Question

(a) What is meant by the term isomer? (b) Among the four alkanes, ethane, propane, butane, and pentane, which is capable of existing in isomeric forms?

Short answer

(a) The term "isomer" refers to a molecule that has the same molecular formula as another molecule, but a different structural or spatial arrangement of its atoms. There are two types of isomers: structural isomers and stereoisomers. (b) Among the given alkanes, ethane ($C_2{H}_6$) and propane ($C_3{H}_8$) have only one possible arrangement, thus cannot exist in isomeric forms. However, butane ($C_4{H}_{10}$) and pentane ($C_5{H}_{12}$) have multiple structural arrangements, making them capable of existing in isomeric forms.

Step-by-step solution

Part (a): Defining the term "isomer"

An isomer is a molecule that has the same molecular formula (i.e., the same number of atoms of each element) as another molecule but a different structural or spatial arrangement of these atoms. Two molecules with the same molecular formula but different atomic arrangements are called isomers of each other. There are two types of isomers: structural isomers and stereoisomers. Structural isomers differ in the order of atomic connections, while stereoisomers differ in the arrangement of atoms in space.

Part (b): Identifying the alkane capable of existing in isomeric forms

To answer this question, let's first write the molecular formula for each of the given alkanes: 1. Ethane: $C_2{H}_6$ 2. Propane: $C_3{H}_8$ 3. Butane: $C_4{H}_{10}$ 4. Pentane: $C_5{H}_{12}$ Next, let's examine the possibility of having different structural arrangements for each alkane: 1. Ethane: With only two carbon atoms, there is only one possible arrangement: a single bond between the two carbons. 2. Propane: With three carbon atoms, there is only one possible arrangement: a linear chain of carbons with single bonds. 3. Butane: With four carbon atoms, there are two structural arrangements: a linear chain or a branched chain with a methyl group on the second carbon. 4. Pentane: With five carbon atoms, there are three structural arrangements: a linear chain, a branched chain with a methyl group on the second carbon, and a branched chain with a methyl group on the third carbon. Based on this analysis, ethane and propane cannot exist in isomeric forms. However, butane and pentane have more than one possible structural arrangement, so they are capable of existing in isomeric forms. So, among the four alkanes (ethane, propane, butane, and pentane), butane and pentane are capable of existing in isomeric forms.