Fundamental Principles of Organic Chemistry

13.3 Isomerism in Organic Compounds

Isomers are different compounds possessing the same molecular formula. Isomerism occurs when the same chemical formula represents distinct substances.

Types of Isomerism:

Isomerism is of two types:

1.       Structural isomerism

2.       Stereoisomerism   

1.       Define isomerism.                                                                               1

13.5 Structural isomerism and its types

Structural isomerism

Structural isomerism is the term used to describe molecules with the same chemical formula but distinct structures. The 5 types of structural isomers are as follows:

 

1.      Chain, nuclear or skeletal isomerism

If two or more compounds have the same molecular formula but different carbon chains or skeletons, they are called chain isomers; the phenomenon is called chain isomerism.

For example, the two isomers of C₄H₁₀ differ only in the length of the carbon chain.

Similarly, butyl alcohol and isobutyl alcohol are chain isomers.

2.      Position/positional isomerism

The compounds with the same molecular formula but different positions of the functional group are termed position isomers; the phenomenon is position isomerism.

For example, the two isomers with molecular formula C₃H₈O are:

3.      Functional isomerism

The compounds with the same molecular formula but different functional groups are termed functional isomers; the phenomenon is functional isomerism.

For example

1.       Aldehydes and ketones, e.g. CH₃CH₂CHO and CH₃COCH₃

2.       Alcohol and ether, e.g. CH₃CH₂OH and CH₃OCH₃

3.       Carboxylic acid and esters, e.g.

4.      Tautomerism

It is a special isomerism in which the isomers are interconvertible due to the change in the position of the hydrogen atom within the molecule. For example:

5.      Metamerism

The compounds of the same family with the same molecular formula, which differ in the arrangement of carbon atoms on either side of the functional group, are metamers; the phenomenon is metamerism. Ketones, Ethers, Esters etc. can exhibit metamerism.

2.       Write a short note on structural isomerism in organic compounds.  5/2

3.       Write the isomers of C₄H₁₀ and give their IUPAC names.                  2

4.       Write down two structure isomers and their IUPAC names of C₄H₈. 2

5.       Write short notes on isomers of C₆H₁₀.                                               5

6.       Give a functional isomer of C₃H₇OH and write its IUPAC name.2

7.       Write down the functional isomers of CH₃-CH₂-CH₂-OH and give IUPAC name.        2

8.       Write down the functional isomer of Propanol and give its IUPAC name. 1

9.       Write down the functional isomers of CH₃-CH₂-OH and give IUPAC name.   2

10.   Give a functional isomer of C₂H₄O₂ and write its IUPAC name.         2

11.   Give the functional isomer of Ethanoic acid with structural formula.       1

12.   Write down the functional isomer of Propanoic acid and give its IUPAC name.          2

13.   Write down the functional isomer of Methyl methanoate and give its IUPAC name.  1

14.   Write down the functional isomers of CH₃-CH₂-COOH and give IUPAC name.         1

15.   Give possible isomers of C₃H₆O₂ and write their IUPAC names.        2

16.    Write down two structure isomers and their IUPAC names of C₃H₆O.    2

17.   Give the functional isomer of Propanone with structural formula.            1

18.   Write the isomers of C₂H₆O and give their IUPAC names.                      2

19.   Write down the functional isomer of Methoxymethane and give its IUPAC name.      1

20.   Give possible isomers of C₃H₆O and write their IUPAC names.              2

Write possible isomeric ethers for the molecular formula C₄H₁₀O.

13.6 Concept of geometrical isomerism (cis & trans) & optical isomerism (d & l form)

Compounds with the same molecular formula and the structural formula but different spatial arrangements of the atoms or groups of atoms in a molecule are called stereoisomers. The phenomenon of the existence of such compounds is called stereoisomerism.

Stereoisomerism is subdivided into geometrical and optical isomerism.

Geometrical Isomerism (cis, trans isomerism)

When the same molecular formula represents two compounds which differ in the spatial arrangement of atoms or groups around the carbon-carbon double bond, these are geometrical isomers. Such isomerism arises due to the inability to rotate around a double bond.

e.g., But-2-ene has two geometrical isomerism i.e., cis-but-2-ene and trans-but-2-ene

1,2-dichloroethene CHCl=CHCl also exhibits such isomerism.

