Chapter 22 Nitrogen containing compounds Part 1- Chemistry free study material by TEACHING CARE online tuition and coaching classes

Chapter 22 Nitrogen containing compounds Part 1- Chemistry free study material by TEACHING CARE online tuition and coaching classes

File name : Chapter-22-Nitrogen-containing-compounds-Part-1.pdf

 

 

The important nitrogen containing organic compounds are alkyl nitrites (RONO), nitro-alkanes (RNO2), aromatic nitro compounds (ArNO2), alkyl cyanides (RCN), alkyl iso cyanides (RNC), amines (– NH2), aryl diazonium salts (ArN2Cl), amides (– CONH2) and oximes (>C = N OH).

Nitrous acid exists in two tautomeric forms.

 

 

HON = O

Nitrite form

H N        O O

Nitro form

 

Corresponding to these two forms, nitrous acid gives two types of derivatives, i.e., alkyl nitrites and nitro alkanes.

 

R O N = O ;

Alkyl nitrite

R N        O O

 

Nitro alkane

It is important to note that nitro alkanes are better regarded as nitro derivatives of alkanes, while alkyl nitrites are regarded as alkyl esters of nitrous acid.

  • Alkyl nitrites : The most important alkyl nitrite is ethyl nitrite.

Ethyl nitrite (C2H5ONO)

  • General methods of preparation : It is prepared
  • By adding concentrated HCl or H2SO4 to aqueous solution of sodium nitrite and ethyl alcohol at very low temperature (0°C).

NaNO2 + HCl ® NaCl + HNO2

 

C2 H5 OH + HNO2  ® C2 H5 ONO

Ethyl nitrite

  • H 2 O

 

  • From Ethyl iodide

C2 H5 I + KONO ® C2 H5 ONO + KI

 

Ethyl iodide

Pot. nitrite

Ethyl nitrite

 

  • By the action

N 2 O3

on ethyl alcohol.

 

2C2 H5 OH + N 2 O3 ® 2C2 H5 ONO + H 2 O

  • Physical properties
  • At ordinary temperature it is a gas which can be liquified on cooling to a colourless liquid (b.p.17°C) having characteristic smell of
  • It is insoluble in water but soluble in alcohol and
    • Chemical properties
  • Hydrolysis : It is hydrolysed by aqueous alkalies or acids into ethyl

CHONO + HO ¾¾NaO¾H ® CHOH + HNO2

  • Reduction : CHONO + 6H ¾¾S¾n ® CHOH + NH 3  + HO

HCl

Small amount of hydroxylamine is also formed.

C2 H5 ONO + 4 H ® C2 H5 OH + NH 2 OH

 

 

 

 

  • Uses
  • Ethyl nitrite dialates the blood vessels and thus accelerates pulse rate and lowers blood pressure, so it is used as a medicine for the treatment of asthma and heart diseases (angina pectoris).
  • Its 4% alcoholic solution (known as sweet spirit of nitre) is used in medicine as a
  • Since it is easily hydrolysed to form nitrous acids, it is used as a source of nitrous acid in organic synthesis.

Note : ® Isoamyl nitrite is used as an antispasmodic in angina pectoris and as a restorative in cardiac failure.

  • Nitro alkanes or Nitroparaffins : Nitro alkanes are regarded as nitro derivatives of
    • Classification : They are classified as primary, secondary and tertiary depending on the nature of carbon atom to which nitro groups is linked.

 

R

RCH 2 NO2   ;  R

R

CHNO2 ; R

C NO2

 

Primary nitro alkane

Secondary nitro alkane             R

Tertiary nitro alkane

 

  • General methods of preparation
  • By heating an alkyl halide with aqueous alcoholic solution of silver nitrite

C2 H5 Br + AgNO2 ® C2 H5 NO2  + AgBr

Some quantity of alkyl nitrite is also formed in this reaction. It can be removed by fractional distillation since alkyl nitrites have much lower boiling points as compared to nitro alkanes.

  • By the direct nitration of paraffins (Vapour phase nitration)

CHCH3  + HONO2 (fuming) ¾¾400¾°¾C ® CHCHNO2  + HO

With higher alkanes, a mixture of different nitro alkanes is formed which can be separated by fractional distillation.

  • By the action of sodium nitrite on ahalo carboxylic acids

 

CH2ClOOH

¾¾NaN¾O¾2  ® CHNO2COOH ¾¾he¾at ® CHNO2  + CO2

 

a – Chloro acetic acid

  • NaCl

α – Nitro acetic acid

Nitro methane

 

  • By the hydrolysis of a–nitro alkene with water or acid or alkali (Recent method)
|

CH3                                                                          CH3

 

CH3 –

|

C = CHNO2

  • HOH ¾¾H+o¾rOH¾–  ® CH   C = O + CH
3

3

NO2

 

O H2

Acetone

Nitro methane

 

2-Methyl, 1-nitro propene

  • Tertiary nitro alkanes are obtained by the oxidation of t-alkyl amines with KMnO4.

