In this post, we will identify all the isomers with the molecular formula C₂H4O₂. One strategy to draw all the isomers with a given molecular formula is to calculate the degree of unsaturation so that we 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:

So, we know that the C₂H₄O₂ isomers contain either/or a one pi bond and a ring. This also tells us the possible functional groups. What functional groups do you recall that contain oxygen atoms with a double bond?

One of the most common functional group units you are going to see in organic chemistry is the carbonyl bond (C=O). Now, considering that there are two oxygen atoms in C₂H₄O₂, the candidates are carbocyclic acids and esters, so let’s list them first:

So, acetic acid and methyl formate are the two constitutional isomers with the molecular formula C₂H₄O₂: one is a carboxylic acid, and the other is an ester.

Now, we all know it does not stop here – you are not going to get a question that is this easy in organic chemistry, are you?

So, what else can we think of? How can we have two oxygen atoms with one degree of unsaturation?

Unstable Molecule with the Molecular Formula of C₂H₄O₂

One of the principles you will learn in organic chemistry is the fact that connecting an OH group to a double-bonded carbon gives an unstable combination. This is because the resulting structure is an enol, which is generally unstable and rapidly rearranges to the corresponding carbonyl compound (aldehyde or ketone) through keto-enol tautomerization, as the carbonyl form is 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.

Let’s, for now, see what structures we can come up with that contain a double-bonded carbon with an oxygen connected to it. These will be cases where we connect one OH group to each CH group, or two of them to the same carbon atom:

Another possibility here is the keto form of the diol we have shown above. That is, we replace one of the OH groups attached to the C=C bond with an aldehyde group, resulting in the molecule glycolaldehyde:

Glycolaldehyde is the simplest molecule that contains both an aldehyde group (–CHO) and a hydroxyl group (–OH). It is a highly reactive compound found in both biological systems and the interstellar medium.

Cyclic Structures with the Molecular Formula of C₂H₄O₂

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:

The first two molecules are 1,2-dioxetane and 1,3-dioxetane, which are highly strained four-membered cyclic systems. Because of this significant ring strain, and in the case of 1,2-dioxetane, also the presence of a weak O–O peroxide bond, both compounds are extremely unstable and tend to decompose readily under mild conditions, often breaking down into carbonyl compounds. One would imagine that the second pair of 3-membered rings is even more unstable because of the ring strain, so keep in mind that these are theoretical answers that fit the category of isomers with the molecular formula of C₂H₄O₂.

At the end, let’s put all the constitutional isomers with the Molecular Formula of C₂H₄O_2:

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