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Organic Compounds Containing Nitrogen

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Organic Compounds Containing Nitrogen PDF Notes, Important Questions and Formulas

Organic Compounds Containing Nitrogen

Amines are the derivatives of ammonia prepared by the replacement of one, two or all the three hydrogen atoms by alkyl and/or aryl groups.

Examples: (i) CH3-NH2

Structure of Amines

  • In amines, the nitrogen atom is trivalent and has an unshared pair of electrons. Hence the nitrogen orbitals are sp3 hybridised with pyramidal geometry.
  • The three sp3 hybrid orbitals of nitrogen overlap with orbitals of hydrogen or carbon depending on the nature of the amines.
  • The fourth orbital of nitrogen in all amines contains an unshared pair of electrons. It is due to the presence of unshared pair of electrons, the angle C-N-E is less than 109.5°.

Preparation of Amines

Reduction of Nitro Compounds

Nitro compounds on reduction with hydrogen gas in the presence of finely divided nickel, palladium or platinum and also on reduction with metals in acidic medium give amines.

Ammonolysis

Alkyl halides or benzyl halide on reaction with an ethanolic solution of ammonia undergoes nucleophilic substitution reaction in which halogen atom is replaced by an amino (-NH2) group. The Process of cleavage of the C-X bond by ammonia molecule is known as ammonolysis.

 begin mathsize 12px style table attributes columnalign left end attributes row cell straight N with • • on top straight H subscript straight 3 space  +  straight R  -  straight X    rightwards arrow space straight R straight N with straight plus on top straight H subscript straight 3 straight X with bar on top end cell row cell Nucleophile            Substitued end cell row cell space                          ammonium space salt end cell end table end style

The primary amine prepared behaves as a nucleophile and reacts with further alkyl halide to form secondary, tertiary amines, and finally quatemary ammonium salt

 begin mathsize 12px style table attributes columnalign left end attributes row cell straight R minus NH subscript straight 2 rightwards arrow with RX on top straight R subscript straight 2 NH rightwards arrow with RX on top straight R subscript straight 3 straight N rightwards arrow with RX on top straight R subscript straight 4 straight N with straight plus on top straight X with straight minus on top end cell row cell left parenthesis 1 to the power of straight o right parenthesis                 ( 2 to the power of straight o right parenthesis             ( 3 to the power of straight o right parenthesis            Quatemary end cell row cell                                                         ammonium space salt end cell end table end style

The free amine can be obtained from the ammoinium salt by treatment with a strong base

 begin mathsize 12px style straight R straight N with straight plus on top straight H subscript straight 3 plus NaOH rightwards arrow straight R minus NH subscript straight 2 plus straight H subscript straight 2 straight O plus Na with straight plus on top straight X with bar on top end style

In this method, a mixture of primary, secondary and teritiary and also a quatemary ammonium salt. However a primary amine is prepared by taking large excess of ammonia

The order of reactivity of halides with amines is RI > RBr > RCl

Reduction of Nitriles

Nitriles in reducing with LiAlH4 or catalytic hydrogenation produce primary amines

 begin mathsize 12px style CH subscript straight 3 straight C º identical to straight N rightwards arrow from Na left parenthesis Hg )/ straight C subscript straight 2 straight H subscript straight 5 OH to straight H subscript straight 2 divided by Ni of CH subscript straight 2 minus CH subscript straight 2 minus NH subscript straight 2 end style

Reduction of Amides

Amides on reducing with LiAlH4 yield amines

 begin mathsize 12px style table attributes columnalign left end attributes row cell        straight o end cell row cell        ll end cell row cell CH subscript straight 3 minus straight C minus NH subscript straight 2 rightwards arrow from left parenthesis ii right parenthesis space straight H subscript straight 2 straight O to left parenthesis straight i right parenthesis LiAlH subscript 4 of CH subscript 3 minus CH subscript 2 minus NH subscript 2 end cell end table end style

Gabriel pthalimide synthesis

Pthalimide on reacting with ethanolic solution of KOH forms potassium salt of pthalimide which on heating with alkyl halide followed by alkaline hydrolysis yields the corresponding primary amine.