RCNH 2  ¾¾KM¾nO¾4  ® RCNO2  + HO

 

  • Physical properties
  • Nitro alkanes are colourless, pleasant smelling
  • These are sparingly soluble in water but readily soluble in organic
  • Their boiling points are much higher than isomeric alkyl nitrites due to polar
  • Again due to polar nature, nitro alkanes are excellent solvents for polar and ionic

 

 

Note : ® and Nitro alkanes are known to exist as tautomeric mixture of nitro-form and aci-form.

 

CH3N = O

¯

O

(nitro-form)

  • Chemical properties

CH2 = N OH

¯

O

(aci-form)

 

  • Reduction : Nitro alkanes are reduced to corresponding primary amines with Sn and HCl or Fe and HCl or catalytic hydrogenation using nickel as

RNO2 + 6H ® RNH 2 + 2H 2O

However, when reduced with a neutral reducing agent (Zinc dust + NH4Cl), nitro alkanes form substituted hydroxylamines.

RNO2  + 4 H ¾¾Zn+¾NH¾4¾Cl ® RNHOH + HO

  • Hydrolysis : Primary nitro alkanes on hydrolysis form hydroxylamine and carboxylic

RCHNO2  + H2O ¾¾HC¾l or ¾80%¾HSO¾4  ® RCOOH + NHOH

secondary nitro alkanes on hydrolysis form ketones.

2RCHNO2  ¾¾H¾Cl ® 2RCO + NO + HO

Ketone

  • Action of nitrous acid : Nitrous acid reacts with primary, secondary and tertiary nitro alkanes

RCH 2 + O = NOH ¾¾ H¾2¾O ® RC = NOH ¾¾NaO¾H ®  RC = NONa

 

|

NO2

Primary

Nitrous acid

|

NO2

Nitrolic acid

|

NO2

Red coloured sodium salt

 

RCH + HON = O ¾¾ HO ® RCNO ¾¾Ethe¾r¾or ®Blue colour

 

|

NO2

Seconary

|

NO2

Pseudo nitrol

NaOH

 

Tertiary nitro alkanes do not react with nitrous acid.

 

  • Thermal decomposition :

R.CH 2 .CHNO2  ¾¾>30¾0°¾C ® R.CH  = CH 2  + HNO2

moderately

 

On rapid heating nitro alkanes decompose with great violence.

 

CH3

NO2

¾¾hea¾t, Ra¾pid¾ly ® 1 N

2    2

  • CO2
  • 3H

2    2

 

  • Halogenation : Primary and secondary nitro alkanes are readily halogentated in the a-position by treatment with chlorine or

 

 

CH 3

  • NO2

¾¾C¾l2  ®

CCl3

NO2

CH3

|

; CH3C H NO2

¾¾Cl2 ¾+ N¾aO¾H  ®

CH3

|

CH3 – CNO2

 

NaOH

Chloropicrin or nitro chloroform (insecticide)

2-Nitropropane                                                    |

Cl

 

  • Condensation with aldehyde : CH3 CHO + CH3 NO2   ® CH3 CH(OH)CH 2 NO2

b -Hydroxy nitropropane

(nitro alcohol)

  • Reaction with grignard reagent : The aci-form of nitroalkane reacts with Grignard reagent forming

 

 

RCH = N

OH + CH MgI ® CH

  • RCH = N

OMgI

 

O
O

3                        4

Methane

 

 

 

Note : ® The nitrogen of –NO2 carrying a positive charge exerts a powerful – I effect and thus activates the hydrogen atom of the a-carbon. Thus the important reactions of nitroalkanes are those which involve

a-hydrogen atom of primary and secondary nitroalkanes (tertiary nitroalkanes have no a– hydrogen atom and hence do not undergo such type of reactions).

  • Acidic character :The a-hydrogen atom of primary and secondary nitroalkanes are weakly acidic and thus can be abstracted by strong alkalies like NaOH. Therefore, 1° and 2° nitroalkanes dissolve in aq. NaOH to form salts. For examples.

+              O                                  –                     +             O                         +                      –

 

CH3 – N

¾¾NaO¾H ® Na + CH2  – N O–            I

O « H2C = N

O N a O

 

Thus 1° and 2° nitroalkanes are acidic mainly due to following two reasons,

  • Strong electron withdrawing effect of the – NO2
  • Resonance stabilisation of the carbanion (I) formed after the removal of

The aci-form of nitroalkanes is relatively more acidic because it produces relatively more conjugate base.

  • Uses : Nitro alkanes are used,
  • As solvents for polar substances such as cellulose acetate, synthetic rubber
  • As an
  • For the preparation of amines, hydroxylamines, chloropicrin

Distinction between Ethyl nitrite and Nitro ethane

 

Test                         Ethyl nitrite (C2H5ONO) (Alkyl nitrite, RONO)

Nitro ethane (C2H5NO2) (Nitro alkane, RNO2)

 

Boiling point        Low, 17°C                                                       Much higher, 115°C

 

Reduction with       metal and acid

Gives alcohol + hydroxyl amine or NH3.

