10 examples of the constitutional-structural isomers of molecular formula C₂H₄O₂ (M_r = 60)

Composition of C₂H₄O₂

Percent composition base on atomic masses C = 12.01  H = 1.01  O = 16.00  and  M_r(C₂H₄O₂) = 60.06

Element composition (to two dp): carbon = 39.99%     hydrogen = 6.73%     oxygen = 53.28%

Empirical formula = CH₂O for molecular formula = C₂H₄O₂

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Introduction to the isomerism and 10 selected isomers of molecular formula C₂H₄O₂

Structural isomerism  - isomers based on different connectivity's of the constituent atoms, so cannot be spatially identical (but can be defined as having the same shape).

This includes (a) carbon chain variation (usually need a minimum of 4 atoms), (b) change in position of a substituent or functional group and (c) functional group isomerism where the atoms have a different connectivity configuration, usually with significant differences in chemical and physical properties e.g.

In terms of isomers of C₂H₄O₂ there are

(a) chain variations based on the C and O atoms, linear and cyclic structures,

(b) positional isomers e.g. the -OH groups in the unsaturated diols,

(c) most are all functional group isomers of each other e.g. there is a carboxylic acid, an ester, enols (unsaturated diols), a hydroxy-aldehyde, peroxides and other heterocyclic structures too!

Stereoisomerism - isomers based on the same connectivity of the atoms (same constitutional formula), but in some way, they are 2D or 3D spatially different non-superimposable images (e.g. E/Z 'geometrical' isomers or mirror image R/S 'optical' isomers)

This is where molecules have the same basic constitutional structural formula, but isomers differ in the 2D/3D arrangement of the atoms.

For stereoisomers, the (CIP) abbreviation means the IUPAC Cahn-Ingold-Prelog priority order rule for assigning E/Z (geometrical) and R/S (optical) stereoisomers.

E/Z stereoisomerism was called 'geometrical isomerism' e.g. cis and trans isomers of alkenes or disubstituted cyclic alkanes where, due to restricted bond rotation, there are 2D/3D spatial variations that are not mirror images and not super imposable.

One isomer exhibits E/Z geometrical isomerism No. (3)

R/S stereoisomerism was called 'optical isomerism', the pairs of isomers are called enantiomers which are 3D non-superimposable mirror image forms of the molecule (enantiomers). The molecule must have a chiral centre (a stereocentre), that is an asymmetric carbon atom with four different atoms/groups attached to it.

One isomer exhibits R/S optical isomerism No. (9)(d)

 

Therefore for isomers of molecular formula C₂H₄O₂

I've identified 10 constitutional-structural isomers of C₂H₄O₂

BUT, including the E/Z and R/S isomers, there are a total of 12 distinct isomers of C₂H₄O₂

If you think there any others, please let me know asap!

(1) ethanoic acid, (acetic acid), structural formula, displayed formula and skeletal formula

, , ,

An aliphatic carboxylic acid, -COOH functional group..

Functional group isomer, but no E/Z or R/S isomerism possible.

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 2 ¹³C (email if disagree?)

¹H NMR ratio of peaks: 3 : 1 (for equivalent protons)

See a comparison of infrared diagnostic spectral peaks for the isomers of C₂H₄O₂

 

(2) methyl methanoate (methyl formate), structural formula, ,

displayed formula  , skeletal formula  

An ester, -COOC- functional group.

Functional group isomer, but no E/Z or R/S isomerism possible.

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 2 ¹³C (email if disagree?)

¹H NMR ratio of peaks: 3 : 1 (for equivalent protons)

See a comparison of infrared diagnostic spectral peaks for the isomers of C₂H₄O₂

Spectra links for isomers (1) and (2)) which can be used to identify a specific isomer

The infrared spectrum of Ethanoic acid (acetic acid)

The infrared spectrum of methyl methanoate (methyl formate)

The mass spectrum of Ethanoic acid (acetic acid)

The mass spectrum of methyl methanoate (methyl formate)

The H-1 NMR spectrum of Ethanoic acid (acetic acid)

The H-1 NMR spectrum of methyl methanoate (methyl formate)

C-13 NMR spectrum of Ethanoic acid (acetic acid)

The C-13 NMR spectrum of methyl methanoate (methyl formate)

 

(3) There is also an enol (alkene-diol) of molecular formula C₂H₄O₂

ethene-1,2-diol (ethen-1,2-diol, 1,2-ethenediol, an enol),

structural formula, HO-CH=CH-OH

The molecule has two different functional groups, alcohol (2 x -OH, hydroxyl, diol) and alkene (>C=C<), hence referred to as an 'enol' molecule.

