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The important nitrogen containing organic
compounds are alkyl nitrites ( RONO ), nitro-alkanes
( RNO 2 ), aromatic nitro compounds ( ArNO 2 ), alkyl
cyanides ( RCN ), alkyl iso cyanides ( RNC ), amines (–
NH 2 ), aryl diazonium salts ( ArN 2 Cl ), amides (–
CONH 2 ) and oximes (> C = N OH ).
Alkyl nitrites and nitro alkanes
Nitrous acid exists in two tautomeric forms.
Nitrite form
H O N O ⇌
Nitroform
O
O H N
Corresponding to these two forms, nitrous acid
gives two types of derivatives, i.e., alkyl nitrites and
nitro alkanes.
Alky lnitrite
R O N O ;
Nitroalkane
O
O R N
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.
(1) Alkyl nitrites : The most important alkyl
nitrite is ethyl nitrite.
Ethyl nitrite ( C 2 H 5 ONO )
(i) General methods of preparation : It is
prepared
(a) By adding concentrated HCl or H 2 SO 4 to
aqueous solution of sodium nitrite and ethyl alcohol at
very low temperature (0° C ).
NaNO (^) 2 HCl NaCl HNO 2
C HOH HNO CHONO H 2 O Ethy lnitrite
(b) From Ethyl iodide
C H I KONO CHONO KI
Ethy lnitrite Pot.nitrite^25 Ethyl iodide
2 5
(c) By the action of N 2 O 3 on ethyl alcohol.
2 C 2 H 5 OH N 2 O 3 2 C 2 H 5 ONO H 2 O
(ii) Physical properties
(a) At ordinary temperature it is a gas which can
be liquified on cooling to a colourless liquid, (boiling
point 17°C) having characteristic smell of apples.
(b) It is insoluble in water but soluble in alcohol
and ether.
(iii) Chemical properties
(a) Hydrolysis : It is hydrolysed by aqueous
alkalies or acids into ethyl alcohol.
C (^) 2 H 5 ONO H 2 O C 2 H 5 OH HNO 2
NaOH
(b) Reduction :
C HONO H CHOH NH HO
HCl
Sn 2 5 ^6 2 5 3 2
Small amount of hydroxylamine is also formed.
C (^) 2 H 5 ONO 4 H C 2 H 5 OH NH 2 OH
(iv) Uses
(a) 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).
(b) Its 4% alcoholic solution (known as sweet
spirit of nitre ) is used in medicine as a diuretic.
(c) Since it is easily hydrolysed to form nitrous
acids, it is used as a source of nitrous acid in organic
synthesis.
Nitrogen Containing Compounds
Chapter
Isoamyl nitrite is used as an antispasmodic in
angina pectoris and as a restorative in cardiac failure.
(2) Nitro alkanes or Nitroparaffins : Nitro
alkanes are regarded as nitro derivatives of
hydrocarbons.
(i) Classification : They are classified as
primary, secondary and tertiary depending on the
nature of carbon atom to which nitro groups is linked.
Primarynitro alkane
RCH 2 NO 2 ;
Secondarynitroalkane
CHNO 2
R
R
Tertiary nitroalkane
C NO 2
R
R
R
(ii) General methods of preparation
(a) By heating an alkyl halide with aqueous
alcoholic solution of silver nitrite
C (^) 2 H 5 Br AgNO 2 C 2 H 5 NO 2 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.
(b) By the direct nitration of paraffins (Vapour
phase nitration)
CH CH HONO CHCHNO HO
C 3 2 2 2
400 3 3 ^2 (fuming^ )^
With higher alkanes, a mixture of different nitro
alkanes is formed which can be separated by fractional
distillation****.
(c) By the action of sodium nitrite on - halo
carboxylic acids
2 2
2
2
α
CH ClOOH CHNO COOH NaCl
NaNO
2 Nitro methane
3 2
heat CH NO CO
(d) By the hydrolysis of –nitro alkene with water
or acid or alkali (Recent method)
Nitro methane
3 2 Acetone
3 |
3
or
2 - Methy l,1-nitropropene
2 2
3 |
3
CH
NO HOH CH C O
O H
CH
CH
CH C
H OH
(e) Tertiary nitro alkanes are obtained by the
oxidation of t- alkyl amines with KMnO 4.
R CNH RCNO HO
KMnO 3 2 3 2 2 ^4
(iii) Physical properties
(a) Nitro alkanes are colourless, pleasant smelling
liquids.
(b) These are sparingly soluble in water but
readily soluble in organic solvents.
(c) Their boiling points are much higher than
isomeric alkyl nitrites due to polar nature.
(d) Again due to polar nature, nitro alkanes are
excellent solvents for polar and ionic compounds.
1° and 2° - Nitro alkanes are known to exist as
tautomeric mixture of nitro-form and aci-form.
( -form)
3
nitro
O
O
CH N
( -form)
2
aci
OH
O
CH N
(iv) Chemical properties
(a) Reduction : Nitro alkanes are reduced to
corresponding primary amines with Sn and HCl or Fe
and HCl or catalytic hydrogenation using nickel as
catalyst.
RNO (^) 2 6 H RNH 2 2 H 2 O
However, when reduced with a neutral reducing
agent ( Zinc dust + NH 4 Cl ) , nitro alkanes form
substituted hydroxylamines****.
R NO H R NHOH HO
ZnNHCl 2 2
(b) Hydrolysis : Primary nitro alkanes on
hydrolysis form hydroxylamine and carboxylic acid.
RCH NO HO RCOOH NHOH
HCl HSO 2
or 80 % 2 2 2 ^2 ^ ^4
secondary nitro alkanes on hydrolysis form
ketones.
R CHNO RCO NO HO Ketone
HCl 2 2 2 2 2 2 2
(c) Action of nitrous acid : Nitrous acid reacts
with primary, secondary and tertiary nitro alkanes
differently.
Nitrolicacid
2
Nitrousacid |
Primary
2
2 |
2
NO
O NOH R C NOH
NO
R CH
H O
Red colouredsodiumsalt
2
| NO
R C NONa
NaOH
Bluecolour
Etheror
Pseudo nitrol
2
(^2) |
Secondary
2
(^2) |
2 NaOH
HO
NO
O R C NO
NO
R CH HON
Tertiary nitro alkanes do not react with nitrous
acid.