 C2H5ONO + 4 H ® C2H5OH + NH2OH

RONO + 6H ® ROH + NH3 + H2O

Gives corresponding primary amine.

 C2H5 NO2 + 6H ® C2H5 NH2 + 2H2O

 RNO  + 6H ® RNH   + 2H  O

 

(Sn/HCl)        or with LiAlH4.

2                            2           2

 

Action          of NaOH

(alkalies).

Readily hydrolysed to give corresponding alcohol and sodium nitrite (decomposition).

 C2H5ONO + NaOH  ® C2H5OH + NaNO2

RONO + NaOH ® ROH + NaNO2

Not decomposed, i.e., alcohols are not produced. But it may form soluble sodium salt, because in presence of alkali the nitro form changes into aci form, which dissolves in alkalies to form sodium salt.

 

CH3

  • CH = N

OH ¾¾NaO¾¾H ® CH O 3

  • CH = N

ONa O

 

Action          of HNO2 (NaNO2+ HCl)

No action with nitrous acid.

Primary nitro alkanes forms nitrolic acid, which dissolve in alkali to give red solution.

Secondary nitro alkane yields pseudo-nitrol, which dissolves in alkali to give blue solution.

 

 

 

Aromatic nitro compounds are the derivatives of aromatic hydrocarbons in which one or more hydrogen atom

  • of the benzene nucleus has been replaced by nitro (– NO2)

(1)  Preparation

  • Nitration (Direct method) : The number of – NO2 groups introduced in benzene nucleus depends upon the nature and concentration of the nitrating agent, temperature of nitration and nature of the compound to be
  • The nature of the nitrating agent : For example,

NO2                                                                                NO2

 

 

 

O2N

NO2

Fuming HNO3

100°C

 

 

Benzene

conc. HNO3

100°C

NO2

 

Syn-Trinitro benzene

  • Temperature of nitration : For example,

NO2

m-Dinitrobenzene

 

NO2

 

 

 

 

NO2

m-Dinitro benzene

HNO3 + H2SO4

60°C

 

 

Benzene

HNO3 + H2SO4

60°C

 

 

Nitrobenzene

 

  • Nature of the compound to be nitrated : Presence of electron-releasing group like –OH, –NH2, –CH3, –OR, , in the nucleus facilitates nitration. Thus aromatic compounds bearing these groups (i.e. phenol, aniline, toluene, etc.) can be nitrated readily as compared to benzene. Thus benzene is not affected by dilute HNO3 while phenol,

aniline and toluene forms the corresponding ortho– and para-nitro compounds.

NO2

 

 

conc. HNO3 H2SO4

dil. HNO3                             No reaction

 

 

 

 

O2N

OH

NO2

 

 

HNO3 H2SO4

OH

 

dil. HNO3

OH                                 OH

NO2

+

 

 

NO2

2, 4, 6-Trinitrophenol

Phenol

o-Nitrophenol

NO2

p-Nitrophenol

 

On the other hand, nitration of aromatic compounds having electron withdrawing groups like – NO2, – SO3 H

requires powerful nitrating agent (like fuming HNO3 + H2SO4) and a high temperature.

  • Indirect method : The aromatic nitro compounds which can not be prepared by direct method may be prepared from the corresponding amino

 

NH2

N2BF4

NO2

 

 

NaNO2

Cu, heat

 

 

NO2

p-Nitroaniline

NO2

NO2

p-Dinitroaniline

 

 

 

(2)  Physical properties

  • Aromatic nitro compounds are insoluble in water but soluble in organic
  • They are either pale yellow liquids or solids having distinct smells. For example, nitro benzene (oil of Mirabane) is a pale yellow liquid having a smell of bitter

(3)  Chemical properties

  • Resonance in nitrobenzene imparts a partial double bond character to the bond between carbon of benzene nucleus and nitrogen of the – NO2 group with the result the – NO2 group is firmly bonded to the ring and therefore cannot be replaced other groups, e., it is very inert.

 

O–          O

N+                              N+

O–          ON+

O–          ON+

d –           d

O           O

N+

 

+                                                     +                                   d+             d+

 

 

+

Resonating structures of nitrobenzene

d+ Resonance hybrid of nitrobenzene

 

  • Displacement of the – NO2 group : Although – NO2 group of nitrobenzene cannot be replaced by other groups, but if a second – NO2 group is present on the benzene ring of nitrobenzene in the o- or p– position, it can be
2

replaced by a nucleophile. For example, NO                                                                                               Nu

 

+ aq. KOH, NH3 or C2H5OK

 

 

NO2

p-Dinitrobenzene

NO2

(Where, Nu = OH, NH2 or OC2H5)

 

  • Reduction : Aromatic nitro compounds can be reduced to a variety of product as shown below in the case of

C6 H5 NO2 ® C6 H5 NO ® C6 H5 NHOH  ® C6 H5 NH2

 

Nitrobenzene

Nitrosobenzene

Phenylhydroxylamine

Aniline

 

The nature of the final product depends mainly on the nature (acidic, basic or neutral) of the reduction medium and the nature of the reducing agent.

