Alkanes: Naming and Structural Formula

 

The Alkanes

The alkanes are hydrocarbons that only contain single covalent bonds between their carbon atoms. This means that they are saturated compounds and are quite unreactive. The simplest alkane has only one carbon atom and is called methane.

 

When you look at the molecular formula for each of the alkanes, you should notice a pattern developing. For each carbon atom that is added to the molecule, two hydrogen atoms are added. In other words, each molecule differs from the one before it by CH2. This is called a homologous series. The alkanes have the general formula CnH2n+2. The alkanes are the most important source of fuel in the world and are used extensively in the chemical industry. Some are gases (e.g. methane and ethane), while others are liquid fuels (e.g. octane, an important component of petrol).

 

Naming the alkanes

In order to give compounds a name, certain rules must be followed. When naming organic compounds, the IUPAC (International Union of Pure and Applied Chemistry) nomenclature is used. We will first look at some of the steps that need to be followed when naming a compound, and then try to apply these rules to some specific examples.

STEP 1: Recognise the functional group in the compound. This will determine the suffix (the ’end’) of the name. For example, if the compound is an alkane, the suffix will be -ane; if the compound is an alkene the suffix will be -ene; if the compound is an alcohol the suffix will be -ol, and so on.

STEP 2: Find the longest continuous carbon chain (it won’t always be a straight chain) and count the number of carbon atoms in this chain. This number will determine the prefix (the ’beginning’) of the compound’s name. These prefixes are shown in table below So, for example, an alkane that has 3 carbon atoms will have the suffix prop and the compound’s name will be propane.

 

The prefix of a compound’s name is determined by the number of carbon atoms in the longest chain

STEP 3: Number the carbons in the longest carbon chain (Important: If there is a double or triple bond, you need to start numbering so that the bond is at the carbon with the lowest number.

STEP 4: Look for any branched groups and name them. Also give them a number to show their position on the carbon chain. If there are no branched groups, this step can be left out.

STEP 5: Combine the elements of the name into a single word in the following order: branched groups; prefix; name ending according to the functional group and its position along the longest carbon chain.

 

 Example 1 

Question: Give the IUPAC name for the following compound:

 

Answer Step 1 : Identify the functional group The compound is an alkane and will have the suffix -ane.

Step 2 : Find the longest carbon chain There are three carbons in the longest chain. The prefix for this compound is -prop.

Step 3 : Number the carbons in the carbon chain If we start at the carbon on the left, we can number the atoms as shown below:

Step 4 : Look for any branched groups, name them and give their position on the carbon chain There is a branched group attached to the second carbon atom. This group has the formula CH3 which is methane. However, because it is not part of the main chain, it is given the suffix -yl (i.e. methyl). The position of the methyl group comes just before its name (see next step).

Step 5 : Combine the elements of the compound’s name into a single word in the order of branched groups; prefix; name ending according to the functional group. The compound’s name is 2-methylpropane.

 

Worked Example 2

Question: Give the IUPAC name for the following compound:

CH3CH(CH3)CH(CH3)CH3

(Remember that the side groups are shown in brackets after the carbon atom to which they are attached.) Answer Step 1 : Draw the compound from its condensed structural formula The structural formula of the compound is:

Step 2 : Identify the functional group The compound is an alkane and will have the suffix -ane.

Step 3 : Find the longest carbon chain There are four carbons in the longest chain. The prefix for this compound is -but.

Step 4 : Number the carbons in the carbon chain If we start at the carbon on the left, carbon atoms are numbered as shown in the diagram below.

Step 5 : Look for any branched groups, name them and give their position on the carbon chain There are two methyl groups attached to the main chain. The first one is attached to the second carbon atom and the second methyl group is attached to the third carbon atom. Notice that in this example it does not matter how you have chosen to number the carbons in the main chain; the methyl groups are still attached to the second and third carbons and so the naming of the compound is not affected.

Step 6 : Combine the elements of the compound’s name into a single word in the order of branched groups; prefix; name ending according to the functional group. The compound’s name is 2,3-dimethyl-butane.

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