The 25 non-cyclic alkene and cycloalkane constitutional-structural isomers of molecular formula C₆H₁₂

The constitutional isomer number of 25 excludes E/Z (geometrical isomers) and R/S (optical isomers) stereoisomers, which, if stereoisomers are included, would bring the total of distinct isomers to at least 36, and probably more.

Introduction to the isomerism for molecular formula C₆H₁₂

Percent composition of C₆H₁₂ based on atomic masses C = 12.01  H = 1.01  and  M_r(C₆H₁₂) = 84.18

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

Empirical formula = CH₂ for molecular formula = C₆H₁₂

Structural isomerism  - isomers of the same specific molecular formula, based on different connectivity's of the constituent atoms (the constitutional isomers), so they cannot be spatially identical (but sometimes 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 C₆H₁₂ there are

(a) chain variations based on the, open chain linear or branched alkenes and lots of saturated cyclic (alicyclic) variations,

(b) positional isomers e.g. position of alkene groups in the open chain compounds.

(c) They are all functional group isomers of each other e.g. in terms e.g. unsaturated alkenes versus saturated cycloalkane molecules.

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 (= Z) and trans (= E) isomers of alkenes or disubstituted cyclic alkanes where there are 2D/3D spatial variations that are not mirror images and not super imposable.

e.g. there are 7 examples of E/Z geometrical isomers, four in open chain alkene compounds via the C=C restricted rotation and three in the alicyclic alkanes.

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.

There 5 examples of R/S optical isomers, one alkene and four of the alicyclic alkane molecules.

This can be complicated stereoisomerism because in three cases E/Z and R/S isomers 'overlap' - need university level for full analysis!

NOTE

The are 25 structural-constitutional isomers of molecular formula C₆H₁₂

Including the stereoisomers, there are at least 34 unique distinct isomers of formula C₆H₁₂.

There are 13 open chain unsaturated aliphatic alkene constitutional isomers, linear and branched

and 12 saturated aliphatic cyclic compounds (alicyclic constitutional isomers) with 3-6 C atom membered rings.

All the isomers of molecular formula C₆H₁₂ are open chain aliphatic compounds (e.g. here unsaturated alkenes) or cyclic aliphatic compounds (e.g. here saturated cycloalkanes), the latter sometimes referred to as alicyclic compounds.

I've identified 13 structural alkene isomers of molecular formula C₆H₁₂ (1)-(13), excluding E/Z and R/S isomers i.e. 13 carbon chain isomers and positional isomers of the C=C double bond in C₆H₁₂ molecules - all open chain unsaturated aliphatic molecules..

I've identified 12 alicyclic saturated alkane molecules, (14) to (25) are cyclic molecules.

There are only 25 constitutional isomers

There are at least 34 distinct isomers because some constitutional isomers exhibit E/Z and R/S isomerism - there can be overlap of these two types of stereoisomerism (university/college level analysis required).

Note the two naming systems allowed i.e. hex-1-ene (e.g. UK) and 1-hexene (e.g. US) etc.

Note that many of the structural formulae of the alkenes are abbreviated or condensed.

e.g. CH₃CH₂CH₂CH₂CH=CH₂ instead of CH₃-CH₂-CH₂-CH₂-CH=CH₂

I've identified 13 open chain aliphatic alkene isomers (1-13) and 12 structural cycloalkane isomers of molecular formula C₆H₁₂ (14-25), excluding E/Z and R/S isomers, where things can get very complicated.

Most of the cycloalkane structures are drawn as their skeletal formula.

There are two groups of functional group isomers i.e. open chain alkenes and cycloalkanes, each group belonging to their own homologous series with their own distinctive chemistries.

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

The chemistry of ALKENES (detailed notes on all aspects of alkene chemistry)

(1) hex-1-ene, , ,   (1-hexene)

Hex-1-ene is an unsaturated linear alkene molecule with a single C=C double bond and does not display E/Z or R/S isomerism.

Hex-2-ene and hex-3-ene are similarly described, with C=C bond in different positions - positional isomers, but both exhibit E/Z (geometric) isomerism.

Number of low resolution NMR chemical shift δ signal peaks: 6 ¹H and 6 ¹³C (email if disagree?)

