Amines📚

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 Amines:

                        Amines are nitrogen containing organic compounds having badic character. Amines are present in structure of proteins, vitamins, hormones and many plant products like nicotine 


Classification of Amines :

  1. Amines are classified as primary,  secondary, and tertiary amines
  2. Their structures are obtained in simple way be replacing one, two or three hydrogen atoms of NH3 molecule by alkyl/ aryl groups.
  3. Secondary and tertiary amines are further classified as simple or symmetrical amines and mixed or unsymmetrical amines.
  4. When all the alkyl or aryl groups on nitrogen are same, it is a simple amine. If these groups are different,  then the amine is a mixed amine.
  5. Amines are also divided into two  major classe, namely, aliphatic and aromatic amines on the basis of nature of the groups attached to the nitrogen atom.


* Physical properties of amines:

1. Intermolecular forces:
                     The N-H bond in amines is polar because the electronegativities of nitrogen and hydrogen are different. Due to polar nature of N-H bond primary and secondary amines have intermolecular hydrogen bonding is to greater extent in primary amine than in secondary amines, because primary amines have two hydrogen atoms bonded to nitrogen for hydrogen bond formation.

Tertiary amines do not have intermolecular hydrogen bonding as there is no hydrogen atim on nitrogen of tertiary amine. But due to polar N-V bonds, tertiary amines are polar molecules,  and gave intermolecular dipole- dipole attraction forces. Thus intermolecular forces of attraction are strongest in primary amines and weakest in tertiary amines.

 2. Boiling points:
                 The boiling point of primary amines is highest and tertiary amines the lowest 
 

3. Solubility:
                      Due to their ability to form hydrigen bond with  water molecule, lower aliphatic amines are soluble in water. Solubility of amines decreases with increase in size of hydrophobic alkyl group. Aromatic amines and higher aliphatic amines are in soluble in water.
   Since N-H bond in amines are less polar than O-H bond in alcohols,  amines and alkanes of comparable molar mass in water are in the decreasing order: 
Alcohols>amines>alkanes.

* Basicity of amines:
      The basic nature of amines is due to presence of a lone pair of electrons on the nitrogen atom. 
A. Lewis theory: In this terms of Lewis theory, amines are bases because they can share a lone pair of electrons on nitrogen atom with an electron deficient species. For example, trimethylamine shares its line pair of electrons with the electron deficeint boron trifluoride.
B. Lowery- Bronsted theory: The basic nature of amines is explaines by the following equilibrium 
N: + H2O ⇌ N-H + OH
In this equilibrium amine accepts H+ , hence an amine is a Lowry- Bronsted base.
For stronger base, this equilibrium shifts towards right, thereby, for stronger base, the Kb value is larger or pKb value is smaller.


* Basic strength  of aliphatic amines:
       The trend is observed pKb calues and basic strength of 1°,2°,3° amunes and NH3 can be represented as 
●Order of pKb values :
NH3>R-NH2>R2NH<R3N
●Order of basic strength:
NH3<R-NH3<R2NH>R3N

       Thus as per the observed pKb values of the aliphatic amines, secondary amines are the strongest bases. Basic strength increases as we move from NH3 to R-NH2 and from R-NH2 to R2NH, but basic strength decreases as we move from R2NH to R3N.
         Basicity idmf amines is related to the structural effects which influence of various species. Greater is the stabilitzation of the protonated amine, that is, the conjugate acid, greater is the basicity of amine.

A. Influence of +I effect :
       An alkyl group exerts electron releasing inductive effect which stabilizes positive charge on atom bonded to it.  As we move from cinjugate acid of ammonia to that of tertiary amine, the number of alkyl groups bonded to Nitrogen goes in increasing steadily. This results in increasing stability of the cinjugate acids and therby an increasing order of basic  sthrenth is expected. 


B. Influence of solvation:
              The solvent water stabilizes the conjugate acid by hydrogen bonding through the H bonded to N. The number of H atoms bonded to the N decreasee from r in NH4 to 1 in R3NH. As a result NH4 is the best stabilized by silvation while the stabilization by solvatuon is very poor in R3NH.


C. Combined effect:
              Combined effect of +I effect and solvation on stabilization if conjugate acids of aliphatic amines decides the observed basic strength and pKb value. These two influencing factors operate in opposite directions.
The net result is that as we move from NH3 toRNH2 to R2NH, the basic strength increases due to better stabilization of the corresponding conjugate acids. But 3° amine is weaker base than 2° amine because the stabilization of conjugate acid if 3°amine by solvation us very poor.

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