 

 

 

  • Reduction in acidic medium

NO2

Nitrobenzene

 

+ 6H        Sn + HCl

NO2

Aniline

 

+ 2H2O

 

Reduction of dinitrobenzene with ammonium sulphide reduces only one – NO2 group (selective reduction)

NO2                                        NO2

 

 

 

NO2

m-Dinitro benzene

(NH4)2S

or Na2S

NH2

  • Nitroaniline

 

 

 

  • Reduction in neutral medium : CHNO2 + 2H ¾¾Znd¾ust¾+ NH¾4¾Cl ® CHNO ® CHNHOH

 

Nitrobenzene

(- H2O)

Nitrosobenzene (intermediate)

Phenylhydroxylamine

 

 

C H NO

¾¾2[H¾] ®

C H  NO

ü ¾¾(- H¾¾O) ® CH5  – NO

 

  • Reduction in alkaline medium : 6   5      2

6     5             ï         2

||

C H

 

Nitrobenzene

4[H]

Nitrosobenzene  ý

6  5    N

 

C6 H5 NO2

Nitrobenzene

¾¾¾®

C6 H5 NHOH ï

Phenylhydroxylamineþ

Azoxybenzene

 

Azoxybenzene on further reduction yields azobenzene and hydrazobenzene.

CH5  – NO ¾¾2[H¾] ® CH5  – N ¾¾2[H¾] ® CH5  – NH

 

||

C6 H5 – N

Azoxybenzene

||

C6 H5 – N

Azobenzene

|

CH5 – NH

Hydrazobenzene

 

  • Electrolytic reduction :
  • Weakly acidic medium of electrolytic reduction gives
  • Strongly acidic medium gives phenylhydroxylamine which rearranges to p

 

NO2

NHOH

NH2

 

 

 

 

 

 

Nitrobenzene

electrolytic reduction in presence

of conc. H2SO4

rearrangement

 

 

Phenylhydroxylamine

 

OH

  • minophenol

 

  • Alkaline medium of electrolytic reduction gives all the mono- and di-nuclear reduction products

mentioned above in point (c) .

  • Electrophilic substitution : Since – NO2 group is deactivating and m-directing, electrophilic substitution (halogenation, nitration and sulphonation) in simple aromatic nitro compounds (g. nitrobenzene) is very difficult as compared to that in benzene. Hence vigorous reaction conditions are used for such reaction and the new group

enters the m-position.

 

 

(a)

NO2

 

+ Cl2

 

AlCl3

NO2

 

 

Cl

 

(b)

NO2

 

conc. HNO3

conc. H2SO4

NO2

 

NO2

 

Nitrobenzene

m-Chloronitrobenzene

Nitrobenzene

m-Dinitrobenzene

 

 

 

 

(c)

NO2

 

+ H2SO4 (fuming)

 

 

 

100°C

NO2

 

 

SO3H

 

Nitrobenzene

m-Nitrobenzene sulphonic acid

 

 

Although nitrobenzene, itself undergoes electrophilic substitution under drastic conditions, nitrobenzene having activating groups like alkyl, – OR, – NH2 etc. undergoes these reactions relatively more readily.

 

 

 

CH3

NO2

 

 

HNO3 H2SO4

CH3

NO2

 

 

HNO3 H2SO4

O2N

CH3

NO2

 

o-Nitrotoluene

NO2

2, 4-Dinitrotoluene

NO2

2, 4, 6-Trinitrotoluene (TNT)

 

 

Sym-trinitrobenzene (TNB) is preferentially prepared from easily obtainable TNT rather than the direct nitration of benzene which even under drastic conditions of nitration gives poor yields.

 

 

O2N

CH3

NO2

 

Na2Cr2O7 H2SO4

O2N

COOH

NO2

 

 

Sodalime (–CO2)

O2N

NO2

 

 

 

NO2

(TNT)

NO2

2, 4, 6-Trinitro benzoic acid

NO2

2, 4, 6-Trinitrophenol (TNB)

 

  • Nucleophilic Substitution : Benzene is inert to nucleophiles, but the presence of – NO2 group in the benzene ring activates the latter in o– and p-positions to

 

NO2

 

 

KOH

fuse

NO2

OH

+

NO2

 

 

Nitro benzene

  • Nitrophenol

OH

  • Nitrophenol

 

  • Effect of the – NO2 group on other nuclear substituents
  • Effect on nuclear halogen : The nuclear halogen is ordinarily inert, but if it carries one or more electron- withdrawing groups (like – NO2) in o– or p-position, the halogen atom becomes active for nucleophilic subtitutions

and hence can be easily replaced by nucleophiles (KOH, NH 3 , NaOC2 H 5 ).

