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 hydrocarbons contain 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.
Unsaturated hydrocarbons that contain one or more double bonds are called alkenes. The name of a specific alkene always ends in –ene and has a prefix indicating the number of carbon atoms. The structural formula in the Figure below represents the smallest alkene, named ethene, which has two carbon atoms and four hydrogen atoms (C2H4). Ethene is produced by most fruits and vegetables. It speeds up ripening. The Figure below show the effects of ethene on bananas.
The bananas on the left were stored in a special bag that absorbs ethene. The bananas on the right were stored without a bag.
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.
Unsaturated hydrocarbons called aromatic hydrocarbons are cyclic hydrocarbons that have double bonds. These compounds have six carbon atoms in a ring with alternating single and double bonds. The smallest aromatic hydrocarbon is benzene, which has just one ring. Its structural formula is shown in the Figure below. Larger aromatic hydrocarbons consist of two or more rings, which are joined together by bonds between their carbon atoms. The name of aromatic hydrocarbons comes from their strong aroma, or scent. That’s why they are used in air fresheners and mothballs.
Q: How many bonds does each carbon atom in benzene form?
A: Each carbon atom forms four covalent bonds. Carbon atoms always form four covalent bonds, regardless of the atoms to which it bonds.
Unsaturated hydrocarbons that contain one or more triple bonds are called alkynes. The names of specific alkynes always end in –yne and have a prefix for the number of carbon atoms. The structural formula in the Figure below represents the smallest alkyne, named ethyne, which has two carbon atoms and two hydrogen atoms (C2H2). Ethyne is also called acetylene. It is burned in acetylene torches, like the one pictured in the Figure below. The flame of an acetylene torch is so hot that it can melt metal.
Cutting metal with an acetylene (ethyne) torch.
Alkynes may form straight or branched chains. They rarely occur in ring shapes. In fact, alkynes of all shapes are relatively rare in nature.
- Unsaturated hydrocarbons are hydrocarbons with at least one double or triple bond between carbon atoms.
- Alkenes are unsaturated hydrocarbons with at least one double bond between carbon atoms.
- Aromatic hydrocarbons are unsaturated cyclic hydrocarbons with alternating single and double bonds between carbon atoms.
- Alkynes are unsaturated hydrocarbons with at least one triple bond between carbon atoms.
- What does it mean for a hydrocarbon to be unsaturated?
- Which chemical formula could represent an unsaturated hydrocarbon?
- Which of the following hydrocarbons has a double bond between carbon atoms?
- Describe an aromatic hydrocarbon