Functional group isomer and positional structural isomer (1,2-OH groups) and ...

HOCH=CHOH (abbreviated formula) exhibits E/Z (geometrical cis and trans isomers) stereoisomerism because of restricted rotation about the >C=C< carbon-carbon double bond.

The E/Z stereoisomers can be named as

(E)-1,2-dihydroxyethene  and  (Z)-1,2-dihydroxyethene

or (E)-ethene-1,2-diol  and  (Z)-ethene-1,2-diol  (E = trans and Z = cis isomers)

, skeletal formula of the two E/Z isomers.

From the CIP priority rule to assign E/Z geometrical isomers: ₈O > ₁H (a nice easy one!)

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 1 ¹³C (email if disagree?)

¹H NMR ratio of peaks: 2 (1+1) : 2 (1+1) (for equivalent protons)

This is an unstable molecule and likely to isomerise to 2-hydroxyethanal.

HO-CH=CH-OH HO-CH₂-CHO

An example of keto-enol tautomerism.

See a comparison of infrared diagnostic spectral peaks for the isomers of C₂H₄O₂

 

(4) Ethene-1,1-diol (ethen-1,1-diol, 1,1-ethenediol),where both hydroxyl groups are attached to the same carbon atom.

H₂C=C(OH)₂ cannot exhibit E/Z isomerism and is a positional (OH group) isomer of ethene-1,2-diol.

Functional group isomer and positional structural isomer (1,1-OH groups)

The skeletal formula of ethene-1,1-diol (1,1-dihydroxyethene), with two functional groups (alkene and alcohol). It is a highly unstable molecule and possibly an intermediate in a synthesis route to ethanoic acid.

This molecule, theoretically, is likely to lose a water molecule and form ketene

H₂C=C(OH)₂  ==> H₂C=C=O  +  H₂O

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 1 ¹³C (email if disagree?)

¹H NMR ratio of peaks: 2 : 2 (1+1), (for equivalent protons)

See a comparison of infrared diagnostic spectral peaks for the isomers of C₂H₄O₂

 

(5) A hydroxy-aldehyde of molecular formula C₂H₄O₂

2-hydroxyethanal, HO-CH₂-CHO , structural formula, very abbreviated formula HOCH₂CHO

, skeletal formula of 2-hydroxyethanal (glycolaldehyde).

Functional groups present: aldehyde (-CHO) and alcohol (-OH, hydroxy group).

(You cannot have a ketone isomer, need three carbon atoms!)

Number of low resolution NMR chemical shift δ signal peaks: 3 ¹H and 2 ¹³C (email if disagree?)

¹H NMR ratio of peaks: 1 : 2 : 1 (for equivalent protons)

See a comparison of infrared diagnostic spectral peaks for the isomers of C₂H₄O₂

 

(6-9) Four other isomers you are unlikely to encounter pre-university.

These are four heterocyclic compounds with C-O-C bonds (cyclic ether link) or -O-O- bonds (peroxide linkage) in them. They are all functional group isomers.

, skeletal formula

(6)(a) 1,2-dioxetane (ethylene peroxide, peroxyethane)

A cyclic organic peroxide -O-O-

Number of low resolution NMR chemical shift δ signal peaks: 1 ¹H and 1 ¹³C

(7)(b) 1,3-dioxetane

A type of cyclic ether (C-O-C).

Number of low resolution NMR chemical shift δ signal peaks: 1 ¹H and 1 ¹³C

(8)(c) 3-methyldioxirane (methyldioxirane)

A cyclic organic peroxide -O-O-

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 2 ¹³C

¹H NMR ratio of peaks: 1 : 3 (for equivalent protons)

(9)(d) oxiranol (2-oxiranol, epoxyethanol, hydoxyoxirane)

Both a cyclic ether (C-O-C) and an alcohol (-OH).