 

Cl

NO2

 

+ KOH, NH3 or C2H5ONa

Nu

NO2

 

 

 

NO2

2, 4-Dinitrochlorobenzene

NO2

(Where, Nu = OH, NH2, OC2H5)

 

  • Effect on phenolic –OH group : The acidity of the phenolic hydroxyl group is markedly increased by the presence of – NO2 group in o– and p-position.

The decreasing order of the acidity of nitrophenols follows following order

 

 

O2N

OH

NO2

OH

NO2

OH                                                 OH

 

NO2

 

 

 

 

NO2

2, 4, 6-Trinitro phenal

NO2

2, 4-Dinitrophenol

o– and p-Nitrophenols

Phenol

 

 

 

Increased acidity of oand p-nitrophenols is because of the fact that the presence of electron-withdrawing

– NO2 group in o-and p-position (s) to phenolic –OH group stabilises the phenoxide ions (recall that acidic nature of phenols is explained by resonance stabilisation of the phenoxide ion) to a greater extent.

 

+

O N = O

+

O – N – O

 

 

 

 

O

Phenoxide ion                          O                                          O

(no –NO2 group)      Extra stabilisation of p-nitrophenate ion due to –NO2 group

Due to increased acidity of nitrophenols, the latter react with phosphorus pentachloride to give good yields of the corresponding chloro derivative, while phenol itself when treated with PCl5 gives poor yield of chlorobenzene.

 

 

 

 

 

 

 

(4)  Uses

OH

NO2

 

 

NO2

2, 4-Dinitrophenol

 

+ PCl5

Cl

NO2

 

NO2

2, 4-Dinitrochlorobenzene

 

  • On account of their high polarity, aromatic nitro compounds are used as
  • Nitro compounds like TNT, picric acid, TNB are widely used as explosives.
  • These are used for the synthesis of aromatic amino
  • Nitro benzene is used in the preparation of shoe polish and scenting of cheap soaps.

Hydrogen cyanide is known to exist as a tautomeric mixture.

H – C N H N        C

Hence, it forms two types of alkyl derivatives which are known as alkyl cyanides and alkyl isocyanides.

R C º N                                 R N       C

 

Alkyl Cyanide

Alkyl isocyanide

 

Nomenclature : According to IUPAC system, cyanides are named as “alkane nitriles“. In naming the hydrocarbon part, carbon of the – CN group is also counted.

FormulaAs cyanideIUPAC name
CH3CNMethyl cyanide(Acetonitrile)Ethane nitrile
C2H5CNEthyl cyanide(Propiononitrile)Propane nitrile
C3H7CNPropyl cyanideButane nitrile
C4H9CNButyl cyanidePentane nitrile

Iso cyanides are named as “Alkyl carbylamine” or “Carbyl amino alkane“.

 

FormulaAs isocyanide(Comman name)IUPAC name
CH3NC

 

C2H5NC

Methyl isocyanide (Methyl isonitrile)

 

Ethyl isocyanide (Ethyl isonitrile)

Methyl        carbylamine       (Carbylamino methane)

Ethyl carbylamine (Carbylamino ethane)

 

 

 

(1)  Alkyl Cyanides

  • Methods of preparation
  • From alkyl halides : The disadvantage of this method is that a mixture of nitrile and isonitrile is

 

RX + KCN(orNaCN) ®

Alkyl

RCN     +

Nitrile

RNC

Isonitrile

 

halide

(Major product)

(Minor product)

 

  • From acid amides :

RCONH 2  ¾¾P2O¾5  ® RCN ;  CH 3 CONH 2 ¾¾P2O¾5  ®  CH 3 CN  + H 2O

 

H2O

Acetamide

Methyl cyanide

 

Industrially, alkyl cyanides are prepared by passing a mixture of carboxylic acid and ammonia over alumina at 500°C.

 

RCOOH+ NH 3  ® RCOONH 4  ¾¾AlO¾3  ® RCONH 2  ¾¾AlO¾3  ®

RCN

 

Acid

Ammonium salt

H2O

Amide

H2O

Alkyl cyanide

 

  • From Grignard reagent

 

RMgX+ ClCN ® RCN + Mg

X ;  CH

 

3

MgBr + ClCN

 

® CH

CN + Mg         Br

 

Grignard reagent

Alkyl cyanide

Cl     Methyl magnesium bromide

Cyanogen chloride

Methylcyanide                Cl

 

3
  • From primary amines : Primary amines are dehydrogenated at high temperature to form alkyl cyanides. This is also a commercial method.

