Isomerism

In the study of organic chemistry we come across many cases when two or more compounds are made of equal number of like atoms. A molecular formula does not tell the nature of organic compound; sometimes several organic compounds may have same molecular formula.  These compounds possess the same molecular formula but differ from each other in physical or chemical properties, are called isomers and the phenomenon is termed isomerism (Greek, isos = equal; meros = parts). Since isomers have the same molecular formula, the difference in their properties must be due to different modes of the combination or arrangement of atoms within the molecule. Broadly speaking, isomerism is of two types.

• Structural Isomerism
• Stereoisomerism

(i) Structural isomerism: When the isomerism is simply due to difference in the arrangement of atoms within the molecule without any reference to space, the phenomenon is termed structural isomerism. In other words, while they have same molecular formulas they possess different structural formulas. This type of isomerism which arises from difference in the structure of molecules, includes:

• Chain or Nuclear Isomerism;
• Positional Isomerism
• Functional Isomerism
• Metamerism and
• Tautomerism

(ii) Stereoisomerism: When isomerism is caused by the different arrangements of atoms or groups in space, the phenomenon is called Stereoisomerism (Greek, Stereos = occupying space). The stereoisomers have the same structural formulas but differ in the spatial arrangement of atoms or groups in the molecule. In other words, stereoisomerism is exhibited by such compounds which have identical molecular structure but different configurations.

Stereoisomerism is of two types:

(a) Geometrical or cis-trans isomerism; and

(b) Optical Isomerism.

Thus various types of isomerism could be summarized as follows.

STRUCTURAL ISOMERISM

Chain or Nuclear Isomerism

This type of isomerism arises from the difference in the structure of carbon chain which forms the nucleus of the molecule. It is, therefore, named as chain, nuclear isomerism or Skeletal isomerism. For example, there are known two butanes which have the same molecular formula (C₄H₁₀) but differ in the structure of the carbon chains in their molecules.

While n-butane has a continuous chain of four carbon atoms, isobutane has a branched chain. These chain isomers have somewhat different physical and chemical properties, n-butane boiling at -0.5o and isobutane at -10.2o. This kind of isomerism is also shown by other classes of compounds. Thus n-butyl alcohol and isobutyl alcohol having the same molecular formula C4H9OH are chain isomers.

It may be understood clearly that the molecules of chain isomers differ only in respect of the linking of the carbon atoms in the alkanes or in the alkyl radicals present in other compounds.

Positional Isomerism

It is the type of isomerism in which the compounds possessing same molecular formula differ in their properties due to the difference in their properties due to difference in the position of either the functional group or the multiple bond or the branched chain attached to the main carbon chain. For example, n-propyl alcohol and isopropyl alcohol are the positional isomers.

         OH

                       |

CH₃–CH₂–CH₂–OH CH₃–CH–CH₃

n-propyl alcohol isopropyl alcohol

Butene also has two positional isomers:

CH₂=CH–CH₂–CH₃ CH₃–CH=CH–CH₃

       1-butene        2-butene

1-Chlorobutane and 3-Chlorobutane are also the positional isomers:

Methylpentane also has two positional isomers:

In the aromatic series, the disubstitution products of benzene also exhibit positional isomerism due to different relative positions occupied by the two substituents on the benzene ring. Thus xylene, C6H4(CH3)2, exists in the following three forms which are positional isomers.

Functional Isomerism

When any two compounds have the same molecular formula but possess different functional groups, they are called functional isomers and the phenomenon is termed functional isomerism. In other words substances with the same molecular formula but belonging to different classes of compounds exhibit functional isomerism. Thus,

(1) Diethyl ether and butyl alcohol both have the molecular formula C₄H₆O, but contain different functional groups.

C₂H₅–O–C₂H₅ C₄H₉–OH

diethyl ether             butyl alcohol

The functional group in diethyl ether is (–O–), while is butyl alcohol it is (–OH).

(2) Acetone and propionaldehyde both with the molecular formula C3H6O are functional isomers.

CH₃–CO–CH₃ CH₃–CH₂–CHO

acetone Propionaldehyde

In acetone the functional group is (–CO–), while in propionaldehyde it is (–CHO).

(3) Cyanides are isomeric with isocyanides:

(4) Carboxylic acids are isomeric with esters.

(5) Nitroalkanes are isomeric with alkyl nitrites:

(6) Sometimes a double bond containing compound may be isomeric with a triple bond containing compound. This also is called as functional isomerism. Thus, butyne is isomeric with butadiene (molecular formula C4H6).

(7) Unsaturated alcohols are isomeric with aldehydes. Thus,

(8) Unsaturated alcohols containing three or more carbon atoms are isomeric to aldehydes as well as ketones:

(9) Aromatic alcohols may be isomeric with phenols:

(10) Primary, secondary and tertiary amines of same molecular formula are also the functional isomers.

(11) Alkenes are isomeric with cycloalkanes:

Such isomers in which one is cyclic and other is open chain, are called ring-chain isomers. Alkynes and alkadienes are isomeric with cycloalkenes.

