26 selected constitutional isomers of molecular formula C₄H₈O (also many stereoisomers)

Percent mass composition of C₄H₈O based on the relative atomic masses

C= 12.01  H = 1.01  O = 16.00  and  M_r(C₄H₈O) = 72.12

Element composition of C₄H₈O by mass: 66.61% carbon * 11.20% hydrogen * 22.19% oxygen

Empirical formula = molecular formula of C₄H₈O

For E/Z (geometrical) or R/S (optical) stereoisomers (CIP) means the IUPAC Cahn-Ingold-Prelog isomer assignment priority order rule.

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.

All three types of structural isomerism applies to most the isomers of C₄H₈O e.g.

(a) Variation of carbon chain in enols, open chain versus cyclic for the carbon atoms and also sequences of carbon and oxygen atoms.

(b) Position of >C=C< and -OH groups in the 'enols', positions of -CH₃ groups in epoxy compounds.

(c) Alkene-alcohols (enols) versus cycloalcohols versus epoxy compounds versus unsaturated ethers.

 

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.

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

I have identified 9 constitutional isomers of C₄H₈ that exhibit stereoisomerism, some of it a bit complex due to overlap of E/Z and R/S isomerism.

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.

E/Z isomerism is seen in 3 of the alkene-alcohols ('enes'), 1 of the unsaturated ethers, and 2 of the cyclic compound isomers of  C₄H₈O

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.

Does apply to a few of the isomers of C₄H₈O, one of the 'enols' and 4 of the cyclic compounds.

 

Note

Two of the examples of stereoisomerism exhibit an overlap of E/Z and R/S isomers, so a bit complex and requires a university level analysis.

I have identified at least 26 constitutional isomers of molecular formula C₄H₈O and 9 of these constitutional isomers of C₄H₈O can exhibit stereoisomerism, so there are at least 35 distinct isomers of C₄H₈O.

So, there maybe more? Let me know if you spot any I have missed!

Some of these isomers are highly reactive and very unstable and some may not exist at all, except theoretically of course using the valency rules for carbon >4<, oxygen >2 and hydrogen -1.

(12) 1-methoxyprop-1-ene, CH₃CH=CHOCH₃, constitutional structural formula (abbreviated)

A linear aliphatic unsaturated ether, skeletal formula

Two functional groups: alkene (>C=C<) and ether linkage (C-O-C).

Exhibits E/Z (geometrical) isomerism: From CIP rule priority is ₈O > ₆C > ₁H

(for atoms and groups about the >C=C< double bond)

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 4 ¹³C (email if disagree?)

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

Infrared spectra comment: Strong absorption for C=C wavenumber, but absence of OH band.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

 

(13) 2-methoxyprop-1-ene, CH₃C(OCH₃)=CH₂, constitutional structural formula (abbreviated)

A branched aliphatic unsaturated ether, skeletal formula

Two functional groups: alkene (>C=C<) and ether linkage (C-O-C).

Number of low resolution NMR chemical shift δ signal peaks: 3 ¹H and 4 ¹³C (email if disagree?)

¹H NMR ratio of integrated chemical shift peak areas: 3 : 3 : 2  (for equivalent protons)

Infrared spectra comment: Strong absorption for C=C wavenumber, but absence of OH band.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

 

(14) 3-methoxyprop-1-ene, CH₃OCH₂CH=CH₂, constitutional structural formula (abbreviated)

A linear aliphatic unsaturated ether, skeletal formula

Two functional groups: alkene (>C=C<) and ether linkage (C-O-C).

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 4 ¹³C (email if disagree?)

¹H NMR ratio of integrated chemical shift peak areas: 3 : 2 : 1 : 2  (for equivalent protons)

Infrared spectra comment: Strong absorption for C=C wavenumber, but absence of OH band.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

 

(15) 1-ethoxyethene, CH₃CH₂OCH=CH₂, constitutional structural formula (abbreviated)

A linear aliphatic unsaturated ether, skeletal formula

Two functional groups: alkene (>C=C<) and ether linkage (C-O-C).

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 4 ¹³C (email if disagree?)

¹H NMR ratio of integrated chemical shift peak areas: 3 : 2 : 1 : 2  (for equivalent protons)

Infrared spectra comment: Strong absorption for C=C wavenumber, but absence of OH band.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

Alicyclic compounds based on cyclobutane and cyclopropane

(16) cyclobutanol, skeletal formula above

One functional group: a secondary cyclic aliphatic alcohol (C-OH)

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 3 ¹³C (email if disagree?)

