Chapter 23 Nitrogen containing compounds Part 2- Chemistry free study material by TEACHING CARE online tuition and coaching classes
Chapter 23 Nitrogen containing compounds Part 2- Chemistry free study material by TEACHING CARE online tuition and coaching classes
- By decarboxylation of a-amino acids
RC HCOOH ¾¾Ba(¾OH¾)2 ® RCH 2 NH 2 ; CH 2 – COOH ¾¾Ba(¾OH¾)2 ® CH3 NH 2
|
NH2
heat
|
NH2
a -amino acetic acid (Glycine)
heat
Methyl amine
- By means of a Grignard regent and chloramine :
- By hydrolysis of Isocyanides or Isocyanates
RMgX + ClNH 2 ® RNH 2 + MgXCl
H
R – N º
OH
|
C + 2H O ¾¾(HC¾l) ® R – NH + HCOOH ; CH – NC+ 2HOH ¾¾H¾+ ® CH – NH + HCOOH
H OH
Alkyl amine
Aceto isonitile
Alkyl isocyanide
H OH CH3 – N = C
H OH
= O + 2KOH ® CH3 – NH 2 + K2 CO3 ;
R – NCO + 2KOH ® R – NH 2 + K2 CO3
Alkyl isocyanate
Methyl isocyanate
- By Schmidt reaction :
R – COOH+
Acid
N 3 H
Hydrazoic acid
¾¾Con¾c.H¾2SO¾4 ® R – NH 2 + N 2 + CO2
Alkyl amine
In this reaction the acyl azide (R – CON3) and alkyl isocyanate (R – NCO) are formed as an intermediate.
R – COOH + N3 H ® RCON3 + H2O ; RCON 3 ® R – N = C = O+ N 2
Acyl azide
Acyl azide
Alkyl isocyanate
R – N = C = O + H2O ® R – NH2 + CO2
Alkyl amine
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.
O O O
|
|| || ||
– – OH ¾¾¾® R – – Cl ¾¾¾® R – – N
C C
Acyl chloride
O
C 3
Acyl azide
||
R – C– N 3
¾¾– N¾2 ® R – N = C = O ¾¾2Na¾O¾H ® R – NH 2
heat
- Na
2CO3
The mechanism of curtius rearrangement is very similar to Hofmann degradation.
|
–
- ·
R N = N = N
C
||
O
R N – N º N
- ·
C
||
O
–N2 C
||
O
Intramolecular alkyl shift
R – N = C = O
|
Schmidt reaction converts R – COOH to R–NH2, which is a modification of curtius degradation. In this reaction a carboxylic acid is warmed with sodium azide (Na+N –) and conc. H SO . The carboxylic acid is directly converted to the primary amine without the necessity of isolating alkyl azide.
O
|
|
R – || – OH ¾¾NaN¾3 +¾H2S¾O4¾(con¾¾c.) ® RNH
heat
- N 2
- CO2
(NaN 3 + H 2 SO4 ® N 3 H + NaHSO4 )
- By Ritter reaction : It is a good method for preparing primary amines having a-tertiary alkyl
(CH 3 )3 C – OH+ H 2 SO4 + HCN ® (CH 3 )3 C – NH 2
Tert- butyl alcohol
Tert -butylamine (1°amine)
|
|
é C – OH ¾¾H¾+ ® H
2O +
R3 C +
¾¾HC¾N ®
+
R3 C N
º CH ¾¾H2¾O ®
|
ë Tert-carboniumion
CHO – R3
O
||
CNH ¾¾OH¾- ® R
C – NH 2
Pri-amine
- HCOO– ù
úû
- Reductive amination of aldehydes and ketones :
R – C– H+ NH 3 + H 2 ¾¾Ni,1¾50°¾C ® R – CH 2 – NH 2 + H 2 O
é H H
Aldehyde
ù
300atm
Primary amine
êR – | = O + H HN ¾¾(–H¾¾O) ®[R – |
2
= NH] ¾¾H¾2 ® RCH – NH ú
ê C 2
êë
O
C
Imine
Ni
|
CH3
2 2 ú
úû
||
R – C– CH3
Ketone
- NH3
- H2
¾¾Ni,1¾50¾°C ® R – | 300atm
- NH2
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
R – CHO + R¢NH2 ¾¾H2 ¾Ni ® RCH2 NHR¢ ;
Sec. amine
RNH 2 + 2H 2 C = O + 2HCOOH ® RN(CH 3 )2 + 2H 2 O + 2CO2
Tert.-amine
- By reduction of azide with NaBH4 :
R – X + NaN 3 ® RN 3 ¾¾NaB¾H¾4 ® RNH 2
Alkyl halide (1°or2°)
Sodium azide
Alkyl azide
H2O
1°amine
- By Leuckart reaction : Aldehydes or ketones react with ammonium formate or with formamide to give formyl derivative of primary
O
||
- C = O + 2HCOONH4 ®> CHNH – C– H + 2H2O + CO2 + NH3 formate
O
||
- C = O + 2HCONH 2 ®> CHNH – C– H + CO2 + NH 3 Formamide
These formyl derivatives are readily hydrolysed by acid to yield primary amine.
