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Doc Brown's
Advanced Chemistry: Part 14.7
Isomers and extra notes on their properties and uses
9
selected constitutional structural isomers of molecular formula C4H6
[Author
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Dr
Phil Brown PhD:
Doc Brown's advanced level organic chemistry exam revision notes
suitable for students of UK A level chemistry courses, IB chemistry & US K12 grade
11, grade 12 and AP honors chemistry courses: Molecular
spectroscopy and analysing the isomers of C4H6
[isomer page updated
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Index of sets of isomers for a given
molecular formula
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9 examples of the constitutional-structural isomers of molecular formula C4H6
(Mr = 54)
Percent composition of
C4H6 based on atomic masses C= 12.01 H =
1.01 and Mr(C4H6) = 54.10
Element composition (to two dp): carbon = 88.80%
hydrogen = 11.20%
Empirical formula =
C2H3 and
molecular formula = C4H6
Introduction
to isomerism for molecular formula
C4H 6
(see also summary
diagram)
details below, can't find any more than 9?
Structural isomerism
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 configuration,
usually with significant differences in chemical and physical properties.
All
three apply to isomers of C4H6 e.g.
(a) methylcyclopropenes versus cyclobutene
(b) but-1-yne versus but-2-yne or buta-1-2-diene versus
buta-1,3-diene
(c) alkynes versus methylcyclopropenes
Stereoisomerism is where
molecules have the same basic constitutional structural formula, but isomers
differ in the 2D/3D arrangement of the atoms.
Neither of these types of
isomerism apply to my
selection of 9 C4H6 isomers.
That is not to say C4H6 has no
stereoisomers?
E/Z
stereoisomerism was called 'geometrical isomerism' e.g. cis
and trans isomers of alkenes or disubstituted cyclic alkanes
where there are 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.
Note
There are at
least 9
structural-constitutional isomers of molecular formula C4H6,
(no E/Z or R/S isomers), 2 alkynes of formula C4H6, 2
diene alkenes of formula C4H6, 3 cycloalkenes of formula C4H6, 1
non-cyclic alkene and 1 bicycloalkane of formula C4H6.
There are NO E/Z (geometrical) or E/S (optical) isomers of the 9 selected
constitutional isomers of C4H6
All the isomers of
molecular formula C4H6 are open chain
aliphatic compounds (e.g. here an unsaturated alkene or
alkyne) OR cyclic aliphatic compounds (e.g. here an
unsaturated cycloalkene), the latter sometimes referred
to as alicyclic compounds.
These molecules are examples of functional group isomerism,
(1-2) alkynes and (3-8) are aliphatic open chain alkenes (dienes), others are
cycloalkenes (alicyclic), each group having a significantly different
chemistry for a particular homologous series. (9) is a 'curious' saturated
bicycloalkane.
The alkenes themselves form two sets of aliphatic unsaturated
molecules, open carbon chain dienes (2 (C=C in chain) and cycloalkenes with a single C=C group.
There will be other isomers for
C4H6,
but I've identified plenty of examples for the moment.
Details of 9 selected
constitutional isomers of molecular formula C4H6
(1) But-1-yne,
CH3-CH2-C≡C-H,
CH3CH2C≡CH,
, 1-butyne, an alkyne.
The abbreviated structural formula and
skeletal formula of the isomer but-1-yne or 1-butyne.
But-1-yne is an unsaturated open chain
aliphatic compound in the homologous series of alkynes, that
is molecules with a carbon-carbon triple bond
C≡C.
No E/Z or R/S isomers.
Number of low resolution
NMR
chemical shift
δ
signal peaks: 3
1H and
4 13C
(email
if disagree?)
1H NMR proton ratio of peaks: 3 : 2 : 1
(for equivalent protons)
Index of 1H NMR spectra organic
compounds and
Index of 13C NMR spectra organic
compounds
(2)
But-2-yne, CH3-C≡C-CH3,
CH3C≡CCH3,
, 2-butyne, linear molecule
The abbreviated structural formula and
skeletal formula of the isomer but-2-yne or 2-butyne.