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

BUT, for the 'end' =CH₂ alkene protons, you can get two chemical shifts very 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 (R = H or alkyl). This causes the two =CH₂ protons to experience slightly different fields. I've usually ignored this unless specific data was available.

Index of ¹H NMR spectra organic compounds

and Index of ¹³C NMR spectra organic compounds

 

(2) hex-2-ene,

CH₃CH₂CH₂CH=CHCH₃, 2-hexene has two E/Z isomers

Other names: (Z)-hex-2-ene, (E)-hex-2-ene, (E)-2-hexene, (Z)-2-hexene, (2Z)-hex-2-ene, (2E)-hex-2-ene, (2E)-2-hexene, (2Z)-2-hexene, cis-hex-2-ene, trans-hex-2-ene, cis-2-hexene, trans-2-hexene.

Skeletal formulae and structural formulae of the E/Z stereoisomers shown below.

CIP assignment priority rule for E/Z isomers, so from the C=C double bond: ₆C₆C > ₆C₁H

Applying the CIP rule for the four atoms/groups around the >C=C< double bond

Z/cis- , , and

E/ trans-, ,

Number of low resolution NMR chemical shift δ signal peaks: 6 ¹H and 6 ¹³C (email if disagree?)

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

 

(3) hex-3-ene, CH₃-CH₂-CH=CH-CH₂-CH₃, structural formula

CH₃CH₂CH=CHCH₂CH₃, 3-hexene, has two E/Z isomers like hex-2-ene above, no R/S isomers.

Other names: (Z)-hex-3-ene, (E)-hex-3-ene, (E)-3-hexene, (Z)-3-hexene, (3Z)-hex-3-ene, (3E)-hex-3-ene, (3E)-3-hexene, (3Z)-3-hexene, cis-hex-3-ene, trans-hex-3-ene, cis-3-hexene, trans-3-hexene.

The old names would be cis-hex-3-ene and trans-hex-3-ene.

Skeletal formulae and structural formulae of the E/Z stereoisomers shown below.

CIP assignment priority rule for E/Z isomers: ₆C₆C  >  ₆C₁H

CIP rule for the four atoms/groups around the >C=C< double bond

 

(4) 2-methylpent-1-ene, ,

The structural and skeletal formula of 2-methyl-1-pentene, no E/Z or R/S isomers.

2-methylpent-1-ene is an unsaturated aliphatic branched alkene molecule with a single C=C double bond.

All the rest of the alkene molecules below can be similarly described, apart from possible E/Z or R/S isomerism.

Number of low resolution NMR chemical shift δ signal peaks: 5 ¹H and 6 ¹³C (email if disagree?)

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

 

(5) 3-methylpent-1-ene, , ,

The structural formula and skeletal formula of 3-methyl-1-pentene

3-methylpent-1-ene no E/Z isomers but has R/S isomers (enantiomers), 3rd C atom in the main chain is chiral (asymmetric carbon atom).

Number of low resolution NMR chemical shift δ signal peaks: 6 ¹H and 6 ¹³C (email if disagree?)

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

 

(6) 4-methylpent-1-ene, ,  

The structural formula and skeletal formula of 4-methyl-1-pentene, no E/Z or R/S isomers.

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

¹H NMR ratio of peaks: 6 (3+3) : 1 : 2 : 1 : 2 (for equivalent protons)

 

(7) 2-methylpent-2-ene, ,

The structural formula and skeletal formula of 2-methyl-2-pentene, no E/Z or R/S isomers

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

¹H NMR ratio of peaks: 3 : 2 : 1 : 6 (3+3) (for equivalent protons)

 

(8) 3-methylpent-2-ene,

3-methyl-2-pentene has E/Z isomers, but no R/S isomers.

Other names: (E)-3-methyl-pent-2-ene, (Z)-3-methylpent-2-ene, (E)-3-methyl-2-pentene, (Z)-3-methyl-2-pentene, (2E)-3-methyl-pent-2-ene, (2Z)-3-methylpent-2-ene, (2E)-3-methyl-2-pentene, (2Z)-3-methyl-2-pentene, cis-3-methylpent-2-ene, trans-3-methylpent-2-ene, cis-3-methyl-2-pentene, trans-3-methyl-2-pentene.

Skeletal formulae and structural formulae of the E/Z stereoisomers shown below.

