Introduction
Functional groups containing nitrogen are present in a variety of naturally occurring and man made organic compounds. These functional groups impart physico- chemical characteristics to these molecules. These groups are responsible for their unique chemical reactivity patterns and play crucial roles in the preparation of drugs, agrochemicals, dyes and molecules of life. There are many functional groups, which contain one or more nitrogen atoms. Some categories of compounds based on these functional groups include nitro compounds, amines, cyanides, isocyanides and diazo compounds.
Nitro Compounds
Nitro compounds are characterized by the presence of nitro group (-NO2) in their molecules. They may be aliphatic or aromatic compounds according to whether the nitro group is attached to alkyl or aryl groups.
Nomenclature of Nitro Compounds
Nitro compounds are named by using the prefix nitro before the name of parent hydrocarbon. Aliphatic nitro compounds are called nitroalkanes while aromatic nitro compounds are called nitroarenes.
Aliphatic Nitro Compounds
Vapour phase nitration of alkanes
Hydrocarbons when heated with forming nitric acid at 693 - 793 K, are converted to nitro alkenes. Nitration of methane end lower members can be carried out by this nitration technique.
Aromatic Nitro Compounds
Aromatic compounds can be directly nitrated using a mixture of concentrated nitric acid and sulphuric acid.
Electronic Structure and Properties of Nitro Compounds
The structure of a nitro group can be represented as a resonance hybrid of two equivalent zwitter ionic polar structures.
Reduction
In aromatic and aliphatic nitro compounds, nitro group undergoes similar reactions. However depending on the attachment of the aryl or alkyl groups, the reactivity may be different.
Reductive Removal of Nitro Group
The nitro group can be removed from an aromatic ring via the following steps:
i) reduction of nitro group to amine
ii) diazotization of amine with HNO2
Electrophilic Substitution
The nitro group strongly deactivates the benzene ring towards electrophilic substitution. Nitro group is electron withdrawing group and thus causes electron deficiency at ortho and para positrons as is clear from the resonating structures of nitro benzene.
Influence on the Reactions of other Functional Groups
The presence of nitro group facilitates nucleophilic substitution of otherwise unreactive aromatic halides by stabilizing the intermediate carbanion.
Acidic Nature of Alpha Hydrogen atom
Alpha hydrogen atom in case of aliphatic nitro compounds becomes acidic due to electron withdrawing nature of nitro group.
Hydrolysis of Aliphatic Nitro Compounds
Primary aliphatic nitro compounds can be converted to aldehydes by treatment of their carbanion salts with sulphuric acid.
Amines - Introduction and Classification
Amines may be regarded as the alkyl or aryl derivatives of ammonia. They are obtained by the replacements of one or more hydrogen atoms of ammonia by alkyl or aryl groups. They can be classified into primary, secondary or tertiary amines depending upon whether two or three H atoms of ammonia have been replaced by alkyl or aryl groups.
Nomenclature of Aliphatic Amines
Aliphatic amines are named by adding the suffix amine to the name of the alkyl group and it is written as one word. E.g., C2H5NH2 is Ethylamine. When two or more alkyl groups in secondary or tertiary amines are some, the prefix di or tri is used. For e.g., (C2H5)2 NH is diethylamine.
Nomenclature of Aromatic Amines
Aromatic amines in the common system they are named by adding suffix amine to the name of aryl group. They are named as derivatives of the parent member aniline. But in some cases other names O/m/p - toluidine for o/m/p methylamiline and o/m/p - anisidine for o/m/p methyl anilines are assigned. Even N-phenyl derivative of aniline is called diphenyl amine.
Structure of Amines
Aliphatic amines have a pyramidal shape that is approximately tetrahedral when we assume the electron pair on nitrogen as a group. An amine which has 3 different groups attached to nitrogen has chiral nitrogen. But such amines cannot be resolved into enantiomers because of rapid inversion of an enantiomer to its mirror image.
Reduction of Nitro Compounds
Nitro compounds are reduced to amines either by catalytic hydrogenation or chemically by using a metal and acid.
