Chapter 21, Problem 78A

Interpretation Introduction

Interpretation:

Whether the following structural formula represents same molecule or not should be explained:

Concept introduction:

The molecules having same molecular formula but differ in structural arrangement of atoms are said to be isomers of each other.

Answer to Problem 78A

The given structural formula does not represent same molecule.

Explanation of Solution

The IUPAC name of both the compounds is determined using rules:

1. The parent (longest) continuous carbon chain containing multiple bonds between the carbon atoms is selected.

2. While writing the name of alkene, the suffix “ane” of the corresponding alkane is replaced by “ene”.

3. Name should be written in alphabetical order and numbering should be done in such a way that the multiple bond and substituent group gets lowest number.

4. Hyphen is used to connect the number to the name.

For number of carbons atoms in alkane chain, the prefix is given as:

Carbon-1   meth

Carbon-2   eth

Carbon-3   prop    

Carbon-4   but      

Carbon-5   pent

Carbon-6   hex

Carbon-7   hept

Carbon-8   oct

Carbon-9   non     

Carbon-10       dec

The numbering is done in such a way that double bond gets lowest number and the compound has longest carbon chain as a parent chain as:

So, the name of first compound is but-2-ene and second compound is 2-methyl-prop-1-ene. Thus, both the compounds have different names and different structural arrangement but the molecular formula of both the compounds is same that is, Hence, both the compounds are structural isomers of each other.