This molecule can exhibit R/S isomerism, bottom right C of ∆ is chiral.

Number of low resolution NMR chemical shift δ signal peaks: 3 ¹H and 2 ¹³C

¹H NMR ratio of peaks: 2 : 1 : 1 (for equivalent protons) (email if disagree?)

 

(10) H₂C=CH-OOH, , vinyl hydroperoxide is a highly unstable unsaturated aliphatic compound.

Vinyl hydroperoxide has two functional groups.

>C=C< alkene and an organic hydro-peroxide C-O-O-H group.

Number of low resolution NMR chemical shift δ signal peaks: 3 ¹H and 2 ¹³C

¹H NMR ratio of peaks: 2 : 1 : 1 (for equivalent protons) (email if disagree?)

There is a possibility that the proton peak for the end H₂C= group, may split into two very close chemical shifts because of the asymmetry of the field experienced due to the asymmetry of the groups at the other end of the double bond =CHOOH.

 

Isomeric variety!

For a simple molecular formula of C₂H₄O₂ there is, perhaps surprising. variety of functional group isomers i.e. molecules with a carboxylic acid, ester, aldehyde, alcohol, ether or an alkene group (some with two functional groups).

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Summary of key points and extra notes on the isomers of molecular formulae C₂H₄O₂

This deceptively small formula, C₂H₄O₂, hides a rich variety of structural and functional possibilities. Below is a full breakdown of the isomerism types, along with differences in physical and chemical properties, uses, and exam-relevant notes and misconceptions.

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Some types of structural Isomerism in C₂H₄O₂ molecules you are likely to come across

Isomer Type	Examples	Explanation
Functional group	Ethanoic acid versus methyl methanoate (methyl formate)	Same molecular formula, different functional groups: carboxylic acid versus ester. There are also 4 cyclic compounds
Structural	Ethanoic acid versus Hydroxyethanal	Same atoms arranged in different connectivity
Tautomerism	Hydroxyethanal ↔ ethene-1,2-diol	Keto-enol equilibrium
Stereoisomerism	Ethene-1,2-diol C2H4O2	E/Z (geometrical) isomerism via C=C
Stereoisomerism	Oxiranol, heterocyclic ring compound	R/S (optical isomerism), C1 carbon atom in the ring is chiral

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Some key isomers of C₂H₄O₂

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Differences in Physical Properties of the isomers of C₂H₄O₂

Exam tip: Differences in boiling point often relate to hydrogen bonding capability. Ethanoic acid forms dimers via hydrogen bonding - worth noting!

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Differences in Chemical Properties of the isomers of C₂H₄O₂

Misconception warning: Students often confuse esters and acids because they share some naming roots (“-oate”, “-ic acid”). But esters are neutral and require hydrolysis to yield acids.

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Common Uses and Applications of some of the isomers of C₂H₄O₂

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A comparison of the prominent peaks in the infrared spectra of the isomers of C₂H₄O₂

A structured comparison of the infrared (IR) absorption bands for nine constitutional isomers of C₂H₄O₂, each identified by their IUPAC name and key functional groups:

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IR Spectral Comparison of infrared wavenumber bands for some of the C₂H₄O₂ Isomers

IUPAC Name	Functional Groups	Key IR Bands (cm⁻¹)	Diagnostic Features
Ethanoic acid	Carboxylic acid	~1710 (C=O), 2500–3300 (broad O–H), ~1200 (C–O)	Broad acid O–H band + sharp C=O stretch
Methyl methanoate	Ester	~1740 (C=O), 1200–1300 (C–O), ~2950 (C–H)	Ester C=O + C–O stretches
2-hydroxyethanal	Aldehyde + Alcohol	~1725 (C=O), 3200–3600 (O–H), 2700–2830 (C–H)	Aldehyde C–H + alcohol O–H
Ethene-1,1-diol (1,1-dihydroxyethene)	Enol (Alcohol + Alkene)	~1600–1650 (C=C), 3200–3600 (O–H)	Alkene + alcohol (predicted)
Ethene-1,2-diol	Enol (Alcohol + Alkene)	~1600–1650 (C=C), 3200–3600 (O–H)	Alkene + alcohol (observed)
Four heterocyclic molecules	ether, peroxide or alcohol	Only oxiranol will give the 2500–3300 (broad O–H),	Broad acid O–H but no sharp C=O stretch or C=C stretch modes, the other three give no OH, C=C or C=O bands

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Notes for Interpretation of the infrared spectra of the isomers of C₂H₄O₂

• Carboxylic acids and alcohols show a very broad O–H stretch overlapping with C–H region.