Ammonolysis is Alkyl halides
An alkyl halides or a benzyl halide reacts with ammonia to form primary secondary and tertiary amines. If ammonia is used in excess primary amine is the major product. Under these conditions in the reaction mixture, an alkyl halide is more likely to encounter and react with a molecule of ammonia than with a molecule of amine which is present in relatively much smaller amount and primary amine is the only product.
Gabriel Synthesis of Primary Amines
Pure primary aliphatic amines are prepared from alkyl halides by this method. In this reaction potassium phthalimide by reaction with KOH. The potassium phthalimide on treatment with alkyl halides gives N - alkyl phthalimide which on hydrolysis with 20% hydrochloric acid gives pure primary amine.
Reduction of Azides
On treatment with sodium azide, alkyl halides undergo nucleophilic substitution to form alkyl azides which on catalytic hydrogenation give primary amines.
Reduction of Nitriles
On treatment with sodium cyanide, alkyl halides undergo nucleophilic substitution to form alkyl nitriles which on reduction with LiAlH4 or Catalytic hydrogenation, form primary amines which have one more carbon than the alkyl halide used. This reaction is used for ascent of series.
Reduction of Amides
Acyl halides on treatment with ammonia or amines form primary, secondary or tertiary amides which are reduced with a strong reducing agent like LiAlH4 to form corresponding amines.
Hoffmann's Bromamide reaction
Acid amides on reaction with bromine in the presence of alkalis at about 343 K give primary amines.
Reduction Amination of Carbonyl Compounds
Aldehydes and Ketones react with ammonia in the presence of reducing agents such as H2/Raney Nickel or Sodium cyano borohydride (NaCNBH3) to give primary amines. The reaction proceeds through the formation of imines which gets further reduced to give amines.
Physical Properties
Lower members of aliphatic amines are gases while higher members are liquids. Among, aryl amines, lower members are liquids but higher members are solids.
Basic Characters of Amines
In amines, there is a lone pair of electrons on nitrogen. The tendency of nitrogen to share these electrons with acids is responsible for the basic character of amines.
Introduction
The reactions of amines are mainly due to participation of unshared pair of electrons of nitrogen which makes them react as a nucleophile or a base (A nucleophile is a species that attacks an electron - deficient carbon and a base is a species that attacks an electron deficient hydrogen i.e., proton)
Salt Formation
Amines being basic react with acids to form salts.
Alkylation
An alkyl halide can react with a primary or secondary amine which acts as a nucleophile. On undergoing nucleophilic substitution, a 1o amine is converted to a 2o amine and a 2o one to a 3o amine. The 2o amine being a more powerful nucleophile again reacts with an alkyl halide to form a tertiary amine.
Acylation
Aliphatic and aromatic primary and secondary amines undergo acylation through nucleophilic substitution of acid derivatives such as acid halides or anhydrides to form amides.
Reaction with Hinsberg's Reagent (Benzene Sulphonyl Chloride)
Primary and secondary amines combine with benzene sulphonyl chloride (C6H5SO2Cl) to form sulphonamides.
Reaction with Carbonyl Compounds
Primary amines react with aldehydes and ketones to yield imines also called Schiff bases.
Carbylamine Reaction
When a primary amine is warmed with chloroform and alcoholic potash, an alkyl isocyanide (carbylamine) is formed which gives an offensive smell. Since this reaction is not given by secondary and tertiary amines, it is used for distinguishing primary amines from other classes of amines.
Oxidation of Amines
Primary amines undergo oxidation to nitro compounds through a sequence of steps. Depending on the reagent, various species such as hydroxylamine, nitro so or nitro compounds are formed.
Reaction with Nitrous Acid
The three classes of amines react differently with nitrous acid. Nitrous acid is a source of electrophilic nitrosonium ion (O = N+), which reacts with amines. Nitrous acid being unstable is generated in situ from a mineral acid and sodium nitrite.
Electrophilic Substitution Reactions
All amino groups -NH2, -NHR, -NR2 are most powerful activating groups in aromatic electrophilic substitution reactions are ortho and para directing.
