JEE Main & Advanced Chemistry GOC Constitutional or structural isomerism

(1) Chain, nuclear or skeleton isomerism : This type of isomerism arises due to the difference in the nature of the carbon chain (i.e., straight or branched) which forms the nucleus of the molecule.

Examples :

(i) C₄H₁₀  \[\underset{n-\text{Butane}}{\mathop{C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}-C{{H}_{3}}}}\,\],\[\underset{\text{Isobutane}}{\mathop{C{{H}_{3}}-\underset{C{{H}_{3}}}{\mathop{\underset{|\,\,\,\,\,\,\,}{\mathop{CH}}\,}}\,-C{{H}_{3}}}}\,\]

(ii) C₅H₁₂ : (Three) \[\underset{n-\text{Pentane}}{\mathop{C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}-C{{H}_{2}}-C{{H}_{3}}}}\,\],

\[\underset{\text{Isopentane}}{\mathop{C{{H}_{3}}-\underset{C{{H}_{3}}}{\mathop{\underset{|\,}{\mathop{C}}\,H}}\,-C{{H}_{2}}-C{{H}_{3}}}}\,\], \[\underset{\text{Neopentane}}{\mathop{C{{H}_{3}}\underset{C{{H}_{3}}\ \ \ \ \ }{\mathop{\overset{C{{H}_{3}}\ \ }{\mathop{\underset{\ \ |}{\mathop{\overset{\ \ \ |}{\mathop{-C}}\,}}\,-C{{H}_{3}}}}\,}}\,}}\,\]

• Except alkynes chain isomerism is observed when the number of carbon atoms is four or more than four.
• Chain isomers differ in the nature of carbon chain, i.e., in the length of carbon chain.
• The isomers showing chain isomerism belong to the same homologous series, i.e., functional group, class of the compound (Cyclic or open) remains unchanged.
• Chain and position isomerism cannot be possible together between two isomeric compounds. If two compounds are chain isomers then these two will not be positional isomers. 

(2) Position isomerism : It is due to the difference in the position of the substiuent atom or group or an unsaturated linkage in the same carbon chain.

(ii) C₃ H₆ Cl₂ : \[\underset{\begin{smallmatrix}  2,2-\text{Dichloro propane,} \\ \text{(gemdihalide)}\end{smallmatrix}}{\mathop{C{{H}_{3}}-CC{{l}_{2}}-C{{H}_{3}}}}\,\],\[\underset{\begin{smallmatrix}1,1-\text{Dichloro propane} \\\text{(gemdihalide)}\end{smallmatrix}}{\mathop{C{{H}_{3}}-C{{H}_{2}}-CH-C{{l}_{2}}}}\,\],

\[\underset{\text{(Vicdihalide)}}{\mathop{\underset{1,2-\text{Dichloro propane}}{\mathop{C{{H}_{3}}-\underset{Cl\,\,}{\mathop{\underset{|}{\mathop{C}}\,H}}\,-\underset{Cl\,\,\,\,\,}{\mathop{\underset{|}{\mathop{C}}\,{{H}_{2}}}}\,}}\,}}\,\], \[\underset{\begin{smallmatrix}1,3-\text{Dichloro propane} \\ \,\,\,\,\,\,\,\,\text{(}\alpha \text{,}\gamma \text{-dihalide)}\end{smallmatrix}}{\mathop{\underset{Cl\,\,\,\,\,}{\mathop{\underset{|}{\mathop{C}}\,{{H}_{2}}}}\,-C{{H}_{2}}-\underset{Cl\,\,\,\,\,}{\mathop{\underset{|}{\mathop{C}}\,{{H}_{2}}}}\,}}\,\]

• Aldehydes, carboxylic acids (and their derivatives) and cyanides do not show position isomerism.
• Monosubstituted alicylic compounds and aromatic compounds do not show position isomerism.
• Structural isomers which differ in the position of the functional group are called regiomers. For example, (i) \[C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}-OH\] (ii) \[C{{H}_{3}}-\underset{OH}{\mathop{\underset{|\,\,\,\,\,}{\mathop{CH}}\,}}\,-C{{H}_{3}}\]

(3) Functional isomerism : This type of isomerism is due to difference in the nature of functional group present in the isomers. The following pairs of compounds always form functional isomers with each other.