CIP assignment priority rule for E/Z isomers from the C=C bond: ₆C₆C  >  ₆C₁H  >  ₁H

CIP rule for the four atoms/groups around the >C=C< double bond

  , , E-3-methylpent-2-ene (trans)

and   , , Z-3-methylpent-2-ene (cis)

Number of low resolution NMR chemical shift δ signal peaks: 5 ¹H and 6 ¹³C (email if disagree?)

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

 

(9) 4-methylpent-2-ene, , 4-methyl-2-pentene, E/Z isomers, no R/S isomers:

Other names: (E)-4-methyl-pent-2-ene, (Z)-4-methylpent-2-ene, (E)-4-methyl-2-pentene, (Z)-4-methyl-2-pentene, (2E)-4-methyl-pent-2-ene, (2Z)-4-methylpent-2-ene, (2E)-4-methyl-2-pentene, (2Z)-4-methyl-2-pentene, cis-4-methylpent-2-ene, trans-4-methylpent-2-ene, cis-4-methyl-2-pentene, trans-4-methyl-2-pentene.

Skeletal formulae and structural formulae of the E/Z stereoisomers shown below.

CIP assignment priority rule for E/Z isomers from off the C=C double bond: ₆C₆C  >  ₆C₁H  >  ₁H

CIP rule for the four atoms/groups around the >C=C< double bond

Z/cis- ,

and E/trans- ,

The old names would be cis-4-methylpent-2-ene and trans-4-methylpent-2-ene.

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

¹H NMR ratio of peaks: 6 (3+3) : 1 : 1 : 1 : 3 (for equivalent protons)

 

(10) 2,3-dimethylbut-1-ene, , ,

structural formula and skeletal formula of 2,3-dimethyl-1-butene, cannot exhibit E/Z or R/S isomerism

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

¹H NMR ratio of peaks: 6 (3+3) : 1 : 3 : 2 (for equivalent protons)

 

(11) 3,3-dimethylbut-1-ene, , ,

structural formula and skeletal formula of 3,3-dimethyl-1-butene, cannot exhibit E/Z or R/S isomerism

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

¹H NMR ratio of peaks: 9 (3x3) : 1 : 2 (for equivalent protons)

The methyl group symmetry reduces the number of chemical shifts.

 

(12) 2,3-dimethylbut-2-ene, ,

structural formula and skeletal formula of 2,3-dimethyl-2-butene,

does not have E/Z isomers, a completely symmetrical alkene, no R/S isomers

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

The minimum number of chemical shifts due to the high symmetry of the molecule.

 

(13) 3-methylenepentane, H₂C=C(CH₂CH₃)₂ , ,

I don't think 2-ethylbut-1-ene would be considered correct, since the longest carbon atom chain is 5.

The structural formula and skeletal formula, no E/Z or R/S isomers

Also called (less correctly) 2-ethylbut-1-ene or 2-ethyl-1-butene.

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

¹H NMR ratio of peaks: 2 : 4 (2+2) : 6 (3+3) = 1 : 2 : 3 (for equivalent protons).

Fewer chemical shifts due to symmetry about both ends of the >C=C< bond.

 

(14) cyclohexane , , the largest possible ring.

It is not a planar molecule but has no E/Z or R/S isomers.

Cyclohexane is a saturated aliphatic cycloalkane molecule (cyclo alkane, alicyclic compound).

All the rest of the cyclic molecules below can be similarly described, apart from possible E/Z or R/S isomers.

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

See also spectra that can distinguish one C6H12 isomer from another

The infrared spectrum of cyclohexane

The mass spectrum of cyclohexane

The H-1 NMR spectrum of cyclohexane

The C-13 NMR spectrum of cyclohexane

 

(15) methylcyclopentane , ,  

not quite planar, does NOT exhibit E/Z (geometric) or R/S (optical) isomerism

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

¹H NMR ratio of peaks: 4 (2+2) : 4 (2+2) : 1 : 3 (for equivalent protons)

 

(16) ethylcyclobutane , ,

does NOT exhibit E/Z (geometric) or R/S (optical) isomerism

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

¹H NMR ratio of peaks: 2 : 4 (2+2) : 1 : 2 : 3 (for equivalent protons)

 

(17) 1,1-dimethylcyclobutane , ,

does NOT exhibit E/Z (geometric) or R/S (optical) isomerism

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

¹H NMR ratio of peaks: 2 : 4 (2+2) : 6 (3+3) = 1 : 2 : 3 (for equivalent protons)

 

(18) 1,2-dimethylcyclobutane , ,  

This molecule exhibits both E/Z and R/S isomerism - complicated - university level analysis.

