2 constitutional isomers of molecular formula C3H6 mass 42, two functional group structural isomers, IUPAC names, carbon chain isomers structural formula, skeletal formula of propene cyclopropane uses properties applications Doc Brown's advanced level chemistry revision notes for AQA Edexcel OCR Salters IB US honors courses

isomers of C₃H₆

Advanced level organic chemistry PART 14.7: Constitutional isomers of molecular formula C₃H₆

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

The constitutional structural carbon chain isomers of molecular formula C₃H₆

[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 & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy and analysing the isomers of C₃H₆ [updated Feb 18th 2026 *]

Associated organic chemistry page links

Index of sets of isomers for a given molecular formula

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The 2 non-cyclic alkene and cycloalkane constitutional structural isomers of molecular formula C₃H₆ (M_r = 42)

more details below

Percent composition of C₃H₆ based on atomic masses C= 12.01 H = 1.01 and M_r(C₃H₆) = 42.09

Element composition (to two dp): carbon = 85.60% hydrogen = 14.40%

Empirical formula = CH₂ and molecular formula = C₃H₆

Introduction to isomerism for molecular formula C₃H₆ (see also summary diagram)

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.

In terms of isomers of C₃H₆ there are

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

(b) they are functional group isomers of each other e.g. an unsaturated alkene and a saturated cycloalkane.

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. Neither possible for C₃H₆

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.

There are only 2 constitutional isomers of molecular formula C₃H₆, 1 alkene and 1 cycloalkane.

All the isomers of molecular formula C₃H₆ are open chain aliphatic compounds (e.g. here an unsaturated alkene) or cyclic aliphatic compounds (e.g. here a saturated cycloalkane), the latter sometimes referred to as an alicyclic compound (cyclo-aliphatic!).

Since one is an alkane and the other an alkene, this is a case of functional group isomerism.

Details of the 2 structural-constitutional isomers of C₃H₆

(1) propene, , , , abbreviated structural formulae

The full displayed formula of propene.

and The skeletal formula of propene, a non-cyclic alkene

The chemistry of ALKENES (detailed notes on all aspects of alkene chemistry)
Number of low resolution NMR chemical shift δ signal peaks: 3 ¹H and 3 ¹³C (email if disagree?)

¹H NMR ratio of peaks: 3 : 1 : 2 (for equivalent protons)

BUT, for the 'end' =CH₂ alkene protons, you can get two chemical shifts close together, if there are two different groups attached to the other carbon of the C=C bond i.e. R"R'C=CH₂ where R" and R' are different. This causes the two =CH₂ protons to experience slightly different fields.

See also a comparison of infrared spectra for the isomers of C₃H₆

See also spectra that can distinguish one C3H6 isomer from another

The infrared spectrum of propene

The mass spectrum of propene

The H-1 NMR spectrum of propene

The C-13 NMR spectrum of propene

and the differences in chemistry between propene and cyclopropane

Index of ¹H NMR spectra organic compounds and Index of ¹³C NMR spectra organic compounds

(2) cyclopropane , , , a saturated cycloalkane

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

See also a comparison of infrared spectra for the isomers of C₃H₆

See also spectra that can distinguish one C₃H₆ isomer from another

The infrared spectrum of cyclopropane

The mass spectrum of cyclopropane

The H-1 NMR of cyclopropane

The C-13 NMR spectrum of cyclopropane

These two molecules are an example of functional group isomerism, (1) an alkene and (2) an alkane, each having a significant different chemistry as members of their own homologous series.

The chemistry of ALKANES (detailed notes on all aspects of alkane chemistry)

Neither propene or cyclopropane molecules can exhibit E/Z or R/S isomerism.

Summary of key points and extra exam revision notes on the isomers of molecular formulae C₃H₆ and their properties and uses

The molecular formula C₃H₆ has quite limited isomeric possibilities.:

Isomerism Types Exhibited by molecular formula C₃H₆

Isomer Type	Description	C₃H₆ Examples
Structural (Chain)	Rearrangement of carbon skeleton	Only if you count linear versus ring, one is cyclic, the other acyclic (linear)
Structural (Functional)	Different functional groups with same formula	Alkene (unsaturated) versus cycloalkane (saturated)
Structural (Positional)	Functional group (e.g. double bond) in different positions	Not possible here, need longer chain or non-hydrogen substituents
Geometrical (E/Z)	Restricted rotation around C=C; needs two different groups on each C of the double bond	Not possible here and neither is R/S optical isomerism.

Differences in Physical Properties of the isomers of C₃H₆ (both smelly colourless hydrocarbon gases at room temperature)

Property	Alkenes (Propene)	Cyclopropane
Boiling point	Lower (–48 °C)	Slightly higher (–33 °C)
Density	Lower	Slightly higher
Bond strain	Minimal	High (angle strain ~60°)

Differences in Chemical Reactivity and Uses of the isomers of C₃H₆

Isomer	Chemical Reactivity	Typical Applications
Propene	Undergoes electrophilic addition; forms polymers	Used in polypropylene plastics
Cyclopropane	Highly strained; undergoes ring-opening reactions	Historically used as anaesthetic (now obsolete)

Common Misconceptions about the isomers of C₃H₆ (see also below)

Geometrical isomerism doesn't always apply: students often assume any alkene can show E/Z isomerism, but identical groups prevent it.
Cyclopropane ≠ alkene: its reactivity may mimic that of unsaturated compounds due to ring strain, but it’s fully saturated.