O
|
R CHNH || H + HOH ¾¾H¾+ ® R CHNH + H O + CO
R C R
2 2 2
This is called Leuckart reaction, i.e.,
R C = O + HCOONH
¾¾180¾-20¾0¾°C ® R
CHNH
- H O + CO
R¢
Ketone
4
Amm. formate
D R¢
2 2 2
Primary amine
Note : ® 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.
CH 2 = CH 2 + NH 3 ¾¾Cob¾alt c¾ata¾ly¾st ® CH 3 CH 2 NH 2
Ethylene
450°C,20atm
- Methods yielding secondary amines
- Reaction of primary amines with alkyl halides
R – NH
+ R – X ¾¾D ® R NH + HX ®
+ – +
;
–
+ NaOH ®
R NH
+ H O + NaX
2 2 R2 NH2 X
dialkyl ammonium salt
R2 N H2 X
2 2
Secondary amine
- Reduction of isonitriles :
R – NC + 4[H] ¾¾Pt ® RNHCH3
Alkyl isnitrile
Sec. amine
Secondary amine formed by this method always possesses one –CH3 group linked directly to nitorgen.
- Reaction of p-nitroso-dialkyl aniline with strong alkali solution :
NH RX NR
HNO2 ON
R NaOH ON
OH + R NH
Aniline
2 heat
2
Dialkyl aniline
2
p-Nitroso-dialkyl aniline
p-Nitroso phenol
2
Sec. amine
This is one of the best method for preparing pure secondary amines.
é ù
- Hydrolysis of dialkyl cyanamide : êCaN – CN ¾¾2Na¾O¾H ® Na2 N – CN ¾¾2R¾X ® R2 N – CN ú
ê Calcium
ëê cyanamide
Sodium cyanamide
Dialkyl ú
cyanamide úû
R N – CN + 2HOH ¾¾H+¾or ®
2 OH –
R2 NH
- CO2
- NH 3
Dialkyl amine
- Reduction of N-substituted amides : Reduction of N-substituted amides with LiAlH4 yields secondary
Alkyl b-amino ketones are formed by the action of ketone with formaldehyde and NH3 (or primry or secondary amines).
The product is referred to as Mannich base and the reaction is called Mannich Reaction.
CH 3 COCH 3 + HCHO + RNH 2 ¾¾he¾at ® CH 3 COCH 2 CH 2 NHR
Which can be reduced to alkyl amines.
R – CONHR¢ + 4[H] ¾¾LiA¾lH¾4 ® RCH 2 NHR¢+ H 2 O
N – Alkyl acid amide
Sec.amine
- Methods yielding tertiary amines
- Reaction of alkylhalides with ammonia
+ – + –
3RX + NH3 ® R3 N + 3HX ®
R3 NH X ;
Trialkyl ammonium salt
R3 N H X+ NaOH ® R3 N + NaX + H2O
- Reduction of N, N-disubstituted amides : The carbonyl group is converted into – CH2
RCONR2¢ ¾¾LiA¾lH¾4 ® RCH 2 NR¢2 + H 2 O
N, N -disubstituted amide
4[H]
ter. amine
- Decomposition of tetra-ammonium hydroxides : The tetra-alkyl ammonium hydroxides are formed when corresponding halides are treated with moist silver
+ + –
R4 N I + AgOH ® R4 N O H+ AgI
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
- 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
+ + –
RNH2.HI or RN H 3 – I + K O H ® RNH2 + KI
Primary amine (Volatile), Distillate
- H2O
+ + –
R2 NH.HI or R2 N H 2 – I + K O H ® R2 NH + KI + H2O
+ + –
R3 N.HI or R3 N H– I + K O H ® R3 N + KI + H2O
+
R4 N I (non-volatile tetra-alkyl ammonium salt) has no reaction with KOH, however remains as residue.
This mixture is separated into primary, secondary and tertiary amines by the application of following methods.
