Question

Write the IUPAC name of the following:

Answer 1

To solve this question, it is required to have knowledge about some of the basic rules of IUPAC naming:
-The parent chain is usually the longest chain or the chain with most of the functional groups or unsaturation (double/triple bond).
-The numbering of chains is started from the side nearest to a substituent group or a double bond carbon atom.
-Double/triple bonds have higher priority than other substituents and are assigned the smallest number possible in that chain.

Complete step by step answer:
IUPAC stands for “International Union of Pure and Applied Chemistry”. It is an endeavour governing chemical nomenclature and terminology, including naming of new elements in the periodic table. For the naming of hydrocarbons, it has a fixed set of rules which are followed. As the compound given in the question has two double bonds, i.e. it is an alkene. The rules governing the naming of alkenes are as:
The ‘ene’ suffix indicates an alkene.
The parent must contain both the carbon atoms of the double bond.
The chain must be numbered from the end nearest to the double bond.
The smaller of the two numbers assigned to the carbons linked by a double bond will be used as the double bond locator.
If any substituent group is also present, it will be written prior to the parent chain.
In the given compound, there are two double bonds present, so the parent chain must contain both the double bonds. Thus, we can say that the parent chain contains 4 carbon atoms, i.e. butyl group. The presence of 2 double bonds means that the suffix diene will be assigned. As both the double bonds are at terminal carbons, so we use the substituent, i.e. the methyl group, to determine the numbering of the chain. So, we can start the numbering from the left side of the compound.

The IUPAC name thus formed will be: 2–methylbuta-1, 3–diene. It is also known as isoprene.

Note:
While mostly we consider the chain with the highest number of carbon atoms as the parent chain, in case of alkenes and alkynes, the chain containing the double/triple bonds is considered as parent chains. If more than one double/triple bond is present on the carbon chain, they are named with prefixes like di, tri, tetra etc.