1.1 Hydrocarbons  (Page 2/6)

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 CH ${}_2$ . This is called a homologous series . The alkanes have the general formula C ${}_n$ H ${}_{2n+2}$ .

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.

1. 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.
2. 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 [link] . So, for example, an alkane that has 3 carbon atoms will have the suffix prop and the compound's name will be propane .

   Carbon atoms	prefix
   1	meth(ane)
   2	eth(ane)
   3	prop(ane)
   4	but(ane)
   5	pent(ane)
   6	hex(ane)
   7	hept(ane)
   8	oct(ane)
   9	non(ane)
   10	dec(ane)

3. 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.
4. 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.
5. 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.

Give the IUPAC name for the following compound:

Note: The numbers attached to the carbon atoms would not normally be shown. The atoms have been numbered to help you to name the compound.

1. Identify the functional group

   The compound is a hydrocarbon with single bonds between the carbon atoms. It is an alkane and will have a suffix of -ane.

2. Find the longest carbon chain

   There are four carbon atoms in the longest chain. The prefix of the compound will be 'but'.

3. Number the carbons in the longest chain

   In this case, it is easy. The carbons are numbered from left to right, from one to four.

4. Look for any branched groups, name them and give their position on the carbon chain

   There are no branched groups in this compound.

5. Combine the elements of the name into a single word

   The name of the compound is butane .

Got questions? Get instant answers now!