21 selected constitutional-structural isomers of molecular formula C₄H₅X (X = halogen)

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 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.

(a) Cyclic versus open chain isomers e.g. cycloalkenes versus alkynes versus dienes.

(b) Positions of the alkene and alkyne groups.

(c) e.g. cyclic alkenes versus open chain alkynes

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.

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.

One selected isomer exhibits E/Z geometrical isomerism No. 8.

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.

Six of the selected isomers exhibits R/S optical isomerism (2, 5, 11, 13, 15 and 17).

Note

Some of the C₄H₅X (X = halogen) 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!).

There will be other isomers for C₄H₅X, but I've identified plenty of examples for the moment.

(1) , CH₃-CH₂-C≡C-X, a substituted alkyne, skeletal formula and structural formula

Functional group and positional isomerism.

Two functional groups: an alkyne (-C≡C-) and a halogen (C-X) group

1-fluorobut-1-yne,  1-chlorobut-1-yne,  1-bromobut-1-yne, 1-iododbut-1-yne

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 4 ¹³C (email if disagree?)

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

 

(2) , CH₃-CHX-≡C-H, a substituted alkyne, skeletal formula and structural formula

Functional group and positional isomerism.

Two functional groups: an alkyne (-C≡C-) and a halogen (C-X) group

These molecules exhibit R/S (optical) isomerism because carbon atom C2 is chiral (asymmetric), 4 different atoms/groups attached to it, so a pair of enantiomers can exist.

3-fluorobut-1-yne,  3-chlorobut-1-yne,  3-bromobut-1-yne, 3-iododbut-1-yne

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 : 1  (for equivalent protons)

 

(3) , XH₂C-CH₂-C≡C-H, a substituted alkyne, skeletal formula and structural formula

Functional group and positional isomerism.

Two functional groups: an alkyne (-C≡C-) and a halogen (C-X) group

4-fluorobut-1-yne,  4-chlorobut-1-yne,  4-bromobut-1-yne, 4-iododbut-1-yne

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

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

 

(4) , XH₂C-C≡C-CH₃, a substituted alkyne, skeletal formula and structural formula

Functional group and positional isomerism.

Two functional groups: an alkyne (-C≡C-) and a halogen (C-X) group

4-fluorobut-2-yne,  4-chlorobut-2-yne,  4-bromobut-2-yne, 4-iododbut-2-yne

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 4 ¹³C (email if disagree?)

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

 

(5) CH₃-CH=C=CH-X, a substituted diene (two alkene groups)

Functional group and positional isomerism.

Two functional groups: a diene (-C=C=C-) and a halogen (C-X) group,

skeletal formula and structural formula for

1-fluorobuta-1,2-diene,  1-chlorobuta-1,2-diene,  1-bromobuta-1,2-diene,  1-iodobuta-1,2-diene

This molecule exhibit R/S (optical) isomerism - but a more unusual case!

These stereoisomers (pairs of enantiomers) exist because the pi orbitals of the two adjacent C=C bonds are at 90^o to each other forming a chiral centre for stereoisomerism.

This allows the formation of non-superimposable mirror image forms (a bit subtle at pre-university, but make the models yourself!).

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 : 1  (for equivalent protons)

 

(6) CH₃CX=C=CH₂, a substituted diene (two alkene groups)

Functional group and positional isomerism.

Two functional groups: a diene (-C=C=C-) and a halogen (C-X) group,

skeletal formula and structural formula for

3-fluorobuta-1,2-diene,  3-chlorobuta-1,2-diene,  3-bromobuta-1,2-diene,  3-iodobuta-1,2-diene

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 4 ¹³C (email if disagree?)

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

 

(7) , XH₂C-CH=C=CH₂, a substituted diene (two alkene groups)

Functional group and positional isomerism.