Structural formulae of the E/Z stereoisomers shown below.

The right carbons of the ring are chiral and E/Z isomers can arise from the two ring methyl groups.

Two of the adjacent -CH- carbon atoms of the cyclobutane ring are asymmetric (stereocentres) but the ring prevents rotation of this C-C bond (as in a >=C< bond, so you can get cis/trans isomers (E/Z isomers).

CIP assignment priority rule for E/Z (cis/trans) isomers: ₆C₆C  >  ₆C₁H  >  ₁H

(Z)-1,2-dimethylcyclobutane, , a = a = CH₃ (cis-1,2-dimethylcyclobutane isomer)

(E)-1,2-dimethylcyclobutane, , a = a = CH₃ (trans-1,2-dimethylcyclobutane isomer)

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

¹H NMR ratio of peaks: 4 (2+2) : 2 (1+1) : 6 (3+3) = 2 : 1 : 3 (for equivalent protons)

 

(19) 1,3-dimethylcyclobutane , ,   ,

This molecule exhibits E/Z (geometric) isomerism arising from the two ring methyl groups, and also R/S isomerism.  Structural formulae of the E/Z stereoisomers shown below.

CIP assignment priority rule for E/Z (cis/trans) isomers: ₆C₆C  >  ₆C₁H  >  ₁H

(E)-1,3-dimethylcyclobutane, , a = a = CH₃ (trans-1,3-dimethylcyclobutane isomer)

(Z)-1,3-dimethylcyclobutane, , a = a = CH₃ (cis-1,3-dimethylcyclobutane isomer)

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

¹H NMR ratio of peaks: 6 (3+3) : 2 (1+1) : 4 (2+2) = 3 : 1 : 2 (for equivalent protons)

 

(20) 1-ethyl-1-methylcyclopropane, ,

No R/S or E/Z isomers

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

¹H NMR ratio of peaks: 4 (2+2) : 3 : 2 : 3 (for equivalent protons)

 

(21) 1-ethyl-2-methylcyclopropane,

This molecule exhibits both E/Z (cis/trans geometric) and R/S (optical) isomerism, two chiral carbon atoms of the ∆ ring, so, very complicated, university level. Structural formulae of the E/Z stereoisomers shown below.

CIP assignment priority rule for E/Z isomers: ₆C₆C  >  ₆C₁H  >  ₁H

(Z)-1-ethyl-2-methylcyclopropane,

a = CH₂CH₃ , b = CH₃  (cis-1-ethyl-2-methylcyclopropane isomer)

(E)-1-ethyl-2-methylcyclopropane,

a = CH₂CH₃ , b = CH₃ (trans-1-ethyl-2-methylcyclopropane isomer)

Number of low resolution NMR chemical shift δ signal peaks: 6 ¹H and 6 ¹³C (email if disagree?)

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

 

(22) 1,1,2-trimethycyclopropane, ,

This exhibits R/S (optical) isomerism, bottom right C of the ∆ ring is a chiral carbon atom, a chiral centre.

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

¹H NMR ratio of peaks: 2 : 6 (3+3) : 1 : 3 (for equivalent protons)

 

(23) 1,2,3-trimethycyclopropane,

Potentially both E/Z (geometric) and R/S (optical) isomerism are possible, complicated, university level.

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

¹H NMR ratio of peaks: 3 (3x1) : 9 (3x3) = 1 : 3 (for equivalent protons)

Fewer chemical shifts due to the high symmetry of the molecule.

This molecule can exhibit E/Z isomerism (cis/trans stereoisomerism) depending on the orientation of the methyl groups attached to the cyclopropane ring.

 

(24) propylcyclopropane , , no E/Z (geometric) or R/S (optical) isomers.

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

¹H NMR ratio of peaks: 4 (2+2) : 1 : 2 : 2 : 3 (for equivalent protons)

 

(25) (1-methylethyl)cyclopropane (isopropylcyclopropane) ,

no E/Z (geometric) and no R/S (optical) isomers

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

¹H NMR ratio of peaks: 4 (2+2) : 1 : 1 : 6 (3+3) (for equivalent protons)

 

Are there any more isomers?

