Definition of hydrocarbons:
Hydrocarbons are chemical compounds that are formed from only hydrogen atoms and carbon atoms.
Categories of hydrocarbons include:
- Alkanes - *saturated hydrocarbons, only single bonds
- Alkenes - *unsaturated hydrocarbons, include a double bond
- Alkynes - *unsaturated hydrocarbons, include a triple bond (more advanced topic, included in A-Level Chemistry)
It is important to understand which hydrocarbon compounds are saturated and which are unsaturated (see below) because this determines and explains key chemical properties including the reactions of these chemicals.
Is an Organic Compound a Hydrocarbon ?
The rule is simple: If an organic compound does not include any elements other than carbon and hydrogen, then it is a hydrocarbon.
Here are some examples:
octane is a hydrocarbon.
1-chloropropane is NOT a hydrocarbon.
pentan-1-ol is NOT a hydrocarbon.
hex-1-yne is a hydrocarbon.
It is not necessary to be able to name a molecule or compound in order to decide if it is, or is not, a hydrocarbon. It is not necessary to know anything about the type of compound represented by that type of molecular structure.
It is sufficient to look at the molecular structure (diagram) of the molecule and notice if any elements other than carbon (C) and hydrogen (H) are present.
More about the examples above:
- octane is an alkane,
- 1-chloropropane is a haloalkane,
- pentan-1-ol is an alcohol, and
- hex-1-yne is an alkyne.
Chemical Bonding of Hydrocarbons
- Carbon and Hydrogen atoms share an electron pair, forming covalent bonds.
This may be explained in terms of the number of electrons in the outer-electron orbits of hydrogen and carbon
(this level of detail may not be required for GCSE but is important for A-Level Chemistry).
First, consider atoms of hydrogen and carbon and their electrons located in orbitals around the nucleus of the atoms:
Next consider the example of the simple hydrocarbon ethane (which is an alkane):
- One of the special properties of carbon is its ability to form double and triple bonds.
When a hydrocarbon molecule includes one or more double or triple bonds between some of the carbon atoms it is not possible for as many hydrogen atoms to be included in the molecule as when all the bonds are single bonds.
There is a special term used to describe to presence of one or more double or triple bonds in a molecule of an organic compound. That term is 'unsaturated' (see more about this below).
The concept of 'saturation' is very important in organic chemistry because the saturation (or unsaturation) of molecules affects the properties of compounds, including their reactions
Saturated and Unsaturated Hydrocarbons
Definition of 'saturation' in the context of organic chemistry:
An organic chemical compound is said to be 'saturated' when it does not contain any double or triple covalent bonds. That is, saturated organic molecules include only single covalent bonds.
Conversely, unsaturated organic molecules include at least one double or triple covalent bond.
Note that the concept of saturation (saturated vs. unsaturated) of organic molecules is not limited to hydrocarbons but may be applied to discussion about many different types of organic molecules. An important example of this concept in the subjects of health and biology is that of saturated and unsaturated fats, i.e. saturated fatty acids have no double bonds between carbons whereas unsaturated fatty acids have at least one double bond.
Hydrocarbons can be classified as saturated or unsaturated.
School-level (e.g. GCSE) chemistry often requires students to be able to give examples of saturated and unsaturated hydrocarbons and to describe their molecular structures in the form of molecular diagrams.
Saturated and unsaturated hydrocarbons are compared in the following table:
More (advanced) information about saturation of organic molecules:
Related terms used to describe some organic compounds are monounsaturated and polyunsaturated.
These terms refer to the number of (usually*) double bonds per molecule of compounds.
'Mono-' generally refers to 'one' or a single unit, whereas 'poly-' generally means 'many'. Hence monounsaturated organic molecules include precisely one double bond, whereas polyunsaturated organic molecules include more than one double bond.
The term degree of unsaturation (of an organic compound) refers to the quantity of double* bonds that could, under certain circumstances, be broken and, instead, form single bonds to a larger total number of atoms, e.g. additional hydrogen atoms, added to the molecule during a chemical reaction. Hence it may be said that the degree of unsaturation of an organic compound specifies the amount of hydrogen (and / or e.g. halogens - but of course the resulting compound would not then be a hydrocarbon) that the compound could 'bind', i.e. attach to the molecule by breaking multiple (double or triple) bonds and forming single bonds instead.
How to Test Hydrocarbons for Saturation
A bromine solution is used to test for unsaturation. On its own bromine solution is orange in colour.
When an alkene (e.g. ethene) is added to a bromine solution, the orange colour disappears resulting in a colourless solution. This is because the bromine in the solution has reacted with the alkene, forming a new (colourless) compound.
This is a useful test to distinguish alkenes from alkanes because alkanes do not react with bromine solution, hence its orange colour remains visible.
More Terminology about Hydrocarbons
Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls.