When you need to identify all the isomers for a given molecular formula, the first thing you want to do is calculate the degree of unsaturation so that you know whether the compounds contain any π bonds or ring structures.

Using the following formula, we determine that there is one degree of unsaturation, which means the isomers are all unsaturated compounds containing two oxygen atoms:

One degree of unsaturation means that all the C₂H₄O isomers contain either/or one pi and a ring. If you have already mastered the functional groups, the first candidate that should come to mind for oxygen with a double bond is the carbonyl group:

Now, many carbonyl-containing functional groups, such as carboxylic acids, esters, acid chlorides, etc., contain two oxygen atoms, and therefore only aldehydes and ketones represent the possible candidates for the formula C₂H₄O. The simplest ketone is acetone, which contains three carbon atoms. Therefore, the aldehyde with the molecular formula C₂H₄O is acetaldehyde, and it is the only carbonyl compound with this formula:

Although you may not be expected to know about enols, let’s also mention that these are the constitutional isomers of the corresponding carbonyl compounds. An enol contains an OH group connected to the C=C double bond:

The enol is always in equilibrium with the carbonyl compound via keto-enol tautomerization, although the latter is generally much more stable:

Do not worry about the instability of the sp² C–OH bond and keto–enol tautomerization for now – it is a topic you will discuss later in the semester.

Cyclic Structures with the Molecular Formula of C₂H₄O

Remember, earlier we mentioned that one degree of unsaturation means either a π bond or a ring structure. Therefore, another possibility for the isomers is to have a ring with no double bonds in the molecule, is ethylene oxide, which is the simplest of epoxides. So let’s put all the constitutional isomers with the Molecular Formula of C₂H₄O:

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