Constitutional vs. Stereoisomers

Isomers are compounds having same molecular formulae but different structural formulae. The molecular formula shows the actual number of atoms whereas the structural formula shows the arrangement of atoms and bonds in a molecule.

There are two main types of isomers.

Constitutional isomers, also known as structural isomers, differ in the arrangements of atoms in molecules. They differ in bond connectivities and the order of attachment of atoms. They also have significantly different physical and chemical properties. Whereas, stereoisomers have the same bond connectivity but differ in the position of identical groups in space (in three dimensions).

Stereoisomers differ from constitutional isomers in the relative spatial orientation of atoms.

Constitutional isomers vs stereoisomers

Constitutional isomerism

Constitutional isomers differ in their structural formulae. They have different bond connectivities e.g. smaller hydrocarbons such as methane, ethane, propane, do not show this type of isomerism because they exist in only one structural form. Generally, isomerism is common in compounds containing more than three carbon atoms.

Butane has two constitutional (chain) isomers, pentane has 3 isomers and hydrocarbons with 30 carbon atoms can have over 4 billion isomers.

Constitutional isomers are classified into the following classes:

1. Chain isomerism (skeletal isomerism)
2. Position isomerism (regioisomers)
3. Functional group isomerism
4. Metamerism
5. Tautomerism (desmotropism)

Chain isomerism (skeletal isomerism)

Compounds having the same molecular formulae but different structural formulae based on the possibility of branching in carbon chains are called chain isomers. They are also known as skeletal isomers.

For example

Pentane having a molecular formula (C₅H₁₂) has three chain isomers.

Position isomerism (Regioisomers)

Compounds posessing the same molecular formula and different positions of the same functional group on the carbon chain are called positional isomers. They are also known as regioisomers.

For example

Bromobutane has two positional isomers

Similarly, pentanol has three positional isomers.

Another example is butene which has 2 positional isomers.

Functional group isomerism

Compounds with the same molecular formula but different functional groups are known to be functional group isomerism. The possibility arises by rearranging the atoms within molecules.

For example

C₂H₆O can have two functional group isomers.

The first structure has an ester functional group while the second has an alcoholic functional group.

Another example is  C₃H₆O which also has two functional group isomers.

The first structure has a ketonic functional group while the second has an aldehyde in it.

Metamerism

The compounds having an unequal distribution of carbon atoms on both sides of the functional group are called metamers. The phenomenon is called metamerism.

Metamers have different alkyl groups on two sides of the functional groups.

For example

C₄H₁₀O has two metamers.

Similarly, C₅H₁₀O has two metamers.

Tautomerism (desmotropism)

Tautomers differ in the position of the hydrogen atom.

The term tautomerism is derived from Greek words “tauto” meaning ‘the same’ and “meros” meaning ‘parts’.  Tautomers are interconvertible structures. In this type of isomerism, substances exist in readily interconvertible structures leading to dynamic equilibrium. It is also known as desmotropism (“desmo” meaning ‘bond’ and “tropos” meaning ‘turn’).

For example

Keto-enol is the most common example of tautomerism. Aldehydes and ketones must have one ∝ hydrogen for keto-enol tautomerism to exist.

Keto form has ketonic functionality whereas enol form has a double bond and alcoholic functional group.

Keto form is much more stable than enol form. The reason for this stability is the bond strength of carbonyl (C=O) which is much stronger than the functional alkene (C=C) bond.

Another example of tautomerism is acetophenone. It also shows keto-enol structures.

Stereoisomerism

Like constitutional isomers, stereoisomers have the same molecular formula but have different orientations of atoms in space.

Stereoisomers are classified into two main classes:

• Conformational isomers
• Configurational isomers

Conformational isomers

Conformational isomers are formed by the free rotation of atoms around a single bond. This type of stereoisomerism can be made possible by interconverting chemical bonds without breaking them.

Conformational structures are readily interconvertible i.e. they can not be separated. Some conformations are more stable than others.

For example

Ethane has two conformations, staggered and eclipsed. Both are interconvertible.

In staggered conformation, all (C-H) bonds are maximum distance apart having minimum repulsions. While in eclipsed conformation, all (C-H) bonds are at a minimum distance, having maximum repulsions. Hence, staggered conformation is more stable than the eclipsed conformation.

Similarly, we have an example of butane that has four different conformations.

These conformations can be explained as:

Anti- conformation

When methyl (CH₃) groups are the farthest from each other. They have minimum repulsion, so, they are the most stable conformers. This type of conformation is termed anti-conformation.

Skew Eclipsed

When methyl (CH₃) group and hydrogen (H) group on adjacent carbon are exactly in front of each other. They have torsional strain as well as steric repulsion. This conformation is termed as skew eclipsed conformation.

Skew Staggered

When methyl (CH₃) groups are farther apart from each other at an angle of 60⁰. The conformation is known as skew staggered conformation also known as ‘Gauche conformation’.

Eclipsed conformation

When methyl (CH₃) groups are exactly in front of each other. They have maximum repulsion. So, this is the least stable of all conformations making it ‘eclipsed conformation’.