Examples :

(i) Alcohols and ethers (C_n H_2n+2O)

C₂H₆O : \[\underset{\text{Ethyl alcohol}}{\mathop{C{{H}_{3}}-C{{H}_{2}}-OH}}\,\] ; \[\underset{\text{Dimethyl}\,\text{ether}}{\mathop{{{H}_{3}}C-O-C{{H}_{3}}}}\,\]  

C₃H₈O : \[\underset{n\,-\,\text{propyl alcohol}}{\mathop{C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}-OH}}\,\] ; \[\underset{\text{Ethyl methyl}\,\text{ether}}{\mathop{{{C}_{2}}{{H}_{5}}-O-C{{H}_{3}}}}\,\]

C₄H₁₀O : \[\underset{n-\text{Butyl alcohol}}{\mathop{C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}-C{{H}_{2}}-OH}}\,\]; \[\underset{\text{Diethyl ether }}{\mathop{{{C}_{2}}{{H}_{5}}-O-{{C}_{2}}{{H}_{5}}}}\,\]

(ii) Aldehydes, ketones and unsaturated alcohols …etc. (C_n H_2nO)

(iii) Acids, esters and hydroxy carbonyl compounds …etc. (C_n H_2nO₂)

C₂H₄O₂ : \[\underset{\text{Acetic acid}}{\mathop{C{{H}_{3}}COOH}}\,\] ; \[\underset{\text{Methyl formate}}{\mathop{HCOOC{{H}_{3}}}}\,\]

C₃H₆O₂ : \[\underset{\text{Propionic acid}}{\mathop{C{{H}_{3}}-C{{H}_{2}}-COOH}}\,\] ; \[\underset{\text{Methyl acetate}}{\mathop{C{{H}_{3}}COOC{{H}_{3}}}}\,\] ;

\[\underset{2-\,\text{Hydroxy propanal}}{\mathop{C{{H}_{3}}\underset{OH\,\,\,\,\,\,\,\,\,\,\,\,}{\mathop{\underset{|}{\mathop{C}}\,HCHO}}\,}}\,\]; \[\underset{1-\,\text{Hydroxy propan-2-one}}{\mathop{C{{H}_{3}}-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-C{{H}_{2}}-OH}}\,\]

(iv) Alkynes and alkadienes (C_n H_2n-2)

C₄H₆ : \[\underset{1-\,\text{Butyne}}{\mathop{C{{H}_{3}}-C{{H}_{2}}-C\equiv CH}}\,\]; \[\underset{1,3-\,\text{Butadiene}}{\mathop{{{H}_{2}}C=CH-CH=C{{H}_{2}}}}\,\];

\[\underset{2-\text{Butyne}}{\mathop{C{{H}_{3}}-C\equiv C-C{{H}_{3}}}}\,\] ; \[\underset{1,2-\text{ Butadiene}}{\mathop{{{H}_{2}}C=C=CH-C{{H}_{3}}}}\,\]

(v) Nitro alkanes and alkyl nitrites (\[-N{{O}_{2}}\]and \[-O-N=O\])

(vi) Amines (Primary, secondary and tertiary)

(vii) Alcohols and phenols

(viii) Oximes and amides

C₂H₅NO : \[\underset{\text{Acetaldoxime}}{\mathop{C{{H}_{3}}-CH=NOH}}\,\]; \[\underset{\text{Acetamide}}{\mathop{C{{H}_{3}}\overset{O}{\mathop{-\overset{||}{\mathop{C}}\,-}}\,N{{H}_{2}}}}\,\]

(4) Ring-chain isomerism : This type of isomerism is due to different modes of linking of carbon atoms, i.e., the isomers possess either open chain or closed chain sturctures.

• Ring - chain isomers are always functional isomers.           

(5) Metamerism : This type of isomerism is due to the difference in the nature of alkyl groups attached to the polyvalent atoms or functional group. Metamers always belong to the same homologous series. Compounds like ethers, thio-ethers ketones, secondary amines, etc. show metamerism.

• If same polyvalent functional group is there in two or more organic compounds, then chain or position isomerism is not possible, there will be metamerism e.g.,

(a) \[\underset{\text{(Pentan}-\text{2}-\text{one)}}{\mathop{C{{H}_{3}}\underset{O}{\mathop{-\underset{|\,|}{\mathop{C}}\,-}}\,C{{H}_{2}}-C{{H}_{2}}-C{{H}_{3}}}}\,\] ;

\[\underset{\text{(Pentan}-\text{3}-\text{one)}}{\mathop{C{{H}_{3}}C{{H}_{2}}\underset{O}{\mathop{-\underset{|\,|}{\mathop{C}}\,-}}\,C{{H}_{2}}C{{H}_{3}}}}\,\] are metamers and not position isomers.

(b) \[\underset{\text{(Pentan}-\text{2}-\text{one)}}{\mathop{C{{H}_{3}}\underset{O}{\mathop{-\underset{|\,|}{\mathop{C}}\,-}}\,C{{H}_{2}}C{{H}_{2}}C{{H}_{3}}}}\,\] ;

\[\underset{\text{(3-Methylbutan-2-one)}}{\mathop{C{{H}_{3}}\underset{O}{\mathop{-\underset{|\,|}{\mathop{C}}\,-}}\,\underset{C{{H}_{3}}\,\,\,\,\,\,\,\,\,\,\,\,\,}{\mathop{\underset{|}{\mathop{C}}\,H-C{{H}_{3}}}}\,}}\,\] are metamers and not chain isomers.

• Alkenes does not show metamerism.

(6) Tautomerism

(i) The type of isomerism in which a substance exist in two readily interconvertible different structures leading to dynamic equilibrium is known as tautomerism and the different forms are called tautomers (or tautomerides).

