Isomerism and Its Types

 

Isomerism and Its Types

 

Definition, Basis and Explanation

The word isomer is a combination of two words, iso means same and mers means unit and this term was invented by Berzelius. The compounds which differ in their properties but have same molecular formula are called isomers and the phenomenon is known as isomerism.

 

Isomerism is the existence of different compounds exhibiting different physical or chemical properties or both having same molecular formula i.e. The phenomenon of existence of two or more compounds possessing the same molecular formula but different properties is known as isomerism. Such compounds are called as isomers.

 

Isomers refer to those compounds which have the same molecular formula but differ in physical or chemical properties or both i.e. Isomers have entirely different physical properties and in many cases also have distantly different chemical properties (except chain isomers, metamers).

 

Isomers refer to two or more compounds having

Same MF, same number of atoms, same nature of atoms, same EF, same degree of unsaturation, same MW, same vapour density, same number of bonds but differ at least in one physical or chemical properties.

 

Reason of Isomerism

Isomerism is due to the different arrangement of atoms or groups in a molecule (structural isomerism) or due to different spatial configuration of the atoms or groups (stereoisomerism).

 

Importance

Isomerism is possible for compounds containing at least four carbon atoms. Number of isomers increases with increase in number of carbon atoms in saturated hydrocarbons.

 

Example # 1

The concept of isomerism can be illustrated by referring to two different compounds, nitromethane and methyl nitrite, both of which have the same molecular formula of CH₃NO₂ but different structural formulae. Nitromethane, used as a high-energy fuel for cars, is a liquid with boiling point of 101°C. Methyl nitrite is a gas with boiling point of -12°C which when inhaled causes dilatation of blood vessels.

Example # 2

Ethyl alcohol and dimethyl ether are two different compounds having entirely different physical and chemical properties due to different structural formulae owing to presence of different functional groups but have same molecular formula. Hence they are referred as isomers of each other.

 

Family or Class ……………      Alcohol                       Ether

Structural formula ………       CH₃CH₂OH               CH₃-O-CH₃

Functional group …………      -OH                               -O-

Reactivity……………… ……  reacts with Na         Do not react with Na

Boiling point ……………….      78℃                           17℃

Vapour density ……………      23                                23

Molar mass ………………….      46                               46       

Molecular formula ………..     C₂H₆O                       C₂H₆O

Empirical formula ………..      C₂H₆O                       C₂H₆O 

Degree of unsaturation …     0                                  0

Schematic Classification of Isomerism

Isomerism is of two types namely structural or constitutional isomerism and stereoisomerism. 

Types of Isomerism

Isomerism is of following two types

1.         Structural or Constitutional isomerism

2.         Stereo or configurational isomerism

 

Structural or Constitutional Isomerism

Definition

When isomerism is caused by the difference in the arrangement of atoms or group of atoms within molecule without any reference to space is called Structural or Constitutional Isomerism.

 

Structural or Constitutional Isomers

Structural or Constitutional Isomers are compounds that have same molecular formula but different structural formulae due to different arrangement of atoms or groups or multiple bonds.

 

Structural Isomers have entirely different physical and in most cases also have distantly different chemical properties. 

 

Reason of Structural Isomerism

In this type of isomerism, compounds possessing same molecular formula differ in their properties due to the difference in the linkages of atoms or groups inside the molecule i.e. due to the difference in their structures. The constitutional isomers differ in the connectivity of carbon atoms i.e. differ in their structural formulae.

 

Six Types of Structural or Constitutional Isomerism

Structural or Constitutional Isomerism is of following six types:

Structural or Constitutional Isomerism is of following six types:

1.  Chain /skeletal/Nuclear isomerism [in all families except MSB]

2.  Position Isomerism  [in all families except alkanes, Aldehydes, acids, acid halides, MSBs]

3.  Functional Group Isomerism  [in alcohol-ether, Aldehydes-ketones, acid-ester etc.]

4.   Metamerism   [in ethers, ketones, esters, secondary amines]

5.  Tautomerism/keto-enol isomerism     [in Aldehydes /ketones with enols]

6.    Ring chain isomerism              

Stereoisomerism

Definition

The prefix stereo- is derived from the Greek word stereos meaning solid. When isomerism is caused by the different spatial configuration (i.e. three-dimensional arrangement) of atoms or groups in space is called Stereoisomerism. Compounds having the same structural formula but different spatial arrangement of atoms or groups in space are called stereoisomers and the phenomenon is called stereoisomerism.

stereoisomers

Two or more compounds having same molecular formula, same structural formula but different arrangements of atoms or groups in space are called stereoisomers. These Compounds have same molecular and structural formulae but different spatial arrangement of atoms or groups

Reason of Stereoisomerism

Stereoisomers have same molecular formula and also the same structural formula but differ in arrangement of the bonds (atoms) in space. Stereochemistry is the term applied to the three-dimensional aspects of molecular structure and reactivity.

