Introduction

Percent composition based on C₅H₁₀ atomic masses C= 12.01  H = 1.01  and  M_r(C₅H₁₀) = 70.15

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

Empirical formula = CH₂ for molecular formula = C₅H₁₀

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 C₅H₁₀ there are

(a) chain variations based on the C atoms, open chain aliphatic linear, branched and cyclic (alicyclic) structures,

(b) positional isomers e.g. the alkene groups or methyl groups in the cyclopropanes,

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

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.

There are two examples E/Z geometric isomers (2) and (10) in an open chain compounds via the C=C restricted rotation or two substituents in a ring where both can be above the 'plane' of a ring or one above and one below the plane of the ring - BUT, can be complicated with overlapping R/S optical isomerism for the same molecule as in the case 1,2-dimethylpropane.

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 is one example of R/S optical isomers, but, as already mentioned, it can be complicated with overlapping E/Z geometric isomers for the same molecule (10).

NOTE

Five non-cyclic alkene and five cycloalkane structural isomers, pent-2-ene and 1,2-dimethylpropane can also exist as E/Z isomers (cis/trans isomerism) an example of stereoisomerism.

There are carbon chain isomers and functional group positional isomers of the C=C double bond molecules.

There are two sets of functional group isomers of open chain alkenes and cycloalkanes and E/Z isomers (cis/trans geometric stereoisomers) of the alkenes.

There are 10 constitutional-structural isomers of molecular formula C₅H₁₀, excluding E/Z or R/S isomers, 5 constitutional alkene isomers and 5 constitutional cycloalkane isomers.

Isomer molecule (2) pent-3-ene, has 2 E/Z stereoisomers and isomer molecule (10) 1,2-dimethylcyclopropane, has 3 stereoisomers from an overlap of E/Z and R/S isomerism.

So in total, there are 13 distinct isomers in total for C₅H₁₀ including the E/Z and R/S stereoisomers.

Details of the 10 constitutional structural isomers of C₅H₁₀ and any resulting E/Z or R/S stereoisomers

Important infrared spectra note: Unsaturated alkenes show an infrared absorption for >C=C< stretching at wavenumber ~1640 cm^-1 (isomers 1 to 5), don't confuse on spectra with the >C=O stretching wavenumber of ~1700 cm^-1, and saturated cyclic alkanes (isomers 6 to 10) will not show either absorption.

(1) pent-1-ene, CH₃CH₂CH₂CH=CH₂, ,

1-pentene, a linear alkene, has no E/Z or R/S isomers.

A positional isomer of the >C=C< alkene functional group.

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

¹H NMR ratio of peaks: 3 : 2 : 2 : 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 spectra that can distinguish one C₅H₁₀ isomer from another

The infrared spectrum of pent-1-ene (1-pentene)

The mass spectrum of Pent-1-ene (1-pentene)

The H-1 NMR spectrum of Pent-1-ene (1-pentene)

The C-13 NMR spectrum of Pent-1-ene (1-pentene)

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

Infrared note: Unsaturated alkenes show an infrared absorption for >C=C< stretching at wavenumber ~1640 cm^-1, don't confuse on spectra with the >C=O stretching wavenumber of ~1700 cm-1, and saturated alkanes will not show either absorption.

 

(2) pent-2-ene, CH₃CH₂CH=CHCH₃, , 2-pentene, E/Z isomers possible

A functional group linear alkene positional isomer of pent-1-ene.

Z/cis-

(Z)-pent-2ene, (Z)-2-pentene, cis-pent-2-ene, cis-2-pentene, an example of stereoisomerism.

E/trans-

(E)-pent-2-ene, (E)-2-pentene, cis-pent-2-ene, cis-2-pentene

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

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

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

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

See also spectra that can distinguish one C₅H₁₀ isomer from another

The infrared spectra of E/Z isomers of pent-2-ene (2-pentene)

The mass spectra of the E/Z isomers of pent-2-ene (2-pentene)

The H-1 NMR spectra of E/Z isomers of pent-2-ene (2-pentene)

The C-13 NMR spectra of E/Z isomers pent-2-ene (2-pentene)

 

(3) 2-methylbut-1-ene, ,  

2-methyl-1-butene, cannot exhibit E/Z or R/S isomerism, although the carbon chain is now branched.

A branched structural isomer of the linear alkenes (1)-(2) above.

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 : 3 : 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 spectra that can distinguish one C5H10 isomer from another

The Infrared spectrum of 2-methylbut-1ene

The mass spectrum of 2-methylbut-1ene

The H-1 NMR spectrum of 2-methylbut-1ene

The C-13 NMR spectrum of 2-methylbut-1ene

 

(4) 3-methylbut-1-ene, , ,

 3-methyl-1-butene, cannot exhibit R/S or E/Z isomerism.

A branched structural isomer of the linear alkenes (1)-(2) above.

