The two constitutional-structural isomers of molecular formula C₂H₂X₂  (X = halogen)

Relative molecular mass and percent composition by mass of C₂H₂F₂ , C₂H₂Cl₂ , C₂H₂Br₂ and C₂H₂I₂ based on atomic masses:

C 12.01,  H 1.01, F 19.00,  Cl 35.45,  Br 79.90, I 126.90

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.

Of these, only substituent (halogen) structural positional isomerism occurs for the isomers of the formulae C₂H₂F₂ C₂H₂Cl₂ C₂H₂Br₂ or C₂H₂I₂

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.

There is no R/S optical isomerism possible for the formulae C₂H₂F₂ C₂H₂Cl₂ C₂H₂Br₂ or C₂H₂I₂, but one of the constitutional structural isomers exhibits E/Z geometrical isomerism.

E/Z (geometrical) stereoisomerism

This 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 due to restricted rotation e.g. about a >C=C< bond (E = trans, Z = cis).

One isomer for each of the four halogen formulae exhibits E/Z (geometrical) isomerism.

R/S (optical) stereoisomerism

This 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), e.g. an asymmetric carbon atom with four different atoms/groups attached to it.

Not applicable here.

Note for isomers of formulae C₂H₂F₂ C₂H₂Cl₂ C₂H₂Br₂ or C₂H₂I₂

Some of the isomers of formulae C₂H₂F₂ C₂H₂Cl₂ C₂H₂Br₂ or C₂H₂I₂ be highly reactive and very unstable and some may not even exist at all (except theoretically of course!), so some isomers presented are theoretical, in the sense that some may be so unstable as not to exist, but you will find most, and sometimes all of them, on the internet.

The are only 2 constitutional-structural isomers possible of molecular formula C₂H₂X₂ (X = F, Cl, Br and I), irrespective of any E/Z or R/S isomerism that may be possible.

If you include the stereoisomeric E/Z geometrical isomers (cis and trans), there are 3 distinct isomers in total. for the four C₂H₂X₂ formulae.

All three of these isomers are derived from the dihalo-substitution of the unsaturated alkene hydrocarbon of formula C₂H₄ (ethene) with fluorine, chlorine, bromine or iodine.

(1) H₂C=CX₂ where X = F, Cl, Br or I,  

The abbreviated structural formula, displayed structural formula and skeletal formula for ...

1,1-difluoroethene, 1,1-dichloroethene, 1,1-dibromethene, 1,1-diiodoethene

You might encounter their old names e.g. 1,1-dichloroethylene.

These molecules cannot exhibit E/Z 'geometrical' isomerism, just positional isomers.

For the same halogen atom, the 1,1-dihalgenated ethene molecules are constitutional structural (positional) isomers along with the 1,2-dihalogenated ethene molecules.

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

The protons have identical chemical environments, but not the two carbon atoms.

 

(2) XHC=CHX where X = F, Cl, Br or I

The abbreviated structural formula, displayed structural formula and skeletal formula for ...

1,2-difluoroethene, 1,2-dichloroethene, 1,2-dibromethene, 1,2-diiodoethene

Apart from positional isomerism, these molecules exhibit E/Z 'geometrical' isomerism, the C=C bond being the stereocentre, the E/Z assignment is quite easy in this case.

From the CIP priority rule: _ZX > ₁H (where X = halogen, and Z = 9, 17, 35 or 53)

Reminder, that for the same halogen atom, the E/Z (cis/trans) geometrical isomers of 1,2-dihalgenated ethene molecules are constitutional structural (positional) isomers along with the 1,1-dihalogenated ethene molecules.

To indicate the E/Z (trans/cis) geometrical isomers, you would be name them as follows:

(E)-1,2-difluoroethene, (E)-1,2-dichloroethene, (E)-1,2-dibromethene, (E)-1,2-diiodoethene

(Z)-1,2-difluoroethene, (Z)-1,2-dichloroethene, (Z)-1,2-dibromethene, (Z)-1,2-diiodoethene

In old notation (E) is trans and (Z) is cis, so all the names can be written as:

trans-1,2-difluoroethene, trans-1,2-dichloroethene, trans-1,2-dibromethene, trans-1,2-diiodoethene

cis-1,2-difluoroethene, cis-1,2-dichloroethene, cis-1,2-dibromethene, cis-1,2-diiodoethene

You might encounter their old names e.g. 1,2-dichloroethylene or trans-1,2-dichloroethylene.