Related Topics

• R/S Nomenclature
• Keto Enol Tautomerism
• Racemic Mixture

Configurational isomers

Configurational isomers are molecules having different arrangements of the same type of groups (molecular groups). They are not readily interconvertible.

They can not be interconverted unless at least one chemical bond is broken.

Configurational isomers are classified into two classes:

1. Optical isomers
2. Geometrical isomers

Optical isomers

Optical isomers have the same bond connectivities but different arrangements of the same groups in space. They are different on the basis of their optical activities.

Optical isomers are further classified as:

• Enantiomers
• Diastereomers

Enantiomers are structures that are non-superimposable mirror images of each other. They have the same physical properties but different stereochemical properties. The identification of enantiomers is the difference in rotation of plane-polarized light.

For example

2-hydroxypropanoic acid (Lactic acid) is capable of forming enantiomers.

Diastereomers are non mirrored images to each other. They are not superimposable and have a different angle of rotation.

Diastereomers can have two or more chiral centers. They have different physical and chemical properties.

For example

2,3-dihydroxybutanoic acid has the capability of forming diastereomers.

Geometrical isomers

The compounds having the same molecular formula but different arrangements of atoms or groups in space around the double bond are called geometrical isomers. Carbon atoms joined by a double bond can not rotate freely. Thus, groups attached get fixed positions in space giving rise to geometrical (cis-trans) isomerism.

The following are the conditions for geometrical isomerism to exist:

1. Presence of double bond.
2. Two different groups must be attached with the same carbon of the double bond.

Geometrical isomers are further classified into:

• cis-trans system
• E-Z system

For the cis-trans system to exist, alkenes must have two similar groups across two sides of the double bond.

For example

The compounds in which similar groups are present on the same side of the double bond are called cis isomers.

For example

• cis-but-2-ene
• cis-1-chloroprop-1-ene

The compounds in which similar groups are present on the opposite side of the double bond are called trans-isomers.

For example

• trans-1,2-dibromoethane
• trans-but-2-ene

Geometrical isomerism is also possible with other double bonds such as (C=N) or (N=N) etc.

The difference in arrangement of same groups across double bond is explained well by the cis-trans system, whereas the presence of different (although similar) groups across double bond is explained by the E-Z system.

E-Z system is somehow an alternate approach to explain geometrical isomerism when the comparison shifts from the same groups to similar groups.

When higher priority groups are present on the opposite sides across the double bond, the E type isomerism exists.

When higher priorities groups are present on the same side of the double bond, the Z type geometrical isomerism exists.

Key Takeaway(s)

Concepts Berg

What are the constitutional isomers? Mention an example.

Molecules having the same molecular formula but different structural formulas are called constitutional isomers. They have different bond connectivities.

For example, n-butane and iso-butane are conformational isomers.

What are stereoisomers’ examples?

Some examples of stereoisomers are:

1. cis-1,2-dibromoethane and trans-1,2-dibromoethane.
2. cis-2-butene and trans-2-butene.
3. (2S)-2-hydroxypropanoic acid and (2R)-2-hydroxypropanoic acid(Lactic acid).
4. Naproxen (a pain reliever) has stereoisomers.

• • (+)-(S)-2-(6-methoxynapthalen-2-yl) propanoic acid.
  • (-)-(R)-2-(6-methoxynapthalen-2-yl) propanoic acid.

What are the types of stereoisomers?

Stereoisomers are classified into two main classes:

1. Configurational isomers
2. Conformational isomers

How are constitutional isomers different?

Constitutional isomers are different in the structural arrangement of the same atoms or groups. They are different in bond connectivities. Constitutional isomers also have differences in physical and chemical properties.

What are examples of constitutional isomers?

Examples of constitutional isomers are:

1. But-1-ene and but-2-ene (position isomerism).
2. Pentan-2-one and pentan-3-one (position isomerism).
3. Dimethyl ether and ethanol (functional group isomerism).
4. Diethyl ether and methyl propyl ether (metamerism).
5. 2-methylbutane and 2,2-dimethylpropane (chain isomerism).

Do stereoisomers have the same physical properties?

Stereoisomers do not have the same physical properties. For example, during the hydrogenation of vegetable oils, both cis and trans products are formed. These products have different melting points. The trans product with a greater melting point is removed from the final edible product because it is not good for health.

What are diastereomers, give an example?

Diastereomers are types of stereoisomers that are not the mirror images of each other. They are non-superimposable. They have different physical and chemical properties and have two or more chiral centers.

For example

1. 2,3-dihydroxybutanoic acid.
2. (1R,2S)-2-chlorocyclohexan-1-ol and (1S,2S)-2-chlorocyclohexan-1-ol.

What are D and L stereoisomers?

Stereoisomer which rotates the plane-polarized light to the right or clockwise direction called D (dextro) isomer. The word D stands for dextrorotatory. It is indicated by sign (+). For example, D-Glucose, D-Alanine, D-glyceraldehyde, etc.

Stereoisomer which rotates plane-polarized light to the left or anticlockwise direction called L (Levo) isomer. The word L stands for levorotatory. It is indicated by sign (-). For example, L-Glucose, L-Alanine, L-glyceraldehyde, etc.