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

¹H NMR ratio of peaks: 6 (3+3) : 1 : 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 spectra that can distinguish one C5H10 isomer from another

The infrared spectrum of 3-methylbut-1-ene

The mass spectrum of 3-methylbut-1-ene

The H-1 NMR spectrum of 3-methylbut-1-ene

The C-13 NMR spectrum of 3-methylbut-1-ene

 

(5) 2-methylbut-2-ene, , ,

2-methyl-2-butene cannot exhibit E/Z or R/S isomerism.

A branched structural isomer of the linear alkenes (1)-(2) above.

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

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

(6) to (8) are cycloalkanes (alicyclic hydrocarbons) which are isomeric with alkenes of the same molecular formula (C₅H₁₀)

This is a case of functional group isomerism, alkenes and cycloalkanes.

See also spectra that can distinguish one C5H10 isomer from another

The infrared spectrum of 2-methylbut-2-ene

The mass spectrum of 2-methylbut-2-ene

The H-1 NMR spectrum of Pent-1-ene (1-pentene)

The C-13 NMR spectrum of Pent-1-ene (1-pentene)

 

(6) cyclopentane, , , cannot exhibit E/Z or R/S isomerism, very symmetrical.

This is a saturated aliphatic compound, a cycloalkane.

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 C₅H₁₀ isomer from another

The infrared spectrum of cyclopentane

The mass spectrum of cyclopentane

The H-1 NMR spectrum of cyclopentane

The C-13 NMR spectrum of cyclopentane

 

(7) methylcyclobutane,  , , cannot exhibit E/Z or R/S isomerism, a cyclic alkane.

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

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

 

(8) ethylcyclopropane, , cannot exhibit E/Z or R/S isomerism, a cyclic alkane.

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 : 2 : 3 (for equivalent protons)

 

(9) 1,1-dimethylcyclopropane, , , cannot exhibit E/Z or R/S isomerism a cyclic alkane.

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

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

 

(10) 1,2-dimethylcyclopropane, , , a cyclic alkane.

However, this molecule has a combination of E/Z and R/S stereoisomerism - very complicated, requires university level analysis. Two of the -CH- carbon atoms of the ∆ are asymmetric (stereocentres) but the ring prevents rotation of this C-C bond (as in a >C=C< bond, so you can get cis/trans isomers (E/Z isomers).

(Z)-1,2-dimethylcyclopropane (cis)  a = a = CH₃, both below plane of ring

and (E)-1,2-dimethylcyclopropane (trans)  a = a = CH₃, one above/below plane of ring

Diagrams illustrating the E/Z (cis/trans) geometric isomerism of 1,2-dimethylcyclopropane

There are three permutations for R/S isomerism, not four, because of the symmetry of the molecule i.e. the chiral centres are adjacent and identical i.e. RR, SS and RS

'Overlapping' are two E (trans) isomers and a Z(cis) isomer to complicate matters, so a full analysis is university level.

Technically the three possibilities are:

cis/Z-1,2-dimethylcyclopropane (the cis or Z isomer of the1R,2S optical isomer)

(1R,2R)-1,2-dimethylcyclopropane: (a trans or E isomer)

(1S,2S)-1,2-dimethylcyclopropane: (a trans or E isomer) 

trans, trans and cis

(adapted from Wikipedia, https://en.wikipedia.org/wiki/C5H10 )

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

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

C₅H₁₀ has a variety of structural possibilities, thanks to its saturation level (degree of unsaturation = 1 for the alkenes)

That opens the door to a full cast of alkenes and cycloalkanes, chain branching, positional shifts, and even stereochemistry.

Here's a summary of the 'isomeric' structural styles:

Types of Isomerism for isomeric C₅H₁₀ compounds

Some physical property differences between the isomers of molecular formulae C₅H₁₀

Chemical Property Differences between the isomers of molecular formulae C₅H₁₀

• Essentially unsaturated alkene chemistry versus saturated cycloalkane chemistry.

• Alkenes:

  • Electrophilic Addition reactions (Br₂, HBr, H₂O).

  • Polymerisation → synthetic rubbers, packaging.

  • More reactive than cycloalkanes because of the alkene C=C bond, especially with respect to electrophilic reagents.

• Cyclopentane:

  • Saturated → undergoes free radical substitution (e.g., halogenation).

  • Does not undergo electrophilic addition reactions like alkenes.

  • Stable, but ring strain can influence reactivity slightly compared to larger rings.

Some typical uses and applications of the isomers of molecular formulae C₅H₁₀

Common Misconceptions about the isomers of molecular formulae C₅H₁₀ (see also below)

• Not all alkenes can show E/Z isomerism: Only when each carbon of the double bond has two different groups.

• Cyclopentane ≠ unsaturated pentenes: Despite same formula, the cycloalkanes don’t readily react with bromine water.

• Don't confuse chain with positional isomerism: They refer to different atoms being rearranged—not the same movement.

• E/Z versus cis/trans: Be cautious—cis/trans only works when identical groups are present (the terms cis/trans are used more widely, but get the E/Z notation correct and you can't go wrong in an exam!).

See also Examples of the effects of isomerism on the similarity or difference in the physical and chemical properties of structural isomers

and Examples of comparing the physical and chemical properties of alkene E/Z isomers

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

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

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