A functional group positional isomer of but-1-yne i.e. the
alkyne group
occupies a different position in the carbon chain giving a more symmetrical
molecule.
Number of low resolution
NMR
chemical shift
δ
signal peaks: 1
1H and
2 13C
(email
if disagree?)
(3) Buta-1,2-diene,
CH3-CH=C=CH2,
CH3CH=C=CH2,
,
1,2-butadiene
The abbreviated structural formula and
skeletal formula of the isomer buta-1,2-diene
(3-4) are open chain aliphatic alkenes,
with two carbon C=C alkene double bonds, a double alkene
(>C=C=C<), hence the term
diene (also known as an allene).
Number of low resolution
NMR
chemical shift
δ
signal peaks: 3
1H and
4 13C
(email
if disagree?)
1H NMR proton ratio of peaks: 3 : 1
: 2
(for equivalent protons)
(4)
Buta-1,3-diene,
CH2=CH-CH=CH2,
CH2=CHCH=CH2,
, 1,3-butadiene
The abbreviated structural formula and
skeletal formula of the isomer buta-1,3-diene
A positional functional group isomer of buta-1,2-diene (diene
- 2 alkene groups C=C-C=C).
There are no stable stereoisomers possible with
buta-1,3-diene i.e. no stable E/Z (cis/trans) isomers - don't be confused by the
shape of the skeletal formula above, there is free rotation about the
central single C-C bond (C2-C3 single bond). At university level you analyse
the structure in terms of cis and trans conformers.
Number of low resolution
NMR
chemical shift
δ
signal peaks: 2
1H and
2 13C
(email
if disagree?)
1H NMR proton ratio of peaks:4
(2+2) : 2 (1+1) = 2 :
1
(for equivalent protons)
(5) Cyclobutene,
 ,
 ,
 ,
, alicyclic compound
The molecular formula, structural formula, full displayed formula
and skeletal formula for cyclobutene. An
unsaturated aliphatic cyclic alkene, with one carbon-carbon C=C
double bond in the ring.Number of low resolution
NMR
chemical shift
δ
signal peaks: 2
1H and
2 13C
(email
if disagree?)
1H NMR proton ratio of peaks: 4 : 2 = 2
: 1
(for equivalent protons)
(6) 1-methylcyclopropene,
 ,
 , an
unsaturated cycloalkene.
The displayed structural formula and
skeletal formula of the isomer 1-methylcyclopropene, an
alicyclic compound
A positional isomer with 3-methylyclopropene, with no E/Z or
R/S isomers.
Note the alkene group ring carbon atoms, >C=C<, are
numbered preferentially C1 and C2.
Number of low resolution
NMR
chemical shift
δ
signal peaks: 3
1H and
4 13C
(email
if disagree?)
1H NMR proton ratio of peaks: 2 : 1
: 3
(for equivalent protons)
(7) 3-methylcyclopropene,
,
, an
unsaturated cycloalkene.
The displayed structural formula and
skeletal formula of the isomer 3-methylcyclopropene
A positional isomer with 1-methylyclopropene, again with no
E/Z or R/S isomers
Note it is NOT 2-methylcyclopropene because the alkene
group ring carbon atoms, >C=C<, are
numbered preferentially C1 and C2.
Number of low resolution
NMR
chemical shift
δ
signal peaks: 3
1H and
3 13C
(email
if disagree?)
1H NMR proton ratio of peaks: 1 : 3 : 2
(1+1)
(for equivalent protons)
(8) Methylenecyclopropane,
, 
The displayed structural formula and
skeletal formula of the isomer methylenecyclopropane
It is essentially an unsaturated alkene molecule (NOT a cycloalkene) with no E/Z or
R/S isomers.
