isomers of C2H4O

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Advanced level organic chemistry PART 14.7: Structural isomers of molecular formula C2H4O

Doc Brown's Advanced Chemistry: Part 14.7: Isomers and extra notes on their properties and uses

Structural isomers of molecular formula C2H4O

[Author ©  Dr WP Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses, IB advanced chemistry and US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy and analysing the isomers of C2H4O [isomerism page updated Feb 9th 2026 *]

 Index of sets of isomers for a given molecular formula

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 Associated organic chemistry page links

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

The three constitutional-structural isomers of molecular formula C2H4O

Percent mass composition of C2H4O based on the relative atomic masses

C= 12.01  H = 1.01  O = 16.00  and  Mr(C2H4O) = 44.06

Element composition of C2H4O by mass: 54.52 % carbon  * 9.17% hydrogen  *  36.31% oxygen

Empirical formula = molecular formula = C2H4O

Introduction to isomerism for molecular formula C2H4O  (see also summary diagram)

isomers of C2H4O names skeletal structural formula types of isomerism how to analysise C2H4O for functional group R/S optical geometrical positional substituent isomers of C2H4O

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 C 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.

Here, only (c) functional group isomerism is applicable to the 3 constitutional isomers of C2H4O

All three isomers of C2H4O are aliphatic compounds with different functional groups.

Two are open chain (aliphatic) and one cyclic (heterocyclic) compounds.

Stereoisomerism - isomers based on the same connectivity of the atoms, but 2D or 3D spatially different, non-superimposable images (e.g. E/Z 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. NOT possible for C2H4O

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

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. The molecule must have a chiral centre (a stereocentre), that is an asymmetric carbon atom with four different atoms/groups attached to it.

Details of the 3 distinct constitutional isomers of molecular formula C2H4O

(1) ethanal (acetaldehyde),  aldehydes and ketones nomenclature (c) doc b, structural formula ethanal aldehydes and ketones nomenclature (c) doc b, displayed formula of ethanal aldehydes and ketones nomenclature (c) doc b, skeletal formula of ethanal aldehydes and ketones nomenclature (c) doc b , an aliphatic aldehyde

Functional group -CHO, functional group isomer.

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

1H NMR ratio of integrated peak areas: 3 : 1  (for equivalent protons)

See a comparison of infrared diagnostic spectral peaks for the isomers of C2H4O

The infrared spectrum of ethanal (acetaldehyde) (no C=C or OH absorptions)

The mass spectrum of ethanal (acetaldehyde)

The H-1 NMR spectrum of ethanal (acetaldehyde) (different in number of peaks and their ratio)

The C-13 NMR spectrum of ethanal (acetaldehyde)

 

(2) ethenol (vinyl alcohol), H2C=CH-OH, two functional groups (enol), functional group isomer.

alkene (>C=C<) and alcohol (-OH), very unstable, readily polymerises or isomerises to ethanal

e.g. this unsaturated compound can isomerise to ethanal.

H2C=CHOH CH3-CHO

An example of keto-enol tautomerism.

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

1H NMR ratio of integrated peak areas: 2 : 1 : 1  (for equivalent protons)

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 =CHOH.

The infrared spectrum would show strong absorption at ~3500 cm-1 and the C=C can be distinguished from the C=O vibrations in the ethanal IR spectrum.

See a comparison of infrared diagnostic spectral peaks for the isomers of C2H4O

(3) oxirane, epoxyethane, 1,2-epoxyethane, ethylene oxide, structural formula of oxirane, epoxyethane, 1,2-epoxyethane, ethylene oxide , (strained bonds)

It is cyclic ether, an example of a heterocyclic comound.

Functional group isomer of (1) and (2) above, a very reactive epoxy compound due to the highly strained ring.

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

See a comparison of infrared diagnostic spectral peaks for the isomers of C2H4O

 

Note that spectral analysis can help distinguish between isomers of C2H4O

Summary of key points and extra notes on the isomers of molecular formulae C2H4O

This is a classic molecular formula that packs a punch in terms of isomerism and exam relevance. Let’s break down the types of isomerism, explore physical and chemical differences, and finish with uses, misconceptions, and exam tips in a structured, exam-friendly format.

Isomers of C2H4O and Types of Isomerism

Isomer

Structure

Functional Group

Type of Isomerism

Ethanal (Acetaldehyde)

CH3–CHO

Aldehyde

Functional group isomerism

Oxirane (Ethylene oxide)

Cyclic three-membered ring

Epoxide (cyclic ether)

Functional group & ring-chain isomerism

Ethenol (Vinyl alcohol)

CH2=CH–OH

Alcohol (enol)

Functional group isomerism + tautomerism

Tautomerism: Ethenol (Vinyl alcohol) readily tautomerizes to acetaldehyde, making it unstable and rarely isolated.

Differences in Physical Properties of the isomers of C2H4O

Property

Ethanal (Acetaldehyde)

Oxirane (Ethylene Oxide)

Ethenol (Vinyl Alcohol)

Boiling Point (°C)

~21

~10.4

Not well-defined (tautomerizes)

State at RT

Liquid

Gas

Unstable

Polarity

Polar

Polar

Polar

Hydrogen Bonding

Weak (no –OH)

None (no –OH)

Strong (–OH group)

Exam Tip: Alcohols generally have higher boiling points due to hydrogen bonding. But vinyl alcohol is unstable—don’t fall into the trap of assuming it behaves like ethanol.