 

Classification of Stereoisomerism

1.   Conformational isomerism

2.   Configurational isomerism

 

There are two types of stereoisomerism:

 

a) Conformational isomerism. Stereo isomers which have following characteristics:

(i) Stereo isomers which cannot interconvert at room temperature due to restricted rotation known as geometrical isomerism

(ii)  Stereo isomers which have different behaviour towards the plane polarized light are known as optical isomers.   

b)  Configurational isomerism.

 

The operational distinction between Conformational and Configurational isomers is that whether they interconvert at room temperature or not. Conformational isomers can interconvert at room temperature so they cannot be separated from the reaction mixture whereas configurational isomers cannot interconvert at room temperature so they can be separated from the reaction mixture.

 

Stereoisomerism is of three types: 

1.    Geometrical/cis-trans Isomerism      

2.    Optical Isomerism                              

3.    Conformational Isomerism                

Chain or skeletal or nuclear isomerism

 

Definition

Isomerism resulting from varying configuration of main carbon skeleton or chain is called skeletal or nuclear isomerism. Here a chain of minimum 4 carbon atoms is necessary to show this type of isomerism.

OR

In this type of isomerism, compounds possessing same molecular formula differ in their properties due to the difference in the arrangement of carbon chain i.e. branched or unbranched chain present in them. 

 

Chain or Skeletal Isomers

Different compounds which have same molecular formula but they differ in the configuration of their main carbon skeleton or chains having different carbon chains are called chain or skeletal isomers.

 

Similar chemical Properties but different physical properties of Chain Isomers

Skeletal isomers are chemically similar because they possess the same functional group belonging to the same homologous series but they differ in physical properties as the van der Waal’s forces between molecules of the straight chain isomer are much stronger than those between molecules of the other two branched isomers.

 

Occurrence

Skeletal isomerism is found in all aliphatic homologous series except mono-substituted benzenes.

 

Starts with C₄

Skeletal isomerism starts with C₄. Methane, ethane and propane do not exhibit chain isomerism.

 

No. of possible isomers of alkane

Examples of Chain Isomers

(i) butane (C₄H₁₀) has two chain isomers. Pentane (C₅H₁₂) has the following 3 chain isomers:

(ii)       Hexane (C₆H₁₄) has the following 5 chain isomers:

(a-b), (a-c), (a-d), (a-e),(b-d), (b-e), (c-d), c-d) are chain isomers

(b-c), (d-e) are position isomers

 

(iii) Pentyl alcohol (C₅H₁₁OH) has the following 4 chain isomers namely primary, iso, isoamyl & neo.

(iv)   Pentene (C₅H₁₀) has two chain isomers namely pent-1-ene and iso-pentylene

(v)       Butanal (C₄H₈O) has two chain isomers

(vi)      Pentanoic acid (C₅H₁₀O₂) has 3 chain isomers

Position Isomerism

Definition

Isomerism resulting from varying position of the functional group in the same carbon skeleton is called positional isomerism.

OR

The compounds which have same molecular formula, same functional group, same parent carbon chain but different position of functional group or multiple bonds or substituents show position isomerism.

 

Positional Isomers

Compounds which have the same structure of a carbon chain but differ only in term of position of the multiple bonds or the functional group or substituents are called position isomers. Position isomers belong to the same homologous series and class of the compound

 

Different compounds that have same molecular formula but their structural formulae are different due to different position of functional group or multiple bonds in the same carbon chain are called positional isomers.

 

Similar chemical Properties but different physical properties of Position Isomers

Positional isomers (belonging to the same homologous series) mostly have same chemical properties owing to the presence of same functional group or substituent in different positions in the same carbon skeleton.

 

The physical properties of positional isomers are different as the physical properties like melting point, boiling point, volatility etc. are affected both by the shapes of molecules and strength of intermolecular forces like hydrogen bonding.

 

Occurrence

Positional isomerism is found in all homologous series except benzene, mono-substituted benzenes (MSBs), alkanes, aldehydes, carboxylic acids and their derivatives (acid halides, acid amides, acid anhydride, sodium carboxylate, nitriles), mono-substituted alicyclic compounds.

 

Minimum No. of Carbons to show Positional isomerism

Positional isomerism starts with C₃ in all functionally substituted alkanes like alkyl halides, alcohols, thioalcohols, primary amines etc.

In alkenes and alkynes, it starts with C₄.

In ketones, it starts with C₅.

 

Families not showing Position Isomerism

Position isomerism is never observed in presence of chain terminating functional group like                       –COOH, –CHO, –COX, –CN, etc. Aldehydes, carboxylic acid and their derivatives do not exhibit position isomerism

 

Chain isomerism and position isomerism can never be possible together

Chain isomerism and position isomerism can never be possible together between two isomeric compounds. If two compounds are chain isomers then these two will not be positional isomers. 

Examples

(i) butene (C₄H₈) has two positional isomers namely 1-butene and 2-butene.