¹H NMR ratio of integrated chemical shift peak areas: 1 : 1 : 4 (2+2) : 2  (for equivalent protons)

Infrared spectra comment: Strong absorption for the C=C wavenumber, absent for OH.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

 

(17) cyclopropylmethanol, skeletal formula above

One functional group: a primary aliphatic alcohol (C-OH)

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 3 ¹³C (email if disagree?)

¹H NMR ratio of integrated chemical shift peak areas: 4 (2+2) : 1 : 2 : 1  (for equivalent protons)

Infrared spectra comment: Strong absorption for the C=C wavenumber, absent for OH.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

 

(18) 1-methylcyclopropanol, 1-methylcyclopropan-1-ol, skeletal formula above

One functional group: a cyclic tertiary aliphatic alcohol (C-OH)

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

¹H NMR ratio of integrated chemical shift peak areas: 4 (2+2) : 3 : 1  (for equivalent protons)

Infrared spectra comment: Strong absorption for the C=C wavenumber, absent for OH.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

 

(19) 2-methylcyclopropanol, 2-methylcyclopropan-1-ol, skeletal formula above

One functional group: a cyclic secondary aliphatic alcohol (C-OH)

Number of low resolution NMR chemical shift δ signal peaks: 5 ¹H and 4 ¹³C (email if disagree?)

Exhibits E/Z (geometrical) isomerism: From CIP rule priority is ₈O > ₆C > ₁H

Exhibits R/S (optical) isomerism:

From CIP rule priority is ₈O₆C > ₈O₁H > ₆C₆C > ₆C > ₁H for the stereoisomers.

This case of stereoisomerism is complex and requires university level analysis.

¹H NMR ratio of integrated chemical shift peak areas: 3 : 1 : 2 : 1 : 1  (for equivalent protons)

Infrared spectra comment: Strong absorption for the C=C wavenumber, absent for OH.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

 

(20) methoxycyclopropane, skeletal formula above

One functional group: an ether linkage (C-O-C)

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

¹H NMR ratio of integrated chemical shift peak areas: 4 (2+2) : 1 : 3  (for equivalent protons)

Infrared spectra comment: Absent absorption for the C=C and OH wavenumbers.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

Heterocyclic compounds including cyclo-ethers e.g. oxetanes and epoxy based compounds

(21) 1,4-epoxybutane, tetrahydrofuran, skeletal formula above

Functional group: cyclic ether linkage (C-O-C)

Exhibits R/S (optical) isomerism: From CIP rule priority is ₈O > ₆C₆C > ₆C > ₁H for the enantiomers.

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

¹H NMR ratio of integrated chemical shift peak areas: 4 (2+2) : 4 (2+2)  (for equivalent protons)

Infrared spectra comment: Absent absorption for the C=C and OH wavenumbers.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

 

(22) 2-methyloxetane,  skeletal formula above

Functional group: cyclic ether linkage (C-O-C)

Exhibits R/S (optical) isomerism:

From CIP rule priority is ₈O₆C > ₈O₁H > ₆C₆C > ₆C > ₁H for the enantiomers.

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 4 ¹³C (email if disagree?)

¹H NMR ratio of integrated chemical shift peak areas: 2 : 2 : 1 : 3  (for equivalent protons)

Infrared spectra comment: Absent absorption for the C=C and OH wavenumbers.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

 

(23) 3-methyloxetane, skeletal formula above

Functional group: cyclic ether linkage (C-O-C)

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

¹H NMR ratio of integrated chemical shift peak areas: 3 : 1 : 4 (2+2)  (for equivalent protons)

Infrared spectra comment: Absent absorption for the C=C and OH wavenumbers.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

 

(24) 1,2-epoxybutane, 2-ethyloxirane, skeletal formula above

Functional group: cyclic ether linkage (C-O-C)

Exhibits R/S (optical) isomerism:

From CIP rule priority is  ₈O > ₆C₈O > ₆C₆C > ₁H for the enantiomers.

This case of stereoisomerism is complex and requires university level analysis.

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 4 ¹³C (email if disagree?)

¹H NMR ratio of integrated chemical shift peak areas: 2 : 1 : 2 : 3  (for equivalent protons)

Infrared spectra comment: Absent absorption for the C=C and OH wavenumbers.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

 

(25) 2,3-epoxybutane, 2,3-dimethyloxirane,  skeletal formula above

Functional group: cyclic ether linkage (C-O-C)

Exhibits E/Z (geometrical) isomerism: From CIP rule priority is ₈O > ₆C > ₁H

Exhibits R/S (optical) isomerism: From CIP rule priority is ₈O > ₆C₆C > ₆C > ₁H for the enantiomers.