Differences in Chemical Properties of the isomers of C2H4O

Isomer

Reactivity

Key Reactions

Ethanal (Acetaldehyde)

Undergoes nucleophilic addition

Aldol condensation, oxidation to acetic acid

Oxirane (Ethylene oxide)

Highly strained; undergoes ring-opening reactions

Acid/base-catalyzed nucleophilic substitution

Ethenol (Vinyl alcohol)

Tautomerizes to Ethanal (acetaldehyde)

Enol chemistry (rarely isolated)

Misconception: Ethylene oxide is not just an ether—it’s a strained epoxide, making it far more reactive than typical ethers.

Some uses of the isomers of the isomers of C2H4O

Isomer

Uses

Ethanal (Acetaldehyde)

Intermediate in synthesis of acetic acid, perfumes, and flavourings

Oxirane (Ethylene oxide)

Sterilizing agent, precursor to ethylene glycol (antifreeze, polymers)

Ethenol (Vinyl alcohol)

Intermediate in polymer chemistry (via vinyl acetate → PVA)

Safety Note: Ethylene oxide is flammable, toxic, and carcinogenic—important for industrial and regulatory contexts.

Common Misconceptions about the isomers of the isomers of C2H4O

  • Ethenol (Vinyl alcohol) is stable: It’s not. It tautomerizes to acetaldehyde almost instantly.

  • Oxirane (Ethylene oxide) is just an ether: It’s a strained cyclic ether (epoxide), much more reactive.

  • All isomers have similar boiling points: Hydrogen bonding and ring strain drastically affect physical properties.

Exam Tips for questions involving the isomers of the isomers of C2H4O

  • Always draw structures when asked about isomers—especially for functional group isomerism.

  • Mention tautomerism when discussing vinyl alcohol.

  • Compare boiling points and reactivity to show understanding of intermolecular forces and functional groups.

  • Don’t confuse constitutional isomers with stereoisomers—C₂H₄O has no stereoisomers.

  • Use reaction examples (e.g., aldol condensation for acetaldehyde) to show chemical behavior.

Comparison of the infrared spectra of the isomers of C2H4O

Certainly! The three isomers of molecular formula C2H4O ethanal, epoxyethane, and ethenol — each exhibit distinct infrared (IR) spectral features due to their unique functional groups. Here's a comparative breakdown using modern IUPAC names:

Isomer Overview for C2H4O

IUPAC Name

Structure Type

Key Functional Group(s)

Ethanal (acetaldehyde)

Linear

Aldehyde (–CHO)

Epoxyethane (ethylene oxide)

Cyclic (3-membered ring)

Epoxide (cyclic ether)

Ethenol (vinyl alcohol)

Linear (unstable)

Alcohol (–OH) + Alkene (C=C)

An infrared Spectral Comparison of wavenumbers for the isomers of C2H4O

IR Region (cm⁻¹)

Ethanal (acetaldehyde)

Epoxyethane (ethylene oxide)

Ethenol (vinyl alcohol)

~1725

Strong C=O stretch (aldehyde)

Absent

Absent

~2700–2830

Weak aldehyde C–H stretch

Absent

Absent

~1050–1150

Weak C–O stretch

Strong C–O stretch (epoxide ring)

Strong C–O stretch (alcohol)

~3200–3600

Absent

Absent

Broad O–H stretch (alcohol)

~1600–1650

Absent

Absent

C=C stretch (alkene)

Fingerprint region

Complex bending vibrations

Ring deformation modes

C–H and O–H bending

 Key Diagnostic Peaks in the infrared spectra of the isomers of C2H4O

  • Ethanal:

    • C=O stretch near 1725 cm⁻¹

    • Aldehyde C–H stretch around 2700–2830 cm⁻¹ (often weak)

  • Epoxyethane:

    • Strong C–O stretch in the 1050–1150 cm⁻¹ region

    • Ring deformation bands in the fingerprint region

  • Ethenol:

    • Broad O–H stretch around 3200–3600 cm⁻¹

    • C=C stretch near 1600–1650 cm⁻¹

    • Note: Ethenol is typically unstable and tautomerizes to ethanal

Learning objectives - questions to be answered?

How many constitutional isomers are there of formula C2H4O?

How do you draw the skeletal structure of isomers of C2H4O?

How do you draw the structural formula of isomers of C2H4O?

Can you recognise the different functional groups in the isomers of C2H4O?

How do you name the isomers of C2H4O?

This page will answer these questions for molecular formulae C2H4O

A summary chart of isomerism

index for all isomerism pages

Associated organic chemistry  links

 Index of sets of isomers for a given molecular formula

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

The molecular structure and naming of ALKENES

The molecular structure and naming of aliphatic ALCOHOLS including isomeric ethers

The molecular structure and Naming of ALDEHYDES and KETONES

Index of all IR, mass, 1H NMR and 13C NMR spectroscopy pages

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

Index of advanced (pre-university) organic chemistry revision notes

 The chemistry of alkanes and the petrochemical industry

 The chemistry of alkenes

 The chemistry of organic halogen compounds

 The chemistry of alcohols

 The chemistry of aldehydes and ketones

  The chemistry of carboxylic acids and derivatives

The chemistry of organo-nitrogen compounds

 The chemistry of aromatic compounds

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