ISOMERISM at a glance

 

ISOMERISM IN ORGANIC COMPOUNDS

Isomerism is defined as the occurrence of two or more compounds with the same molecular formular but different molecular structures. 

The different molecular structures are known as Isomers.

Isomers may belong to the same homologous series ( having the same functional group) or may belong to different homologous series (having different functional group)

Isomers with the same functional group, (i.e. belonging to the same homologous series) have similar chemical properties while isomers having different functional group (i.e. belonging to different homologous series) have different physical and chemical properties.

Example:- the formula C2H6O has two isomers 

1. Ethanol  an Alkanol with molecular formula  CH3CH2OH has a melting point of 78 degree celçius and is a liquid at room twmperature 

     H   H

H--C--C--OH

     H. H

and 

2. Methoxymethane an Alkoxy compound with molecular formula CH3OCH3 which is a gaseous at room temperature

     H       H

H--C--O--C--H

     H       H

TYPES OF ISOMERISM

(i) Structural isomerism

(ii) Stereo or geometric Isomerism

(iii) Optical isomerism

 

(I) STRUCTURAL ISOMERISM

This is the occurrence of two or more compounds with the same molecular formular but different molecular structures 

TYPES OF STRUCTURAL ISOMERISM

(a) Chain isomerism: in this case, the isomer differs in the way the carbon atoms are arranged in the molecule. Eg

 CH₃CH₂CH₂CH₃           CH₃CHCH₃    
 n-butane                                 |                                                                    
                                               CH₃   
                             2-methylpropane (isobutane)

(‘n’ stands for the normal compound while ‘iso’ means the isomer of the normal compound).

NOTE: - Isomerism begins from the 4th member of the alkane homologous series, that is, butane, and as the number of carbon atoms increases the number of isomers also increases. For example, butane has two isomers normal-butane and 2-methyl propane, pentane (C₅H₁₂) has three isomers:  n-pentane, 2-methylbutane and 2,2-dimethylpropane.            

  I.  CH₃CH₂CH₂CH₂CH3.  (n-pentane)

              CH₃
               |
 II.  CH₃CHCH₂CH3  (2-methylbutane)

                                     

III.              CH₃
                    |         
          CH₃-C-CH₃
                    |
                   CH₃        2,2-dimethylpropane

Three isomers for pentane 

Another example is hexane (C6H14)

I.  CH₃CH₂CH₂CH₂CH2CH3

               n-hexane

          

              CH₃
               |
 II.  CH₃CHCH₂CH2CH3    2-methylpentane

III.                        CH₃
                              |
              CH₃CH2CHCH2CH3          3-methylpentane

                   CH₃
                       |
 IV.      CH₃CCH₂CH3    2,2-dimethylbutane
                   |
                  CH3

                                     

                  CH₃
                    |
 V.      CH₃CHCHCH3    2,3-dimethylbutane
                        |
                       CH3  

Hexane has 5 isomers 

(b) position isomerism: Position isomers are those that have a substituent in different position on the same carbon skeleton. Example

                CH₃CH₂CH₂OH                 CH₃CHCH₃ 
                    propan-1-ol                             |
                                                                    OH 
                                                             propan-2-ol

(c) Functional Group isomerism: These are isomers having the same molecular formular but different functional groups. They belong to different homologous series.

e.g.
                         H
                        /
(i)  CH₃CH₂C                          CH₃-CCH₃ 
                       \\                                 ‖‖    
                        O                               O
     Propanal  (an aldehyde)       propan-2-one (a ketone)

(ii) CH₃CH₂OH                                  CH₃-O-CH₃ 

      Ethanol  ( Alkanol)                Methoxymethane (dimethyl ether) (Alkoxy compounds)

(2) STEREO ISOMERISM OR GEOMETRIC ISOMERISM

In this type of isomerism, compounds have the same molecular formular but differ only in the way their atoms are arranged in space.

TYPES   STEREO ISOMERISM

(a) Geometrical isomerism: This type of isomerism is found in compounds having either a double bond, triple bond or a ring structure. These multiple bonds prevent the free rotation about a carbon=carbon atom. that is, the carbon atoms are not cylindrically symmetrical. There are two forms of geometric isomers, the Cis and Trans isomer. For example, but-2-ene has two geometric isomer which are trans-but-2-ene and cis-but-2-ene as shown below.

when naming geometric isomers, you look at the substituents, where the substituents (group other than Carbon atoms) are on the same side of the double bond, that isomer is named the Cis isomer and when the substituents are on opposite sides of the double bond, we put a trans- isomer, but the names are the same

   H               CH₃                       H             H          
        \           /                                 \           /
           C=C                                       C=C
         /         \                                    ∕         ∖
  CH₃          H                            CH₃         CH₃

Trans-but-2-ene                          Cis-but-2-ene

 

  H                Br               H             H
      \               /                    \          /
          C=C                            C=C
      /             \                      /          \   
  Br              H                 Br          Br
Trans-1,2-dibromoethene       Cis-1,2-dibromoethene                    

   C1OOH.  C4OOH            H           COOH
     \           /                             \         /       
        C2=C3                              C=C    
       ∕         ∖                            /        \
COOH       H                      H             H
Trans-butenedioic acid            Cis-butenedioic acid

(b) Optical isomerism: Optical isomers   are isomers that have the same molecular and structural formular but cannot be superimposed on each other. In other words, an optical isomer is one which is not super-imposable on its mirror image. An optical isomer has at least one carbon atom which has four different groups or atoms attached to it. Such a carbon atom (surrounded by four different atoms /group) is called chiral carbon or chiral center. E.g. 2-hydroxylpropanoic acid (Lactic acid).

                           CH₃
                             |
              HO — *C— H
                             |
                            COOH

                  _{The Asterix C-atom is called a chiral C-atom because it is surrounded by four (4) different groups.}

            In glucose, there are several chiral carbon atoms.

                                          H
                                           ∣
                                          C = O
                                           |
                                H — *C —OH
                                            |
                              OH —*C —H
                                            |
                                H — *C— OH
                                           |
                                 H —*C —OH
                                           |
                                 H — C—H
                                           |
                                          OH

Optical isomers are also called Enantiomers.

Isomers that belong to the same homologous series (same functional group) have similar chemical properties but may have different physical properties. But isomers which belong to different homologous series (different functional group) have different chemical and physical properties.