What is the difference between position isomerism and stereoisomerism?

Positional isomers are the type of constitutional isomers which differ in the position of the same functional group.

For example, 1-bromobutane and 2-bromobutane.

While stereoisomers have the same connectivities but differ in three-dimensional arrangements of the same atoms in space. For example, cis-1,4-dimethylcyclohexane and trans-1,4-dimethylcyclohexane.

What is the difference between isomers and enantiomers?

Isomers differ in structure, their molecular formula is the same. For example, n-pentane, isopentane, and neopentane are isomers.

Enantiomers are the type of stereoisomers that are non-superimposable mirror images. They have the same structure but have different arrangements of atoms around the central atom in space. They differ in optical activities.

For example, 2-hydroxypropanoic acid has 2 enantiomers which are (2S)-2-hydroxypropanoic acid and (2R)-2-hydroxypropanoic acid.

What is the difference between geometrical and structural isomerism?

The isomers having the same structural formula but the different positions of identical groups in space due to restricted rotation across double bonds are called geometrical isomers. For example, cis-2-butene and trans-2-butene, etc.

While structural isomers are molecules having the same molecular formula but different structural formulas. They have different arrangements of the same groups. For example, n-butane and 2-methyl propane (isobutane), etc.

What is the difference between (-) and (+) isomers?

(-) isomers are also called levorotatory because they rotate plane-polarized light to the left or in an anticlockwise direction. While (+) isomers are also called dextrorotatory because they rotate plane-polarized light to the right or in the clockwise direction. This rotation of plane-polarized light makes such molecules, optical isomers.

What are isomers of heptane?

Structural isomers of heptane:

• n-heptane
• 2-methylhexane.
• 3-methylhexane.
• 2,2-dimethylpentane.
• 2,3-dimethylpentane.
• 2,4-dimethylpentane.
• 2,2,3-dimethylbutane.
• 3-ethylpentane.
• 3,3-dimethylpentane.

What is the difference between allotropes and isomers?

The existence of an element in two or more forms in the same physical state is called allotropy and those different forms of an element are called allotropes. Carbon, oxygen, sulfur, phosphorus, tin, etc all have allotropic forms. For example, diamond, graphite, graphene, and buckyballs are the different allotopes of carbon.

While compounds having different structural arrangements of atoms are called isomers. For example, n-butane and isobutane, etc.

Can epimers be called isomers?

Epimers can be called isomers because they are in fact isomers with one carbon atom fixed for different configurations called the stereogenic centers.

What are the possible isomers of C5H8?

Isomers of (C₅H₈) are:

As pentyne (C₅H₈):

1. 1-pentyne.
2. 2-pentyne
3. 3-methyl-1-butyne

As pentadiene (C₅H₈):

1. 1,2-pentadiene.
2. 1,3-pentadiene.
3. 1,4-pentadiene.
4. 2,3-pentadiene.

As Butadiene (C₅H₈):

1. 3-methyl-1,2-butadiene.
2. 2-methyl-1,3-butadiene.

As Cyclopentene (C₅H₈):

1. Cyclopentene.

As cyclobutane (C₅H₈):

1. methylenecyclobutane.

As Cyclobutene (C₅H₈):

1. 1-methylcyclobutene.
2. 3-methylcyclobutene.

As Cyclopropane (C₅H₈):

1. Ethylenecyclopropane.
2. Ethylidienecyclopropane.
3. 2-methyl-1-methylenecyclopropane.

As Cyclopropene (C₅H₈):

1. 1-ethylcyclopropene.
2. 3-ethylcyclopropene.
3. 1,2-dimethylcyclopropene.
4. 1,3-dimethylcyclopropene.

As Bicyclopentane(C₅H₈):

1. Bicyclo (1,1,1)pentane.
2. Bicyclo (2,1,0)pentane.

As spiropentane(C₅H₈):

1. Spiro (2,2) pentane.

What is the difference between epimers and enantiomers?

Epimers are types of diastereomers that have two or more chiral centers but differ only in absolute configuration at one chiral center called the stereogenic center. 2,3,4-trihydrocypentanoic acid is an example of diastereomers which can be called epimers.

While enantiomers are non-superimposable mirror images the two forms of the same compound. They have the same physical and chemical properties. They differ in optical activities.

For example, (2S)-2-hydroxypropanoic acid (Lactic acid) is levorotatory while (2R)-2-hydroxypropanoic acid (Lactic acid) is a dextrorotatory compound.

What is the difference between mesocompounds and enantiomers?

Differences between mesocompounds and enantiomers are as follows:

• Mesocompounds are achiral while enantiomers are chiral.
• They are optically active while enantiomers are not.
• They have a plane of symmetry whereas enantiomers have no plane of symmetry.
• Superimposable images are mesocompounds whereas non-superimposable images are enantiomers.

What are the isomers of ethanol?

Ethanol has many types of isomers, one type of the isomers of ethanol are functional group isomers e.g. dimethyl ether and ethanol.

References

• D- and L- Notation For Sugars – Master Organic Chemistry
• Solomons’ Organic Chemistry, 12th Edition, Global Edition