15.4 Notes

ISOMERISM

The compounds having the same molecular formula but having different physical & chemical properties are known as Isomers and the phenomenon is called Isomerism.

Since the isomers have the same molecular formula, the difference in their properties must be due to different modes of combination or arrangement of atoms with in the molecule. There are two main types of Isomerism.

1) Structural Isomerism.

The isomerism is due to difference in the arrangement of atom with in the molecule, without any reference to space, the phenomenon is known as structural isomerism. and the compounds are called as structural isomers.

Structural isomerism is of five types.

I.      ) Chain Isomerism.

Chain isomers have same molecular formula but different in order in which the carbon atoms are bonded to each other. This phenomenon is called Chain isomerism

 for example.

                                                                       1       2       3

1)   CH₃ - CH₂ - CH₂ - CH₃                            CH₃ - CH - CH₃

              n butane                                                   |      

                                                                                           CH₃                                                                                                                                         

                                                                             Iso butane 2- methyl propane.

                                                                                                  CH₃

                                                                               |

2)   CH₃ - CH₂ - CH₂ - CH₂ - CH₃                CH₃ - C - CH₃

            n pentane                                              |

                                                                            CH₃

                                                                              2, 2- diamethyl propane neopentane

II.  ) Positional isomerism.

Positional isomers have the same molecular formula but different in the position of a functional group on the carbon chain.

 For example.                           

OH

3        2            1                                   |

a) CH₃ - CH₂ - CH₂OH                  CH₃- CH - CH₃

1- propanol                                    2- propanol

1        2       3       4                      1       2       3       4

b) CH₂= CH₂- CH₂- CH₂                     CH₃- CH = CH - CH₃

1- butene                                              2- butene

iii) Functional isomerism.

The phenomenon in which compounds have same molecular formula but having different functional groups said to be functional isomers and the compound is called functional isomers.

For example.

                O                                                              O

                II .                                                            II            

a)  CH₃ - C - CH₃                                       CH₃ - CH₂ - C - H

     2- Propane one (kelone)                       propanal (aldehyde)

b)  CH₃ - O - CH₃                                 CH₃ - CH₂OH      

          Dimethyl ether                                        Ethyl alcohol

iv) Metamerism.

 This type of isomerism is due to the distribution of carbon atoms on either side of the functional group.

For example.

a)  CH₃ - CH₂ - O - CH₂ - CH₃                      CH₃ - O - CH₂ - CH₂ - CH₃

             Diethyl ether                                                   Methyl propyl ether

b)  CH₃ - C - CH₂ - CH₂ - CH₃                    CH₃ - CH₂ - CO - CH₂ - CH₃

      Methyl propyl ketone                                                                     Diethyl ketone

v)  Tautomerism.

 It is a special type of functional isomerism in which the isomers are is dynamic equilibrium with each other.

        O                    O                                               OH            O                                 

         II                      II                                                 I             II

CH₃ - C - CH₂ - C - O - C₂H₅                         CH₃ - C = CH - C -OC₂H₅

             Keto –Form                                                        enol form

·          One functional group change itself into another functional group.

2)           Stereoisomerism.

The isomerism which is caused by the different arrangements of atoms or groups in space, the phenomenon is called Stereoisomerism. The stereo isomers have the same structural formulas but different is arrangement of atoms in space stereoisomerism is of two types.

                      i.            Geometrical or cis-Trans isomerism.

                   ii.            Optical isomerism.

i)  Geometrical isomerism.

Geometrical isomerism results from a restriction in the rotation about

double bonds, or about single bonds in cyclic compound.

In alkenes the carbon atoms of the carbon-carbon double bond is Sp² hybridized consists of a 6 bond and a p bond. The s bond is formed by the overlap of sp² hybrid orbital. The p bond is formed by the overlap of p orbital. The presence of p bond locks the in molecule in one position. The two carbon atoms of c = c bond and four atoms that are attached to them lie in the same plane due to sp² hybridization and their position in space are fixed because rotation around the c = c bond is not possible because rotation would break the bond.

 The Cis isomer is one in which two similar groups are on the same side of the double bond. Cis isomer are unstable because they have bulky groups are on the same side of double bond. The stearic repulsion of the group is maximum. Cis isomers are optically active.

