Chemistry. Organic compounds with functional groups containing nitrogen-I Session.

Chemistry

Organic compounds with functional groups containing nitrogen-I

Session

Session Objectives

1. Introduction nitro compounds

2. Nomenclature

3. Structure and physical properties

4. Preparation of nitro compounds

5. Chemical reactions

6. Cyanides and isocyanides

General method of preparation

Physical and chemical properties

7. Diazo compounds

General method of preparation

Physical and chemical properties

Preparation of nitro compounds

1. Aliphatic nitro compounds

Vapour phase nitration of alkanes

Treatment of alkyl halides with alcoholic AgNO3

Oxidation of t-alkyl amines with KMnO4.

2. Aromatic nitro compounds

Vapour phase nitration of alkanes

Hydrocarbons on heating with fuming nitric acid at 693-793 K are converted into nitroalkanes.

CH — CH 3 3 + HNO 3

CH — CH — NO3 2 2 + H O2

( Fum ing) ( low yield)

This method is important in the commercial production of nitro compounds.

Treatment of alkyl halides with alcoholic AgNO3

Iodoalkanes on treatment with alcoholic AgNO2 are converted into nitroalkanes besides alkylnitriles.

CH — CH — I + AgNO3 2 2CH — CH — NO n itroethane +C H — O— N— O ethyl nitrile

3 2 2

2 5

Limitations

(i) Aromatic nitro compounds cannot be prepared by this method because of the less reactivity of aryl halide towards nucleophilic substitution.

(ii) This method is not suitable for the large scale preparation of nitro compounds.

Oxidation of t-alkyl amines with KMnO4

The amine must be primary and —NH2 group should be attached to a tertiary carbon.

CH — C— NH 3 2

CH 3

CH 3

K M nO 4

CH — C— NO3 2

CH 3

CH 3

Aromatic nitro compounds

Nitration is performed with a mixture of concentrated nitric and sulphuric acid (source of nitronium ion).

conc. HNO 3

conc. H S O2 4

NO 2

+ HNO 3

Conc. H SO2 4

300 K

CH3CH 3

NO 2

CH3

NO 2

+

Reactions of nitro compounds

In aromatic and aliphatic nitro compounds, notro group undergoes similar reactions.

Reduction

(i) Catalytic reduction: easily reduced by catalytic hydrogenation using Pd/C catalyst in ethanol.

H / Pd- C2

ethanol

NO 2NH 2

Reduction in acidic medium

(ii) By metal in acidic solutions: Metals(Fe, Sn and Zn) and HCl are used for reducing a nitro group to an amino group.

S n

H C l

NO 2 NH 2

Aniline hydrochloride

O H —

+ S n salts2+

Fe

H C l

NO 2 NH 2

Aniline hydrochloride

O H —

+ Fe salts2+

Reduction in neutral medium

(iii) Reduction in neutral medium: Zinc dust and ammonium chloride convert nitro benzene to corresponding hydroxylamine.

H O2

Zn/ NH Cl4

NO 2 NHOH

N- pheylhydroxylam ine

+ ZnO

4Zn /NH Cl3 2 3 2

N-methyl hydroxyl amineCH NO 4 H CH — NHOH H O

Reduction with LiAlH4

Aliphatic nitro-compounds are reduced to primary amines with LiAlH4

.

4LiAlH3 2 3 2ether

CH NO CH NH

Aromatic nitro-comopunds on reduction with LiAlH4 give azo compounds.

4LiAlH6 5 2 6 5 6 5

Azo benzene2C H NO C H — N == N — C H

Reduction in basic medium

Forms different products depending on reducing reagent.

H O2

M eO HNO 2 N

azobenzene

+ 4Na ZnO + 4H O2 2 2

+ 4Zn + 8NaOH N

NO 2 N

azoxybenzene

+ 6Na AsO + 9H O3 4 2

+ 3As O + 8NaOH2 3

N

O —

+

Selective reduction

One nitro group can be reduced without affecting the second group on benzene ring using ammonium sulphide orsodium polysulphide.

+ 3 (NH ) S4 2

NH2NO 2

+ 6 NH 3 + 2 H O + 3S2

NO2 NO2

Reductive removal of nitro group

Nitro group can be removed from aromatic ring via reduction to amine followed by deoxidization with HNO2 and then reductive removal of the diazonium group using sodium borohydride or hypo phosphorus acid/Cu+ mixture.

NO 2 S n + H C lNH 2 N Cl2

NaN O + H C l2

H PO ,CuC l

3 2

Electrophilic substitution

The nitro group strongly deactivates the benzene ring towards electrophilic substitution.

Required strong conditions.

HNO , H SO3 2 4

N O 2

383 K

N O 2

N O 2

Nucleophilic substitution

Nitro group facilitates the nucleophilic substitution by stabilising the intermediate carbanion as depicted below.

