Constitutional isomers, also called structural isomers, are compounds that have the same molecular formula but differ in the connectivity of the atoms. Connectivity means which atoms are bonded to which within the molecule. ^[1–4]

Constitutional isomers have distinct physical and chemical properties, such as molecular shape, boiling point, polarity, and reactivity. For example, butane and 2-methylpropane (isobutane) both are C₄H₁₀, but have boiling points of approximately –0.5 °C and –11.7 °C, respectively.

Types

1. Chain or Skeletal Isomers

Chain isomers differ in the arrangement of the carbon skeleton. One may have a straight chain, while another has a branched chain. ^[1,2,6,7]

There is no universal formula for calculating the number of constitutional isomers for every molecular formula, but small alkanes follow a well-known pattern:

• C₄H₁₀ – 2 isomers
• C₅H₁₂ – 3 isomers
• C₆H₁₄ – 5 isomers
• C₇H₁₆ – 9 isomers

Examples

Methane, ethane, and propane cannot form constitutional isomers because alkanes need at least four carbons for chain isomerism. These molecules have only one carbon skeleton each.

As far as the physical properties are concerned, branched alkane structures generally have lower boiling points than their straight-chain counterparts. It is because branching reduces the surface area available for intermolecular attractions, especially London dispersion forces.

2. Positional Isomers

Positional isomers have the same carbon skeleton, but the position of the functional group, substituent, or multiple bond changes within the molecule.

Examples

3. Functional Group Isomers

Functional group isomers have the same molecular formula but different functional groups.

Examples

How to Draw Constitutional Isomers

To draw constitutional isomers systematically, follow this process: ^[5]

1. Write the molecular formula.
2. Choose the longest possible carbon chain.
3. Draw one valid structure.
4. Change the branching or the position of the functional group.
5. Check that all atoms satisfy their correct valency.
6. Remove any redrawn duplicates.
7. Stop when no new connectivity is possible.

Constitutional isomers demonstrate that a molecular formula alone is insufficient to predict how a compound will behave. This idea is a foundation of organic chemistry and helps explain why substances with the same atoms can have different properties, uses, and reactions in both laboratory and real-world applications.

Multiple Choice Questions

Question 1

Which of the following is a constitutional isomer of heptane?
A. CH₃CH(CH₃)CH₂CH₂CH₂CH₃
B. CH₃CH₂CH₂CH₂CH₂CH₃
C. CH₃CH₂CH₂CH=CHCH₂CH₃
D. CH₃CH₂CH₂CH₂CH₂CH₂CH₂OH

Answer

Heptane has the molecular formula C₇H₁₆. Its condensed structural formula is CH₃CH₂CH₂CH₂CH₂CH₂CH₃.

Constitutional isomers must have:

• the same molecular formula
• different connectivity

Now check each option:

A. CH₃CH(CH₃)CH₂CH₂CH₂CH₃ is 2–methylhexane. It has the same molecular formula as heptane, but a different connectivity. Therefore, it is a constitutional isomer of heptane.
B. CH₃CH₂CH₂CH₂CH₂CH₃ is hexane. Its molecular formula is C₆H₁₄, not C₇H₁₆. Therefore, it is not a constitutional isomer of heptane.
C. CH₃CH₂CH₂CH=CHCH₂CH₃ is hept–3–ene. Its molecular formula is C₇H₁₄, not C₇H₁₆. Therefore, it is not a constitutional isomer of heptane.
D. CH₃CH₂CH₂CH₂CH₂CH₂CH₂OH is heptan–1–ol. Its molecular formula is C₇H₁₆O, not C₇H₁₆. Therefore, it is not a constitutional isomer of heptane.

The correct answer is A.

Question 2

The following are the condensed structural formulas of two different compounds.
i: CH₃CH₂CH₂OH
ii: CH₃CH(OH)CH₃

Are they constitutional isomers? Choose the correct answer.

A. Yes, they have the same molecular formula and connectivity.

B. Yes, they have the same molecular formula, but their atomic connectivities are different.

C. No, they do not have the same molecular formula and connectivity.

D. No, they have different molecular formulas but have the same connectivity.

Answer

To decide whether these are constitutional isomers, follow the steps below.
Step 1: Check the molecular formula
CH₃CH₂CH₂OH has the formula C₃H₈O
CH₃CH(OH)CH₃ also has the formula C₃H₈O

Both compounds have the same molecular formula.

Step 2: Check whether each atom is connected to the same neighbors
In CH₃CH₂CH₂OH, the OH group is attached to the first carbon.
In CH₃CH(OH)CH₃ , the OH group is attached to the second carbon.

The atoms are not connected in the same way. Their connectivity differs.

Since the two compounds have the same molecular formula but different connectivity, they are constitutional isomers.

The correct answer is B.