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Stereoisomerism



Definition of Stereoisomerism

Compounds/molecules that have some connectivity but different arrangements of atoms in space, so those compounds posses this situation is called stereoisomer and this phenomenon is said to be stereoisomerism.

OR

Stereoisomerism is caused by different arrangements of atoms or groups in Space.. This phenomenon is known as stereo isomerism.

Stereo=space
Isomerism=same molecular formula but differ in chemical/physical properties.
Stereoisomerism is exhibited by such compounds which have same structural formula but differ in configuration.

The term configuration refers to three dimensional arrangements of atoms, that characterize a particular compound.

Types of Stereoisomerism:

There are two types of stereo-isomerism:
1= Geometrical isomerism (cis-trans isomerism)
2= Optical isomerism

Geometrical Isomerism:

Those compounds having same molecular formula but a different geometric configuration as when atom or groups of atoms are attached in different special arrangements on either side of a bond or ring are said to be geometrical isomers and the phenomenon is known as geometrical isomerism.

Optical Isomerism:

It is a type of stereoisomerism, in which an optically active compounds can exist in two isomeric form which rotate plane-polarised light in opposite direction. These are called optical isomers and phenomenon is said to be optical isomerism.


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Tautomerism and Tautomerization




Type of functional isomerism in which the isomers are in dynamic equilibrium with each other. commonly they are also called as Keto or Enol form
keto= ketone group
enol= alcohol group
So the process in which keto form is converted to enol form is said to be Tautomerization. this interconversion takes place by the movement of hydrogen.

1. Acetone Tautomerization









2. Tautomerization in Lactam-Lactim




Explanation:

When writing the resonance structures of a compound. It was observed that the relative position of atomic nuclei remained unchanged. The structures which differ in the relative position of their atoms are not resonance form but different compounds which are isomeric with each other and are capable independent existence. The most common example of Tautomerism are provided buy carbonyl compounds containing alpha-hydrogen, in which the proton (hydrogen) from alpha-carbon shifts to carbonyl oxygen, associated with appropriate movement of electron pair form an ENOL , this process is called ENOLIZATION. The hydrogen then moves back to convert the enol form to keto form.




           Bond energy= 1502 kj/mol                                                                1433kj/mol 


Keto form is therefore more stable by 69kj/mol. 

However, enol form would be more stable than its corresponding ketone due to its resonance stabilization. 

Similarly in certain molecules like acetoacetic ester the enol form is also stabilized by internal hydrogen bonding which isn’t possible in the keto form.

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Stereochemistry Introduction



HISTORY:

During early days of development of organic chemistry, the chemists frequently encountered two or more compounds that had same molecular formula yet different physical/chemical properties.
It was later realize that the difference in their properties of compound was due to their difference in their structure.

Isomerism:

          Such compounds which posses the same molecular formula but different structures are known as isomers and this phenomenon is known as isomerism.




     1= Constitutional Isomerism                                               2= Stereoisomerism
  • Chain Isomerism
  • Position Isomerism
  • Functional Isomerism
  • Metamerism Isomerism
  • Tautomerism                                    

Constitutional  Isomerism or structural isomers is shown by the compounds that have the same molecular formula but different connectivity.

i.e sequence in which the atoms are bonded to each other in a molecule.

  1. Chain Isomerism:

Those with Same molecular formula but differ in order in which the carbons are bonded to other is called chain isomers and this phenomena is known as Chain isomerism.




  1. Position Isomerism:

 Same molecular formula but differ in the position of functional group is said to be position isomers and this phenomenon is called position isomerism



Functional Isomerism:

Same molecular formula but different functional group.
CH3CH2OH                                             CH3-O-CH3
Ethylalcohol                                                 Dimethyl Ether


Metamerism:

Unequal distribution of carbon atoms on either side of functional group.

CH3CH2-O-CH2CH3                                         CH3-O-CH2CH2CH3
Diethylether                                                                Methylpropyl ether

                 H                                                                                  H
CH3CH2-N-CH2CH3                                          CH3CH2CH2-N-CH3
 Diethylamine                                                              Methyl ropyl amine



Tautomerism:

Type of functional isomerism in which the isomers are in dynamic equilibrium with each other.. Most coomonly they are also called as Keto or Enol form
keto= ketone group
enol= alcohol group
Example:




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Hybridization



Definition:

When atomic orbitals mix to form new atomic orbitals is called hybridization.

Valence bond theory explains bond is formed by overlapping of two half-filled valence atomic orbitals



The electrons in overlapping get paired and confined b/w nuclei of two atoms and electron density can be increased

Types of Covalent Bonds

There are 2 types of covalent bonds based on pattern of overlapping 
1; SIGMA BONDS 
2; PI BONDS


1: SIGMA BONDS

  • A sigma bond is formed by the linear or end to end overlap of orbitals
  • STRONG in nature

(A) Overlapping Of 2s Orbitals 



(B) By Overlapping Of Px And S Orbitals
  


(C) By Overlapping Of 2 P Orbitals

2: PI BONDS

  • it is formed by parallel or side to side overlap of orbitals



Characteristics;
  • pi-electrons are loosely held then a pair of electrons in sigma bond
  • highly reactive
  • cis-trans isomers, rotation of atoms is not possible around pi-bonds

ILLUSTRATIONS AND EXAMPLES


1. H2 MOLECULES
Electronic configuration of hydrogen is 1S1 two half filled S-orbitals of hydrogen overlaps.





CHLORINE
The electronic configuration of chlorine is (1S2, 2S2, 2P6, 3S2, 3P5)    
The valence electrons are (3S2 3P5) that makes   ———> 7 ELECTRONS, so in case of Hydrochloric Acid we do hybridization as follows

 2. HCL MOLECULE

  • hydrogen = 1s1  1
  • chlorine = 1s2, 2s2, 2p6, 3s2, 3p5  17



Updates:
Last update 10 Feb 2015

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