N O 2

O H —

N O 2

N O 2

C l

N O 2

N O 2

C l

N O 2

O H

–

O H

Acidic nature of alpha hydrogen atom

Alpha hydrogen atom in aliphatic nitro compounds becomes acidic due to the electron withdrawing nature of nitro group.

Gives aldol condensation with carbonyl compounds which on dehydration gives unsaturated nitro compound.

H C — N3

O

O —

O H —

H C — N2

O

O —

— +H C — N2

O —

+O —

CH CHO3

H O — C — C — N O 2

H

HC H 3

H

C H — N O 2

H O2

Hydrolysis of aliphatic nitro compounds

Primary nitro-compounds are hydrolysed by boiling HCl or by 85% H2SO4 to a carboxylic acid and hydroxylamine.

2 2 2 2Carboxylic1 Nitro alkane Hydroxylamineacid hydrochloride

RCH NO H O HCl RCOOH NH OHHCl

Secondary nitro-compounds are hydrolysed by boiling hydrochloric acid to ketones and nitrous oxide.

Boiling HCl2 2 2 2 2

ketone Nitrousoxide

2R CHNO 2R CO N O H O

Tertiary nitro-compounds are generally unaffected by HCl.

Diazonium Salts

The diazonium salts are represented by general formula ArN2+X–,

where Ar stands for the aryl group and X may be any anion such as

4 4Cl , Br , HSO , BF etc.

Nomenclature

obtained by adding the suffix diazonium to the parent compound and is further followed by the name of the anion, e.g.

N 2+

Cl–

Benzenediazoniumchloride

Benzenediazoniumhydrogen sulphate

N 2+

HSO 4

p - to luenediazoniumhydrogen sulphate

N 2+

HSO 4– –

CH 3

p -nitrobenzenediazoniumfluoroborate

N2+

BF4–

NO 2

Preparation of diazonium salts

1. Obtained by treatment of primary aromatic amine dissolved in cold aqueous mineral acid with sodium nitrite.

The conversion of primary amine into a diazonium salt is called diazotisation.

Cold

2 2 2A diazonium salt1 Aromatic amine

Ar NH NaNO 2HX Ar N N X NaX 2H O

2. Nitrite esters formed from alcohols and nitrous acid are also used to generate diazonium salt on treatment with aromatic primary amines.

273 278 K5 11 6 5 2 6 5 5 11 2C H O N O HCl C H NH C H N NCl C H OH H O

Chemical properties of diazonium salts

Arenediazonium salts are highly reactive compounds due to excellent leaving ability of the diazo group as nitrogen gas, N2.

Their chemical reactions may be classified into two types.

(i). Reactions in which the –N2X is completely replaced.

(ii). Reactions in which the nitrogen atoms are retained.

Reactions in which the –N2X is completely replaced

1. Replacement by –Cl, –Br and –CN (Sandmeyer reaction)

Ar — Br + N + X2–

Ar — C l + N + X2

–

Ar — C N + N + X2

–

CuBr/HBr

CuCl/HCl

CuCN/Cu

ArN X2

––

2. Replacement by iodine

warm2 2ArN Cl KI ArI N KCl

Reactions in which the –N2X is completely replaced

3. Replacement by fluorine — Balz-Schiemann reaction

HCl2 4 2 4 2 3ArN Cl HBF ArN BF Ar F N BF

4. Replacement by nitro group

N 2+

Cl–

HBF4

–HCl

N 2+

BF 4–

NaNO 2

Cu

NO 2

5. Replacement by hydroxyl group

2 4H SO

2 2 2Boiling

ArN X H O Ar OH N HX

Reactions in which the –N2X is completely replaced

6. Replacement by hydrogen atom

Cu2 3 2 2 3 3 2ArN X H PO H O Ar H H PO N HX

8. Replacement by an aryl group Gomberg-Bachmann reaction

N C l2

–+B r + C H 66

N a O HB r + N + H C l2

Reactions in which the nitrogen atoms are retained

1. Reduction to arylhydrazines

This reduction can be brought about by a number of reagents such as stannous chloride-hydrochloric acid, sodium sulphite, sodium hydrosulphide and even electrolytically.

2 3

NaOH2 2

or Na SOArN X Ar NH NH

Vigorous reducing agent such as Zn/HCl is used, the product is an aromatic amine.

Zn /HCl Zn /HCl6 5 2 6 5 2 6 5 2 3C H N Cl C H NHNH C H NH NH

Reactions in which the nitrogen atoms are retained2. Coupling reactions

Diazonium salts are weak electrophiles and they react with highly reactive aromatic compounds such as phenols, napthols and aromatic amines to form highly coloured azo compounds which are used as dyes.