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EXTRA NOTES on the isomers of C₆H₁₂ and some multiple choice practice questions with answers

There are 25 constitutional isomers of C₆H₁₂, including both saturated cycloalkanes and unsaturated open chain alkenes.

These isomers exhibit chain, positional, ring and functional group isomerism, with varied physical and chemical properties due to differences in structure, saturation, and substitution.

Some basic multiple choice practice questions

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Molecular Formula: C₆H₁₂

• Degrees of unsaturation: 1

• Possible structural classes:

  • Alkenes (one C=C double bond)

  • Cycloalkanes (one ring)

  • Branched and substituted variants of both

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Types of Isomerism in C₆H₁₂

Type of Isomerism	Description
Constitutional	Different connectivity (e.g. hex-1-ene versus methylcyclopentane)
Positional	Location of double bond or substituent (e.g. hex-1-ene versus hex-2-ene)
Chain (skeletal)	Straight versus branched carbon skeletons (e.g. hexene versus methylpentene)
Ring isomerism	Different ring sizes and substituents (e.g. cyclohexane versus dimethylcyclobutane)
Geometric (E/Z)	For alkenes with two different groups on each C=C (e.g. hex-2-ene)
Conformational	Chair/boat forms in cyclohexane; rotamers in branched chains

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Representative Isomers of C₆H₁₂

Alkenes

• Hex-1-ene, Hex-2-ene (E/Z)

• 2-Methylpent-1-ene, 3-Methylpent-1-ene

• 3-Methylpent-2-ene (E/Z)

• 2,3-Dimethylbut-1-ene, 2,3-Dimethylbut-2-ene

Cycloalkanes

• Cyclohexane

• Methylcyclopentane, Ethylcyclobutane

• 1,1-Dimethylcyclobutane, 1,2-Dimethylcyclobutane

• Propylcyclopropane, Isopropylcyclopropane

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Differences in Physical Properties of the isomers of C₆H₁₂

Property	Alkenes	Cycloalkanes
Boiling Point	Higher in straight-chain alkenes	Lower in branched or small rings
Melting Point	Higher in symmetrical isomers	Cyclohexane has unusually high m.p.
Density	Slightly higher in cyclic isomers	Compact packing increases density
Spectroscopy	C=C stretch ~1640 cm⁻¹ (IR); vinylic H ~5–6 ppm (NMR)	Cycloalkanes show saturated C–H signals

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Differences in Chemical Reactivity of the isomers of C₆H₁₂

Reaction Type	Alkenes	Cycloalkanes
Electrophilic Addition	Undergo addition (e.g. HBr, H2O)	Generally inert unless activated
Hydrogenation	Converts to alkane	Cyclohexane is already saturated
Polymerisation	Terminal alkenes polymerise readily	Cycloalkanes do not polymerise

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Uses and Applications of the isomers of C₆H₁₂

• Alkenes: Precursors in polymer production (e.g. polyethylene), intermediates in organic synthesis, fuel additives.

• Cycloalkanes: Solvents, reference compounds in spectroscopy, intermediates in ring-opening reactions.

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Student Misconceptions about the isomers of C₆H₁₂

• “All C₆H₁₂ isomers are alkenes” — incorrect: many are cycloalkanes.

• “Double bond position doesn’t affect reactivity” — False: affects regioselectivity and stability.

• “Cyclohexane is aromatic” — incorrect: it’s fully saturated and non-aromatic.

• “E/Z isomers are the same compound” — False: they are distinct stereoisomers and can have both similar or different physical and chemical properties.

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Exam Revision Tips for questions involving the isomers of C₆H₁₂

• Draw and name all major isomers: practice IUPAC naming and skeletal structures.

• Spectroscopy practice: identify IR stretches for C=C and NMR shifts for different proton environment.

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Basic practice questions and answers

Practice multiple-choice questions designed around the isomers of molecular formula C₆H₁₂, focusing on structural and spectroscopic differences.

These are tailored for advanced pre-university chemistry courses (A-level, IB, AP), with accessible language and subtle technical depth.

