NEET General Organic Chemistry Isomerism Notes

Isomerism

The existence of two or more compounds with the same molecular formula but different physical and chemical properties is known as isomerism and the molecules themselves are called as isomers.

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The term was given by Berzelius. The difference in properties of the two isomers is due to differences in the arrangement of atoms within their molecules.

Isomerism is mainly classified into structural isomerism and stereoisomerism.

Structural Isomerism

Structural isomerism is due to the differences in structures of the isomers. Structural isomerism is further classified into 5 types.

1. Chain isomerism (nuclear isomerism): Compounds with same molecular formula but differ in the arrangement and number of carbon atoms within the molecule are called chain isomers and the phenomenon as chain isomerism.

2. Butane (C₄H₁₀) has two isomers – normal butane and isobutene. One isomer has a straight chain and the other has a branched chain.

3. Cyclohexane and methyl cyclopentane are nuclear isomers.

4. C₅H₁₂ has three chain isomers.

5. C₄H₉NH₂ also shows two chain isomers.

Solve 1: How many chain isomers does butane have?
Solution:

Solve 2: How many chain isomers does propylbenzene have?
Solution:

Functional Isomerism

Compounds with same molecular formula but differ in functional groups are called functional isomers and the phenomenon is known as functional isomerism. For example

Diethyl ether butyl alcohol both have the molecular formation C₄H₆O but contains different functional groups. Thus, functional group in diethyl ether is (-O-), while is butyl alcohol it is (-OH).

C₂H₅-O-C₂H₅ (diethyl ether); C₄H₉-OH (butyl alochol)

Acetone and propionaldehyde both with the molecular formula are functional isomers. In acetone the functional group is (-CO-) while in acetaldehyde it is (-CHO)

CH₃-CO-CH₃ (acetone); CH₃-CH₂-CHO (acetaldehyde)

Cyanides are isomeric with isocyanides

RCN (Alkyl cyanide); RNC (Alkyl isocyanide)

Carboxylic acids are isomeric with esters.

CH₃CH₂COOH (Propanoic acid); CH₃COOCH₃ (Methyl ethanoate)

Nitroalkanes are isomeric with alkyl nitrites:

R-O-N = O (Alkyl nitrite)

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 C₆H₆).

CH₃-CH₆C ≡ CH (1-Butyne);

CH₆ = CH-CH = CH₂ (1, 3-Butadiene)

Unsaturated alcohols are isomeric with aldehydes. Thus,

CH₂=CH-OH (Vinyl alochol); CH₃CHO (Acetaldehyde)

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

CH₂ = CH-CH₂OH (Alkul alochol);

CH₃CH₂CHO (Propionaldehyde);

CH₃COCH₃ (Acetone)

Aromatic alcohols may be isomeric with phenols

Primary, secondary, and tertiary amines of same molecular formula are also functional isomers.

Alkenes are isomeric with cycloalkanes:

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

CH₃CH₂C ≡ CH (1-Butyne);

CH₂ =CH-CH = CH₂ (1,3 -Butadinene)

Position isomerism

Compounds which have the same structure (arrangement) of carbon chain (carbon skeleton) but differ in the position of the multiple bond or the functional group are called position isomers and the phenomenon is known as position isomerism.

Thus the following compounds can exhibit position isomerism:

1. Alkenes
2. Alkynes
3. Arenes
4. Alkyl halides
5. Aryl halides
6. Alcohols
7. Amines and
8. Nitro compounds

Alkenes containing four or more carbon atoms can exhibit position isomerism due to the difference in the position of the double bond on the same carbon skeleton, For example:

1. CH₃CH₂CH = CH₂ (but-1-ene) ; CH₃CH =CHCH₃ (But-2-ene)
2. CH₂CH₂CH₂CH = CH₂ (Pent-l-ene); CH₃CH₂CH = CHCH₃ (Pent-2-ene)
3. CH₃CH₂CH₂CH = CH₂ (Hex-l-ene); CH₃CH₂CH₂CH = CHCH₃ (Hex-2-ene);  CH₃CH₂CH = CHCH₂CH₃(Hex-3-ene)

Alkynes containing four or more carbon atoms can exhibit position isomerism due to the difference in the position of triple bond on the same carbon skeleton. For example:

1. CH₃CH₂C = CH₂ (But-l-yne); CH₃C = CCH₃ (But-2-yne)
2. CH₃CH₂CH₂C (Pent-l-ene) = CH; CH₃CH₂C = CCH₃

Arenes containing eight or more carbon atoms exhibit position isomerism due to the difference in the position of alkyl groups on the benzene ring. For example:

