ISOMERISMClass 11

Types of isomerism

STRUCTURAL ISOMERISM - INTRODUCTIONCOMPOUNDS HAVE THE SAME MOLECULAR FORMULABUT DIFFERENT STRUCTURAL FORMULA

Chain - they have different arrangements of the carbon skeletonsimilar chemical propertiesslightly different physical propertiesmore branching = lower boiling point

Positional -same carbon skeletonsame functional groupfunctional group is in a different positionsimilar chemical properties - slightly different physical

properties

Functional Group - different functional group different chemical properties different physical properties

Structural isomerismCHAIN 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 three pentanes which have the same molecular formula (C5H12) but differ in the structure of the carbon chains in their molecules.

While n-pentane has a continuous chain of five carbon atoms, isopentane and neo-pentane has  branched chains. These chain isomers have somewhat different physical and chemical properties. 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.

There are two structural isomers of C4H10. One is a straight chain molecule where all the carbon atoms are in a single row. The other is a branched molecule where three carbon atoms are in a row and one carbon atom sticks out of the main chain.

BUTANEstraight chain

2-METHYLPROPANEbranched

C4H10

EXAMPLE OF CHAIN ISOMERISM

POSITIONAL ISOMERISM:-It is the type of isomerism in which the compounds possessing same molecular formula differ 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.                                                     n-propyl alcohol :CH3–CH2–CH2–OH and isopropyl alcohol:  CH3–CH(OH)–CH3

Butene also  has two positional isomers that is 1- Butene: CH2=CH–CH2–CH3  and 2-Butene: CH3–CH=CH–CH3

1-Chlrobutane: CH3CH2CH2CH2Cl and 2- Chlorobutane: CH3CH2CH(Cl)CH3 are also the 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 GROUP 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, Diethyl ether ( C2H5–O–C2H5) and butyl alcohol(C4H9–OH) both have the molecular formula C4H6O, but contain different  functional groups. The functional group in diethyl ether is (–O–), while is butyl alcohol it is (–OH).Acetone(CH3–CO–CH3 )  and propionaldehyde( CH3–CH2–CHO) both with the molecular formula C3H6O are functional  isomers. In acetone the functional group is (–CO–), while in acetaldehyde it is (–CHO).Cyanides(R-CN) are isomeric with isocyanides(R-NC).Carboxylic( R-COOH) acids are isomeric with esters (R’-COOR”).

Nitroalkanes are functionally isomeric with alkyl nitrites:

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).

Unsaturated alcohols  are isomeric with aldehydes. e.g: CH2=CH-O & CH3CHO

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

Aromatic alcohols are functionally isomeric with phenols

METAMERISMThis type of isomerism is due to the different alkyl groups on eithe rsides of the carbon atoms.For example, methyl propyl ether and diethyl ether both have the same molecular formulaExamples: Methyl propyl ether CH3–CO–C3H7  and  Diethyl ether C2H5–O–C2H5

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.

Eg:- methoxyethane and ethoxyethane

QUESTIONS ON ISOMERISM Give the possible chain isomers for ethyl benzene.

ANS

Q.2) C4H8 can have so many isomers. Write their structures.

ANS. C4H8 can have following structures:

(a) CH3—CH2—CH=CH2 (1-butene)

(b) CH3—CH=CH—CH3 (2-butene)

(c)  CH3—C(CH3)=CH2 (2-methyl-1-propene)

(d) Cyclobutane(e) methyl cyclopropane    

Revision Questions What do you understand by functional group isomerism ? Explain.Write down any two isomers of propanol.Why is more branching considered better ?Draw isomers of p-cyclohexachloride.An isomer A had a melting point of 44.8C whereas an isomer B had a melting point of -115.6. which one was more stable ? Why ? How does branching effect the boiling point in this case ?