This case of stereoisomerism is complex and requires university level analysis.

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

¹H NMR ratio of integrated chemical shift peak areas: 6 (3+3) : 2 (1+1)  (for equivalent protons)

Infrared spectra comment: Absent absorption for the C=C and OH wavenumbers.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

 

(26) 2-methyl-1,2-epoxypropane, 2,2-dimethyloxirane, skeletal formula above

Functional group: cyclic ether linkage (C-O-C)

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

¹H NMR ratio of integrated chemical shift peak areas: 2 : 6 (3+3) (for equivalent protons)

Infrared spectra comment: Absent absorption for the C=C and OH wavenumbers.

See also infrared spectra

a comparison of diagnostic infrared absorption wavenumbers of selected isomers of C₄H₈O

(c) Extra information about the isomers of C₄H₈O

Note that spectral analysis can help distinguish between isomers of C₄H₈O

Summary of key points and extra notes on the isomers of molecular formula C₄H₈O

The molecular formula C₄H₈O, with a degree of unsaturation of one, gives rise to a diverse array of structural (constitutional) isomers.

These isomers can be broadly categorized into several functional groups, including aldehydes, ketones, alcohols (both unsaturated and cyclic), and ethers (both unsaturated and cyclic).

Carbonyl Compounds: Aldehydes and Ketones

The simplest isomers of C₄H₈O are the carbonyl compounds.

Aldehydes: There are two aldehyde isomers.

(1) Butanal: A straight-chain aldehyde.

(2) 2-Methylpropanal: A branched-chain aldehyde.

Ketones: There is one ketone isomer.

(3) Butan-2-one (butanone): Also known as methyl ethyl ketone (MEK).

Alcohols: Unsaturated and Cyclic

A significant number of isomers are alcohols, which can be either acyclic (containing a double bond) or cyclic.

Unsaturated Alcohols

These isomers contain both a hydroxyl (-OH) group and a carbon-carbon double bond. The stable forms are:

(4) But-1-en-1-ol (I added) cis and trans stereoisomers

(11) 2-Methylprop-2-en-1-ol

Cyclic Alcohols

These are saturated alcohols with a ring structure.

(16) Cyclobutanol: A four-membered ring with a hydroxyl group.

(17) Cyclopropylmethanol: A three-membered ring with a -CH₂OH group attached.

(18) 1-Methylcyclopropanol: A three-membered ring with a methyl and a hydroxyl group on the same carbon.

(19) 2-Methylcyclopropanol: A three-membered ring with a methyl and a hydroxyl group on adjacent carbons. This isomer has multiple stereoisomers.

Ethers: Unsaturated and Cyclic

The remaining isomers are ethers, featuring an oxygen atom connected to two alkyl or vinyl groups.

Unsaturated Ethers

These ethers contain a carbon-carbon double bond.

(15) Ethoxyethene (Ethyl vinyl ether)

(12) 1-Methoxypropene: Exists as *cis* and *trans* stereoisomers.

(14) 3-Methoxyprop-1-ene (Allyl methyl ether)

(13) 2-Methoxypropene

Cyclic Ethers

In these isomers, the ether oxygen is part of a ring structure.

(21) Tetrahydrofuran (THF): A five-membered ring containing one oxygen atom.

(22) 2-Methyloxetane: A four-membered ring with a methyl substituent. It is chiral.

(23) 3-Methyloxetane: A four-membered ring with a methyl substituent.

(24) 2-Ethyloxirane: A three-membered ring (epoxide) with an ethyl substituent. It is chiral.

(26) 2,2-Dimethyloxirane: A three-membered ring with two methyl groups on the same carbon.

(25) 2,3-Dimethyloxirane: A three-membered ring with methyl groups on different carbons. It exists as *cis* and *trans* stereoisomers. Can also exhibit R/S (optical) isomerism.

In summary, the formula C₄H₈O corresponds to a large number of constitutional isomers, each with unique chemical and physical properties, distributed across aldehydes, ketones, unsaturated alcohols, cyclic alcohols, unsaturated ethers, and cyclic ethers.

Many of these also exhibit stereoisomerism, further increasing the total number of possible unique molecules.

Infrared Spectrum of C₄H₈O: Key Points, Prominent Wavenumbers, Misconceptions, and Revision Tips

Key Points of the infrared spectra of the isomers of C₄H₈O

C₄H₈O has one degree of unsaturation, so expect either a carbonyl (C=O), an alkene (C=C) or a ring.

Ketones and aldehydes give a strong C=O stretch around 1700–1750 cm⁻¹.