RCHNH 2  ¾¾Cu o¾r¾Ni ® RCN + 2H 2 ;  CHCHNH2  ¾¾Cuo¾r¾Ni ®  CHCN  + 2H2

 

Primary amine

500°C

 

H

Ethylamine

500°C

Methyl cyanide

 

|

  • From oximes : R

= NOH ¾¾P O¾® RCN + H  O

 

C                          2   5                                                 2

 

Aldoxime

  • Physical properties

H2O

Alkyl cyanide

 

  • Alkyl cyanides are neutral substance with pleasant odour, similar to bitter
  • Lower members containing upto 15 carbon atoms are liquids, while higher members are
  • They are soluble in The solulbility decreases with the increase in number of carbon atoms in the molecule.
  • They are soluble in organic
  • They are poisonous but less poisonous than HCN
    • Chemical properties
  • Hydrolysis

RCN ¾¾HO ® RCONH 2  ¾¾HO ® RCOOH+ NH 3

 

Alkyl         H +

cyanide

Amide             H +

Acid

 

CHCN ¾¾HO ® CHCONH 2  ¾¾HO ® CHCOOH+ NH 3

 

Methyl           H +

cyanide

Acetamide

H +                     Acetic acid

 

  • Reduction : When reduced with hydrogen in presence of Pt or Ni, or LiAlH4 (Lithium aluminium hydride) or sodium and alcohol, alkyl cyanides yield primary

 

 

 

 

RCN

Alkyl cyanide

¾¾4¾H ® RCHNH 2

Primary amine

 

However, when a solution of alkyl cyanides in ether is reduced with stannous chloride and hydrochloric acid and then steam distilled, an aldehyde is formed (Stephen’s reaction).

RC º N ¾¾SnC¾l2   ¾H¾Cl ® RCH  = NH.HCl ¾¾HO ® RCHO + NHCl

 

[2H]

Imine hydrochloride

Aldehyde

 

  • Reaction with Grignard reagent : With grignard’s reagent, an alkyl cyanide forms a ketone which further reacts to form a tertiary alcohol.

 

 

R C º N + RMgX ® R

R¢                                    R¢

C
|

C  = NMgX ¾¾2H¾2O ® R –  |

= O+ NH3

  • Mg OH

X

 

 

R¢                                  R¢

|                                                |

¢

 

 

H2O

Ketone

R¢

|                                    OH

 

R C = O + R MgX ® R C OMgX ¾¾¾® R C

= OH + Mg          X

 

|

R¢

 

é       +

ê      N H2

|

R¢

Tertiary alcohol

 

ù

ú

 

  • Alcoholysis :

RCN + R¢OH+ HCl ® ê   – || –

¢ú     –  ¾¾HO ® RCOOR¢ + NH  Cl

 

 

Alkyl cyanide

Alcohol

êR    C

ê

ëê

OR ú Cl

ú

úû

Ester                  4

 

imido ester

  • Uses : Alkyl cyanides are important intermediates in the laboratory synthesis of a large number of compounds like acids, amides, esters, amines

(2)  Alkyl Isocyanides

  • Methods of preparation

 

  • From alkyl halides :

R X + AgCN ®

Alkyl halide

RNC  +

Isocyanide (Isonitrile) Main product

RCN     ;

Cyanide (Nitrile) Minor product

CH 3 Cl

Methyl chloride

+ AgCN ®

CH 3 NC

Methyl isocyanide (Main product)

+ CH 3 CN

 

  • From primary amines (Carbylamine reaction) :

 

C

O

RNH 2

Primary amine

+ CHCl3 + 3KOH ®

Chloroform

RNC + 3KCl + 3H 2O

Isocyanide

 

  • From N-alkyl formamides :

RNH – || – H ¾¾POC¾¾l3  ® RN

C+ H 2 O

 

 

  • Physical properties

N -alkyl formamide

Pyridine

Isocyanide

 

  • Alkyl isocyanides are colourless, unpleasant smelling
  • They are insoluble in water but freely soluble in organic
  • Isonitriles are much more poisonous than isomeric
    • Chemical properties

 

  • Hydrolysis :

RN =r C +  2H

Alkyl isocyanide

O ¾¾H¾+  ® RNH

  • HCOOH

Formic acid

 

 

  • Reduction :

Primary amine

2
2

RN =r C + 4 H ¾¾Ni ® RNHCH

 

Alkyl isocyanide

3

secondary amine

 

 

 

  • Action of heat : When heated for sometime at 250°C, a small amount of isonitrile changes into isomeric

RNC ¾¾he¾at ® RCN

  • Addition reaction : Alkyl isocyanide give addition reactions due to presence of unshared electron pair on carbon

+            –

R : N ::: C : or R N º C

The following are some of the addition reactions shown by alkyl iscoyanides.

 

RNC

+    X2      ®

(Halogen)

RNCX2         ;

Alkyl iminocarbonyl halide

RNC + S ®

RNCS  ;

Alkyl isothiocyanate

RNC + HgO ® RNCO+ Hg

Alkyl isocyanate

 

  • Uses : Due to their unpleasant smell, alkyl isocyanides are used in detection of very minute Carbylamine reaction is used as a test for the detection of primary amino group.

Note : ® Methyl isocyanate (MIC)gas was responsible for Bhopal gas tragedy in Dec. 1984.

