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Doc Brown's
Advanced Chemistry: Part 14.7:
Isomers and extra notes on their properties and uses
Structural
isomers of molecular formula
C3H3F
C3H3Cl
C3H3Br C3H3I
[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 chemistry & US K12 grade
11, grade 12 and AP honors chemistry courses: Molecular
spectroscopy and analysing the isomers of C3H3X
(X = halogen atom)
[isomerism page updated Feb 11th 2026 *]
*
Associated organic chemistry page links
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Index of sets of isomers for a given
molecular formula
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to explore it
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The 5 structural
isomers of molecular formula
C3H3F,
C3H3Cl, C3H3Br
and
C3H3I
Relative molecular mass and
percent composition by mass of
C3H3F ,
C3H3Cl, C3H3Br
and C3H3I
based on atomic masses:
C 12.01,
H 1.01, F 19.00, Cl 35.45,
Br 79.90, I 126.90
|
Formula of compound |
Relative molecular
mass |
%
carbon |
%
hydrogen |
%
halogen |
|
C3H3F |
58.06 |
62.06 |
5.22 |
32.72 |
|
C3H3Cl |
74.51 |
48.36 |
4.06 |
47.58 |
|
C3H3Br |
118.96 |
30.29 |
2.55 |
67.16 |
|
C3H3I |
165.96 |
21.71 |
1.83 |
76.46 |
Empirical formula
=
molecular formula =
C3H3X
Introduction to isomerism in molecules of formulae
C3H3F
C3H3Cl C3H3Br
and C3H3I
If applicable
(see isomerism
summary at the end of the page)
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 C 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
C3H3X
there are
(a) The carbon atoms can be a linear chain
(aliphatic) or cyclic (alicyclic)
(b) Various position isomers based on the halogen
substituent
-X
(c) There is functional group isomerism - alkynes, diene and
cycloalkenes.
Stereoisomerism - isomers
based on the same connectivity of the atoms, but 2D or 3D spatially different
and
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
C3H3X
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.
Note
Some of the
C3H3F , C3H3Cl
, C3H3Br and
C3H3I isomers
described may be highly reactive
and very thermodynamically unstable e.g. due to weak highly
strained bonds and some may not even exist at all (except
theoretically of course!).
The images of C3H3X
(X = F, Cl, Br and I)
presented are theoretical, in the sense that some may be
so unstable not to exist, but you will find most, and sometimes all of them,
on the internet.
The are 5
constitutional-structural isomers of molecular formula
C3H3X (X =
F, Cl, Br and I),
irrespective of any E/Z or R/S isomerism that may be possible,
not possible here.
So, there are only 5 distinct isomers of formula
C3H3X
(where X = halogen atom)
Details of the
5 constitutional structural isomers
C3H3F C3H3Cl C3H3Br
and
C3H3I
(1)
X-C≡C-CH3,
structural formula of 1-fluoropropyne, 1-chloropropyne,
1-bromopropyne and 1-iodopropyne
Skeletal formula
 ,
no E/Z or R/S isomerism possible.
Functional group isomer
(e.g. open chain alkyne versus cycloalkene) and positional isomer (-X).
Functional groups:
alkyne (-C≡C-)
and halogen (-X, where X = F, Cl. Br or I)
There is no need
specify a number for the ...yne group, i.e. no need for ...prop-1-yne or
...-1-propyne,
i.e. 1-chloroprop-1-yne or 1-chloro-1-propyne, but you can if you want too!
No
E/Z or R/S isomerism possible.
Number of low resolution
NMR chemical shift
δ
signal peaks: 1 1H
and 3 13C
(email
if disagree?)
Index of 1H NMR spectra organic
compounds and
Index of 13C NMR spectra organic
compounds
(2)
H-C≡C-CH2-X,
structural formula of 3-fluoropropyne, 3-chloropropyne,
3-bromopropyne and 3-iodopropyne
Skeletal formula
,
no E/Z or R/S isomerism
possible.
Functional group isomer
(e.g. open chain alkyne versus cycloalkene) and positional isomer (-X).
Functional groups:
alkyne (-C≡C-)
and halogen (-X, where X = F, Cl. Br or I)
Again, there is no need
specify a number for the ...yne group, i.e. no need for ...prop-1-yne or
...-1-propyne, i.e. 3-chloroprop-1-yne or 3-chloro-1-propyne but you can if you want too!
There are no
2-halopropynes, no spare valency on the middle carbon atom.
No
E/Z or R/S isomerism possible.
Number of low resolution
NMR chemical shift
δ
signal peaks: 2 1H
and 3 13C
(email
if disagree?)
1H NMR ratio of peaks: 1 : 2 (for equivalent protons)
(3) H2C=C=CHX,
structural formula of
1-fluoropropa-1,2-diene,
1-chloropropa-1,2-diene,
1-bromopropa-1,2-diene and
1-iodopropa-1,2-diene.
Functional group isomer
(e.g. open chain diene versus cycloalkene).