• Esters have sharp C=O and C–O stretches, often with minimal O–H.

• Aldehydes exhibit strong C=O, and a unique C–H stretch around 2700–2830 cm⁻¹.

• Alcohols show a very broad O–H stretch overlapping with C–H region and a sharp band for C=C if they are an enol..

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Some Exam Tips and Misconceptions about the isomers of C₂H₄O₂

• Confusion between functional group and positional isomerism: In C₂H₄O₂, most isomerism is functional or structural.

• Be cautious when drawing esters and acids: Swapped atoms can invalidate the functional group.

• Naming errors: Students often name methyl formate incorrectly due to reversed ester naming conventions.

• Remember tautomerism is a dynamic equilibrium, not permanent: vinyl alcohol isn’t isolable under normal conditions - it rapidly converts to acetaldehyde.

• Practice distinguishing IR/NMR spectra: e.g., ethanoic acid shows broad OH peak, ester shows characteristic C=O stretch.

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Learning objectives - questions to be answered?

How do you work out the structure of the isomers of molecular formula C2H4O2?

How do you draw the structural formula and skeletal formula of the isomers of molecular formula C2H4O2?

How do you name the isomers of molecular formula C2H4O2?

How many aliphatic structural isomers are there of molecular formula C2H4O2?

How many aliphatic carbon chain isomers are there of molecular formula C2H4O2?

How many positional isomers are there of molecular formula C2H4O2?

Are there any aliphatic open chain alkene isomers of molecular formula C2H4O2?

Are there any carboxylic acid isomers of molecular formula C2H4O2?

Are there any ester isomers of molecular formula C2H4O2?

Are there any enol isomers of molecular formula C2H4O2?

Are there any alkene isomers of molecular formula C2H4O2?

Are there any alcohol isomers of molecular formula C2H4O2?

Are there any aldehyde isomers of molecular formula C2H4O2?

Are there any ketone isomers of molecular formula C2H4O2?

Are there any functional group isomers with a molecular formula C2H4O2?

Does C2H4O2 have any stereoisomers?

Are there any E/Z (geometrical) isomers with a molecular formula C2H4O2?

Are there any R/S (optical) isomers (enantiomers) with a molecular formula C2H4O2?

How many E/Z (geometrical) isomers are there of molecular formula C2H4O2?

How many R/S (optical) isomers (enantiomers) of molecular formula C2H4O2?

This page will answer these questions for molecular formula C₂H₄O₂

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Associated organic chemistry  links

 Advanced Level pre-university organic chemistry notes

 IR, mass and H-1 & C-13 NMR spectra of organic compounds

 Index of sets of isomers for a given molecular formula

Molecular structure and naming of CARBOXYLIC ACIDS and DERIVATIVES

Examples of effects of isomerism on similarity or difference in the physical & chemical properties of structural isomers

Examples of comparing the physical and chemical properties of R/S (optical) isomers (enantiomers)

Comparison of the ir, mass, 1H and 13C NMR spectra of the isomers of C₂H₄O₂ (via a 1H NMR spectrum page)

INDEX of ALL revision notes on the chemistry of CARBOXYLIC ACIDS and DERIVATIVES

For isomerism in organic chemistry, see also the notes

Isomerism: introduction, structural isomerism - chain, positional, functional group, tautomerism

Stereoisomerism: introduction, definition, priority rules, E/Z isomerism (cis/trans isomerism)

Stereoisomerism - R/S isomerism (optical isomerism) - definition - examples explained

 This is a big chemistry website, please allow time to explore it

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