The alkenes and alkynes are hydrocarbons that contain double or triple covalent bonds, respectively.  The presence of a double or triple bond has a direct effect on the chemical and physical properties of these hydrocarbons.

• Because of the double or triple bond, alkenes and alkynes have less hydrogen atoms than alkanes.
• Empirical evidence has demonstrated that alkenes and alkynes are much more reactive than alkanes.  You will look much more closely at this in Module 2.

Alkenes and alkynes occur in low quantities in crude oil and natural gas, but they are very important because they represent the starting materials for many hydrocarbon derivatives (which you will study in Module 2).  Because they exist only naturally in small quantities, alkenes and alkynes are often manufactured during chemical refining processes.

The alkenes contain at least one double bond.  Alkenes possess the general formula C_nH_2n. Alkenes have less hydrogen atoms per carbon than alkanes. This is because a double bond requires two pairs of electrons.

For example, consider ethene, the simplest alkene.

Ethene (C₂H₄) only contains four hydrogen atoms. Therefore in order for each carbon to still form four bonds, each carbon must bond to two hydrogen atoms and form a double bond with the other carbon atom.

Note that some alkenes form closed rings. In this case, the molecules are known as cycloalkenes and have the general formula C_nH_2n-2. Compared to the straight chain alkenes, the cycloalkenes do not have as many hydrogen atoms per carbon atom. This is due to the ring structure. For example, consider the structure of cyclohexene shown below.

Count the carbons and hydrogens in the molecule.  Notice that the compound reflects the general formula C_nH_2n-2.

The alkynes contain at least one triple bond. Alkynes possess the general formula C_nH_2n-2. Alkynes have less hydrogen atoms per carbon than alkenes. This is because a triple bond is composed of three pairs of electrons.

For example, consider ethyne (C₂H₂), the simplest alkyne.

Ethyne only contains two hydrogen atoms. Therefore, in order for each carbon to still form four bonds, each carbon atom must bond to one hydrogen atom and form a triple bond with the other carbon atom.

Note that some alkynes form closed rings. In this case, the molecules are known as cycloalkynes and have the general formula C_nH_2n-4. Compared to the straight chain alkynes, the cycloalkynes do not have as many hydrogens per carbon atom. This is due to the ring structure. For example, consider the structure of cyclooctyne shown below.  Count the carbons and the hydrogens in the molecule.  You will see that the compound reflects the general formula, C_nH_2n-4

Because alkenes and alkynes have fewer hydrogen atoms attached than alkanes, these molecules are classified as unsaturated.

Check Your Understanding

Go to your textbook and complete Practice question 1 on page 377.  Check your answers by clicking the link beneath.