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) : 2 = 2 : 1
(for equivalent protons)
(9) Bicyclobutane,
![displayed structural formula of bicyclobutane bicyclo[1.1.0]butane isomers of C4H6 molecular mass 54](../addhoc06/bicyclobutane.gif) ,
![bicyclo[1.1.0]butane structural skeletal formula of bicyclobutane isomer of molecular formula C4H6](../addhoc06/0isomc4h6-09.gif) ,
The displayed formula and skeletal formula of
bicyclo[1.1.0]butane, a sort of saturated double ringed cycloalkane molecule.
It has effective functional group.
This is a saturated cycloalkane molecule, unlike all
the other unsaturated molecules of formula C4H6
Number of low resolution
NMR
chemical shift
δ
signal peaks: 2
1H and
2 13C
(email
if disagree?)
1H NMR ratio of peaks: 4
(2+2) : 2 (1+1) = 2 : 1
(for equivalent protons)
Summary of isomerism in C4H6 and extra comments on their
physical and chemical properties and uses
Overview of isomers of C4H6
C4H6 has nine well characterized
structural isomers, spanning alkynes, dienes, cycloalkenes, cycloalkanes and
bicyclic frameworks.
Types
of Isomerism of isomers of C4H6
Constitutional (Structural) Isomerism for the isomers of C4H6
NO
Stereoisomerism
-
Conformational isomerism of 1,3-butadiene: s-cis versus
s-trans rotamers about the central C–C single bond.
-
No classical cis/trans around an isolated C=C in
unsubstituted dienes or alkynes.
Summary Table of Structural Isomers of C4H6
|
Isomer Name |
Structural
Type |
Notes |
|
1-Butyne |
Alkyne (terminal) |
Linear triple bond
at C1 |
|
2-Butyne
(Dimethylacetylene) |
Alkyne (internal) |
Triple bond
between C2–C3 |
|
1,2-Butadiene
(Allene) |
Cumulated diene |
C=C=C sequence |
|
1,3-Butadiene
(Conjugated) |
Conjugated diene |
Widely used
monomer |
|
Cyclobutene |
Cycloalkene |
Four-membered ring |
|
Methylenecyclopropane |
Cycloalkene |
Exocyclic C=CH2 |
|
1-Methylcyclopropene |
Cycloalkene |
Methyl on sp˛
carbon |
|
3-Methylcyclopropene |
Cycloalkene |
Methyl on spł
carbon |
|
Bicyclo[1.1.0]butane |
Bicyclic
hydrocarbons |
Highly strained
ring system |
Physical Property Trends for the isomers of C4H6
-
Boiling points generally rise with increased ring strain and
decreased branching because of stronger van der Waals interactions.
-
Linear alkynes/dienes are gaseous at room temperature
(lowest boiling points around –34 °C to +8 °C for 1,2-butadiene and
1-butyne).
-
Cycloalkenes pack more efficiently, elevating boiling points
into the +10 °C to +20 °C range.
-
More branching or cumulative π-systems tends to lower
boiling points by reducing surface contact.
Chemical Reactivity and Applications of the isomers of C4H6
Alkynes (1-Butyne, 2-Butyne)
-
Undergo electrophilic addition (halogenation, hydrogenation,
hydrohalogenation).
-
Deprotonation to form acetylide anions – key in C–C
bond-forming reactions.
-
Industrial feedstock for specialty polymers and alkylation
agents.
Dienes
(1,2-Butadiene, 1,3-Butadiene)
-
1,3-Butadiene: major monomer for synthetic
rubber (SBR, polybutadiene) and ABS plastics.
-
Conjugation lowers polymerization threshold, enabling
Diels–Alder syntheses.
Cycloalkenes &
Bicycles
Common
Misconceptions about the isomers of C4H6
(see also below)
-
Assuming only alkynes and forgetting cyclic isomers under C4H6.
