3.0 Organic Chemistry 1

3.1.3 Nomenclature (systematic naming) of alkanes

The examples of alkanes we have seen so far are simple unbranched alkanes (one open chain). They are called normal alkanes (n-alkanes). But many possible arrangements emerge with branching and increasing number of carbon atoms, hence the need for a system of naming (nomenclature). The naming system adopted is called the International Union of Pure and Applied Chemistry (IUPAC) system.

Naming according to IUPAC system

To name a branched alkane:

1. Identify the longest continuous chain and its name (parent chain name).
2. Number the carbon atoms 1, 2, 3 etc beginning, at the end closer to a branch.
3. Name the compound in the form x-branchalkane, where x is the position of the branch.

(courtesy Youtube-Naming (nomenclature) of alkanes by ormalearn)

Example

In this example, the longest chain is butane, and the branch is methyl (from methane). So the name is 2-Methylbutane (one word).

NB: A branch should not be confused with a bend. A branch occurs where a carbon atom is bonded to three other carbon atoms.

In a structural formula, a branch is normally indicated using brackets. For example, in CH₃CH(CH₃)CH₂CH₃, (CH₃) is the branch. It is attached to the second carbon atom from the left. How do we know this? (Hint: Compare the number of hydrogen atoms on the second and third carbon atoms.)

Branches are named derive their names from their parent names. For example, CH₃- is from methane (CH₄) so its name is methyl. CH₃CH₂- is ethyl, CH₃CH₂CH₂- is propyl and so on. They are called alkyl groups.

Questions 3.1.3(a)

1. Name the following alkanes.
   
   

   Figure 3.1.3(c): Some alkanes with single branches and no branches



2. Draw an open structure of 4-ethyldecane.
3. Write the structural formula of 4-ethyldecane
4. A student named a compound as 1-methylpropane. What was the error in this name? Give the correct name of the compound.

Answers to Questions 3.1.3(a)

If there are two or more repeated branches in an alkane, we add the prefixes di-, tri- tetra- and so on. If the branches are different, their names appear in alphabetical order, such as 3-ethyl-2-methyloctane.

Questions 3.1.3(b)

1. Draw the structure of (a) 2,2-dimethylpropane (b) 3-ethyl-2-methyloctane
2. Name the branched alkanes represented below.
   

   Figure 3.1.3(d): Some alkanes with double branches




Answers to Questions 3.1.3(b)