How to Name Carbon Compounds: Alkanes, Alkenes, and Esters
Why is Life "Carbon-Based"?
Carbon is the basis of life on Earth. Carbon forms long chains and rings and has four free electrons in its outer electron shell. Carbon atoms make four bonds each due to the four free electrons in its outer shell. Where electron pairs are shared between two atoms, a covalent bond is formed. These bonds are very stable but are still reactive. These two properties combined allow for complex molecules to be formed based on a carbon skeleton:
- Nucleic Acids
The simplest carbon compounds are hydrocarbons - molecules made of hydrogen and carbon only. These are further broken down into alkanes and alkenes.
Naming Carbon compounds forms a large part of GCSE examinations - these questions are often worth several marks for a rather straightforward question. In my experience teaching chemistry, however, I have found that students often struggle with the specific nomenclature related to naming these molecules.
Naming Carbon Compounds 1
Number of Carbon Atoms
How to Name Alkanes
When the spine of a hydrocarbon contains single covalent bonds only, it is known as an alkane. The name of an alkane always ends with the suffix '-ane.' The start of the name depends on how many carbon atoms are found in the spine of the molecule. For example, a hydrocarbon with two carbon atoms containing only single covalent bonds would be called ethane.
Alkanes are described as saturated hydrocarbons because they only have single bonds. The molecule contains the maximum possible number of hydrogen atoms per carbon atom.
Naming Carbon Compounds 2
How to Name Alkenes
When the spine of a hydrocarbon contains one or more double covalent bonds between two adjacent carbon atoms, the molecule is known as an alkene. The name of an alkene always ends with the suffix '-ene.' Once again, the start of the name depends on how many carbon atoms are found in the spine of the molecule. For example, a hydrocarbon with 4 carbon atoms containing a double covalent bond would be called butene.
Alkenes are described as unsaturated hydrocarbons because they have one or more double bond (two pairs of electrons shared between two atoms.) The carbon atoms are not bonded to the maximum possible number of hydrogen atoms.
Alkenes are very good at joining together to produce a new substance. This process, known as polymerisation, is due to the reactivity of the the Carbon-Carbon double bond (C=C). This process is how compounds such as plastics are made.
Alkenes can be distinguished from alkanes by testing with Bromine water. Due to their double bond, alkenes react with the bromine water and decolourise it; alkanes have no effect:
"If it's an alkane, it stays the same. If it's an alkene, no colour is seen"
How to Name Alcohols
Alcohols are another group of Carbon Compounds: the alcohol consumed by humans is also known as ethanol. You recognise alcohols due to the presence of the hydroxyl functional group (-OH) at the end of a carbon chain. Naming this carbon compound follows the (now familiar) rules according to IUPAC. Count the longest continuous carbon chain to gain the prefix, then add the suffix -anol. If the longest carbon chain was 6 atoms long with a hydroxyl group at the end of the chain, the compound would be called hexanol.
The location of the International Union of Applied and Pure Chemistry - the body responsible for naming chemical compounds
How to Name Esters
Esters are pleasant smelling compounds created by reacting an alcohol with a carboxylic acid. These compounds are used in perfumes and are found naturally in fruits - pears are particularly ester-rich. The ester that gives that wonderful pear smell is ethyl ethanoate, and is made by reacting ethanol with ethanoic acid:
ethanol + ethanoic acid → ethyl ethanoate + water
Esters are tricky to name if you do not know how they are made. The first part of the name comes from the alcohol (____-yl), the second part of the name comes from the acid used (______-anoate). For example, if you react methanol and ethanoic acid, the resultant ester is methyl ethanoate.
If you react propanol and butanoic acid, the resulting ester is named - you guessed it - propyl butanoate!
(The name can be deduced from its structure - but that's the subject of another hub)
A Festive Review: Naming Carbon Compounds
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Where Next? Carbon Compounds
- Chemistry Tutorial : Summary Tables for Naming Carbon Compounds
A summary table for the naming of carbon compounds (organic molecules) suitable for high school chemistry students
- Chem4Kids.com: Atoms: Compound Naming
A basic look at nomenclature, but well worth a look for anyone needing a refresher course in naming chemical compounds
- Understanding the names of organic compounds
Explains how to write the formula for an organic compound from its name, and vice versa.