-
Expecting cis/trans nomenclature for unsubstituted dienes or
alkynes; none apply without asymmetric substitution.
-
Overlooking conformers (s-cis/s-trans) in conjugated dienes
when considering stereochemistry.
-
Confusing positional versus chain isomerism—practice
identifying the longest continuous carbon chain first.
Exam
Revision Tips for questions that may involve the isomers of C4H6
(see also above)
-
Sketch Skeletons First: Draw all linear
backbones, then cyclic frameworks, then place unsaturations.
-
Classify Each Isomer: Label functional
group, chain, positional, ring-chain, stereoisomer type.
-
Use Tables: Compare structure, boiling
point trend, reactivity, and industrial use in a two-column table for fast
recall.
-
Practice Naming: IUPAC for allenes versus
conjugated dienes versus cycloalkenes; confirm numbering gives lowest
locants.
-
Memorize Key Applications: Link
1,3-butadiene to synthetic rubber and 2-butyne to acetylide chemistry –
evokes industrial context.
Learning objectives - questions to be answered?
How do you work out the isomers of
molecular formula C4H6?
How do you draw the structural formula
and skeletal formula of the isomers of molecular formula C4H6?
How many aliphatic structural isomers
are there of molecular formula C4H6?
How many aliphatic carbon chain isomers
are there of molecular formula C4H6?
How many positional isomers are there
of molecular formula C4H6?
How many E/Z (geometrical) isomers are
there of molecular formula C4H6?
How many R/S (optical) isomers
(enantiomers) of molecular formula C4H6?
Are there any aliphatic open chain
alkene isomers of molecular formula C4H6?
Are there any diene isomers of
molecular formula C4H6?
Are there any alkyne isomers of
molecular formula C4H6?
Are there any alicyclic cycloalkane
isomers of molecular formula C4H6?
Are there any alicyclic cycloalkene
isomers of molecular formula C4H6?
Are there any functional group isomers
with a molecular formula C4H6?
Are there any E/Z (geometrical) isomers
with a molecular formula C4H6?
Are there any R/S (optical) isomers
(enantiomers) with a molecular formula C4H6?
Does C4H6 have any stereoisomers?
Be able to work out the number of different 1H
proton NMR chemical shift signals for C4H6 isomers
Be able to work out the number of different 13C
NMR chemical shift signals for C4H6 isomers
This page
will answer these questions for molecular formula C4H6.
Associated organic chemistry links
This is a BIG
website, you need to take time to explore it
[SEARCH
BOX]
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 All my advanced Level pre-university
organic chemistry notes
IR, mass and H-1 & C-13 NMR
spectra of organic compounds
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
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
Summary diagram of isomerism which
links to details of the types of isomerism
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The isomerism of molecular formula C4H6.
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Keywords or phrases: how many isomers are
there of molecular formula C4H6? how do you name the isomers of
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and alkynes of molecular
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cis trans stereoisomers of C4H6
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isomers) alkene molecules isomeric of molecular formula C4H6,
molecules isomeric with molecular formula C4H6, structural isomers of molecular
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which types of isomerism are exhibited by molecules isomeric with
molecular formula C4H6 alkyl positional isomers of C4H6 branched
carbon chain alkene isomers of C4H6 cycloalkanes of molecular formula
C4H6 which are isomeric with alkenes of molecular formula C4H6
stereoisomers of molecular formula C4H6
cycloalkene molecules isomeric of molecular formula C4H6,
cycloalkenes and alkynes molecules isomeric with molecular formula C4H6, structural isomers of molecular
formula C4H6, optical isomers R/S enantiomers isomeric with molecular
formula C4H6, positional isomers isomeric with molecular formula C4H6,
which types of isomerism are exhibited by molecules isomeric with
molecular formula C4H6 alkyl positional isomers of C4H6 cycloalkenes
and alkynes branched
carbon chain cycloalkene isomers of molecular formula C4H6
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