(d) Thermal decomposition :.
2 2 moderately
300 R. CH 2. CH 2 NO 2 R. CH CH HNO
C
On rapid heating nitro alkanes decompose with
great violence.
2 2 2
heat,Rapidly 3 2 2
3
2
1 CH NO N CO H
(e) Halogenation : Primary and secondary nitro
alkanes are readily halogenated in the -position by
treatment with chlorine or bromine.
Chloropicr inornitrochloroform(insecticide)
3 2 3 2 CH NO^2 CClNO NaOH
Cl
(b) Temperature of nitration : For example,
(c) Nature of the compound to be nitrated :
Presence of electron-releasing group like – OH, – NH 2 , –
CH 3 , – OR, etc., 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 HNO 3 while phenol, aniline and toluene forms
the corresponding ortho - and para-nitro compounds.
On the other hand, nitration of aromatic
compounds having electron withdrawing groups like –
NO 2 , – SO 3 H requires powerful nitrating agent (like
fuming HNO 3 + conc. H 2 SO 4 ) and a high temperature.
(ii) Indirect method : The aromatic nitro
compounds which can not be prepared by direct method
may be prepared from the corresponding amino
compound.
(2) Physical properties
(i) Aromatic nitro compounds are insoluble in
water but soluble in organic solvents.
(ii) 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 almonds.
(3) Chemical properties
(i) Resonance in nitrobenzene imparts a partial
double bond character to the bond between carbon of
benzene nucleus and nitrogen of the – NO 2 group with
the result the – NO 2 group is firmly bonded to the ring
and therefore cannot be replaced other groups, i.e., it is
very inert.
(ii) Displacement of the – NO 2 group : Although –
NO 2 group of nitrobenzene cannot be replaced by other
groups, but if a second – NO 2 group is present on the
benzene ring of nitrobenzene in the o- or p - position, it
can be replaced by a nucleophile. For example,
(iii) Reduction : Aromatic nitro compounds can
be reduced to a variety of product as shown below in
the case of nitrobenzene.
Aniline
6 5 2 Pheny lhy droxy lamine
6 5 Nitrosobenzene
6 5 Nitrobenze ne
C 6 H 5 NO 2 CHNO CH NHOH CHNH
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.
(a) Reduction in acidic medium
Reduction of dinitrobenzene with ammonium
sulphide reduces only one – NO 2 group ( selective
reduction )
NO 2
NO 2
m - Dinitro benzene
Benzen e
HNO 3 + H 2 SO 4 100° C
NO 2
Nitrobenzene
HNO 3 + H 2 SO 4 60° C
NO 2
conc. HNO 3 H 2 SO 4
dil. HNO 3 No reaction
conc. HNOconc. 3 H 2 SO 4
O 2 N^ NO^2
NO 2
OH
2, 4, 6- Trinitrophenol
OH
Phenol
dil. HNO 3
NO 2
OH
o - Nitrophenol
NO 2
OH
p - Nitrophenol
NO 2
NH 2
p - Nitroaniline
NO 2
N 2 BF 4
NaNO 2 Cu , heat
NO 2
NO 2
p - Dinitroanilin e
NaNO 2 HBF 4
N
O^ O
+^ +
+
Resonance hybrid of nitrobenzene
N
O O
N
O
O
N
O
O
N
O
O
Resonating structures of nitrobenzene
NO 2
NO 2
p - Dinitrobenzene
NO 2
Nu
(Where, Nu = OH , NH 2 or OC 2 H 5 )
- aq. KOH , NH 3 or C 2 H 5 OK
NO 2
Nitrobenzen e
NH 2
Aniline
+ 2 H 2 O
(b) Reduction in neutral medium :
Phenylhydroxylamine
6 5
(intermedi ate)
Nitrosobenzene
6 5 Nitrobenze ne ( )
6 5 2 2
C H NO 2 H^4 CH NO CH NHOH
HO
Zn dust NHCl
(c) Reduction in alkaline medium :
Azoxybenzene
6 5
||
6 5
Pheny lhy droxy lamine
6 5
Nitroso benzene
2 []^65
Nitrobenze ne
(^6 ) C CH N
CH N O HNHOH
CH NO C HNO HO
H
Azoxybenzene on further reduction yields
azobenzene and hydrazobenzene.
Hy drazobenzene
6 5
|
6 5
2 []
Azobenzene
6 5
||
6 5
2 []
Azoxy benzene
6 5
||
6 5 – – –
CH NH
CH NH
CH N
CH N
CH N
C H N O
H H
(d) Electrolytic reduction :
Weakly acidic medium of electrolytic reduction
gives aniline.
Strongly acidic medium gives
phenylhydroxylamine which rearranges to p -
aminophenol.
Alkaline medium of electrolytic reduction gives
all the mono- and di-nuclear reduction products
mentioned above in point (c).
(iv) Electrophilic substitution : Since – NO 2
group is deactivating and m-directing , electrophilic
substitution (halogenation, nitration and sulphonation)
in simple aromatic nitro compounds ( e.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)
(b)
(c)
Although nitrobenzene, itself undergoes
electrophilic substitution under drastic conditions,
nitrobenzene having activating groups like alkyl, – OR ,
- NH 2 etc. undergoes these reactions relatively more
readily.
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.
(v) Nucleophilic Substitution : Benzene is inert
to nucleophiles, but the presence of – NO 2 group in the
benzene ring activates the latter in o - and p - positions
to nucleophiles.
(vi) Effect of the – NO 2 group on other nuclear
substituents
(a) Effect on nuclear halogen : The nuclear
halogen is ordinarily inert, but if it carries one or more
electron-withdrawing groups (like – NO 2 ) in o - or p -
position, the halogen atom becomes active for
nucleophilic substitutions and hence can be easily
replaced by nucleophiles KOH , NH 3 , NaOC 2 H 5 .