Exam Tips for questions involving the isomers of C₃H₆ (see also above)

Highlight which type of isomerism is shown and justify with a structural sketch.
Link structural features to reactivity (e.g., ring strain → cyclopropane's reactivity).
For synoptic questions, compare bond angles: 120^o (for C-C=C) in alkenes versus 60^o (for C-C-C) in cyclopropane (huge energy difference!).

An infrared spectra comparison of the isomers of C₃H₆

A structured comparison of the infrared (IR) spectra of propene (CH₂=CH–CH₃) and cyclopropane (cyclo-C₃H₆), highlighting how molecular geometry and bonding influence their vibrational modes:

Structural Overview of the isomers of C₃H₆

Molecule	Structure Type	Key Features
Propene	Linear alkene	C=C double bond, sp² & sp³ C–H bonds
Cyclopropane	Cyclic alkane	Strained 3-membered ring, all sp³ C–H bonds

IR Spectral Comparison of the isomers of C₃H₆

Region (cm⁻¹)	Propene Features	Cyclopropane Features
3100–3000	=C–H stretch (sp²) — sharp, medium intensity	C–H stretch (sp³) — multiple overlapping bands
3000–2850	C–H stretch (sp³) — methyl group	C–H stretch (sp³) — ring CH₂ groups
1650–1600	C=C stretch — medium intensity	Absent — no double bonds
1450–1350	CH₃/CH₂ bending — scissoring & deformation	CH₂ scissoring and wagging
1000–650	=C–H out-of-plane bending — diagnostic	Ring deformation, CH₂ rocking & twisting
Fingerprint region	Complex due to conjugation and bending modes	Complex due to ring strain and symmetry

Notable IR spectrum Differences of the isomers of C₃H₆

Propene shows alkene-specific bands:
- =C–H stretch above 3000 cm⁻¹
- C=C stretch around 1650 cm⁻¹
- Out-of-plane =C–H bending near 900 cm⁻¹
Cyclopropane lacks unsaturation but exhibits:
- Unique ring deformation modes (e.g. ~850–750 cm⁻¹)
- Strain-induced shifts in CH₂ wagging and twisting

Spectroscopic Insight for the infrared spectrum of the isomers of C₃H₆

The planar sp² hybridization in propene leads to sharper, more intense =C–H stretches and bending vibrations.
Cyclopropane’s ring strain and symmetry cause splitting and shifting of CH₂ vibrations, often making its fingerprint region more congested and less intuitive.

Learning objectives - questions to be answered?

Be able to describe and write out the isomers of C3H6

What are the possible isomers of C3H6?

How do you work out the isomers of molecular formula C3H6?

How do you draw the structural formula and skeletal formula of the isomers of molecular formula C3H6?

How many aliphatic structural isomers are there of molecular formula C3H6?

How many aliphatic carbon chain isomers are there of molecular formula C3H6?

How many positional isomers are there of molecular formula C3H6?

How many E/Z (geometrical) isomers are there of molecular formula C3H6?

How many R/S (optical) isomers (enantiomers) of molecular formula C3H6?

Are there any aliphatic open chain alkene isomers of molecular formula C3H6?

Are there any diene isomers of molecular formula C3H6?

Are there any alkyne isomers of molecular formula C3H6?

Are there any alicyclic cycloalkene isomers of molecular formula C3H6?

Are there any functional group isomers with a molecular formula C3H6?

Are there any E/Z (geometrical) isomers with a molecular formula C3H6?

Are there any R/S (optical) isomers (enantiomers) with a molecular formula C3H6?

Does C3H6 have any stereoisomers?

This page will answer these questions for molecular formula C3H6.

Associated organic chemistry links

Advanced Level pre-university organic chemistry notes

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

Index of sets of isomers for a given molecular formula

Comparison of the ir, mass, 1H and 13C NMR spectra of the isomers of C₃H₆ (via the 1H NMR spectrum page)

Index of sets of isomers for a given molecular formula

The molecular structure and naming of ALKANES (how to name and draw alkane structures)

The molecular structure and naming of ALKENES (how to name and draw alkene structures)

Index of revision notes on the chemistry of ALKANES and the petrochemical industry

INDEX of ALL revision notes on the chemistry ALKENES including reactions and polymers

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

Summary diagram of isomerism which links to details of the types of isomerism

Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of Doc Brown's pre-university advanced level chemistry website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial. These organic chemistry revision notes on isomerism are suitable for use of pre-university students studying AQA advanced level chemistry constitutional isomers of C3H6, Edexcel advanced level chemistry constitutional isomers of C3H6, OCR advanced level chemistry constitutional isomers of C3H6, IB advanced level chemistry constitutional isomers of C3H6, WJEC (Eduqas) advanced level chemistry constitutional isomers of C3H6, CIE Cambridge advanced level chemistry constitutional isomers of C3H6, US grade 11-12 AP honors chemistry courses constitutional isomers of C3H6 and they will also prove useful to 1st year undergraduate students of chemistry including constitutional isomers of C3H6.

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Structural Overview of the isomers of C3H6

IR Spectral Comparison of the isomers of C3H6

Notable IR spectrum Differences of the isomers of C3H6

Spectroscopic Insight for the infrared spectrum of the isomers of C3H6

Structural Overview of the isomers of C₃H₆

IR Spectral Comparison of the isomers of C₃H₆

Notable IR spectrum Differences of the isomers of C₃H₆

Spectroscopic Insight for the infrared spectrum of the isomers of C₃H₆