The trans isomers are that in which two similar groups are on the opposite sides of the double bond. Tran’s isomers are more stable than the corresponding cis isomers. This is because, they have bulky groups are on the opposite side of the double bond due to this they have minimum stearic repulsion

Cis and trans isomerism is only valid for those alkane in which two identical groups are attached to carbons have double bond. This method does not work when there are no identical groups attached to the carbons have double bond. For example

BrClC = CFI

In the above example Br has greater mass than Cl and I has greater mass than F. The group priorities for number is given to heavier mass are on opposite sides of double bond therefore the configuration is E (German word entgegen = opposite)

In the above one, one are on same side of double bond therefore the configuration is Z (German word Zusammen = logether )

Optical Isomerism .

The compounds which have same molecular formula but rotate the plane polarized light in opposite directions are known as optical Isomers and the phenomenon is called Optical Isomerism. Optclal isomer have the same physical properties, ie. melting  point , boiling point , density , etc. They have same specific rotation but opposite signs

The isomers which rotates plane polarized light to the right ( clockwise direction ) is called Dexorotatory Isomer or ( + ) isomers.

The isomer which rotates plane polarized light to the left ( anticlockwise direction ) is called laevorotatory Isomers or  ( - ) isomers.

An equimolor mixture of two isomers , will not rotate the plane polarized light at all because optical rotatory power of two isomers are equal in magnitude but opposite sign is said to be Racemic mixture.

Optical isomerism have same chemical & Physical property like as melting and boiling point the of the compound has asymmetric carbon atom or asymmetric molecule or chiral molecules. For example   lactic acid

(2- hydroxypropanoic acid ) shows optical isomerism. It contains one asymmetric carbon atom. Two three dimensional structures are possible for lactic acid. Fischer projection

  

  COOH                                                             COOH

          |                                                                        |

HO - C - H                                                          H - C - OH   

          |                                                                        |

   CH₃                                                                                    CH₃

                                         

 MIRROR

( + ) acid mp = 26 ^oC                           ( - ) - acid mp = 26 ^oC

These structures are not identical because they cannot be superimposed on esch other. One is mirror image of the other. Such non superimpose mirror image forms are optical isomers and are called enantiomers.

Enantiomers are stable isolable compound have identical properties in all respect such as melting point , density, solubility colour and reactivity towards acids and bases except in their interaction with plane polarized light because they are different from one another in three dimensional spatial arrangement the same extent ( same angle ) the other one rotates the plane to the left ( anticlockwise )

In general, each asymmetric carbon atom in a molecule doluble the number of the theoritically possible isomers. A molecule with n chiral center should have 2ⁿ stereoisomer.

Chloro-2-butanol have two chiral center , therefore it has four stereoisomers and two racemic forms because the number racemic forms is obtained by 2^n-1

        CH₃                         CH₃                                   CH₃                         CH₃

          |                               |                                     |                             |        

 H -  C - OH           HO - C - H                       H - C - OH         HO - C - H

          |                               |                                     |                             |

CL - C - H                H -  C - CL                    H - C -CL           CL -  C - H

          |                               |                                     |                             |

        CH₃                         CH₃                                    CH₃                       CH₃

         ( I )      Mirror       ( II )                               ( III )    Mirror      ( IV )

                 Enantiomers                                                Enantiomers

               Racemic form 1                                           Racemic form 2

Notice that I is the mirror image but not super impose of II ,III is also the mirror image but not super impose of IV. Thus four isomers are two pairs of enantiomers.

Now I is not the mirror image of III & IV,II is also not a mirror image of III & IV . The stereoisomer which are neither super impose nor they are mirror image are called Diastereoisomers.

Diastereomers have different properties. Two diastereomers have different melting points, boiling point and solubility. They will different chemical reactivity toward most reagent.

A compound with two or more asymmetric carbon atom having a plane of symmetry is called meso compound . For example.

  COOH                                                          CH₃

       |                                                                  |

H - C - OH                                                H - C - OH

       |            plane of symmetry                                 |               plane of symmetry

H - C - OH                                                H - C - OH

       |                                                                  |

  COOH                                                          CH₃

Meso tartaric acid                                   Meso 2,3 - batondiol

These molecules have plane of symmetry dividing them midway between the two asymmetric carbons in each. One half of the molecule is the mirror image of the other. Meso compounds are optically in active , even though each has two asymmetric center neither rotates the plane polarized light.