NO 2
NO 2
m - Dinitro benzene
( NH 4 ) 2 S or Na 2 S NH 2
NO 2
m - Nitroaniline
NO 2
Nitrobenzene
electrolyti reduction in c presence of conc. H 2 SO 4
NHOH
Phenylhydroxylami ne
rearrangeme nt
OH
NH 2
p - Aminophenol
NO 2
Nitrobenze ne
AlCl 3
Cl
NO 2
m - Chloronitrobenzene
NO 2
Nitrobenze ne
NO 2
NO 2
m - Dinitrobenzene
conc. HNO 3 conc. H 2 SO 4
NO 2
Nitrobenze ne
+ H 2 SO 4 100° C
(fuming) (^) SO 3 H
NO 2
m - Nitrobenzene sulphonic acid
o - Nitrotoluene
NO 2
CH 3
HNO 3 H 2 SO 4
NO 2
NO 2
CH 3
2, 4- Dinitrotoluene
O 2 N^ NO^2
NO 2
CH 3
2, 4, 6- Trinitrotoluene (TNT)
HNO 3 H 2 SO 4
Na 2 Cr 2 O 7 H 2 SO 4
O 2 N^ NO^2
NO 2
1, 3, 5- TrinitroBenzene (TNB)
O 2 N^ NO^2
NO 2
CH 3
(TNT )
O 2 N^ NO^2
NO 2
COOH
2, 4, 6-Trinitro benzoic acid
Sodalime (– CO 2 )
KOH fuse
NO 2
Nitro benzene
OH
NO 2
o - Nitrophenol OH
NO 2
p - Nitrophenol
500 2 Primary amine
RCH 2 NH 2 C RCN 2 H
Cu orNi
2 Methylcyanide
3 Ethylamine 500
CH 3 CH 2 NH 2 CH CN 2 H
C
Cu orNi
(e) From oximes :
NOH R CN HO
H
R C HO
PO 2 Aldoxime Alky lcy anide
|
2
^2 5
(ii) Physical properties
(a) Alkyl cyanides are neutral substance with
pleasant odour, similar to bitter almonds.
(b) Lower members containing upto 15 carbon
atoms are liquids, while higher members are solids.
(c) They are soluble in water. The solubility
decreases with the increase in number of carbon atoms
in the molecule.
(d) They are soluble in organic solvents.
(e) They are poisonous but less poisonous than
HCN
(iii) Chemical properties
(a) Hydrolysis
Acid^3 Amide
2 cy anide
Alky l
RCN^2 RCONH^2 RCOOH NH
H
HO
H
H O
Acetamide
3 2
cy anide
Methy l
3 CH CN^2 CHCONH H
HO
3 Aceticacid
3
2 CHCOOH NH
H
H O
(b) Reduction : When reduced with hydrogen in
presence of Pt or Ni , or LiAlH 4 (Lithium aluminium
hydride) or sodium and alcohol, alkyl cyanides yield
primary amines.
Primaryamine
2 2
4 Alkyl cyanide
RCN RCH NH
H
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 ).
R C N RCH NHHCl RCHO NH Cl
HO H
SnCl HCl [ 2 ] Iminehy drochloride Aldehy de^4
2 . ^2
(c) Reaction with Grignard reagent : With
grignard's reagent, an alkyl cyanide forms a ketone
which further reacts to form a tertiary alcohol.
NMgX
R
R C N RMgX R C
| '
X
OH
O NH Mg
R
R C
H O
Ketone
| (^2 )
OMgX
R
R
O RMgX R C
R
R C
|
|
|
X
OH
OH Mg
R
R
R C
H O
Tertiary alcohol
|
|
2
(d) Alcohololysis :
Cl
NH
RCN ROH HCl R C OR
imido ester
2 ||
Alcohol cy anide
Alky l
RCOOR NH Cl
HO Ester^4
^2 ^
(iv) Uses : Alkyl cyanides are important
intermediates in the organic synthesis of a large
number of compounds like acids, amides, esters,
amines etc.
(2) Alkyl Isocyanides
(i) Methods of preparation
(a) From alkyl halides :
Minor product
(Nitrile)
Cy anide
Main product
(Isonitrile)
Alky lhalide Isocy anide
R X AgCN RNC RCN
CH Cl AgCN CHNC CH 3 CN
(Main product)
Methy l isocy anide
3 Methy l chloride
(b) From primary amines ( Carbylamine reaction ) :
RNH CHCl KOH RNC KCl H 2 O Chloroform Isocy anide
3 Primary amine
2 ^3 ^3 ^3
(c) From N-alkyl formamides :
H R N C HO
O
R NH C
POCl N
2 alky lformamide Py ridine Isocy anide
| | ^3
(ii) Physical properties
(a) Alkyl isocyanides are colourless, unpleasant
smelling liquids.
(b) They are insoluble in water but freely soluble
in organic solvents.
(c) Isonitriles are much more poisonous than
isomeric cyanides.
(iii) Chemical properties
(a) Hydrolysis :
Alky lisocy anide (^2) Primary amine^2 Formicacid
RN C 2 HO RNH HCOOH
H
(b) Reduction : secondary amine
3 Alkylisocyanide (^300)
R N C 4 H RNHCH
C
Ni o
(c) Action of heat : When heated for sometime at
250° C , a small amount of isonitrile changes into
isomeric nitrile.
RNC RCN
heat
(d) Addition reaction : Alkyl isocyanide give
addition reactions due to presence of unshared electron
pair on carbon atom.
R : N ::: C : or
R N C
The following are some of the addition reactions
shown by alkyl isocyanides.
halide
Alky liminocarbo ny l
2 (Halogen)
RNC X 2 RNCX
isothiocy anate
Alky l
RNC S RNCS ; RNC HgO RNCO Hg
isocy anate
Alky l
(iv) Uses : Due to their unpleasant smell, alkyl
isocyanides are used in detection of very minute
leakage. Carbylamine reaction is used as a test for the
detection of primary amino group.
Methyl isocyanate (MIC)gas was responsible
for Bhopal gas tragedy in Dec. 1984.
Cyanides have more polar character than
isocyanides. Hence cyanides have high boiling points and
are more soluble in water. 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–^ ions.