Functional groups:
alkene (-C=C=C-, actually
diene or allene) and halogen (-X,
where X = F, Cl. Br or I)
Can also
be named as: 1-fluoro-1,2-propadiene, 1-chloro-1,2-propadiene,
1-bromo-1,2-propadiene
and 1-iodo-1,2-propadiene.
Also old names: 1-fluoroallene, 1-chloroallene,
1-bromoallene, 1-iodoallene
Skeletal formula
, no E/Z or R/S isomerism
possible.
Functional groups:
alkene (-C=C=C-, actually diene) and halogen (-X,
where X = F, Cl. Br or I)
There are no
2-halopropadienes, no spare valency on the middle carbon atom.
Number of low resolution
NMR chemical shift
δ
signal peaks: 2 1H
and 3 13C
(email
if disagree?)
1H NMR ratio of peaks: 1 : 2 (for equivalent protons)
(4)
 ,
skeletal formula of ...
1-fluorocyclopropene, 1-chlorocyclopropene,
1-bromocyclopropene and 1-iodocyclopropene.
Functional group isomer
(e.g. open chain alkyne versus cycloalkene) and positional isomer (-X).
Functional groups:
cycloalkene (-C=C-) and halogen (-X, where X =
F, Cl. Br or I)
No
E/Z or R/S isomerism possible.
Number of low resolution
NMR chemical shift
δ
signal peaks: 2 1H
and 3 13C
(email
if disagree?)
1H NMR ratio of peaks: 2 : 1 (for equivalent protons)
(5)
 ,
skeletal formula of 3-fluorocyclopropene, 3-chlorocyclopropene,
3-bromocyclopropene and 3-iodocyclopropene
Functional group isomer
(e.g. open chain alkyne versus cycloalkene) and positional isomer (-X).
Functional groups:
cycloalkene (-C=C-) and halogen (-X, where X =
F, Cl. Br or I)
No E/Z or R/S isomerism is possible.
There are no
2-halocyclopropenes, because the C=C atoms are preferentially numbered 1 and
2.
Number of low resolution
NMR chemical shift
δ
signal peaks: 2 1H
and 2 13C
(email
if disagree?)
1H NMR ratio of peaks: 2
(1+1) : 1 (for equivalent protons)
Summary of key points and extra notes on the isomers of molecular
formulae
C3H3F, C3H3Cl,
C3H3Br
and
C3H3I
Molecular Overview of isomers of molecular
formulae C3H3F, C3H3Cl, C3H3Br
and
C3H3I
Each compound has the formula C3H3X,
where X = F, Cl, Br, or I. These are halogenated alkynes,
typically halo-propynes, with a triple bond and a halogen
substituent.
Examples of types of isomerism of isomers of molecular
formulae C3H3F, C3H3Cl, C3H3Br
and
C3H3I
|
Isomerism Type |
Explanation |
Examples |
|
Structural
(Constitutional) |
Same molecular formula,
different connectivity of atoms |
1-propynyl halide (CH≡C–CH2X)
versus 3-halopropyne (X–CH2–C≡CH) |
|
Positional
Isomerism |
Functional group (halogen)
attached to different carbon atoms |
CH≡C–CH2X
versus CH2X–C≡CH |
|
Functional Group
Isomerism |
Rare here, but possible if
rearranged to form vinyl halides or allenes |
CH2=C=CHX
(allene-type) versus CH≡C–CH2X (alkyne) versus
cycloalkene |
|
Geometrical
Isomerism |
Not applicable due to
linearity of triple bond and lack of restricted rotation |
Same applies to the dienes
and no disubstituent groups. |
|
Optical Isomerism |
Not applicable unless a
chiral center is introduced (not present here) |
No chiral carbons
possible. |
Physical Property Differences halogen trends for molecular
formulae C3H3F, C3H3Cl, C3H3Br
and
C3H3I
|
Property |
Trend
Across C3H3X Series |
Explanation |
|
Boiling
Point |
Increases from F <
Cl < Br < I |
Larger halogens →
stronger London dispersion forces |
|
Dipole
Moment |
Decreases
from F to I |
Fluorine is most
electronegative, but bond length affects net dipole |
|
Density |
Increases with
halogen mass |
Heavier halogens
contribute more to molecular weight |
|
Solubility
in Water |
Decreases from F
to I |
Polar molecules
interact better with water than less polar ones, and molecules
get more bulky disrupting the hydrogen bonding in water. |
Common
Misconceptions about the isomers of molecular
formulae C3H3F, C3H3Cl, C3H3Br
and
C3H3I
-
“Triple bonds can show E/Z isomerism” →
False. No restricted rotation and only two substituents.
-
“All halogenated alkynes are equally reactive”
→ False. Reactivity depends on C–X bond strength and leaving group ability.
-
“Halogen position doesn’t affect properties”
→ False. Positional isomers differ in boiling point, dipole moment, and
reactivity.
Exam
Tips for questions involving isomers of molecular
formulae C3H3F, C3H3Cl, C3H3Br
and
C3H3I
-
Draw all structural isomers: e.g. X-C≡C–CH3
versus XCH2–C≡CH.