Hoffmann bromamide degradation reaction

In this method, primary amines are prepared by treating an amide with bromine in an aqueous or ethanolic solution of NaOH.

The amine formed has one carbon atom less than the starting amide.

 

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Physical Properties of Amines

  • Solubility

Lower aliphatic amines are soluble in water because they can form a hydrogen bond with water. Solubility decreases with increase in molar mass of amines due to an increase in the size of the hydrophobic group.

  • Boiling points

Among the isomeric amines, primary and secondary amines have a high boiling point because they can form hydrogen bonds. Tertiary amines cannot form hydrogen bonds due to the absence of a hydrogen atom for hydrogen bond formation.

Hence, the order of boiling points of isomeric amines is Primary > Secondary > Tertiary

Chemical Properties of Amines

a)     Basic character of amines

*        Amines have an unshared pair of electrons on the nitrogen atom due to which they act as a Lewis base.

*        The basic character of amines can be better understood in terms of their Kb and pKb values

R – NH2 + H2O ⇋ R -

 begin mathsize 12px style table attributes columnalign left end attributes row cell straight K equals fraction numerator left square bracket straight R minus straight N with straight plus on top straight H subscript straight 3 ][ OH to the power of straight minus straight right square bracket over denominator left square bracket straight R minus NH subscript straight 2 ][ straight H subscript straight 2 straight O right square bracket end fraction end cell row cell straight K left square bracket straight H subscript straight 2 straight O ]= fraction numerator left square bracket straight R minus straight N with straight plus on top straight H subscript straight 3 ][ OH to the power of straight minus straight right square bracket over denominator left square bracket straight R minus NH subscript straight 2 straight right square bracket end fraction end cell row cell straight K subscript straight b equals fraction numerator left square bracket straight R minus straight N with straight plus on top straight H subscript straight 3 ][ OH to the power of straight minus straight right square bracket over denominator left square bracket straight R minus NH subscript straight 2 end fraction end cell row cell pK subscript straight b equals negative logK subscript straight b end cell end table end style

*        Greater Kh value or smaller pKh indicated that a base is strong

b)     Comparision of basic strength of aliphatic amines and ammonia

Aliphatic amines are stronger bases than ammonia due to the +l effect of alkyl groups, leading to high electron on the nitrogen atom.

 

 

c)     Comparison of basic strength of primary, secondary and tertiary amines

  • The order of basicity of amines in the gaseous phase follows the expected order on the basis of the +I effect:
  • In aqueous solution, tertiary amines are less basic than either primary or secondary amines. This can be explained on the basis of the following factors:

Tertiary amine > Secondary amine > Primary amine > NH3

  • Solvation effect
  • Greater the stability of the substituted ammonium cation formed, stronger is the corresponding amine as a base.
  • The tertiary ammonium ion is less hydrated than the secondary ammonium ion, which is less hydrated than the primary amine. Thus, tertiary amines have less tendency to form ammonium ion and consequently are least basic.
  • On the basis of the solvation effect, the order of basicity of aliphatic amines should be: Primary amine > Secondary amine > Tertiary amine
  • Steric factor
    • As the crowding of the alkyl group increases from primary to tertiary, amine hindrance to hydrogen bonding increases which eventually decreases the basic strength. Thus, there is a subtle interplay of the inductive effect, solvation effect and steric hindrance of the alkyl group which decides the basic strength of alkyl amines in the aqueous state.
  • Comparison of basic strength of aryl amines and alkanamines
    • Generally, aryl amines are considerably less basic than alkyl amines. Example: Ethyl amine is more basic than aniline.
    • In aniline, the –NH2 group is directly attached to the benzene ring. Hence, the unshared pair of electrons on nitrogen is less available for protonation because of resonance.
    • In the above resonating structures, there is a positive charge on the nitrogen atom making the lone pair less available for protonation. Hence, aniline is less basic than ethyl amine which has no resonating structures.
    • Less basicity of aniline can also be explained by comparing the relative stability of aniline and anilinium ion obtained by accepting a proton.
    • Greater the number of resonating structures, greater is the stability of that species.
    • Aniline is a resonance hybrid of five resonating structures, whereas anilinium ion has only two resonating structures.
    • Thus, aniline has less tendency to accept a proton to form the aniliniumion.

