Structural Isomerism: Chain and Position Isomers

Structural isomers are compounds with the same molecular formula but different connections between their atoms; chain isomers differ in the branching of the carbon skeleton, and position isomers differ in where a group or multiple bond is attached. In brief: C₄H₁₀ has two chain isomers (butane and 2-methylpropane), and C₃H₇OH has two position isomers (propan-1-ol and propan-2-ol).

Isomerism is the reason why organic chemistry has so many compounds, and it is a recurring theme in questions about naming, properties and reactivity. A molecular formula is not enough to identify a compound. This article explains the types of structural isomerism, shows how to draw all isomers of a formula systematically, gives data that show how isomers differ, and covers the history of the concept.

What are structural isomers?

Isomers are different compounds with the same molecular formula. Structural (constitutional) isomers differ in the order in which their atoms are bonded. There are three kinds:

• Chain isomerism: a different arrangement of the carbon skeleton.
• Position isomerism: the same carbon skeleton, but a different position of the functional group or substituent.
• Functional group isomerism: a different functional group (the topic of a separate article).

Stereoisomers, which have the same connections but a different arrangement in space, form a different class.

What are chain isomers?

Chain isomers have the same functional group, but the carbon atoms are arranged in a straight chain in one isomer and are branched in the other. The alkane C₄H₁₀ has two isomers:

• butane, CH₃CH₂CH₂CH₃,
• 2-methylpropane, (CH₃)₃CH.

C₅H₁₂ has three isomers: pentane, 2-methylbutane and 2,2-dimethylpropane. C₆H₁₄ has five: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane.

What are position isomers?

Position isomers have the same carbon skeleton and the same functional group, but the group is bonded to different carbon atoms:

• Alkenes: but-1-ene (CH₂=CHCH₂CH₃) and but-2-ene (CH₃CH=CHCH₃).
• Alcohols: propan-1-ol (CH₃CH₂CH₂OH) and propan-2-ol (CH₃CH(OH)CH₃).
• Haloalkanes: 1-chloropropane and 2-chloropropane.
• Substituted benzenes: the three isomers of dimethylbenzene (xylene): 1,2- (ortho), 1,3- (meta) and 1,4- (para).

How many isomers are there?

The count grows very fast because there are more ways to branch a longer chain. These numbers apply to constitutional isomers only, without stereoisomers.

How do you draw all isomers of a formula?

1. Find the degree of unsaturation to see how many multiple bonds or rings are needed.
2. Start with the longest straight chain.
3. Shorten the main chain by one carbon, and attach that carbon as a branch at every different position.
4. Continue with shorter main chains and larger or multiple branches.
5. Check for duplicates by naming each structure with IUPAC rules: two structures with the same name are the same compound.

Worked example: the isomers of C₅H₁₂

1. Longest chain with 5 carbons: pentane.
2. A chain of 4 carbons with a methyl branch: on carbon 2 gives 2-methylbutane. A methyl on carbon 3 gives the same compound (numbering from the other end), so it is not a new isomer.
3. A chain of 3 carbons with two methyl groups: both on carbon 2 give 2,2-dimethylpropane. Methyl groups on carbons 1 and 3 would extend the chain, so they are not valid.
4. The total is 3 isomers.

Worked example: the isomers of C₄H₉Cl

The two C₄ skeletons (butane and 2-methylpropane) give four isomers. From butane: 1-chlorobutane and 2-chlorobutane. From 2-methylpropane: 1-chloro-2-methylpropane and 2-chloro-2-methylpropane. Note that 2-chlorobutane also exists as two mirror-image stereoisomers.

How do isomers differ in properties?

Branching reduces the surface area of the molecule and thus the dispersion forces, so branched isomers boil at lower temperatures. In the alcohols, the OH group in the middle of the chain is more shielded than the one at the end, which weakens the hydrogen bonding a little. The chemical properties also differ: propan-1-ol is oxidized to an aldehyde and then an acid, while propan-2-ol is oxidized to a ketone, propanone.

How was isomerism discovered?

In the 1820s, Friedrich Wöhler showed that silver cyanate and Justus von Liebig’s silver fulminate had the same composition but different properties. In 1830 Jöns Jacob Berzelius proposed the term isomerism for the existence of compounds of the same composition and different properties. The explanation had to wait for structural theory: in 1861, Aleksandr Butlerov argued that the properties of a compound depend on the way its atoms are connected, and he predicted and prepared isobutane (2-methylpropane), the isomer of butane, in 1864.

Which mistakes are common?

• Counting the same compound twice: rotating or flipping a chain does not make a new isomer.
• Treating a bend in the drawing as a branch: the longest chain may turn corners.
• Forgetting the numbering rule: 3-methylpentane is not “4-methylpentane”.
• Mixing up isomers and homologues: homologues differ by CH₂, isomers have the same formula.
• Ignoring position isomers: the position of a functional group changes the name and the properties.