The term tautomerism (Greek: tauto = same; meros = parts) was used by Laar in 1885 to describe the phenomenon of a substance reacting chemically according to two possible structures.

(ii) It is caused by the wandering of hydrogen atom between two polyvalent atoms. It is also known as Desmotropism (Desmos = bond and tropos = turn). If the hydrogen atom oscillates between two polyvalent atoms linked together, the system is a dyad and if the hydrogen atom travels from first to third atom in a chain, the system is a triad.

(a) Dyad system : Hydrocyanic acid is an example of dyad system in which hydrogen atom oscillates between carbon and nitrogen atoms \[H-C\equiv N\rightleftharpoons C\overset{\scriptscriptstyle\leftarrow}{=}N-H\]

(b) Triad system

Keto-enol system : Polyvalent atoms are oxygen and two carbon atoms.

Examples :

\[\underset{\text{(Keto)}}{\mathop{\overset{\,\,O}{\mathop{-\overset{||}{\mathop{C}}\,-}}\,\overset{H}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,}}\,}}\,\rightleftharpoons \underset{\text{(Enol)}}{\mathop{\overset{\,\,\,\,\,OH}{\mathop{-\overset{|}{\mathop{C}}\,=}}\,\underset{|}{\mathop{C}}\,-}}\,\]

Acetoacetic ester (Ethyl acetoacetate) :

Acetoacetic ester gives certain reactions showing the presence of keto group (Reactions with \[HCN\], \[{{H}_{2}}NOH\],\[\,{{H}_{2}}NNH{{C}_{6}}{{H}_{5}}\], etc.) and certain reactions showing the presence of enolic group (Reactions with \[Na,\,C{{H}_{3}}COCl,\,N{{H}_{3}},\,PC{{l}_{5}},\,B{{r}_{2}}\]water and colour with neutral \[FeC{{l}_{3}},\] etc.). Enolisation is in order \[C{{H}_{3}}COC{{H}_{3}}<C{{H}_{3}}COC{{H}_{2}}COO{{C}_{2}}{{H}_{5}}<{{C}_{6}}{{H}_{5}}COC{{H}_{2}}COO{{C}_{2}}{{H}_{5}}\]\[<C{{H}_{3}}COC{{H}_{2}}COC{{H}_{3}}\]\[<C{{H}_{3}}COC{{H}_{2}}CHO\] Acid catalysed conversion   \[\underset{\text{(Intermediate)}}{\mathop{C{{H}_{3}}\overset{\,\,\,\,\,OH}{\mathop{-\underset{\oplus }{\mathop{\overset{|}{\mathop{C}}\,}}\,-}}\,\underset{H\,\,\,\,\,}{\mathop{\underset{|}{\mathop{C}}\,H}}\,-R}}\,\xrightarrow{-{{H}^{\oplus }}}\underset{\text{(Enol form)}}{\mathop{C{{H}_{3}}\overset{\,\,\,\,OH}{\mathop{-\overset{|}{\mathop{C}}\,=}}\,CH-R}}\,\]  Base catalysed conversion                 (c) Triad system containing nitrogen : Examples Nitrous acid exists in 2 forms                            Nitro acinitro system                                       (iii) Characteristics of tautomerism             (a) Tautomerism (cationotropy) is caused by the oscillation of hydrogen atom between two polyvalent atoms present in the molecule. The change is accompanied by the necessary rearrangement of single and double bonds.             (b) It is a reversible intramolecular change.             (c) The tautomeric forms remain in dynamic equilibrium. Hence, their separation is a bit difficult. Although their separation can be done by special methods, yet they form a separate series of stable derivatives.             (d) The two tautomeric forms differ in their stability. The less stable form is called the labile form. The relative proportion of two forms varies from compound to compound and also with temperature, solvent etc. The change of one form into another is also catalysed by acids and bases.             (e) Tautomers are in dynamic equilibrium with each other and interconvertible\[(\rightleftharpoons ).\]             (f) Two tautomers have different functional groups.             (g) Tautomerism has no effect on bond length.             (h) Tautomerism has no contribution in stabilising the molecule and does not lower its energy.             (i) Tautomerism may occur in planar or nonplanar molecules.             r Keto=enol tautomerism is exhibited only by such aldehydes and ketones which contain at least one \[\alpha \]-hydrogen.             For example             \[C{{H}_{3}}CHO,\,C{{H}_{3}}C{{H}_{2}}CHO,\,C{{H}_{3}}COC{{H}_{2}}COC{{H}_{3}}\] etc,.             r Tautomerism is not possible in benzaldehyde \[({{C}_{6}}{{H}_{5}}CHO)\], benzophenone \[({{C}_{6}}{{H}_{5}}CO{{C}_{6}}{{H}_{5}})\], tri methyl acetaldehyde, \[{{(C{{H}_{3}})}_{3}}C-CHO\] and chloral \[CC{{l}_{3}}-CHO\] as they do not have \[\alpha -H\].   Number of structural isomers