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 Multiple Choice Practice based on, but not exclusively, the isomers of C₆H₁₂

Jot down your responses and then check out the answers

Q1. Which molecule is most likely to show a strong IR absorption near 1640 cm⁻¹?

1. Cyclohexane

2. 2-Methylpentane

3. Hex-1-ene

4. Hexane

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Q2. Which molecule would show a singlet in its ¹H NMR spectrum due to equivalent protons in a symmetrical ring?

1. 2-Methylpentane

2. Cyclohexane

3. 3-Methylpentene

4. 1-Methylcyclopentane

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Q3. Which molecule can exist as E/Z (cis-trans geometrical) isomers?

1. Hexane

2. Cyclohexane

3. 2-Methylpentane

4. Hex-2-ene

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Q4. Which molecule would show four ¹H chemical shifts in its NMR spectrum?

1. Cyclohexane

2. Hexane

3. Methylcyclopentane

4. Pent-2ene

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Q5. Which molecule would show a strong IR absorption near 1700 cm⁻¹?

1. Cyclohexanone

2. Hex-1-ene

3. Cyclohexane

4. 2-Methylpentane

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Q6. Which molecule contains a tertiary carbon and would show reduced C–H stretching intensity in IR?

1. 2-Methylpentane

2. Cyclohexane

3. Hexane

4. Hex-1-ene

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Q7. Which molecule would show five signals in its ¹³C NMR spectrum?

1. 2-Methylpentane

2. Cyclohexane

3. Hexane

4. Hex-1-ene

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Q8. Which molecule has 3 different hydrogen environments in its ¹H NMR spectrum?

1. Cyclohexane

2. Hex-1-ene

3. 2-Methylpentane

4. Hex-3-ene

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Q9 Which molecule will react the fastest with bromine water?

1. Hex-1-ene

2. Cyclohexane

3. Hexane

4. 2-Methylpentane

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Q10. Which molecule will give four ¹H chemical shifts in its NMR spectrum?

1. Cyclohexane

2. 1-Hexene

3. 3-Methylpentane

4. Hex-2-ene

   Jot down your responses and then check out the answers

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Learning objectives - questions to be answered?

How do you work out the structure of the isomers of molecular formula C6H12?

How do you draw the structural formula and skeletal formula of the isomers of molecular formula C6H12?

How many aliphatic structural isomers are there of molecular formula C6H12?

How many aliphatic carbon chain isomers are there of molecular formula C6H12?

How many positional isomers are there of molecular formula C6H12?

How many E/Z (geometrical) isomers are there of molecular formula C6H12?

How many R/S (optical) isomers (enantiomers) of molecular formula C6H12?

Are there any aliphatic open chain alkene isomers of molecular formula C6H12?

Are there any diene isomers of molecular formula C6H12?

Are there any alkyne isomers of molecular formula C6H12?

Are there any alicyclic cycloalkane isomers of molecular formula C6H12?

Are there any alicyclic cycloalkene isomers of molecular formula C6H12?

Are there any functional group isomers with a molecular formula C6H12?

Are there any E/Z (geometrical) isomers with a molecular formula C6H12?

Are there any R/S (optical) isomers (enantiomers) with a molecular formula C6H12?

Does C6H12 have any stereoisomers?

Be able to work out the number of different 1H proton NMR chemical shift signals for C6H12 isomers

Be able to work out the number of different 13C NMR chemical shift signals for C6H12 isomers

This page will answer these questions for molecular formula C₆H₁₂

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ANSWERS

Q1. Which molecule is most likely to show a strong IR absorption near 1640 cm⁻¹?

1. Cyclohexane

2. 2-Methylpentane

3. Hex-1-ene

4. Hexane

Q1. Correct: C. Hex-1-ene
Explanation: The C=C stretch appears near 1640 cm⁻¹ in IR spectra.
Tips:

• Saturated alkanes like Cyclohexane and Hexane show no C=C stretch.

• 2-Methylpentane is branched but still saturated.

---

Q2. Which molecule would show a singlet in its ¹H NMR spectrum due to equivalent protons in a symmetrical ring?

1. 2-Methylpentane

2. Cyclohexane

3. 3-Methylpentene

4. 1-Methylcyclopentane

Q2. Correct: B. Cyclohexane
Explanation: All protons are equivalent due to ring symmetry and rapid flipping.
Tips:

• Branched and unsaturated isomers show multiple signals.