Table : 29.2 Comparison of Alkyl Cyanides and Alkyl
Isocyanides
Test Ethyl cyanide Ethyl isocyanide
Smell Strong but pleasant Extremely unpleasant
Dipole
moment
More ( 4D) Less ( 3D)
B.P. 98° C (i.e. High) 78° C (i.e. low)
Solubilit
y in
water.
Soluble Insoluble
Hydrolys
is with
acids
Gives propionic acid
(Acid, in general)
Give ethyl amine (1°
amine, in general)
Hydrolys
is with
alkalies
Same as above No action
Reductio
n
Gives propylamine (1°
amine, in general)
Gives ethylmethyl
amine (2° amine, in
general)
Stephen'
s
reaction
Gives
propionaldehyde
(Aldehyde, in general)
Does not occur
Heating
(250° C )
No effect Ethyl cyanide is
formed
Amines
Amines are regarded as derivatives of ammonia
in which one, two or all three hydrogen atoms are
replaced by alkyl or aryl group.
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: (amino)
– NH 2 ;
|
(imino)
– NH ;
( nitrogen)
|
|
tert
N
In addition to above amines, tetra-alkyl
derivatives similar to ammonium salts also exist which
are called quaternary ammonium compounds****.
NH 4 I^ ;
ammoniumiodide
Quaternary
R 4 NI ;
ammonium iodide
Tetramethyl
( CH 3 ) 4 NI or
R X
R
R
R N
am m onium salt
Tetra-alky l
|
|
(1) 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 : Dimethy lamine
( CH 3 ) 2 NH ;
Triethy lamine
( CH 3 CH 2 ) 3 N
Mixed amines :
Ethy lmethylamine
3
|
2 5
CH
C H NH ;
Methy laniline
3
|
6 5
CH
C H NH
The aliphatic amines have pyramidal shape with
one electron pair. In amines, N undergoes sp
3
hybridisation.
(2) General methods of preparation
(i) Methods yielding mixture of amines
(Primary, secondary and tertiary)
(a) Hofmann's method :The mixture of amines (1°,
2° and 3°) is formed by the alkylation of ammonia
with alkyl halides****.
(2 )
Dimethy lamine
32
(1 )
Methy lamine
3 3 2 Methy liodide
3 ( )
3
CH I NH CHNH CH NH
CHI
ammonium iodide
Tetramethyl
34
(3 )
Trimethylamine
CH^ 3 I CH N CH ^3 I CH NI
R 2 NH
(Secondary )
RNH 2
(Primary)
R 3 N
(Tertiar y)
NH 3
(g) By decarboxylation of - amino acids
2 2 heat
( )
2
|
OOH^2 RCH NH
NH
R CHC
Ba OH
Methy l amine
3 2 heat
( )
(Gly cine)
2
2 |
COOH^2 CH NH
NH
CH
Ba OH
(h) By means of a Grignard reagent and chloramine
:
RMgX ClNH 2 RNH 2 MgXCl
(i) By hydrolysis of Isocyanides or Isocyanates
HO R NH HCOOH
OH
OH
C
H
H
R N
HCl Alky lamine
2
( ) 2
Alky lisocy anide
2
CH NC HOH CH NH HCOOH
H
3 2 methy lisonitile
3 2
3 2 2 3
Methy lisocy anate
3 O^2 KOH CH NH KCO
OH
OH
C
H
H
CH N
2 2 3 Alky l isocy anate
R NCO 2 KOH R NH KCO
(j) By Schmidt reaction :
2 2
amine
Alkyl
2
.
acid
Hydrazoic Acid^3
R COOH NH Conc ^ H ^2 SO ^4 R NH N CO
In this reaction the acyl azide ( R – CON 3 ) and
alkyl isocyanate ( R – NCO ) are formed as an
intermediate.
R COOH NH RCON H 2 O Acy lazide
Alky lisocy anate^2 Acylazide
RCON 3 R N C O N
2 Alky lamine
R N C O H 2 O R NH 2 CO
The overall reaction which proceeds by the
elimination of nitrogen from acyl azide followed by
acidic or alkaline hydrolysis to yield primary amine
containing one carbonless, is called Curtius
Degradation.
The method uses acid chloride to prepare primary
amine through acyl azide.
Acy lazide
3
||
Acy lchloride
|| || (^2 3) N
O
Cl R C
O
OH R C
O
R C
SOCl NaN
2 2 3
2 3
|| N^2 R N C O R NH NaCO
O
R C heat
N NaOH
The mechanism of curtius rearrangement is very
similar to Hofmann degradation.
Schmidt reaction converts R – COOH to R – NH 2 ,
which is a modification of curtius degradation. In this
reaction a carboxylic acid is warmed with sodium azide
( Na + N 3 – ) and conc. H 2 SO 4. The carboxylic acid is
directly converted to the primary amine without the
necessity of isolating alkyl azide.
2 2 2
( .)
|| OH^3 24 RNH N CO
O
R C heat
NaN HSO conc
( NaN (^) 3 H 2 SO 4 N 3 H NaHSO 4 )
(k) By Ritter reaction : It is a good method for
preparing primary amines having -tertiary alkyl
group.
(1 amine)
Tert buty lamine
2 4 33 2 Tert- buty lalcohol
( 3 ) 3 ( )
CH C OH H SO HCN CH C NH
R C OH HO RC RCN CH
H HCN
3 Tert- carboniumion
3 2 3
CHO RCNH RC NH HCOO
HO OH
Pri- amine
3 3 2
2
(l) Reductive amination of aldehydes and ketones :
H NH H R CH NH HO
O
R C
atm
Ni C 2 Primaryamine
2 2 300
, 150 3 2 Aldehy de
||
[ ] Imine
| ( ) 2
| (^2) NH
H
O HHN R C
H
R C
HO
2 2
2 RCH NH
Ni
H
2
3 |
300
, 150 3 2 Ketone
3
|| NH
CH
CH NH H R CH
O
R C atm
Ni C
This reaction probably takes place through the
formation of an imine (Schiff's base).
