Functional-Group Isomerism

Functional group isomerism occurs when compounds have the same molecular formula but different functional groups, so they belong to different families and have different chemical properties. In brief: C₂H₆O can be ethanol (an alcohol, boiling point 78 °C) or methoxymethane (an ether, boiling point −25 °C), and C₃H₆O can be propanal (an aldehyde) or propanone (a ketone).

Functional group isomers are the most striking examples of isomerism, because the isomers can differ completely in their reactions. They are often used in exercises on identification, and in the laboratory, simple tests and spectra are used to tell them apart. This article lists the common pairs, shows how to find them from a formula, gives physical data and describes the tests that distinguish them.

What is functional group isomerism?

In structural isomerism, the atoms are connected in different ways. When the different connection creates a new functional group, the isomers are called functional group isomers. They have the same molecular formula and molar mass but may differ strongly in boiling point, solubility and reactivity, since the intermolecular forces and the reactive sites are not the same.

Which families are isomeric?

Some families have the same general formula, so their members can be isomers of each other:

• Alcohols and ethers: C_nH_2n+2O. Example: ethanol and methoxymethane (C₂H₆O).
• Aldehydes and ketones: C_nH_2nO. Example: propanal and propanone (C₃H₆O).
• Carboxylic acids and esters: C_nH_2nO₂. Example: propanoic acid and methyl ethanoate (C₃H₆O₂).
• Alkenes and cycloalkanes: C_nH_2n. Example: but-1-ene and cyclobutane (C₄H₈).
• Alkynes and dienes: C_nH_2n−2. Example: but-1-yne and buta-1,3-diene (C₄H₆).
• Amines of different type: primary, secondary and tertiary amines, such as propylamine, ethylmethylamine and trimethylamine (C₃H₉N).

Examples with physical data

The pairs with an O–H group (ethanol and propanoic acid) form hydrogen bonds with themselves and have much higher boiling points than their isomers. The aldehyde and ketone are both polar but cannot form hydrogen bonds with themselves, so their boiling points are close.

How do you find functional group isomers?

1. Calculate the degree of unsaturation: DoU = (2C + 2 − H) / 2, where oxygen is ignored.
2. Decide what could account for it: a double bond, a ring, or a C=O group.
3. List the families that have this formula type.
4. Draw the isomers for each family, and use the IUPAC rules to name them.

Worked example: isomers of C₃H₈O

DoU = (2 × 3 + 2 − 8) / 2 = 0, so there are no double bonds or rings. The oxygen must be in an –OH group or in an ether link. The isomers are:

• propan-1-ol, CH₃CH₂CH₂OH (alcohol),
• propan-2-ol, CH₃CH(OH)CH₃ (alcohol),
• methoxyethane, CH₃OCH₂CH₃ (ether).

There are three isomers in total: two alcohols (position isomers) and one ether (functional group isomer of both).

Worked example: isomers of C₃H₆O₂ with C=O

DoU = (8 − 6) / 2 = 1, which corresponds to a C=O group. With two oxygen atoms, the carboxylic acid and ester families are possible: propanoic acid (CH₃CH₂COOH), methyl ethanoate (CH₃COOCH₃) and ethyl methanoate (HCOOCH₂CH₃). Hydroxy-aldehydes and hydroxy-ketones are also possible isomers of the same formula.

How can you tell them apart?

Which spectra distinguish them?

• Infrared spectroscopy: alcohols and acids show a broad O–H absorption at about 3200–3600 cm⁻¹, which ethers and esters do not have. A C=O group shows a strong absorption at about 1650–1800 cm⁻¹, found in aldehydes, ketones, acids and esters, but not in alcohols or ethers.
• NMR: an aldehyde H gives a signal at about 9–10 ppm, and an acid O–H signal is found at 10–12 ppm.
• Mass spectrometry: isomers have the same molecular ion but different fragmentation patterns.

Where was this first observed?

The case of ethanol and methoxymethane (then called “alcohol” and “methyl ether”) was among the first used to support the idea that atoms in a molecule can be arranged in different ways. Alexander Williamson used the synthesis of ethers from alkoxides and haloalkanes in 1850 to establish the structure of ethers, and the knowledge that ethanol and its isomer have different properties supported the structural theory of Kekulé, Couper and Butlerov in the following decade.

Which mistakes are common?

• Counting position isomers as functional group isomers: propan-1-ol and propan-2-ol have the same functional group.
• Forgetting that aldehydes and ketones are isomers: both have a C=O group, but at different positions.
• Assuming that isomers have similar boiling points: hydrogen bonding can cause large differences.
• Mixing up esters and carboxylic acids: both have the formula C_nH_2nO₂, but the ester has no O–H group.
• Ignoring rings: a ring can replace a double bond in the formula.