Metamerism

This type of isomerism is due to the unequal distribution of carbon atoms on either side of the functional group in the molecule of compounds belonging to the same class. For example, methyl propyl ether and diethyl ether both have the same molecular formula.

CH₃–O–C₃H₇ C₂H₅–O–C₂H₅

methyl propyl ether diethyl ether

In methyl propyl ether the chain is 1 and 3, while in diethyl ether it is 2 and 2. This isomerism known as Metamerism is shown by members of classes such as ethers, and amines where the central functional group is flanked by two chains. The individual isomers are known as Metamers.

Examples:

Tautomerism

It is the type of isomerism in which two functional isomers exist together in equilibrium. The two forms existing in equilibrium are called as tautomers. For example, the compound acetoacetic ester has two tautomers – one has a keto group and other has an enol group:

Out of the two tautomeric forms, one is more stable and exists in larger proportion. In above, normally 93% of the keto form (more stable) and only 7% of the enol form (less stable i.e. labile) exist.

The equilibrium between the two forms is dynamic, i.e., if one form is somehow removed by making a reaction, some of the amount of the other form changes into the first form so that similar equilibrium exists again. Thus, whole of the acetoacetic ester shows the properties of both ketonic group as well as the enolic group. Thus, it adds on HCN, NaHSO3 etc. due to the presence of •C=O group and it decolourises bromine water and gives dark colouration with FeCl3 due the presence of >C—OH group. Due to the presence of keto and enol form this type of tautomerism is known as keto-enol tautomerism. It is the most commonly observed type of tautomerism.

Keto-enol tautomerism is generally observed in those compounds in which either a methyl
(CH₃—), methylene (—CH₂—), or a methyne () group is present adjacent to a carbonyl
(—CO—) group as in acetoacetic ester above. In other words, it can be said that keto-enol tautomerism is possible in only those carbonyl compounds in which atleast one α-hydrogen atom is present so that it may convert the carbonyl group to enol group.

Another example of keto-enol tautomerism is:

It is found that if the α-hydrogen atoms are present on both the carbons attached to carbonyl group, more stable is the enol form and hence more its content. Thus, larger the number of α-hydrogens in a ketone, more is enol content. Also, if number of α-hydrogen containing carbonyl groups is more, again more is the enol content. Thus the order of enol content is:

CH₃CHO • CH₃COCH₃ • CH₃COCH₂CHO • CH₃COCH₂COCH₃

CH₃COCH₂COH₃ has about 75% enol content. Moreover, the enol form shows acidic nature due to the tendency to liberate proton (H+) from the enol ( C—OH) group. This H is the α-hydrogen in keto – form. Therefore, α-H in carbonyl compound is acidic in nature. More the enol content, more is the acidic nature of α-hydrogens in a carbonyl compound. Thus above is also the increasing order of acidity of α-hydrogens.

An interesting observation about the enol content in acetoacetic ester is the fact that above mentioned percentage ratio (93:7) is in its aqueous solution. In liquid state this ratio is nearly   25 : 75. It is because in liquid state the enol form is much stabilised by intramolecular H-bonding.

In aqueous solution the intermolecular H-bonding with water takes place and it dominates the intramolecular H-bonding, resulting in lower enol content.

Keto-enol tautomerism exists in cyclic carbonyl compounds also if they fulfil the condition of presence of α-H. Thus, we have

Tautomerism is also termed as desmotropism (desomo= bond, tropis = turn) because in tautomers the bonding changes.

Every compound having skeleton has its tautomer skeleton e.g., in the preparation of a very small amount of enol isomers also forms which can be isolated. The two exist in equilibrium with each other and can be separated by suitable methods.

A hydroxy group attached to a carbon which is itself attached to another carbon atom by a double bond is known as enolic (en for double bond, ol for alcohol). Its nature becomes acidic as in phenol and unlike OH group in alcohol which is neutral or only very slightly acidic (C₂H₅OH + Na → C₂H₅ONa + 1/2H2). In the above two examples migration of a proton from one carbon atom to another takes place with simultaneous shifting of bonds.

Hydrocyanic acid, H – C ≡ N and Isohydrocyanic acid H — N ≡ C are also tautomeric isomers or tautomers.

Difference between Tautomerism & Resonance

(a) In tautomerism, an atom changes place but resonance involves a change of position of pi-electrons or unshared electrons.

(b) Tautomers are different compounds and they can be separated by suitable methods but resonating structures cannot be separated as they are imaginary structures of the same compound.

(c) Two tautomers have different functional groups but there is same functional group in all canonical structures of a resonance hybrid.

(d) Two tautomers are in dynamic equilibrium but in resonance only one compound exists.

(e) Resonance in a molecule lowers the energy and thus stabilises a compound and decreases its reactivity. But no such effects occur in tautomerism.

(f) In resonance, bond length of single bond decreases and that of double bond increases e.g. all six C—C bonds in benzene are equal and length is in between the length of a single and a double bond.

(g) Resonance occurs in planar molecule but atoms of tautomers may remain in different planes as well.

(h) Tautomers are indicated by double arrow in between the two isomers but double headed single arrow ←⎯→ is put between the canonical (resonating) structures of a resonating molecule.