Two functional groups: a diene (-C=C=C-) and a halogen (C-X) group,

skeletal formula and structural formula for

4-fluorobuta-1,2-diene,  4-chlorobuta-1,2-diene,  4-bromobuta-1,2-diene,  4-iodobuta-1,2-diene

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

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

 

(8) , H₂C=CH-CH=CH–X, a substituted diene (two alkene groups)

Functional group and positional isomerism.

skeletal formula and structural formula for

1-fluorobuta-1,3-diene,  1-chlorobuta-1,3-diene,  1-bromobuta-1,3-diene,  1-iodobuta-1,3-diene

Two functional groups: a diene (-C=C–C=C-) and a halogen (C-X) group.

These molecules exhibit E/Z (geometrical) isomerism from the halogenated -1-ene group

From the Cahn-Ingold-Prelog (CIP) priority assignment rule:

₆C, ₉F, ₁₇Cl, ₃₅Br and ₅₃I have a higher priority than ₁H.

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

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

 

(9) H₂C=C-CHX=CH₂, a diene (two alkene groups)

Functional group and positional isomerism.

Two functional groups: a diene (-C=C–C=C-) and a halogen (C-X) group.

skeletal formula and structural formula for

2-fluorobuta-1,3-diene,  2-chlorobuta-1,3-diene,  2-bromobuta-1,3-diene,  2-iodobuta-1,3-diene

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

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

 

(10) a substituted cycloalkene, skeletal formula

Positional and functional group isomerism.

Two functional groups: a cycloalkene (-C=C-) and a halogen (C-X) group.

1-fluorocyclobutene,  1-chlorocyclobutene,  1-bromocyclobutene,  1-iodocyclobutene

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

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

 

(11) a substituted cycloalkene, skeletal formula

Positional and functional group isomerism.

Two functional groups: a cycloalkene (-C=C-) and a halogen (C-X) group (secondary haloalkane).

This molecule exhibits R/S optical isomerism, top right carbon atom is a chiral centre (asymmetric carbon atom).

3-fluorocyclobutene,  3-chlorocyclobutene,  3-bromocyclobutene,  3-iodocyclobutene

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

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

 

(12) a substituted cycloalkene, skeletal formula

Positional and functional group isomerism.

Two functional groups: a cycloalkene (-C=C-) and a halogen (C-X) group.

1-fluoro-2-methylcyclpropene,  1-chloro-2-methylcyclopropene,  1-bromo-2-methylcycloprpene,  1-iodo-2-methylcyclopropene

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 4 ¹³C (email if disagree?)

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

 

(13) a substituted cycloalkene, skeletal formula

Positional and functional group isomerism.

Two functional groups: a cycloalkene (-C=C-) and a halogen (C-X) group.

This molecule exhibits R/S optical isomerism, top carbon atom is chiral (asymmetric carbon atom).

3-fluoro-1-methylcyclopropene,  3-chloro-1-methylcyclopropene,  3-bromo-1-methylcycloprpene,  3-iodo-1-methylcyclopropene

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

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

 

(14a) a substituted cycloalkene, skeletal formula

Positional and functional group isomerism.

Two functional groups: a cycloalkene (-C=C-) and a halogen (C-X) group (a primary haloalkane).

1-(fluoromethyl)cyclopropene,  1-(chloromethyl)cyclopropene,  1-(bromomethyl)cycloprpene,  1-(iodomethyl)cyclopropene

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

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

 

(14b)   a substituted cycloalkene, skeletal formula

Positional and functional group isomerism.

Two functional groups: a cycloalkene (-C=C-) and a halogen (C-X) group (a primary haloalkane).

3-(fluoromethyl)cyclopropene,  3-(chloromethyl)cyclopropene,  3-(bromomethyl)cycloprpene,  3-(iodomethyl)cyclopropene

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

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

 

(15) a substituted cycloalkene, skeletal formula

Positional and functional group isomerism.

Two functional groups: a cycloalkene (-C=C-) and a halogen (C-X) group.

This molecule exhibits R/S optical isomerism, top carbon atom of the ring is a chiral centre (asymmetric carbon atom).