C H N Cl5 2

+ –

6

C H NM e5 26

H+ C H — N66 N NM e2

C H OH56

OH–

C H — 5 6 N N O H

N Cl2

–+

+

OH

CH 3

OH

CH 3

N N

Cyanides

Cyanides

Cynanides are considered to be the derivatives of hydrogen cyanide (HCN) in which H atom is replaced by alkyl or aryl group. These are also known as nitriles or carbonitriles.

Nomenclature

General Methods of Preparation

1. From alkyl halide

2 5 2C H OH H OR X NaCN R C N Na X

Aryl cyanides cannot be prepared by this method since aryl halides are almost unreactive towards nucleophillic substitution reaction.

2. By dehydration of primary amides

2 5P O2 2RCONH R C N H O

In this reaction, ammonium salts of carboxylic acids can be used instead of amides.

2 5 2 5

2

P O , P O4 2 2

H OR COONH R CONH R C N H O

General Methods of Preparation

3. By dehydration of aldoximes with P2O5 or acetic anhydride

3 2

2

CH CO O

H ORCH NOH RC N

4. From Grignard reagent

etherRMgX Cl CN R CN MgXCl

5. From aryl diazonium salt

CuCN2 2

or Cu powderArN X KCN Ar C N N KCl

This reaction is a special case of Sandmeyer reaction or Gattermann reaction.

Physical properties

1. Physical state and smell:

The lower members of the family are colourless liquids whereas the higher members are crystalline solids. They are stable compounds with pleasant smell.

2. Boiling point:

Due to the presence of polar group in the molecules, they have high dipole moment and consequently high melting and boiling points.

Physical properties

Solubility: The lower cyanides are soluble in water due to the tendency of their molecules to form hydrogen bond with water molecules.

R C N H OH N C R

With the increase in molecular mass, the bulk of the non-polar portion (i.e. R–) increases and consequently, solubility in water decreases.

However, the cyanides are fairly soluble in organic solvents.

Chemical properties

1. Hydrolysis:

Hydrolysed in acidic as well as in basic solution to give amides as the initial products. Exhaustive hydrolysis form carboxylic acid.

C H CN + HCl(g) in CH OH6 5 3

reflux

C H6 5

NH Cl2

OCH 3

H O2

C H COOCH6 5 3

By using alcohol under acidic conditions, an ester is obtained.

R — C NH O /H2

+

R — C — N H2

H O /H2

+

R — C — O H + NH4+

O

Am m oniumsalt

Acid

O

Chemical properties

R C NR'MgXether

R C NMgX

R'H O/H2

+

–Mg(OH)X–NH 3

R C O

R'

Ketone

4. Alcoholysis

R C N + R 'O H + H O2H

+

R C O R ' + N H 4

+

O

Reaction with Grignard reagent

Chemical properties

Reduction

Nitriles need relatively stronger reagents for reduction. Catalytic hydrogenation in presence of Raneys nickel or on reduction with LiAlH4 nitriles are reduced to primary amines.

R C N + 2H 2Ni or Pt RCH NH2 2

R C N + 4[H]L iA lH o r4 RCH NH2 2Na /C H O H52

Isocyanides

Isocyanides are structural isomers of alkyl cyanides. In these compounds, the group is linked to the alkyl or aryl group through N atom. They have general formula

R N CA lky l

iso cya n id e

o r A r N CA ry l

iso cya n id e

NomenclatureIn common system, these are generally named by adding a prefix iso before the name of the isomeric alkyl nitriles.

They are also called alkyl carbylamines.

In IUPAC system, isocyanides are named as alkyl isocyanides.

CH3CH2NC Ethyl isocyanide

General Methods of Preparation

1. From alkyl halide

R X

R C N + KCN (a lc.)

AgCN (alc.)

R N C(major) (m inor)

R N C + R C N(major) (m inor)

2. From primary amines (Carbylamine reaction)

2 3R NH CHCl 3KOH alc. R N C + 3KCl + 3H2O

Physical properties

Physical state, smell:

Isocyanides are colourless liquids with very unpleasant smell.

Boiling point:

They are relatively less polar in comparison with isomeric cyanides. Consequently, their melting and boiling points are relatively low in comparison with the cyanides of same molecular mass.

Solubility:

They are not very soluble in water. This is due to the reason that nitrogen atom does not have a lone pair of electrons and hence cannot form H-bonds.

Chemical properties

1. Addition of water: Acid catalysed addition of water gives alkyl formamide derivative.

2RNC H O RN CHOH RNHCHO

2. Reduction with LAH: reduced to N-methyl amines.

LAH3RNC RNHCH

3. Oxidation: On reaction with HgO or with ozone as well as with halogen and dimethylsulphoxide, oxidised to isocyanates

RNC + Cl + CH — S — CH2 3 3

O

RNCO + CH — S — CH3 3

Cl

Cl

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