Distinction between Primary, Secondary and Tertiary Amines
1o, 2o and 3o can be distinguished by carbyl amine test. Primary amines form carbylamine with an offensive odor. Secondary amines give no reaction with chloroform and alcoholic KOH. Tertiary amines do not give carbylamine reaction.
Introduction
Cyanides are characterized by the presence of cyano (-CN) group in their molecules. They are considered to be the derivatives of hydrogen cyanide (HCN) in which H atom is replaced by alkyl or aryl group. If -CN group is linked to alkyl group, the compound is called alkyl cyanide, whereas if cyano group is linked to aryl group, the compound is an aryl cyanide.
Nomenclature
By adding the suffix o-nitrile in place of ic - acid in the common name of the corresponding acid produced by the hydrolysis of the cyanide compound.
Structure
Organic cyanides has a triple bond between the C and N atoms i.e., one sigma and two pi bonds are present. Thus nitrogen here is sp hybridized.
From Alkyl halides
Alkyl halides on treatment with ethanolic solution of sodium cyanide or potassium cyanide give alkyl cyanides.
Substitution of Diazonium Salts
This is a convenient method of preparation of aryl cyanides only. It is not suitable for preparing alkyl cyanides because alkyl diazonium salts are quite unstable even at low temperatures. It involves the treatment of benzene diazonium salt with a mixture of copper (I) cyanide and sodium or potassium cyanide. The aryl diazonium salts are formed through replacement of diazonium group by cyanide (CN). This is called the sand meyer's reaction.
Dehydration of Primary Amides
The dehydration of primary amides can be achieved by phosphorus pentoxide (P2O5), thionyl chloride (SOCl2), or acetic anhydride etc. The primary amide is converted to the corresponding nitrile.
Physical Properties
The lower members of the family are colorless liquids whereas the higher members are crystalline solids. They are stable compounds with pleasant smell.
Chemical Properties
In organic nitrile, nitrogen of the nitrile group can react with the electrophilic reagent and the carbon of the nitrile group can react with a nucleophilic reagent. The unsaturated cyano group can also be reduced.
Introduction
Isocyanides are structural isomers of alkyl cyanides. In these compounds, the -C 
N group is linked to the alkyl or aryl group through N atom. The group is -NC. Thus they have the general formula is
Nomenclature
In the common system, the word iso is prefixed to the name of the corresponding cyano nitrile compound. Thus CH3NC is named as methyl isocyanide or acetoisonitrile.
Methods of Preparation of Isocyanides
Alkyl halides, on treating with ethanolic solution of silver cyanide, give alkyl isocyanides as the major products. Ag+ facilitates the removal of halide ion.
Physical Properties
They are relatively less polar in comparison with isomeric cyanides. Consequently, their melting and boiling points are relatively low in comparison with cyanides of same molecular mass.
Chemical Properties
In the reactions shown by isocyanide, first an electro phile and then a nucleophile add at the carbon to form a species which usually undergoes further transformations.
Introduction
Methods of Preparation
Primary aromatic armines react with nitrous acid at low temperature (273 - 278 K) to give aromatic diazonium salts. This reaction is known as diazotisation. Nitrous acid being unstable, is prepared in situ by the reaction of sodium nitrite and dilute mineral acid.
Replacement Reactions
Aromatic diazonium salts undergo a large number of reactions. These reactions may be divided into two categories.
i) Replacement reactions and
ii) Coupling reactions
Coupling Reactions
The reactions in which diazonium salt acts as electrophile and brings about substitution in some electron rich aromatic ring such as in phenols and in amines.
Importance of Diazonium Salt in Synthetic Organic Chemistry
Aryl diazonium salts constitute intermediates that can be used to synthesis many classes of organic compounds especially aryl halides in the pure state.
Some Commercially Important Compounds
The chief commercial use of amines is as intermediates in the synthesis of dyes, synthetic fibers and as medicines due to physiological and psychlological effects.
Summary
Reduction of -NO2 group proceeds through intermediate stages such as nitroso -(-NO), substituted hydroxylamine (-NHOH) with final products being amines (-NH2). The extent of reduction and product, depends on the nature of the reducing agent and pH of the medium.