  • Cyanides have more polar character than isocyanides. Hence cyanides have high b.p., and are more soluble in However, both isomers are more polar than alkylhalides, hence their boiling points are higher than the corresponding alkyl halides.
  • Being less polar, isocyanides are not attacked by OH

Comparison of Alkyl Cyanides and Alkyl Isocyanides

 

Amines are regarded as derivatives of ammonia in which one, two or all three hydrogen atoms are replaced

 

by alkyl or aryl group.

NH3

 

 

 

 

RNH2

(Primary)

R2NH

(Secondary)

R3N

(Tertiary)

 

 

 

Amines are classified as primary, secondary or tertiary depending on the number of alkyl groups attached to nitrogen atom.

 

The characteristic groups in primary, secondary and tertiary amines are: – NH2 ;

(amino)

|

  • NH ;

(imino)

|

  • N

|

(tert -nitrogen)

 

 

 

In addition to above amines, tetra-alkyl derivatives similar to ammonium salts also exist which are called

quaternary ammonium compounds.

|

é       R        ù +

 

NH  I ;       R  NI         ;  (CH  )  NI   or

êR – | – Rú   X

 

4                 4                            3 4

ê      N        ú

 

Quaternary ammonium iodide

Tetramethyl ammonium iodide

ëê     R        úû

Tetra-alkyl

 

ammonium salt

 

  • Simple and mixed amines : Secondary and tertiary amines may be classified as simple or mixed

amines according as all the alkyl or aryl groups attached to the nitrogen atom are same or different. For example,

Simple amines : (CH3 )2 NH ; (CH3 CH 2 )3 N ; (C6 H5 )2 NH

 

Dimethylamine

Triethylamine

Diphenylamine

 

Mixed amines : C2 H5N H ; C6 H5N H ; C3 H7 – N C H3

 

|

CH3

Ethylmethylamine

|

CH3

Methylaniline

|

C2H5

Ethylmethyl-n– propylamine

 

The aliphatic amines have pyramidal shape with one electron pair. In amines, N undergoes sp3 hybridisation.

  • Nomenclature : In common system, amines are named by naming the alkyl groups attached to nitrogen atom followed by suffix-amine.

CH3 NH 2 ; C2 H5 NH 2 ; CH3 CH 2 CH 2 NH 2

 

Methylamine

 

CH3

Ethylamine

n-Propylamine

CH3

CH3

CH 3

 

|

CH  

  • NH

; CH3          NH ; C2 H5

NH ;

CH3           NH ; CH

N ;  CH

N ;    C  H           N

 

3    C H

2   CH                       C H                       C  H

3                           3                           2     5

 

Isopropylamine                            3                        2     5

2   5                CH                     C  H                        C  H

 

Dimethylamine

Diethylamine

Ethylmethylamine                     3                      2     5                          3     7

 

Trimethylamine           Ethyldimethylamine   Ethylmethylpropylamine

In IUPAC system, amino group is considered as substituent and amines are named as amino derivatives of alkanes (Amino alkanes).

 

 

CH3 NH 2  ;

Aminomethane

C2 H5 NH 2 ;

Aminoethane

CH3

|

CH3C H NH2

  • Aminopropane

 

Secondary amines are named as alkyl aminoalkanes and tertiary as dialkyl amino alkanes with highest rank to the amino alkane (primary amine).

 

 

 

 

CH3

 

NH ;

C2 H5

 

NH ;

CH3

CH3          N

 

CH3

N -Methyl amino

CH3

N -Methyl amino

CH3

 

methane

ethane

N, N -Dimethyl amino methane

 

Alternatively, in IUPAC system, primary amines are named by replacing the final-e of the parent alkane by

-amine (Alkanamine). A number is added to indicate the position of – NH2 group.

CH3

|

CH3 NH 2 ; CH3 CH 2 NH 2 ; CH3 – C H NH 2

 

Methanamine

Ethanamine

2-Propanamine

 

When two or more amino groups are present, words di, tri- etc., are used with position numbers.

 

H 2 NCH 2 – CH 2 NH 2 ;

1,2-Ethane-di-amine (1,2-di-amino ethane)

H 2 NCH 2 CH 2 CHCH2 CH3

|

N

1,3-Pentane-di-amine

 

Secondary or tertiary amines are named as N-substituted derivatives of primary amines. The largest group attached to nitrogen is taken as the alkyl group of the primary amine.

 

NH3

|

C2H5

|

CH3

|

 

CH3CH 2 NHCH3 ; CH3CH 2N CH 2CH 2CH3 ; C2 H5 – N C H 2 – C H C H 2  – C H 2  – C H3

 

N -Methylethanamine

N -Ethyl- N -methylpropanamine

1             2           3              4              5

 

N,N -Diethyl-2-methyl-pentanamine

  • Isomerism : Amines are represented by a general formula, CnH2n+3N and exhibit following types of isomerism,
  • Functional isomerism : This is due to the presence of different functional

Molecular formula C3H9N represents three functional isomers.