Number of low resolution NMR chemical shift δ signal peaks: 1 ¹H and 1 ¹³C

because of the symmetry of both E/Z isomer molecules. (email if disagree?)

Both pairs of protons and carbon atoms have their own identical chemical environments, though there might be small difference between the chemical shifts of the E (trans) and Z (cis) isomers.

 

There are three distinct isomers in total, counting the two E/Z isomers.

There are only 2 possible constitutional structural isomers.

These dihalogenated ethene molecules pack a surprising variety of isomerism types into a deceptively simple formula.

General Molecular Structure

Types of Isomerism Exhibited

Example Breakdown e.g. for the dichloroethenes: C₂H₂Cl₂

Common Misconceptions

Key Factors Influencing Boiling Points

• Polarity: Z/cis isomers are generally more polar than E/trans, leading to stronger dipole - dipole interactions and higher boiling points.

• Symmetry: E/trans isomers are more symmetrical, often resulting in lower boiling points due to reduced polarity.

• Molecular Mass: Heavier halogens (I > Br > Cl > F) with more electrons (more polarizable) increase London dispersion forces, increasing intermolecular bonding force, hence raising boiling points across the series.

• Isomer Type: 1,1-isomers (geminal) tend to have intermediate boiling points due to their unique dipole arrangements.

Uses of these halogenated ethenes and their isomers.

Here’s a detailed overview of the isomers and their uses for the molecular formulas, all of which are halogenated ethenes with geometric (cis/trans) and positional isomerism.

General Overview of Isomers

Each of these compounds has:

• 1,1-isomer: both halogen atoms on the same carbon

• cis-1,2-isomer: halogens on adjacent carbons, same side

• trans-1,2-isomer: halogens on adjacent carbons, opposite sides

These isomers differ in polarity, boiling points, reactivity, and industrial utility.

C₂H₂F₂ (Difluoroethene)

Isomers:

• 1,1-Difluoroethene

• cis-1,2-Difluoroethene

• trans-1,2-Difluoroethene

Applications:

• Refrigerants: Used in hydrofluoroolefins (HFOs), especially in low-global-warming refrigerants.

• Polymer precursors: Building blocks for fluoropolymers like Teflon.

• Electronic materials: Used in plasma etching and semiconductor processing.

• Research: Studied for unique dipole interactions and cis-trans stability inversion.

C₂H₂Cl₂ (Dichloroethene)

Isomers:

• 1,1-Dichloroethene

• cis-1,2-Dichloroethene

• trans-1,2-Dichloroethene

Applications:

• Solvents: Used in degreasing and cleaning agents, especially trans-isomer.

• Intermediate in vinyl chloride production: A precursor for PVC plastics.

• Chemical synthesis: Participates in coupling and cycloaddition reactions.

• Environmental studies: Found as a breakdown product of trichloroethene in polluted sites.

C₂H₂Br₂ (Dibromoethene)

Isomers:

• 1,1-Dibromoethene

• cis-1,2-Dibromoethene

• trans-1,2-Dibromoethene

Applications:

• Flame retardants: Used in fire-resistant materials.

• Organic synthesis: Intermediate for pharmaceuticals and agrochemicals.

• Material science: Studied for molecular geometry and dipole behavior.

• Nonpolar solvents: Trans-isomer is nonpolar and used in specialized solvent systems.

C₂H₂I₂ (Diiodoethene)

Isomers:

• 1,1-Diiodoethene

• cis-1,2-Diiodoethene

• trans-1,2-Diiodoethene

Applications:

• Radiopaque agents: Iodine-rich compounds are used in medical imaging.

• Synthetic intermediates: Useful in coupling reactions due to iodine’s leaving group ability.

• Research: Valuable in studying halogen effects on molecular stability and reactivity.

Comparative Table of Isomer Uses

Index of sets of isomers for a given molecular formula

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

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