 

  • Effect of substituent on the basic character of amines
    • An electron-donating or electron-releasing group (EDG) increases basic strength.
    • An electron-withdrawing group (EWG) decreases basic strength.
    • Acylation reaction
    • Aliphatic and aromatic primary and secondary amines (which contain replaceable hydrogen atoms) react with acid chlorides, anhydrides and esters to form substituted amide.
    • The process of introducing an acyl group (R–CO–) into the molecule is called acylation.
    • The reaction is carried out in the presence of a stronger base than the amine, such as pyridine, which removes HCl formed and shifts the equilibrium to the product side.
    • Carbylamine reaction
      • On heating aliphatic and aromatic primary amines with chloroform and ethanolic KOH they form isocyanides or carbylamines which have foul odour.
      • Secondary and tertiary amines do not show this reaction.
      • This reaction is used as a test for primary amines.
      • Reaction with Nitrous acid
        • Primary aliphatic amines react with nitrous acid to form aliphatic diazonium salts. Being unstable diazonium salts liberate nitrogen gas quantitatively which is used in the estimation of amino acids and proteins.
        • Aromatic amines on treating with nitrous acid at low temperatures to form diazonium salts which are used in the synthesis of a variety of aromatic compounds.
        • Secondary and tertiary amines react with nitrous acid in a different manner.
        • Reaction with arylsulphonyl chloride
          • Hinsberg’s reagent or benzenesulphonyl chloride (C6H5SO2Cl) reacts with primary amines and secondary amines to form sulphonamides.
          • Primary amine reacts with benzenesulphonyl chloride to form N-ethylbenzenesulphonyl amide.
          • The hydrogen bonded to nitrogen is strongly acidic due to the presence of strong electron withdrawing sulphonyl group and is hence soluble in alkali.
          • With secondary amine,N,N-diethyl-benzenesulphonamide is formed.
          • Tertiary amines do not react with benzenesulphonyl chloride.
        • Electrophilic substitution
          • Ortho- and para-positions to the -NH2 group become centres of high electron density. So -NH2 group is ortho and para directing and a powerful activating group.

(a)  Bromination

  • Aniline reacts with bromine water at room temperature to give a white precipitate of 2, 4,6- tribromoaniline.
  • Due to the high reactivity of aromatic amines, electrophilic substitution takes place at ortho- and parapositions.
  • For preparing monosubstituted aniline derivative, the -NH2 group is protected by acetylation with acetic anhydride then carrying out the desired substitution followed by the hydrolysis of the substituted amide to the substituted amine.

(b)  Nitration

  • Nitric acid is a nitrating agent plus a good oxidising agent. So direct oxidation of aromatic amines is not useful since it gives tarry oxidation products along with some nitro derivatives.
  • In strong acidic medium, aniline is protonated to form the anilinium ion which is meta directing. Hence besides the ortho and para derivatives, significant amount of meta derivative is also formed.

(c)   Sulphonation

Aniline on reacting with sulphuric acid forms anilinium hydrogen sulphate which on heating with sulphuric acid at 453-473K gives p-aminobenzene sulphonic acid as the major product.

Aniline does not undergo Friedel-Crafts reaction due to salt formation with Lewis acid aluminium chloride which is used as a catalyst. As a result, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction.

  • Diazonium salts have the general formula

Where R = Aryl group

X-ion = Cl- Br- HSO , Bf  Etc.