Alcohols show a broad O–H band near 3200–3600 cm⁻¹, while alkenes display a medium C=C stretch around 1600–1680 cm⁻¹.

All isomers exhibit sp³ C–H stretches between 2850–2960 cm⁻¹. Aldehydes uniquely feature weak C–H stretches at 2720–2820 cm⁻¹. The fingerprint region below 1500 cm⁻¹ confirms identity but is too complex for primary assignments.

Prominent Wavenumbers of the isomers of C₄H₈O

Common Misconceptions about the infrared spectra of the isomers of C₄H₈O (see also below)

• Treating the fingerprint region as the first place to look; it's for confirmation only after identifying major functional bands.

• Confusing broad O–H with N–H; C₄H₈O has no nitrogen, and O–H is broader and stronger.

• Assuming every C=O absorption sits exactly at one value; conjugation, ring strain, and H-bonding can shift it by 20–30 cm⁻¹.

• Overlooking the weak aldehyde C–H stretches at 2720–2820 cm⁻¹ because they can merge with baseline noise.

• Misassigning C=C bands when a weak shoulder near 1680 cm⁻¹ actually comes from conjugation or impurities.

Exam Revision Tips for questions that may involve the infrared spectra of  the isomers of C₄H₈O (see also above)

• Know the degree of unsaturation first to predict possible functional groups.

• Scan in order: 3600–3200 cm⁻¹ (O–H), 3000–2850 cm⁻¹ (C–H), 1750–1700 cm⁻¹ (C=O), 1680–1600 cm⁻¹ (C=C).

• Quote exact wavenumbers and describe band shape: “sharp C=O stretch at 1740 cm⁻¹ indicates a ketone.”

• Use an elimination approach: absence of O–H rules out alcohols; absence of C=O rules out carbonyls.

• In your answer structure:

  1. State unsaturation index.

  2. List key peaks (wavenumber, shape, intensity).

  3. Assign each band to a functional group.

  4. Deduce the most likely isomer.

• For structured practice, overlay spectra of known isomers and test yourself on peak assignments under timed conditions.

• Cross-check with table of common vibrational bands from your data sheet to build confidence in ranges and shifts.

Selected Properties and Applications of C₄H₈O Isomers

Some key Isomers of C₄H₈O (but remember there at least 26 constitutional isomers theoretically possible)

1. Butanal (an aldehyde)

2. 2-Butanone, butanone, butan-2-one (methyl ethyl ketone)

3. 1-Butanol, butan-1-ol (primary alcohol)

4. 2-Butanol, butan-2-ol (secondary alcohol)

5. Tetrahydrofuran (cyclic ether)

6. Cyclobutanol (cyclic alcohol)

Comparative Physical Properties of isomers of C₄H₈O

Uses and applications of selected isomers of C₄H₈O

• Butanal
  • Precursor in synthesis of perfumes, flavourings, and rubber accelerators.
  • Intermediate in pharmaceutical manufacture.

• 2-Butanone (MEK)
  • Industrial solvent for paints, plastics, and adhesives.
  • Extraction agent in formulations requiring high solvency.

• Tetrahydrofuran (THF)
  • Polar aprotic solvent widely used in polymerization (e.g., PVC) and organometallic chemistry.
  • Reaction medium for Grignard reagents and lithium aluminium hydride reductions.

• Cyclobutanol
  • Specialist intermediate in synthetic organic chemistry.
  • Research compound for ring‐expansion and photochemical studies.

Structure–Property–Use Relationships

The presence and type of functional group govern each isomer's intermolecular forces, which in turn dictate boiling point, miscibility, and solvent behaviour.

Aldehydes and ketones, with strong dipoles and no H-donor capability, excel as moderate‐boiling solvents and synthesis intermediates.

Primary and secondary alcohols engage in hydrogen bonding, boosting boiling points and water miscibility - traits leveraged in coatings, biofuels, and cosmetic formulations.

Cyclic ethers like THF combine polarity with no H-donation, making them ideal aprotic solvents for sensitive organometallic reactions.

Cyclobutanols ring strain and H-bonding profile suit it to niche synthetic applications, particularly ring‐transformation methodologies.

How many constitutional structural isomers are there of molecular formula C4H8O?

Are there any positional isomers of molecular formula of C4H8O?

Are there any E/Z (cis/trans) geometrical isomers of molecular formula C4H8O?

How do you draw the skeletal structure of isomers of molecular formula C4H8O?

How do you draw the structural formula of the isomers of molecular formula C4H8O?

Can you recognise the different functional groups in the isomers of molecular formula C4H8O?

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