http://www.ck12.org Chapter 3. Chemical Interactions
3.35 Unsaturated Hydrocarbons
- Define unsaturated hydrocarbon.
- Describe alkenes, aromatic hydrocarbons, and alkynes.
Look at these colored lozenges. Notice how the unsaturated form of each color is dimmer and less colorful than
the saturated form. Compounds called hydrocarbons can also be saturated or unsaturated. Can you guess what an
unsaturated hydrocarbon is?
What Are Unsaturated Hydrocarbons?
Hydrocarbons are compounds that contain only carbon and hydrogen. The carbon atoms in hydrocarbons may share
single, double, or triple covalent bonds. Unsaturated hydrocarbonscontain at least one double or triple bond
between carbon atoms. They are classified on the basis of their bonds as alkenes, aromatic hydrocarbons, or alkynes.
Q:Why do you suppose hydrocarbons with double or triple bonds are called unsaturated?
A:A carbon atom always forms four covalent bonds. Carbon atoms with double or triple bonds are unable to bond
with as many hydrogen atoms as they could if they were joined only by single bonds. This makes them unsaturated
with hydrogen atoms.
Alkenes
Unsaturated hydrocarbons that contain one or more double bonds are calledalkenes. The name of a specific alkene
always ends in–eneand has a prefix indicating the number of carbon atoms. The structural formula in theFigure
3.64 represents the smallest alkene, named ethene, which has two carbon atoms and four hydrogen atoms (C 2 H 4 ).
Ethene is produced by most fruits and vegetables. It speeds up ripening. TheFigure3.64 show the effects of ethene
on bananas.
Alkenes can have different shapes. They can form straight chains, branched chains, or rings. Alkenes with the same
atoms but different shapes are called isomers. Smaller alkenes have relatively high boiling and melting points, so
they are gases at room temperature. Larger alkenes have lower boiling and melting points, so they are liquids or
waxy solids at room temperature.
Aromatic Hydrocarbons
Unsaturated hydrocarbons calledaromatic hydrocarbonsare cyclic hydrocarbons that have double bonds. These
compounds have six carbon atoms in a ring with alternating single and double bonds. The smallest aromatic