                  H₃C–CH₂–CH=CH₂                        H₃C–CH=CH–CH₃

           1-butene/ α-butylene)                2-butene/β-butylene 

(ii) Propyl alcohol (C₃H₇OH) has two position isomers namely primary and secondary;

(iii) Butyl alcohol (C₄H₉OH) has three positional isomers namely primary, secondary and tertiary:

(iv) 2-pentanone and 3-pentanone are two positional isomers of five carbon ketone.

(v) Di-substituted benzenes (DSB) have three positional isomers namely ortho, meta and para e.g. xylene (a DSB) has three positional isomers.           

                               

(vi) Cresol (methylphenol) has three positional isomers which show different physical properties. 

(vii)Dihydric phenols have three positional isomers which show different physical properties. 

(viii)  There are three positional isomers of benzdioic acids (a DSB) which have different physical and  chemical properties. e.g. only benz-1,2-dioic acid forms an acid anhydride, this being “sterically” impossible for the other two isomers as the – COOH groups are too distant from one another.

Functional Group Isomerism

Definition

Isomerism resulting from the presence of different functional group is called functional isomerism.

 

Functional isomers

Different compounds which have same molecular formula but their structural formulae are different due to the presence of  different functional groups are called functional group isomers.

Functional isomers belong to different homologous series and different class

 

different chemical Properties and different physical properties of Functional Isomers

Functional isomers (belonging to the different homologous series) have different chemical properties owing to the presence of different functional groups. Functional isomers also have entirely different physical properties.

Minimum No. of Carbons to show Positional isomerism

It starts with C₂ in alcohol-ethers, acid-esters and aldehydes-oxiranes,

it starts with C₃ in aldehydes-ketones, ketones-oxirane.

 

Families not showing Position Isomerism

Alkyl halides do not show functional isomerism.

 

Functional and chain isomerism and functional and positional isomerism can never be possible together

 

Occurrence

Important functional group isomers are given below:

 

Functional Group Isomerism in Alcohol and Ethers (C_nH_2n+2O)

1. C₂H₆O is the molecular formula of two functional isomers namely ethyl alcohol and dimethyl ether.

2.  C₃H₈O is the molecular formula of two functional isomers namely primarypropyl alcohol (or secondarypropyl alcohol) and ethyl methyl ether.

No. of possible isomers from given molecular formula

Functional Group Isomerism in Aldehydes and Ketones, alkenol, oxirane and oxolane (C_nH_2nO)

1. C₃H₆O is the molecular formula of various functional isomers namely propionaldehyde (having aldehydic group), acetone (having keto group), propenol (unsaturated alcohol, allyl alcohol, methyl oxirane (oxirane) and oxopropane (oxolane) obeying general formula C_nH_2nO.

Functional Group Isomerism in Carboxylic acids and Esters and hydroxycarbonyl compounds etc. (C_nH_2nO₂) 

1.  C₂H₄O₂ is the molecular formula of two functional isomers namely acetic acid (having carboxylic group) and methyl formate (having ester group).

2. Propanoic acid and methyl acetate (or ethyl formate) are functional isomers having same molecular formula (C₃H₆O₂).

Functional Group Isomerism in Aldehydes-Ketones and Oxiranes 

C₃H₆O is the molecular formula of three functional isomers namely propanoic acid, propanone and propylene epoxide.

Aromatic alcohols and phenolic compounds are functional isomers.

Functional Group Isomerism in Alkynes and alkadiene (C_nH_2n-2)   

But-1-yne and 1,3-butadiene are functional isomers.

Functional Group Isomerism in cyanides and isocyanides   

Functional Group Isomerism in Nitro and nitrite     

Metamerism

Definition

Isomerism resulting from unequal distribution of carbon atoms (alkyl or aryl groups) on either side of the polyvalent functional group is called metamerism.

OR

It is the type of isomerism in which compounds possessing same molecular formula differ in their properties due to the difference in the alkyl groups present in them i.e. same functional group but different alkyl groups attached to it. It is a type of chain isomerism.

metamers

Different compounds which have same molecular formula having same functional group in which polyvalent atom of the same functional group joins different combinations of alkyl or aryl radicals are called metamers. 

OR

Compounds having same molecular formula, same polyvalent functional group, same class but different alkyl groups attached to the polyvalent functional group are metamers.

Functional groups Showing Metamerism

Polyvalent functional groups (having more than one valency) are:

 

Same chemical Properties and different physical properties of Metamers

Metamers have same chemical properties due to the presence of same functional group and belong to the same homologous series but have different physical properties. Metamerism occurs among the members of the same homologous series i.e. metameric pairs.

Families showing Metamerism

Metamerism is found in ethers, thioethers, ketones and secondary amines, tertiary amines, alkenes esters etc.

 

Minimum No. of Carbons to show Positional isomerism

In ethers and secondary amines metamerism starts with C₄. In esters metamerism starts with C₃ but in ketones it starts with C₅