-
Compare boiling points and dipole moments
using halogen trends.
-
Use CIP rules only when stereoisomerism is
relevant (not here).
-
Don’t confuse constitutional isomers with
stereoisomers in this context.
-
Practice naming: e.g., 3-chloropropyne versus
1-chloropropyne.
Learning objectives - questions to be answered?
How do you draw the structural formula
and skeletal formula of the isomers of molecular formula C3H3Cl C3H3Br
C3H3F C3H3I?
How many aliphatic structural isomers
are there of molecular formula C3H3Cl C3H3Br C3H3F C3H3I?
How many aliphatic carbon chain isomers
are there of molecular formula C3H3Cl C3H3Br C3H3F C3H3I?
How many positional isomers are there
of molecular formula C3H3Cl C3H3Br C3H3F C3H3I?
How many E/Z (geometrical) isomers are
there of molecular formula C3H3Cl C3H3Br C3H3F C3H3I?
How many R/S (optical) isomers
(enantiomers) of molecular formula C3H3Cl C3H3Br C3H3F C3H3I?
Are there any cycloalkene isomers of
formula C3H3Cl C3H3Br C3H3F C3H3I?
Are there any cyclic haloalkene isomers
of formula C3H3Cl C3H3Br C3H3F C3H3I?
Are there any cyclo haloalkane isomers
of formula C3H3Cl C3H3Br C3H3F C3H3I?
Are there any diene isomers of
molecular formula C3H3Cl C3H3Br C3H3F C3H3I?
Are there any alkyne isomers of
molecular formula C3H3Cl C3H3Br C3H3F C3H3I?
Are there any functional group isomers
with a molecular formula C3H3Cl C3H3Br C3H3F C3H3I?
Do C3H3Cl C3H3Br C3H3F or C3H3I have any stereoisomers?
Are there any E/Z (geometrical) isomers
with a molecular formula C3H3Cl C3H3Br C3H3F C3H3I?
Are there any R/S (optical) isomers
(enantiomers) with a molecular formula C3H3Cl C3H3Br C3H3F C3H3I?
Be able to deduce the isomers of
organic halogen molecules with the formula C3H3Cl C3H3Br C3H3I
C3H3F.
Be able to draw and name the
substituted halogen compounds
isomers of molecular formula C3H3Cl C3H3Br C3H3I C3H3F.
Be able to deduce if
the isomers of C3H3Cl C3H3Br C3H3I C3H3F organic halogen compounds can exhibit R/S optical isomerism.
Be able to deduce if the isomers of
C3H3Cl C3H3Br C3H3I C3H3F can exhibit E/Z geometrical isomerism (do
C3H3Cl C3H3Br C3H3I C3H3F have cis/trans geometric
isomers).
This page
will answer these questions for molecular formula
C3H3F C3H3Cl
C3H3Br and
C3H3I
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
The molecular structure and naming of
HALOALKANES (how
to name and draw alkane structures)
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
HALOALKANES
including reactions
Index of revision notes
on the chemistry of ALKANES and the petrochemical
industry
Index of 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
A summary chart of isomerism
| Website content © Dr
Phil Brown 2000+. All copyrights reserved on revision notes, images,
quizzes, worksheets etc. Copying of website material is NOT
permitted. Exam revision summaries & references to science course specifications
are unofficial. |
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which types of isomerism are exhibited by molecules of formula
C3H3Cl C3H3Br C3H3F C3H3I alkyl positional isomers of C3H3Cl
C3H3Br C3H3F C3H3I branched
carbon chain isomers of C3H3Cl C3H3Br C3H3F C3H3I cycloalkanes of molecular formula
C3H3Cl C3H3Br C3H3F C3H3I which are isomeric with alkenes of molecular formula
C3H3Cl C3H3Br C3H3F C3H3I stereoisomers of molecular formula
C3H3Cl C3H3Br C3H3F C3H3I,
optical isomers R/S enantiomers isomeric with molecular
formula C3H3Cl C3H3Br C3H3F C3H3I, positional isomers isomeric with molecular formula
C3H3Cl C3H3Br C3H3F C3H3I,
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alkyl positional isomers of C3H3Cl C3H3Br C3H3F C3H3I, how to
work out the isomers of C3H3Cl C3H3Br C3H3F C3H3I
How to deduce the isomers of
organic halogen molecules with the formula C3H3Cl C3H3Br C3H3I
C3H3F, How to write, draw and name the substituted halogen
compounds isomers of molecular formula C3H3Cl C3H3Br C3H3I
C3H3F, How to deduce if the isomers of C3H3Cl C3H3Br C3H3I C3H3F
organic halogen compounds can exhibit R/S optical isomerism, How to deduce if the isomers of
C3H3Cl C3H3Br C3H3I C3H3F can exhibit E/Z geometrical isomerism (do
C3H3Cl C3H3Br C3H3I C3H3F have cis/trans geometric
isomers?), number of structural isomers of C3H3Cl C3H3Br C3H3I
C3H3F, number of stereoisomers of C3H3Cl C3H3Br C3H3I C3H3F
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