The primary amine can also be converted into sec.
or tert. amines by the following steps
Sec.amine
2 2 R CHO R NH H^^2 Ni RCHNHR
RNH 2 2 H 2 C O 2 HCOOH
2 2 Tert.- amine
RN ( CH 3 ) 2 2 HO 2 CO
(m) By reduction of azide with NaBH 4
1 amine
2
azide
Alky l
3
azide
Sodium
3 (1 or2)
Alky lhalide (^2)
4
^
R X NaN RN RNH HO
NaBH
(n) By Leuckart reaction : Aldehydes or ketones
react with ammonium formate or with formamide to
give formyl derivative of primary amine.
C || O
R N = N = N
C || O
R N – N N
|| O
R N
Intramolecu alkyl lar shift
R – N = C = O
H
O
C O 2 HCOONH CHNH – C –
||
Amm.formate
4
2 H 2 O CO 2 NH 3
2 3
||
Formamide
2 2 H CO NH
O
C O HCONH CHNH C
These formyl derivatives are readily hydrolysed
by acid to yield primary amine.
H H HOH
O
CHNH C
R
R ||
CHNH 2 H 2 O CO 2 R
R
This is called Leuckart reaction, i.e.,
C C O HCOONH R
R (^) 180200
Amm.formate
4
Ketone
2 2
Primaryamine
CHNH 2 HO CO
R
R
On commercial scale, ethylamine is obtained by
heating a mixture of ethylene and ammonia at 450°C
under 20 atmospheric pressure in presence of cobalt
catalyst.
3 2 2 450 , 20
Cobaltcatalyst 3 Ethylene
CH 2 CH 2 NH CHCHNH
C atm
(iii) Methods yielding secondary amines
(a) Reaction of primary amines with alkyl halides
dialky l ammoniumsalt
2 2 2 2
R NH R X RNH HX R NH X
R NH X NaOH RNH HO NaX
2 Secondaryamine
2 2 2
(b) Reduction of isonitriles :
Sec. amine Alky l R^ isonitrile NC^4 [ H ] RNHCH^3
Pt
Secondary amine formed by this method always
possesses one – CH 3 group linked directly to nitrogen.
(c) Reaction of p-nitroso-dialkyl aniline with
strong alkali solution :
This is one of the best method for preparing pure
secondary amines.
(d) Hydrolysis of dialkyl cyanamide
cy anamide
Dialky l
2
2
cy anamide
Sodium
2
2
cy anamide
Calcium
CaN CN NaN CN RN CN
NaOH RX
2 3 Dialky lamine
R (^) 2 N CN 2 HOH R 2 NH CO NH OH
H or
(e) Reduction of N-substituted amides : Reduction
of N- substituted amides with LiAlH 4 yields secondary
amines.
Alkyl -amino ketones are formed by the action of
ketone with formaldehyde and NH 3 (or primary or
secondary amines).
The product is referred to as Mannich base and
the reaction is called Mannich Reaction.
CH COCH HCHO RNH CHCOCH CHNHR
heat 3 3 2 3 2 2
Which can be reduced to alkyl amines.
R CONHR H RCH NHR HO
LiAlH N - Alky lacidamide Sec.amine^22
4 [ ] ^4
(iv) Methods yielding tertiary amines
(a) Reaction of alkylhalides with ammonia
Trialky lammoniumsalt
(^3 3 )
RX NH RN HX RNHX
R (^) 3 NHX NaOH R 3 N NaX H 2 O
(b) Reduction of N, N-disubstituted amides : The
carbonyl group is converted into – CH 2 group.
RCON R RCH NR HO
H
LiAlH
N N
2 ter. amine
2 2 4 [] amide
, - disubstituted
2
^4
(c) Decomposition of tetra-ammonium hydroxides :
The tetra-alkyl ammonium hydroxides are formed when
corresponding halides are treated with moist silver
oxide.
R NI AgOH R NOH AgI
4 4
The hydroxides thus formed on heating decompose into
tertiary amines. Tetramethyl ammonium hydroxide
gives methyl alcohol as one of the products while all
other tetra-alkyl ammonium hydroxides give an olefin
and water besides tertiary amines.
( CH (^) 3 ) 4 NOH ( CH 3 ) 3 N CH 3 OH
( R ) (^) 4 NOH ( R ) 3 N olefin H 2 O
(3) Separation of mixture of amines : When the
mixture consists of salts of primary, secondary and
tertiary amines along with quaternary salt, it is first
distilled with KOH solution. The mixture of three
amines distils over leaving behind non-volatile
quaternary salt.
NH 2
Anilin e
RX heat
NR 2
Dialkyl aniline
HNO 2
NR 2
p - Nitroso-dialkyl aniline
ON
OH H
NaOH (^) OH + R 2 NH
Sec. p - Nitroso amine phenol
ON
(iii) Amines are soluble in water. This is due to
hydrogen bonding between amine and water molecules.
Amines are also soluble in benzene and ether.
Hy drogenbonding betweenamineandwatermolecules
|
| |
|
| |
- : – : – : – :
H
R
H N
H
H O
H
R
H N
H
H O
Solubility decreases with increase of molecular
mass.
(5) Chemical properties : The main reactions of
amines are due to the presence of a lone pair of
electrons on nitrogen atom. Amines are electrophilic
reagents as the lone pair of electrons can be donated to
electron seeking reagents, ( i.e. , electrophiles).
Except the amines containing tertiary butyl
group, all lower aliphatic amines are stronger bases than
ammonia because of + I (inductive) effect. The alkyl
groups, which are electron releasing groups, increase
the electron density around the nitrogen thereby
increasing the availability of the lone pair of electrons
to proton or Lewis acids and making the amine more
basic (larger Kb ). Thus, it is expected that the basic
nature of amines should be in the order tertiary >
secondary > primary, but the observed order in the case
of lower members is found to be as secondary >
primary > tertiary****. This anomalous behaviour of
tertiary amines is due to steric factors , i.e. , crowding
of alkyl groups cover nitrogen atom from all sides and
thus makes the approach and bonding by a proton
relatively difficult which results the maximum steric
strain in tertiary amines. The electrons are there but
the path is blocked, resulting the reduced in its
basicity.
(i) The order of basic nature of various amines
has been found to vary with nature of alkyl groups****.