---

Q3. Which molecule can exist as E/Z (cis-trans, geometrical) isomers?

1. Hexane

2. Cyclohexane

3. 2-Methylpentane

4. Hex-2-ene

Q3. Correct: D. Hex-2-ene
Explanation: Geometric isomerism requires a C=C with different groups on each carbon.
Tips:

• Alkanes and cyclic saturated compounds do not show cis-trans isomerism.

---

Q4. Which molecule would show four ¹H chemical shifts in its NMR spectrum?

1. Cyclohexane

2. Hexane

3. Methylcyclopentane

4. Pent-2-ene

Q4. Correct: C. Hexane
Explanation: Four different proton environments, 3 in the ring and 1 for the methyl group

A 1, B 3 and D 5 different proton environments

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Q5. Which molecule would show a strong IR absorption near 1700 cm⁻¹?

1. Cyclohexanone

2. Hex-1-ene

3. Cyclohexane

4. 2-Methylpentane

Q5. Correct: A. Cyclohexanone
Explanation: The C=O stretch in ketones appears near 1700 cm⁻¹.
Tips:

• Alkenes absorb lower (~1640 cm⁻¹).

• Alkanes lack double bonds.

• You should get a wavenumber table in exams.

---

Q6. Which molecule contains a tertiary carbon and would show reduced C–H stretching intensity in IR?

1. 2-Methylpentane

2. Cyclohexane

3. Hexane

4. Hex-1-ene

Q6. Correct: A. 2-Methylpentane
Explanation: Tertiary carbons have fewer C–H bonds, reducing IR intensity.
Tips:

• Hexane and Cyclohexane have primary/secondary C–H.

• Hex-1-ene has vinylic C–H (-CH=CH₂).

---

Q7. Which molecule would show five signals in its ¹³C NMR spectrum?

1. 2-Methylpentane 5 ¹³C signals

2. Cyclohexane 1 ¹³C

3. Hexane 3 ¹³C

4. Hex-1-ene 6 ¹³C

Q7. Correct: A. 2-Methylpentane
Explanation: Branching creates distinct carbon environments.
Tips:

• Cyclohexane shows one signal.

• Hexane is linear but less branched.

---

Q8. Which molecule has 3 different hydrogen environments in its ¹H NMR spectrum?

1. Cyclohexane 1 ¹H

2. Hex-1-ene 6 ¹H

3. 2-Methylpentane 5 ¹H

4. Hex-3-ene 3 ¹H signals

Q8. Correct: D hex-3-ene
Explanation: symmetrical molecule with 3 pairs of equivalent protons H₃C-CH₂-CH=CH-CH₂-CH₃  (proton ratio of 6 : 4 : 2)
Tips:

• Alkenes and branched alkanes show more environments than e.g. cyclohexane, but hex-3-ene is more symmetrical than B or C.

• A 1, B 6, C 4, D 3 ¹H principal chemical shifts respectively

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Q9 Which molecule will react the fastest with bromine water?

1. Hex-1-ene

2. Cyclohexane

3. Hexane

4. 2-Methylpentane

Q9. Correct: A. 1-Hexene
Explanation: Reactive C=C bond

---

Q10. Which molecule will give four ¹H chemical shifts in its NMR spectrum?

1. Cyclohexane 1 ¹H

2. 1-Hexene 6 ¹H

3. 3-Methylpentane 4 ¹H signals

4. Hex-2-ene 6 ¹H

Q10. Correct: C. 3-methylpantene
Explanation:   A1, B 6, C 4, D 6

---

Associated organic chemistry  links

 Combined multiple choice and type in name quiz on the structure and naming of alkenes

 Combined multiple choice and type in name quiz on the structure and naming of alkanes

 Advanced Level pre-university organic chemistry notes

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

INDEX of nuclear magnetic resonance spectroscopy: ¹H NMR spectra of organic compounds

INDEX of nuclear magnetic resonance spectroscopy: ¹³C NMR spectra of organic compounds

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 ALL 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

 email doc brown - comments - query?

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

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

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, Edexcel advanced level chemistry, OCR advanced level chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level chemistry, CIE advanced level chemistry, US grade 11-12 AP honors chemistry courses and they will also prove useful to 1st year undergraduate students of chemistry.

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