  • A suffix diazonium is added to the parent hydrocarbon from which they are formed followed by the name of the anion

ü  Anion = chloride, hydrogensulphate, etc

ü  Diazonium group =

  • Primary aliphatic amines from highly unstable alkyadiazonium whereas primary aromatic amines from arenediazonium salts which are stable for a short time in a solution at low temperatures.
  • The stability of arenediazonium ion is explained on the basis of resonance.

 

 

Preparation of Diazonium Salts

  • Benzenediazonium chloride is prepared by the action of aniline with nitrous acid at 273-278K.
  • The conversion of primary aromatic amines into diazonium salts is known as diazotisation.

 

 Physical Properties

  • Benzenediazonium chloride is a colourless crystalline solid.
  • It is readily soluble in water and is stable in cold but reacts with warm water.
  • It decomposes easily in the dry state.
  • Benzenediazonium fluoborate is water insoluble and stable at room temperature.

 

Chemical Reactions

Reactions involving displacement of Nitrogen

-          Replacement by halide or cyanide ion: This reaction is called Sandmeyer reaction in which nucleophiles like Cl- ,Brand CN can be easily introduced in the benzene ring in the presence of Cu(I) ion.

-          Alternatively, chlorine or bromine can also be introduced in the benzene ring by treating the diazonium salt solution with corresponding halogen acid in the presence of Cu powder. This is referred as Gatterman reaction.

  • Replacement by iodide ion :

Iodobenzene is formed on treating diazium salt solution with potassium iodide

 begin mathsize 12px style Ar straight N with straight plus on top subscript straight 2 straight X with bar on top straight space rightwards arrow Arl plus KCl plus straight N subscript straight 2 end style

  • Replacement by H :

Hypophosphorus acid or ethanol are mild reducing agents and reduce diazonium salts to arenes and themselves get oxidized to phosphorus acid and ethanal respectively

 begin mathsize 12px style table attributes columnalign left end attributes row cell Ar straight N with straight plus on top subscript straight 2 Cl with bar on top  +  straight H subscript straight 3 PO subscript straight 3 plus straight H subscript straight 2 straight O rightwards arrow ArH plus straight N subscript straight 2 plus straight H subscript straight 3 PO subscript straight 3 plus HCl end cell row cell Ar straight N with straight plus on top subscript straight 2 Cl with bar on top plus CH subscript straight 3 CH subscript straight 2 OH rightwards arrow ArH plus straight N subscript straight 2 plus CH subscript straight 3 CHO plus HCl end cell end table end style

  • Replacement by hydroxyl group

Diazonium salt solution gets hydrolysed to phenol when the temperature is allowed to rise up to 283 K.

 begin mathsize 12px style Ar straight N with straight plus on top subscript straight 2 Cl with bar on top  +  straight H subscript straight 2 straight O rightwards arrow ArOH plus straight N subscript straight 2 plus HCl end style

  • Replacement by –NO2 group:

On heating diaozonium fluoroborate with aqueous sodium nitrate solution in the presence of copper, the diazonium group is replaced by – NO2 group

 

Reactions involving retention of diazo group coupling reactions

  • Benzene diazonium chloride reacts with phenol in which the phenol at its para position is coupled with the diazonium salt to form orange colour dye called p-hydroxyazobenzene.
  • The reaction of diazonium salt with aniline gives yellow dye p-aminoazobenzene.
  • The reaction is known as coupling reaction and it is an example of electrophilic substitution reaction.

Importance of Diazonium Salts in Synthesis of Organic Compounds

  • Diazonium salts are very good intermediates for introducing –F, –Cl, –Br, –I, –CN, –OH,–NO2 groups into the aromatic ring.
  • Direct halogenation method cannot be used for preparing aryl fluorides and iodides.
  • Cyanobenzene can be easily prepared from diazonium salt.
  • Thus the replacement of diazo group by other groups is useful in preparing substituted aromatic compounds which cannot be prepared by direct substitution in benzene or substituted benzene.
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