Alkyl group Relative strength
CH 3 – R 2 NH > RNH 2 > R 3 N > NH 3
C 2 H 5 – R 2 NH > RNH 2 > NH 3 > R 3 N
( CH 3 ) 2 CH – RNH 2 > NH 3 > R 2 NH >
R 3 N
( CH 3 ) 3 C – NH 3 > RNH 2 > R 2 NH > R 3 N
(ii) Basic nature of aromatic amines : In aniline
or other aromatic amines, the lone pair present on
nitrogen atom is delocalized with benzene ring by
resonance.
But anilinium ion is less resonance stabilized than
aniline.
Thus, electron density is less on N atom due to
which aniline or other aromatic amines are less basic
than aliphatic amines.
However, any group which when present on
benzene ring has electron withdrawing effect (– NO 2 , –
CN, – SO 3 H, – COOH – Cl, C 6 H 5 , etc.) decreases basicity
of aniline (Nitroaniline is less basic than aniline as
nitro group is electron withdrawing group (– I group)
and aniline is more basic than diphenyl amine), while a
group which has electron repelling effect (– NH 2 , – OR, R
- , etc.) increases basicity of aniline. Toluidine is more
basic than aniline as – CH 3 group is electron repelling
group (+ I group).
Further greater the value of Kb or lower the value
of pKb, stronger will be the base. The basic character of
some amines have the following order,
R 2 (^) NH RNH 2 C 6 H 5 CH 2 NH 2 NH 3 C 6 H 5 NH 2
N-alkylated anilines are stronger bases than
aniline because of steric effect. Ethyl group being bigger
than methyl has more steric effect, so N - ethyl aniline is
stronger base than N- methyl aniline. Thus, basic
character is,
C 6 (^) H 5 N ( C 2 H 5 ) 2 C 6 H 5 NHC 2 H 5 C 6 H 5 N ( CH 3 ) 2
C 6 H 5 NHCH 3 C 6 H 5 NH 2 NH 3 C 6 H 5 NHC 2 H 5
C 6 H 5 NHCH 3 C 6 H 5 NH 2 C 6 H 5 NHC 6 H 5
In Toluidines – p- isomer > m - > o-
Chloroanilines– p- isomer> m- > o-
Phenylene diamines – p - isomer > m- > o-
Nitroanilines– m - isomer > p - > o-
Aniline is less basic than ammonia. The phenyl
group exerts – I (inductive) effect, i.e., it withdraws
electrons. This results to the lower availability of
electrons on nitrogen for protonation.
Ethylamine and acetamide both contain an
amino group but acetamide does not show basic nature.
This is because lone pair of electrons on nitrogen is
delocalised by resonance with the carbonyl group which
makes it less available for protonation.
2
|
2 3
||
3 –^ NH
O
NH CH C
O
CH C
Not available due to delocalization
–
: NH 2 + NH 2
–
–
NH 3
NH 3
No other resonating structure possible
N – H | H
Resonance hybrid
+ N – H | H
–
The compounds with least 's' character (sp^3 -
hybridized) is most basic and with more ‘s’ character
(sp-hybridized) is least basic. Examples in decreasing
order of basicity are,
CH NH CH N CHCH CH C N
sp sp sp
( )
3 3 ( )
3 ( )
3 2 –^2 –
3
CH 3 CH 2 CH 2 NH 2 H 2 C CHCH 2 NH 2 HC CCH 2 NH 2
( CH 3 ) 2 NH CH 3 NH 2 NH 3 C 6 H 5 NH 2
Electron withdrawing (C 6 H 5 – ) groups decrease
electron density on nitrogen atom and thereby
decreasing basicity.
( CH 3 ) 2 NH CH 3 NH 2 C 6 H 5 NHCH 3 C 6 H 5 NH 2
CH 3 CH 2 NH 2 HO ( CH 2 ) 3 NH 2 HO ( CH 2 ) 2 NH 2
Electron withdrawing inductive effect of the – OH
group decreases the electron density on nitrogen. This
effect diminishes with distance from the amino group.
CH 3 CH 2 NH 2 C 6 H 5 CONH 2 CH 3 CONH 2
(iii) Salt formation : Amines being basic in
nature, combine with mineral acids to form salts.
chloride
Alky lammonium
R NH 2 HCl RNH 3 Cl
Alky lammoniumsulphate
2 R – NH 2 H 2 SO 4 ( RNH 3 ) 2 SO 4
(iv) Nature of aqueous solution : Solutions of
amines are alkaline in nature.
RNH 2 HOH ⇌
R NH 3 OH
⇌
[ RNH (^) 3 ] OH
R 2 NH HOH^ ⇌
R 2 (^) NH 2 OH
⇌[ R (^) 2 NH 2 ] OH –
R 3 N HOH ⇌
R 3 (^) NHOH
⇌
[ R 3 (^) NH ] OH
The aqueous solutions of amines behaves like
NH 4 OH and give ferric hydroxide precipitate with ferric
chloride and blue solution with copper sulphate.
3 RNH (^) 3 OH FeCl 3 Fe ( OH ) 3 3 RNH 3 Cl
(v) Reaction with alkyl halides (Alkylation)
Quaternarysalt
3 Tert. amine
2 Pri.amine Sec.amine
RNH (^) 2 RNHR R – NR ( R – NR ) X
RX HX
RX HX
R X
(vi) Reaction with acetyl chloride (Acylation)
3
3 Pri.a mine
2 N
HCl RNH ClOCCH RNHOCCH
, - Dialkylacetamide
2 3
3 Sec.amine
2 NN
HCl R NH ClOCCH RNOCCH
Tertiary amines do not react since they do not
have replaceable hydrogen on nitrogen.
Therefore, all these above reactions are used to
distinguish between
o o 1 , 2 and
o 3 - amines.
(vii) Action of sodium
2 Sod.salt
1 amine
2 RNH 2 2 Na 2 [ RNH ] Na H o
2 Sod.salt
2 2 amine
2 R 2 NH 2 Na 2 [ RN ] Na H o
(viii) Action of halogens
amine
Dihalo- alkyl
2 Alkylamine
2
RNH^2 RNHX^2 RNX
NaOH
X NaOH
X
amine
Halo-dialkyl
2 Dia lkylamine
2
R NH^2 RNX
NaOH
X
(ix) Reaction with Grignard reagent
CH RNH Mg I I
CH RNH Mg 4
3 2
R (^) 2 NH CH 3 – Mg – I CH 4 R 2 N – Mg – I
(x) Carbylamine reaction : This reaction is shown
by only primary amines****. This is a test of primary
amines and is used to distinguish primary amines from
secondary and tertiary amines.