 

 

CH3 CH 2 CH 2 NH 2 ;

CH3          NH ;

CH 3

CH3           N

 

n-Propylamine (Primary) 1°

C2 H5

Ethylmethylamine

CH3

 

(Secondary)2°

Trimethylamine (Tertiary)3°

 

  • Chain isomerism : This is due to the difference in the carbon skeleton of the alkyl group attached to the amino

 

 

CH3 CH 2 CH 2 CH 2 NH 2    ;

n-Butylamine

CH3

|

CH 3 C HCH2 NH 2 (C4 H11 N)

Isobutylamine

 

  • Position isomerism : This is due to the difference in the position of amino group in the carbon

CH3

|

 

CH3 CH 2 CH 2 NH 2  ;

n-Propylamine

(I-amino propane)

CH3  – C H NH 2 ; (C3 H9 N)

Isopropylamine

(2-amino propane)

 

  • Metamerism : This is due to different alkyl groups attached to the same polyvalent functional

 

CH3          NH C3 H7

CH5 C2 H5

NH ; (C4

H11 N)

 

Methyl propylamine                  Diethylamine

 

 

 

(4)  General methods of preparation

  • Methods yielding mixture of amines (Primary, secondary and tertiary)
  • Hofmann’s method :The mixture of amines (1°, 2° and 3°) is formed by the alkylation of ammonia with alkyl halides.

 

CH3 I

+ NH3  ® CHNH2  ¾¾CH¾3 I  ®(CH3 )2 NH ¾¾CH¾3 I  ®( CH3 )3 N

¾¾CH¾3 I  ®(

CH3 )4 NI

 

Methyliodide

Methylamine (1°)

Dimethylamine (2°)

Trimethylamine (3°)

Tetramethyl ammonium iodide

 

The primary amine may be obtained in a good yield by using a large excess of ammonia. The process is also termed as ammonolysis of alkyl halides. It is a nucleophilic substitution reaction.

  • Ammonolysis of alcohols : CH 3OH + NH 3  ¾¾AlO¾3  ® CHNH 2  ¾¾CH¾3O¾H ®(CH 3 )2 NH ¾¾CH¾3O¾H ®(CH 3 )3 N

350°C

Primary amine may be obtained in a good yield by using a large excess of ammonia.

  • Methods yielding primary amines
  • Reduction of nitro compounds

RNO2  + 6[H] ¾¾S¾n H¾Cl ¾or¾® RNH2  + 2H2O ;  CH 5  – NO2  + 6[H] ® CHNH 2  + 2HO

Zn HCl or Ni or LiAlH4

  • Reduction of nitriles (Mendius reaction)

R C º N + 4[H] ® R CH 2 NH 2 ; CH3 C º N + 4[H] ® CH3 – CH 2 NH 2

 

Methyl cyanide

The start can be made from alcohol or alkyl halide.

Ethylamine

 

ROH ¾¾SOC¾l2  ®

R Cl

¾¾KC¾N ® RCN ¾¾LiA¾lHor ® RCHNH2

 

Alcohol

Alkyl chloride

Alkyl nitrile

Na + C2H5OH

Primary amine

 

This sequence gives an amine containing one more carbon atom than alcohol.

  • By reduction of amides with LiAlH4

RCONH2  ¾¾LiA¾lH¾4  ® RCHNH2 ;  CH 3 CONH 2 ¾¾LiA¾lH¾4  ® CH 3 CH 2 NH 2

Acetamide                                    Ethylamine

  • By reduction of oximes : The start can be made from an aldehyde or

RCHO ¾¾HN¾O¾H ® RCH = NOH ¾¾LiA¾lH¾4  ® RCHNH 2

 

Aldehyde

Oxime

orH2  Ni

Primary amine

 

R       C = O + H

R                            2

Ketone

NOH ® R

R

C = NOH ¾¾LiA¾lH¾4  ® R

R

Oxime

CH NH 2

 

  • Hofmann’s bromamide reaction or degradation (Laboratory method) : By this method the amide (–CONH2) group is converted into primary amino (– NH2) group.

R CO NH 2 + Br2 + 4 KOH ® R NH 2 + 2KBr + K 2 CO3  + 2H 2 O

Amide                                                Pri-amine

This is the most convenient method for preparing primary amines.

This method gives an amine containing one carbon atom less than amide.

  • Gabriel phthalimide synthesis : This method involves the following three
    • Phthalimide is reacted with KOH to form potassium
    • The potassium salt is treated with an alkyl

 

 

 

  • The product N-alkyl phthalimide is put to hydrolysis with hydrochloric acid when primary amine is

 

 

CO                                             CO

NH                                              NK

CO

NC H

 

HOH

C H NH +

COOH

 

CO

Phthalimide

CO

Potassium phthalimide

CO

N-Ethyl phthalimide

2  5    HCl

2  5        2

COOH

Phthalic acid

 

When hydrolysis is difficult, the N-alkyl phthalimide can be treated with hydrazine to give the required amine.

 

CO

NH +

CO

 

NH2

| NH2

Hydrazine

 

heat

 

CO –NH

|       + CO –NH

 

RNH2

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