1-fluoro-3-methylcyclpropene,  1-chloro-3-methylcyclopropene,  1-bromo-3-methylcycloprpene,  1-iodo-3-methylcyclopropene

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

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

 

(16) a substituted cycloalkene, skeletal formula

Positional and functional group isomerism.

Two functional groups: a cycloalkene (-C=C-) and a halogen (C-X) group (a tertiary haloalkane).

3-fluoro-3-methylcyclpropene,  3-chloro-3-methylcyclopropene,  3-bromo-3-methylcycloprpene,  3-iodo-3-methylcyclopropene

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+1) : 3  (for equivalent protons)

 

(18) can be classed as an open chain alkene or a cycloalkane, skeletal formula

Positional and functional group isomerism.

Two functional groups: a alkene (-C=C-) and a halogen (C-X) group.

(fluoromethylidene)cyclopropane,  (chloromethylidene)cyclopropane,  (bromomethylidene)cyclopropane,  (iodoromethylidene)cyclopropane

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 4 ¹³C (email if disagree?)

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

Not shown, but theoretically you can reposition the halogen atom and make:

1-fluoro-2-methylenecyclopropane,  1-chloro-2-methylenecyclopropane,  1-bromo-2-methylenecyclopropane,  1-iodo-2-methylenecyclopropane

 

(19) and (20) bicyclo-alkane molecules, saturated cycloalkanes

These are two theoretical isomeric versions of: fluorobicyclobutane,  chlorobicyclobutane,  bromobicyclobutane, iodobicyclobutane

The only functional group is the halogen are a secondary and tertiary haloalkane, no C=C or C≡C.

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

Positional and functional group isomerism.

One functional group: halogen (C-X) group (a secondary haloalkane).

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

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

Positional and functional group isomerism.

One functional group: halogen (C-X) group (a tertiary haloalkane).

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

Note how spectra can be used to distinguish which halogen compound and which isomer

• Trend: As halogen atomic mass increases (F → I), boiling point and density increase due to stronger London dispersion forces.

• Solubility: Generally low in water, but increases in organic solvents.

Chemical Reactivity Differences - halogen trend for C₄H₅F , C₄H₅Cl , C₄H₅Br and C₄H₅I compounds

• Fluoroalkenes are less reactive due to strong C–F bonds.

• Iodoalkenes are highly reactive in substitution and elimination due to weak C–I bonds.

• For each formula, general speaking the reactivity trend for non >C=CH-Cl is: tertiary > secondary > primary for SN1 mechanisms and primary > secondary > tertiary for SN2 mechanisms.

• The presence of a >C=C-X group reduces the activity of the halogen leaving group.

Uses and Applications of C₄H₅F , C₄H₅Cl , C₄H₅Br and C₄H₅I compounds

Common Misconceptions about C₄H₅F , C₄H₅Cl , C₄H₅Br and C₄H₅I compounds (see also below)

• “All isomers have similar properties”: False. Even positional isomers can differ significantly in boiling points and reactivity.

• “Halogens behave the same in reactions”: Not true—reactivity varies widely due to bond strength and leaving group ability.

• “Geometrical isomerism only occurs in cyclic compounds”: Incorrect—alkenes like C₄H₅Cl can show E/Z isomerism.

Exam Revision Tips for questions involving C₄H₅F , C₄H₅Cl , C₄H₅Br and C₄H₅I compounds (see also above)

• Draw all possible isomers: Use skeletal structures and label positions clearly.

• Compare boiling points and reactivity: Link trends to halogen size and electronegativity.

• Use reaction mechanisms: SN1 versus SN2, E1 versus E2 - know which halogen favours which.

• Practice naming: IUPAC names help distinguish positional and stereoisomers.

• Highlight functional group behavior: Especially in substitution and elimination.

 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

Index of all IR, mass, 1H NMR and 13C NMR spectroscopy pages