RNH CHCl KOH RNC KCl H 2 O
(carby lamine)
(Alc.) Alky lisocy anide
2 3 ^3 ^3 ^3
Isocyanides are bad smelling compounds and can
be easily detected.
(xi) Reaction with nitrous acid
(a) Primary amines form alcohols with nitrous
acid ( NaNO 2 + HCl ). Nitrogen is eliminated.
RNH HONO ROH N 2 H 2 O Pri.amine 2 Alcohol
Methyl amine is an exception to this reaction, i.e.,
CH NH HONO CH O N O N 2 H 2 O (^3 23) Methy lnitrite 2 – – 2
CH NH HONO CH O CH N 2 H 2 O
Dimethyl ether
(b) Secondary amines form nitrosoamines which
are water insoluble yellow oily liquids.
R NH HONO RNNO H 2 O
nitrosoamine
Dialky l
2 Sec.amine
2
Nitrosoamine on warming with phenol and conc.
H 2 SO 4 give a brown or red colour which soon changes
to blue green. The colour changes to red on dilution and
further changes to blue or violet with alkali. This
colour change is referred to Liebermann's nitroso
reaction and is used for the test of secondary amines.
The hydrolysis of nitroacetanilides removes the
protecting acyl group and gives back amines.
(c) Sulphonation
The sulphanilic acid exists as a dipolar ion
(structure II) which has acidic and basic groups in the
same molecule. Such ions are called Zwitter ions or
inner salts****.
(6) Uses
(i) Ethylamine is used in solvent extraction
processes in petroleum refining and as a stabiliser for
rubber latex****.
(ii) The quaternary ammonium salts derived from
long chain aliphatic tertiary amines are widely used as
detergents.
(iii) Aliphatic amines of low molecular mass are
used as solvents.
Table : 29.3 Distinction between primary, secondary and tertiary amines
Test Primary amine Secondary amine Tertiary amine
Action of CHCl 3 and
alcoholic KOH.
(Carbylamine test)
Bad smelling carbylamine
(Isocyanide) is formed.
No action. No action.
Action of CS 2 and HgCl 2.
(Mustard oil test)
Alkyl isothiocyanate is
formed which has pungent
smell like mustard oil.
No action. No action
Action of nitrous acid. Alcohol is formed with
evolution of nitrogen.
Forms nitrosoamine which
gives green colour with
phenol and conc. H 2 SO 4
(Liebermann's test).
Forms nitrite in cold
which on heating gives
nitrosoa- mine which
responds to
Liebermann's test.
Action of acetyl chloride. Acetyl derivative is formed. Acetyl derivative is formed. No action.
Action of Hinsberg's
reagent.
Monoalkyl sulphonamide is
formed which is soluble in
KOH.
Dialkyl sulphonamide is
formed which is insoluble
in KOH.
No action.
Action of methyl iodide. 3 molecules (moles) of CH 3 I
to form quaternary salt
with one mole of primary
amine.
2 moles of CH 3 I to form
quaternary salt with one
mole of secondary amine.
One mole of CH 3 I to
form quaternary salt
with one mole of
tertiary amine.
Aniline does not form alcohol with nitrous acid
but it forms benzene diazonium chloride which shows
dye test.
Aniline
Aniline was first prepared by Unverdorben
(1826) by dry distillation of indigo****. In the laboratory,
NH 2
Aniline
O ||
- Cl – C – CH 3 Acetyl chloride
NHCOCH 3
Acetanilide
HNO 3 , H 2 SO 4 288 K
NHCOCH 3
NO 2
o - Nitroacetanilide
CHH 23 OCOOH , H +
NHCOCH 3
NO 2
p - Nitroacetanilid e
NH 2
NO 2
o - Nitroaniline (minor)
NH 2
NO 2
p - Nitroaniline (major)
NH 2
Aniline
NH 3 +^ HSO 4 –
Anilinium hydrogen sulphate
Heat 453 - 473 K
NH 2
SO 3 H
Sulphanilic acid (I)
NH 3 +
SO 3 –
Zwitter ion structure (II)
it can be prepared by the reduction of nitrobenzene
with tin and hydrochloric acid.
C HNO H CH NH H O
SnHCl 2 Aniline
6 5 2
,
Nitrobenze ne
6 5 2 ^6 ^ ^2
Aniline produced combines with
H (^) 2 SnCl 6 ( SnCl 4 2 HCl ) to form a double salt.
6 Double salt
2 C 6 (^) H 5 NH 2 SnCl 4 2 HCl ( C 6 H 5 NH 3 ) 2 SnCl
From double salt, aniline is obtained by treating
with conc. caustic soda solution.
( C 6 (^) H 5 NH 3 ) 2 SnCl 6 8 NaOH 2 C 6 H 5 NH 2
6 NaCl Na 2 SnO 3 5 H 2 O
On a commercial scale , aniline is obtained by
reducing nitrobenzene with iron filings and
hydrochloric acid.
Aniline is also obtained on a large scale by the
action of amine on chlorobenzene at 200° C under 300-
400 atm pressure in presence of cuprous catalyst.
C HCl NH CuO CHNH CuCl HO atm
C 300400 6 5 2 2 2 2
200 2 6 5 2 3 2 2
Properties Aniline when freshly prepared is a
colourless oily liquid (b.p. 184°C)****. It has a
characteristic unpleasant odour and is not poisonous in
nature. It is heavier than water and is only slightly
soluble. It is soluble in alcohol, ether and benzene. Its
colour changes to dark brown on standing.
It shows all the characteristic reactions discussed
earlier.
Uses : (1) It is used in the preparation of
diazonium compounds which are used in dye industry.
(2) Anils (Schiff's bases from aniline) are used as
antioxidants in rubber industry.