            Optical isomerism

·         Optical isomerism is the phenomenon in which isomers have different arrangements of atoms or groups in three-dimensional space.

·         Such compounds resemble in their chemical and most physical properties but differ in optical rotation.

·         Optical rotation is a phenomenon where optically active compounds rotate the plane of plane-polarized light in a clockwise or anticlockwise direction.

·         Polarimeter measures the optical rotation.

·         The optical isomers that are mirror images of each other are called enantiomers.

·         The enantiomers which rotate the plane of the polarized light to a clockwise direction (right) and anticlockwise direction are called dextrorotatory and laevorotatory respectively, designated as + or d and - or l respectively.

Only compounds having a chiral carbon atom exhibit enantiomerism. A carbon atom bonded with 4 different groups is called chiral carbon. In the above four examples, the central atom is chiral.

 

1.       Compounds with the same molecular formula but possessing different properties are

a.       Isotopes         e. isomers              c. isobars               d. isochors

b.       isotherms

2.       Compounds having the same molecular formula but different structural formulas are

a.       Isotopes                         c. Structural isomers

b.       Isobars                           d. polymers

3.       Isomers must have

a.       Same chemical properties         c. Same structural formula

b.       Same molecular formula            d. Same functional group

4.       Structural isomers have the same molecular formula but different

a.       Structure                                       c. Functional group

b.       Physical properties                      d. Chemical properties

5.       Two isomeric forms of saturated hydrocarbons have

a.       Same structure

b.       Same molecular formula

c.        Same properties

d.       Different compositions of elements

6.       CH₃-O-CH₃ and C₂H₅OH are

a.       Isomers          b. Allotropes         c. Isobars               d. Isomorphs

7.       Which of the following compound is a functional isomer of C₂H₅OH?

a.       CH₃CHO         b. CH₃COOH          c. (CH₃)₂O              d. (C₂H₅)₂O

8.       How many structural isomers does pentane have?

a.       4                      b. 5                         c. 6                          d. 7

9.       Chain isomers differ in

a.       Molecular formula                       c. Empirical formula

b.       Molecular weight                         d. Length of the carbon chain

10.    Which compound is not an isomer of the other three?

a.       Pentane                                         c. 2-methylbutane

b.       2,2-dimethylpropane                  d. 2,3-dimethylbutane

11.    But-1-yne and But-2-yne are

a.       Chain isomers                               c. Metamers

b.       Functional isomers                      d. Positional isomers

12.    Metamerism is shown by

a.       Ketones         b. Alkanes              c. Aldehydes         d. Alcohols

13.    The number of possible alcoholic isomers of C₄H₁₀O is

a.       2                      b. 3                         c. 4                          d. 8

14.    How many isomeric ethers are possible from C₄H₁₀O?

a.       2                      b. 3                         c. 4                          d. 5

15.    The maximum  number of isomeric alkenes with molecular formula C₄H₈ is

a.       2                      b. 3                         c. 4                          d. 5

16.    Which of the following are isomers?

a.       Propanone and Ethanal

b.       Ethanol and Methoxymethane

c.        Propanone and Propanoic acid

d.       Methanol and Methoxymethane

17.    Isomerism where a single compound exists in two or more readily interconvertible structures is

a.       Chain isomerism                          c. Metamerism

b.       Tautomerism                                 d. Stereoisomerism

18.    …………….. generally exhibit geometrical isomerism.

a.       Alkanes          b. alkenes              c. alcohols             d. alkynes

19.    cis-but-2-ene and trans-but-2-ene are

a.       position isomers                          c. optical isomers

b.       conformational isomers             d. geometrical isomers

20.    Optical activity is measured by

a.       Nicol prism    b. Analyzer            c. Polarimeter       d. Polaroid

21.    Which of the following compound is chiral?

a.       2-chloropropane         c. 2-methoxybutane

b.       Propan-2-ol                   d. Methanal

22.    But-1-ene exhibits

a.       Geometrical isomerism              c. optical isomerism

b.       Position isomerism                      d. none of these

 

***This is not a complete note. It is to guide you. Study the prescribed textbooks along with this material. ***