Organic compounds with functional groups containing Nitrogen (Nitro, amino, cyano, and diazo compounds)

The compounds containing the functional group, –NO₂ are called nitro compounds. They may be aliphatic and aromatic compounds according to as the nitro group is attached to alkyl or aryl group. e.g.

R—NO₂                                              Ar—NO₂

e.g. CH₃—NO₂                                                        C₆H₅—NO₂

Where R= alkyl group and Ar = aryl group.

Nitroalkanes are further be classified as primary, secondary or tertiary depending upon whether the nitro group is attached to primary, secondary or tertiary carbon atom.

Preparation of nitro compound:

1. From alkyl halides: Nitroalkane can be prepared by reaction between alkyl halide with silver nitrate solution in presence of alcohol.

R—I       +   AgNO₃———–>  R—NO₂    +     AgI

e.g.    CH₃CH₂I   +   AgNO₃ ———->   CH₃CH₂NO₂   +    AgI.

Iodoethane                                      Nitroethane

N.B. Aromatic nitro compounds can’t be prepared by this method.

2. By nitration of alkane: When a gaseous mixture of an alkane and nitric acid is passed through a metal tube at about 400ºc, nitroalkane is produced.

CH₄     +  HNO₃  ————> CH₃—NO₂      +    H₂O

CH₃CH₃  +  HNO₃ ————>   CH₃CH₂NO₂    +            H₂O.

Ethane                                           Nitroethane

3. Preparation of aromatic nitro compounds: Nitrobenzene can be prepared by heating a mixture of conc. HNO₃ and H₂SO₄ . e.g.

Physical properties of nitro compounds:

Lower nitroalkanes are colourless liquids with pleasant smell. Aromatic nitro compounds are either pale yellow liquids or solids having distinct smell. Both aliphatic and aromatic nitro compounds have high boiling points as they are highly polar in nature. They are sparingly soluble in water but readily dissolve in organic solvents. Most of the nitroalkanes are stable and therefore, can be distilled easily without any decomposition under atmospheric pressure.

Chemical properties of nitro alkanes:

The important chemical properties of nitro alkanes are as follows:

1. Reduction: The nitroalkanes cab be reduced to primary amines and it is occur through the following stages.

R—NO₂   +  2H —–>  R—N=O   +  2H ——> R—NHOH   +  2H  —-> R—NH_2.

Nitroalkane                    Nitroso comp.                     subs. Hydroxylaine             1º-amine.  (a) Catalytic reduction: Both aromatic and aliphatic nitro compounds are reduced to primary amines with hydrogen in the presence of raney Ni, Pt or Pd.

_{Raney Ni/Pt}

CH₃CH₂NO₂      +  3H₂  ————>  CH₃CH₂NH₂    +   2H₂O

Nitroethane                                         Ethyl amine

C₆H₅-NO₂           + 3H₂   ————> C₆H₅-NH₂       +   2H₂O

Nitrobenzene                                         Aniline

(b) Reduction with metal hydrides: Nitroalkanes can be reduced to corresponding primary amines with lithium aluminium hydride and that of aromatic nitro compounds to azo compounds and not primary amine.

CH₃CH₂NO₂      ——^LiAlH4—–>  CH₃CH₂NH₂

C₆H₅-NO₂           ——^LiAlH4—–>  C₆H₅-NH₂

(c) Reduction with neutral medium: The reduction with zinc dust and ammonium chloride provides neutral medium. Both aromatic and aliphatic nitro compounds are reduced to N-alkyl hydroxylamine.

(d) Reduction in acidic medium: Aliphatic and aromatic nitro compounds can be reduced to corresponding primary amines by active metals like Zn, Fe, or Sn in the presence of conc. HCl. e.g.

(e) Reduction in alkaline medium: Aromatic nitro compounds give different products in alkaline medium depending upon the reducing agent used. e.g.

2. Hydrolysis:

• Primary nitroalkanes can be hydrolysed with HCl or H₂SO₄ to give carboxylic acids and hydroxyl amine.
• Secondary nitro compounds are hydrolysed to ketones and nitrous oxide in presence of hydrochloric acid.
•  Tertiary nitroalkanes do not undergo hydrolysis with boiling hydrochloric acid.

*3.Reaction with nitrous acid: The reaction is used to distinguish between primary, secondary or tertiary nitroalkanes. With nitrous acid, α-hydrogen is replaced by nitroso group.

• Primary nitroalkanes react with nitrous acid to form nitrolic acid which gives red colouration with alkali (NaOH).
• Secondary nitroalkanes gives blue coloured, pseudonitroles. These are insoluble in water as well as in alkali.

(CH₃)₂—CH—NO₂  +  HONO ——> (CH₃)₂—CO—N=O                                                                                                                           Pseudonitrole (blue coloured)

• Tertiary nitroalkanes do not react with nitrous acid because there is no H-atom on the carbon atom carrying the NO₂ group.

4. Halogenation: Primary and secondary nitroalkanes are readily halogenated in the α-position by treatment with chlorine or bromine in alkaline solution.

CH₃—NO₂ —-^Cl/NaOH——–> C—Cl—NO₂

Nitro methane                            Chloropicrin (an important insecticide).

Ring substitution reaction of nitrobenzene: Nitrobenzene may be represented by the following resonance structures.

It is clear from the above structures that because of electron withdrawing nature of NO₂ group, the electron density is reduced at o-and p-positions (because of +ve charge). Therefore, electron density is comparatively more at m-position than o- and p-positions and consequently nitro group is meta directing. Moreover, –NO₂ is electron withdrawing group and it deactivates the benzene ring. Therefore, nitrobenzene is less reactive towards electrophilic substitution reactions and it undergoes reactions only under drastic conditions. e.g.

• Nitration:
• Sulphonation:
• Halogenation:
• Trinitrotoluene (TNT): When toluene is heated with a mixture of fuming HNO₃ and H₂SO₄, trinitrotoluene is formed.

                                 Toluene                                           Trinitrotoluene (TNT)