(3) It is used for the manufacture of its some
derivatives such as acetamide, sulphanilic acid and
sulpha drugs, etc.
(4) It is used as an accelerator in vulcanizing
rubber.
Some important conversions
(1) Conversion of methylamine to ethylamine
(Ascent)
Methy l iodide
3 Methy lalcohol
3 Methy lamine
3 2
CH NH HNO ^^2 CHOH PI ^3 CHI
Ethy lamine
3 2 2 Methy lcy anide
3 NaCN ^ CH CN LiAlH ^4 CHCHNH
(2) Conversion of ethylamine to methylamine
(Descent)
Acetaldehyde
3
[]
Ethanol
3 2 Ethy lamine
3 2 2 2 2 7 2 4
CH CHNH^2 CHCHOH CHCHO
KCrO HSO
HNO O
Acety lchloride
3 Aceticacid
3
[ ] 2 CH COOH CHCOCl
O SOCl
Methylamine
3 2 Acetamide
3 2
3 CH CONH 2 CH NH
KOH
NH Br
(3) Conversion of ethylamine to acetone
2 4
2 2 27
Ethy l alcohol
2 5 Ethy lamine
2 5 2 HSO
HNO KCrO C HNH CHOH
Calcium acetate
3 2
( )
Aceticacid
3 Acetaldehyde
2 2 4
CH CHO^2 27 CHCOOH CaOH CHCOO Ca HSO
K CrO
Acetone
CH 3 COCH 3
heat
(4) Conversion of propionic acid to
(i) Ethylamine, (ii) n - Butylamine.
(i) ^2 3 Propiony l chloride
3 2 Propionicaicd
3 2
SOCl NH CHCHCOOH CHCHCOCl
Ethy lamine
3 2 2 Propionamide
3 2 2
CH CHCONH^2 CHCHNH
KOH
Br
or 2 5 2 ( .)
2 5 2 4
C HCOOH^3 CH NH
HSO conc
N H
(ii) ^4 5
3 2 2 Propionicacid
3 2
PBr Ether n
LiAlH CHCHCOOH CHCHCHOH
Propylcyanide
3 2 2 Propyl bromide
CH (^) 3 CH 2 CH 2 Br CHCHCHCN
KCN
2 5 rLiAlH n
Na CHOH CHCHCH CHNH
(5) Conversion of ethylene to 1,4-
diaminobutane
NaCN CCl
Br CH CH CHBrCHBr Ethy lene bromide
2 2 Ethy lene
4
2
1,4 - Diaminobutane
2 2 2 2 2 2 Ethy lene cy anide
2 2 NCCH CHCN LiAlH ^4 NHCHCHCHCHNH
Diazonium salts
The diazonium salts have the general formula
ArN (^) 2 X
, where X–^ may be an anion like Cl–, Br–^ etc. and
the group 2 ( )
N N N is called diazonium ion group.
(1) Nomenclature : The diazonium salts are
named by adding the word diazonium to the name of
the parent aromatic compound to which they are
related followed by the name of the anion. For example,
NO 2
Fe 3 / HCl 30%
NH 3 + Cl –
Na 2 CO 3
NH 2
N +^ NCl –
Benzenediazonium chloride
N +^ NCl –
p - Toluenediazonium chloride
CH 3
N +^ NCl –
o - chlorobenzenediazonium chloride
Cl
N +^ NBr –
m - Hydroxybenzenediazonium bromide
HO
(h) Replacement by thio (–SH) group
(ii) Coupling reactions : The diazonium ion acts
as an electrophile because there is positive charge on
terminal nitrogen. It can react with nucleophilic
aromatic compounds ( Ar – H ) activated by electron
donating groups (– OH and – NH 2 ), which as strong
nucleophiles react with aromatic diazonium salts.
Therefore, benzene diazonium chloride couples with
electron rich aromatic compounds like phenols and
anilines to give azo compounds. The azo compounds
contain – N = N – bond and the reaction is called
coupling reaction.
Coupling occurs para to hydroxy or amino group.
All azo compounds are strongly coloured and are used as
dyes. Methyl orange is an important dye obtained by
coupling the diazonium salt of sulphanilic acid with N ,
N - dimethylaniline.
Diazonium salts are highly useful
intermediates in the synthesis of large variety of
aromatic compounds. These can be used to prepare many
classes of organic compounds especially aryl halides in
pure state. For example,. 1, 2, 3-tribromo benzene is not
formed in the pure state by direct bromination of
benzene. However, it can be prepared by the following
sequence of reaction starting from p-nitroaniline
through the formation of diazonium salts as :
(5) Uses of diazonium salts
(i) For the manufacture of azo dyes.
(ii) For the industrial preparation of important
organic compounds like m - bromotoluene, m-
bromophenol, etc.
(iii) For the preparation of a variety of useful
halogen substituted arenes.
SH
Thiophenol
N 2 + Cl –
N +^ NCl –^ + OH
Phenol
N = N
p - Hydroxyazobenzene (yellow)
OH
Base ( pH 9 - 273 10)- 278 K
N +^ NCl –^ + NH 2
N = N
p - Aminoazobenzene (orange)
NH 2
H +( pH 273 4.5)- 278 K
N +^ NCl –^ + N
CH 3
CH 3
NaNO 2 , HCl 273 - 278 K
NH 2
Sod. Salt of sulphanilic acid
Na + O 3 – S
Na + O 3 – S N NCl
OH – 273 - 278 K Methyl orange
Na + O 3 – S N = N N ( CH 3 ) 2
Na + O 3 – S N N N ( CH 3 ) 2
N , N - Dimethylaniline
Cl + H
NH 2
NO 2
p - Nitroaniline
Br 2
NH 2
Br Br
NO 2
Diazotisati on
N 2 + Cl –
Br Br
NO 2
CuBr
Br
Br Br
NO 2
Sn , HCl
Br
Br Br
NH 2
Diazotisati on
Br
Br Br
N 2 + Cl –
H 3 PO 2
Br
Br Br
H +( pH ^ 1, 2, 3-Tribromo benzene 273 4.5)- 278 K
N = N
(orange )
N
CH 3
CH 3 N,N-dimethyl-p- aminoagobenzene
