Structural Isomerism

Isomerism

Two or more compounds having the same molecular formula but different chemical and physical properties are called isomers and the phenomenon is known as isomerism.

It is of 2 types:

1) Structural isomerism

2) Stereoisomerism

1) Structural Isomerism : Compounds having the same molecular formula but different structures i.e. different arrangement of atoms within the molecules are called structural isomers and the phenomenon is called structural isomerism.

It is of 6 types:

1)Chain or nuclear isomerism: Compounds having the same molecular formula but different arrangement of carbon chain within the molecule are called chain or nuclear isomers and the phenomenon is called chain or nuclear isomerism.

2) Position isomerism: Compounds which have the same structure of the carbon chain but differ only in the position of the multiple bond or functional group are called position isomers and the phenomenon is called position isomerism.

3) Functional isomerism

Compounds having same molecular formula but different functional group are called functional isomers and the phenomenon is called functional isomerism.

1) Alcohols and ethers

2) Carboxylic acid and esters

3) Aldehydes , ketones , unsaturated alcohols and unsaturated ethers

CH₃-CO-CH₃ Propanone

CH₃CH₂-CHO Propanal

CH₂=CH-CH₂OH Prop-2-en-1-ol

CH₂=CH-O-CH₃ Methoxyethene

4) Aromatic alcohols, phenols, ethers

5) Dienes , allenes and alkynes

CH₂=CH-CH=CH₂ Buta-1,3-diene

CH₂=C=CH-CH₃ Buta-1,2-diene

CH₃CH₂C≡CH But-1-yne

CH₃CH₂C≡CCH₃ But-2-yne

6) Nitroalkanes ,alkyl nitrites and amino acids (C_nH_2n+1NO₂)

CH₃CH₂NO₂ Nitroethane

CH₃-CH₂-O-N=O Ethyl nitrite

H₂N-CH₂-COOH 2-Aminoethanoic acid

7) 1° , 2° and 3° amines

CH₃CH₂CH₂NH₂ Propan-1-amine

CH₃CH₂-NH-CH₃ N-Methylethanamine

CH₃-N-(CH3)₂ 

N,N-Dimethylmethanamine

8) Cyanides and Isocyanides

CH₃C≡N Ethanenitrile

CH₃-NC Methyl isocyanide

4) Metamerism

Compounds having the same molecular formula but different number of carbon atoms on either side of the functional group are called metamers and the phenomenon is called metamerism. Metamerism occurs among the members of the same homologous family.

1) CH₃CH₂-O-CH₂CH₃  (Ethoxyethane) is a metamer of

CH₃-O-CH₂CH₂CH₃ (1-Methoxypropane)  or CH₃O-CH(CH3)₂ (2-Methoxypropane)

2) CH₃CH₂-S-CH₂CH₃ (Diethyl thioether) is a metamer of

CH₃-S-CH₂CH₂CH₃ (Methyl propyl thioether) or CH₃S-CH(CH₃)₂ (Isopropyl methyl thioether)

3) CH₃CH₂-NH-CH₂CH₃ (Diethylamine)

is a metamer of CH₃-NH-CH₂CH₂CH₃ (Methyl propylamine) or CH₃-NH-CH(CH₃)₂ (Isopropyl methylamine)

4) CH₃COOCH₂CH₃ (Ethyl acetate) and CH₃CH₂COOCH₃ (Methyl propionoate) are metamers.

5) Tautomerism

In this isomerism the isomers exist in dynamic equilibrium with each other.It arises due to migration of a hydrogen atom from one polyvalent atom to the other within the same molecule with necessary rearrangement of linkages.The isomers thus obtained are called tautomers and the phenomenon is called tautomerism.

Tautomerism is also called desmotropism.

The two types of tautomerism are:

Dyad system : This type of tautomerism involves 1,2- migration of a proton from one polyvalent atom to the other within the same molecule with necessary rearrangement of linkages.

HCN ⇔ HNC

Hydrogen migrates from carbon to nitrogen.The alkyl derivative of these tautomers are called cyanide and isocyanide.

RCN Alkyl cyanide

RNC Alkyl isocyanide

Triad system

Hydrogen migrates from one polyvalent atom to the third polyvalent atom within the same molecule.

Keto-enol tautomerism is just one type of triad system.One form contains keto group while the other contain the enolic group.

In all the monocarbonyl compounds, the greater stability of the keto form w.r.t the enol form is due to the greater strength of the carbon -oxygen π bond as compared to carbon-carbon π bond.

Factors affecting the relative amount of keto and enol forms in keto-enol tautomerism

1. Stability of the enol form: In simple aldehydes and ketones the amount of enolic form is negligibly small.If the enolic form is stabilized by intramolecular hydrogen – bonding or resonance, such as in 1,3-dicarbonyl compounds, the amount of enolic form is much greater than in acetaldehyde or acetone.

Acetylacetone also exhibit also exhibit keto-enol tautomerism but the amount of enolic form here is much higher than even in acetoacetic ester .This is due to the reason that keto group is a much better electron withdrawing group than the ester group.

Higher the stability of the enol form , greater is the enol content.

2) Steric hindrance 

Acetylacetone

α-Methylacetylacetone

Both the enols are stabilized by H-bonding , the enol form of α-methylacetylacetone is destabilized to some extent by the steric repulsion due to the presence of the α-methyl group.

As a result α-methylacetylacetone has lower enol content as compared to that of acetylacetone.

3)Effect of polarity of solvent

Polar protic solvents such as water, methanol , acetic acid which form H-bonds with the carbonyl group of the keto form decrease the enol content. Aprotic solvents such as hexane, benzene increase the enol content.

Essential condition

1. The compound must have a electronegative atom (N,O,S) bonded by a double or a triple bond.

2 The compound must have atleast one acidic α-hydrogen present on a saturated carbon. Acetophenone , butan-2-one  and propionaldehyde all contain acidic α-hydrogen atoms and hence show keto-enol tautomerism.

C₆H₅COCH₃ Aectophenone

CH₃COCH₂CH₃ Butan-2-one

CH₃CH₂CHO Propionaldehyde

p-benzoquinone contain α-hydrogens but they are not acidic because they are present on a double bond.Therefore it does not show keto-enol tautomerism.

Benzaldehyde

Benzophenone

Benzophenone and benzaldehyde do not show keto-enol tautomerism because they do not contain α-hydrogen atoms.

Other types of tautomerism involving 1,3-hydrogen shifts are:

1. Nitro- acinitro tautomerism : 1° and 2° nitroalkanes which contain acidic α- hydrogen show tautomerism.

2 Nitroso- oximino system : 

3 Imine-enamine system

6 Ring-Chain isomerism

Compounds having the same molecular formula but possessing open chain and cyclic structures are called ring chain isomers and the phenomenon is called ring-chain isomerism.

1. Alkenes and cycloalkanes 

2 Alkynes and cycloalkenes