Alkyl halides containing three or more carbon atoms can exhibit position isomerism due to the difference in the position of halogen atom on the same carbon skeleton. For example

1. C₃H₇Cl has two position isomers:

2. n – Pentane on monochlorination gives three isomeric chloromethanes:

Polyhalogen derivatives containing two or more carbon atoms can also exhibit position isomerism. For example:

CH₃CHCl₂ (1, 1- Dichloroethane)

ClCH₂CH₂Cl (1, 2-Dichloroethane)

Aryl halides containing two or more benzene rings can exhibit position isomerism due to the difference in the position of halogen atom. For example:

Polyhalogen compounds containing just one benzene ring can also exhibit position isomerism. For example:

Alcohols containing three or more carbon atoms exhibit position isomerism due to the difference in the position of functional group (-OH). For example:

1. The molecular formula C₃H₈O represents two isomeric alcohols:

CH₃CH₂CH₂OH (Propan 1-ol)

2. Four monohydric alcohols differing in the position of -OH group can be derived from isopentane:

Primary amines (RNH₂) containing three or more carbon atoms can exhibit position isomerism due to the difference in the position of amino group on the same carbon skeleton. For example:

Nitro compounds (RNO₂) containing three or more carbon atoms can exhibit position isomerism due to the difference in the position of nitro group on the same carbon skeleton. For example:

CH₃CH₂CH₂CH₂NO₂ (1-Nitobutane)

Metamerism

Here two or more different compounds having the same molecular formula but different number and arrangements of carbon atoms on either side of the functional group is called metamerism. Such compounds are known as metamers.

Metamerism is never possible in compounds possessing the univalent functional group. Metamerism is due to the difference in the nature of alkyl groups attached to the same polyvalent is functional group such is Metamerism is exhibited by compounds of the same homologous series.

Ethers, R-Q-R, exhibit metamerism due to differences in the nature of the alkyl groups attached to the oxygen atom. Thus, the molecular formula, C₄H₁₀O, represents the following metamers:

Ketones  , exhibit metamerism due to the difference in the nature of the alkyl groups attached to the carbonyl group. Thus, the molecular formula, C₅H₁₀O, represents the following metamers:

Thioethers, R-S-R’, exhibit metamerism due to the difference in the nature of the alkyl groups attached to the sulfur atom. Thus, the molecular formula, C₄H₁₀S, represents the following metamers:

Secondary and tertiary amines exhibit metamerism due to the difference in the nature of the alkyl groups attached to the – NH – group and the atom respectively. Thus the molecular formula, C₄H₁₁N, represents the following metamers:

Esters,  exhibit metamerism due to the difference in the nature of the alkyl groups attached to the -C – 0 – group. Thus, the following esters are metamers:

Note – If same polyvalent functional group is present in two or more organic compounds, then instead of chain or position isomerism, treat the phenomenon as metamerism.

1. Pentan – 2- one and pentan – 3- one are metamers and not position isomers. They can be included in position isomerism, if metamerism is not mentioned.
2. Similarly, pentan – 2- one and 3 – methylbutan-2-one are metamers and not chain isomers.

Metamers may be considered as position isomers. For instance, pentan – 2- one and penta-3- one may be regarded as position isomers as well as metamers.

Tautomerism

Here a single compound exists in two readily interconvertible structures that differ in position of the hydrogen atom. Tautomer exhibits dynamic equilibrium with each other. A very common form of tautomerism is that between a carbonyl compound containing an a – hydrogen and its enol form. This type of isomerism is also known as keto-enol isomerism.

The percentage of enol form increases in the order simple aldehydes and ketones <β-keto esters <β-diketones <β-diketones having phenyl group < phenols. This increase in the enol content is due to the fact that the enol form of the above type of compounds is increasingly stabilized by resonance and hydrogen bonding than the corresponding keto form.

Ring Chain Isomerism

The phenomenon of existence of two or more compounds having the same molecular formula but possessing open chain and closed chain (cyclic structure) is called ring-chain isomerism. This type of isomerism arises due to different modes of linking of carbon atoms. Thus ring-chain isomers possess open chain or closed-chain structures as illustrated by the following examples:

Two ring-chain isomers are possible corresponding to the molecular formula C₃H₆:

Six pairs of ring-chain isomers are possible for the molecular formula C₄H₈:

The molecular formula C₃H₄ represents the two ring-chain isomers:

Ring chain isomerism can be included in functional isomerism, if not considered separately.