What are isomers? Draw the structures of two isomers of butane, $C_4H_{10}$. Why can’t we have isomers of first three members of alkane series?

1. Definition of Isomers

Compounds that have the identical molecular formula but different structural arrangements of atoms are called structural isomers.

2. Structures of the Two Isomers of Butane ($C_4H_{10}$)

Butane has two structural isomers:

• $n$-Butane (Straight-chain skeleton):

$$\begin{array}{cccc} \text{H} & \text{H} & \text{H} & \text{H} \\ | & | & | & | \\ \text{H}-\text{C}-\text{C}-\text{C}-\text{C}-\text{H} \\ | & | & | & | \\ \text{H} & \text{H} & \text{H} & \text{H} \end{array}$$

Condensed Formula: $CH_3-CH_2-CH_2-CH_3$

• Iso-butane / 2-Methylpropane (Branched-chain skeleton):

$$\begin{array}{ccc} \text{H} & \text{H} & \text{H} \\ | & | & | \\ \text{H}-\text{C} & -\text{C}- & \text{C}-\text{H} \\ | & | & | \\ \text{H} & | & \text{H} \\ & \text{C}-\text{H} & \\ & / \ \ \backslash & \\ & \text{H} \ \ \ \text{H} & \end{array}$$

Condensed Formula: $CH_3-CH(CH_3)-CH_3$

3. Why the first three alkanes do not have isomers

The first three members of the alkane series are methane ($CH_4$), ethane ($C_2H_6$), and propane ($C_3H_8$).

• Isomerism requires the possibility of branching the carbon skeleton.
• With $1$, $2$, or $3$ carbon atoms, any arrangement of carbon atoms will always result in a continuous straight chain.
• No branching is physically possible because bending a chain of three carbons does not create a new structural arrangement; it remains a continuous path of carbons connected in a single line.