University of Manitoba
Teacher Background Information and Applications:
Hydrocarbons are compounds containing only carbon and hydrogen atoms and are the building blocks of organic molecules.
Saturated hydrocarbons are molecules made entirely of single carbon-carbon bonds; they cannot incorporate additional atoms into their structure, thus they are said to be saturated. These molecules, called alkanes, are stable and not very reactive.
Unsaturated hydrocarbons, the alkenes and alkynes, are molecules that contain at least one double or triple carbon-carbon bond within their structure. These molecules are highly reactive, and thus can incorporate other atoms into their structure.
Note: The terms "saturated" and "unsaturated" also refer to the concentration of solutions. These have a much different meaning and context.
For carbon, the maximum (and ideal) number of bonds is four.
Lewis Dot Structure for Carbon:
[pic]
Recall that the carbon atom has four electrons in the outer shell which are available for bonding. To reach electronic stability, carbon atoms must share four electrons from other atoms. Carbon, therefore, forms four bonds to other atoms. The bonds between carbon atoms can be single, double or triple bonds, as long as the total bond count around each carbon atom is four.
Saturated Hydrocarbons: Alkanes
Below is a table that gives the names of the straight chain alkanes. The general formula for an alkane is CnH2n+2 where n is the number of carbon atoms in the molecule. There are two ways of writing a condensed structural formula. For example, butane may be written as CH3CH2CH2CH3 or CH3(CH2)2CH3.
|# Carbons |Name |Molecular |Structural |
| | |Formula |Formula |
|1 |Methane |CH4 |CH4 |
|2 |Ethane |C2H6 |CH3CH3 |
|3 |Propane |C3H8 |CH3CH2CH3 |
|4 |Butane |C4H10 |CH3CH2CH2CH3 |
|5 |Pentane |C5H12 |CH3CH2CH2CH2CH3 |
|6 |Hexane |C6H14 |CH3(CH2)4CH3 |
|7 |Heptane |C7H16 |CH3(CH2)5CH3 |
|8 |Octane |C8H18 |CH3(CH2)6CH3 |
|9 |Nonane |C9H20 |CH3(CH2)7CH3 |
|10 |Decane |C10H22 |CH3(CH2)8CH3 |
n CnH2n+2
Unsaturated Hydrocarbons: Alkenes and Alkynes
The alkenes and alkynes follow the same general pattern as the alkanes except that, in the case of the alkenes, there is a double bond between two carbon atoms. That gives the alkenes the general formula CnH2n, while for alkynes, containing one triple bond, it is CnH2n-2.
The structures and molecular models below show the bonding differences in ethane, a saturated hydrocarbon, and ethene and ethyne, unsaturated hydrocarbons:
Ethane Ethene Ethyne
CH3-CH3 CH2=CH2 CH≡CH
[pic] [pic] [pic]
Single C-C bond Double C=C bond Triple C≡C bond
As another example, 3-hexene is unsaturated because the carbon atoms shown in red have two bonds connecting them and are bound to only three different atoms. If we chemically add hydrogen atoms to the red carbons, this reduces the carbon-carbon double bond to a single bond and gives the red carbons four single bonds to other atoms. The resulting molecule, n-hexane, is saturated because we can not add any more atoms to it without removing bonds to existing atoms.
H H
H2
CH3 – CH2 – C = C - CH2 – CH3 CH3 - CH2 - C C – CH2 – CH3
H H H H
3-hexene n-hexane
Common Hydrocarbons:
Examples of common saturated hydrocarbons are: plastics, gasoline, diesel fuels, lighter fluid, propane, home heating oil (kerosene/diesel mixture), marine and motor oil, fuels, and cleaning solvents.
Note: Kerosene is a mixture of hydrocarbon chains containing 12 to 15 carbon atoms. It is one of the products distilled from crude oil. The hydrocarbons in crude oil are mostly alkanes, cycloalkanes and various aromatic hydrocarbons while the other organic compounds contain nitrogen, oxygen and sulfur, and trace amounts of metals such as iron, nickel, copper and vanadium. The exact molecular composition varies widely from formation to formation but the proportions of chemical elements vary as follows: Carbon: 83-87%; Hydrogen: 10-14%; Nitrogen: 0.1-2%; Oxygen: 0.1-1.5%; Sulfur: 0.5-6%; metals: ................
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