ethylene also reacts with bromine to form a new compound, 1,2-dibromoethane, which results from the addition of bromine across the carbon-carbon double bond.
C2H4 + Br2 → C2H4Br2
In the unsaturated hydrocarbon reactions, two substances react to give a single product. This is called an addition reaction.
Compounds that contain two carbon atoms joined by a double bond belong to a group of hydrocarbons called alkenes. Alkenes are much more reactive than the alkanes. There is yet another group of hydrocarbons that contain carbon atoms joined by a triple bond, called the alkynes. Look at the following equation involving an example of a compound in the alkyne group, the hydrocarbon acetylene.
C2H2 + H2 → C2H4
Can you determine the types of bonding present in this equation? Which hydrocarbon would be saturated? Unsaturated? What is the name of the product of the acetylene and hydrogen reaction? What could you predict would happen when additional hydrogen or other atoms are introduced to the end product of the reaction?
Remember that the alkanes participate in substitution reactions, while the double-bonded and triple-bonded carbon compounds undergo addition reactions. In substitution reactions there are two end products, whereas addition reactions form only one end product. These two types of reactions—addition and substitution—are among the most important reactions in organic chemistry.