- Fractional distillation : The boiling points of primary, secondary and tertiary amines are quite different,
i.e., the boiling point of C2H5NH2 is 17°C, (C2H5)2NH is 56°C and
(C2 H5 )3 N
is 95°C and thus, these can be
separated by fractional distillation. This method is used satisfactorily in industry.
- Hofmann’s method : The mixture of three amines is treated with diethyl oxalate. The primary amine forms a soild oxamide, a secondary amine gives a liquid oxamic ester while tertiary amine does not
CO OC2H5
|
H NHR
+
¾¾-2C¾2H5¾O¾H ® CONHR
; COOC2 H5 + HNR2 ¾¾–C2¾H5O¾¾H ®
CONH2
CO OC2 H5
Diethyl oxalate
H NHR
Primary amine
|
CONHR
Dialkyl oxamide (Solid)
|
COOC2H5
Diethyl oxalate
Secondary amine
|
COOC2H5
Dialkyl oxamic ester (liquid)
Primary amine is recovered when solid oxamide is heated with caustic potash solution and collected as distillate on distilling the reaction mixture.
CO OK
| + ®
COOK
|
- 2RNH 2
CO OK
COOK Primary amine
Pot.oxalate (Distillate)
The liquid (mixture of oxamic ester+ tertiary amine) is subjected to fractional distillation when tertiary amine distils over.
The remaining liquid is distilled with KOH to recover secondary amine.
CONR2
|
HOK
+
COOK
® R2 NH + |
- C2 H 5 OH
COOC2H5
HOK
Secondary amine
COOK
Pot. oxalate
- Hinsberg’s method : It involves the treatment of the mixture with benzene sulphonyl chloride, e., Hinsberg’s reagent (C6H5SO2Cl). The solution is then made alkaline with aqueous alkali to form sodium or potassium salt of monoalkyl benzene sulphonamide (soluble in water).
C6 H 5 SO2 Cl + HNHR ® C6 H 5 SO2 NHR ¾¾NaO¾H ® C6 H 5 SO2 N(Na)R
Primary amine
N – Alkyl benzene sulphonamide
Soluble salt
The secondary amine forms N,N-dialkyl benzene sulphonamide which does not form any salt with NaOH and remains as insoluble in alkali solution.
C6 H 5 SO2 Cl + HNR2 ® C6 H 5 SO2 NR2 ¾¾NaO¾H ® No reaction
Sec. amine
Tertiary amine does not react.
(Insoluble in water, soluble in ether)
The above alkaline mixture of the amines is extracted with ether.
Two distinct layers are formed. Lower layer, the aqueous layer consists of sodium salt of N-alkyl benzene sulphonamide (primary amine) and upper layer, the ether layer consists of N,N-dialkyl benzene sulphonamide (secondary amine) and tertiary amine.
Two layers are separated. The upper layer is fractionally distilled. One fraction obtained is tertiary amine and the other fraction is treated with concentrated HCl to recover secondary amine hydrochloride which gives free secondary amine on distillation with NaOH.
C6 H 5 SO2 NR2 + HCl + H 2 O ® C6 H 5 SO2 .OH + R2 NH.HCl ;
R2 NH.HCl + NaOH ® R2 NH + NaCl + H 2 O
Sec. amine
The aqueous layer is acidified and hydrolysed with dilute HCl. The hydrochloride formed is then distilled with
NaOH when primary amine distils over.
C6 H 5 SO2 N(Na)R + HCl ® C6 H 5 SO2 NHR + NaCl
Sulphonamide of primary amine
C6 H 5 SO2 NHR + HCl + H 2 O ® C6 H 5 SO2 .OH + RNH 2 .HCl ;
Primary amine hydrochloride
RNH 2 .HCl + NaOH ® RNH 2 + NaCl + H 2 O
(6) Physical properties
- Lower amines are gases or low boiling point liquids and possess a characteristic ammonia like smell (fishy odour). Higher members are solid
- The boiling points rise gradually with increase of molecular mass. Amines are polar compounds like NH3 and have comparatively higher boiling points than non-polar compounds of similar molecular This due to the presence of intermolecular hydrogen bonding.
H H H
| | |
H – N : – – –H – N : – – –H – N : – – –
| | |
R R R
Hydrogen bonding in amines
- Amines are soluble in This is due to hydrogen bonding between amine and water molecules. Amines are also soluble in benzene and ether.
H O&
H H
| & |
– : – – –H – N : – – –H – O : – – –H – N : – – –
| | | |
H R H R
Hydrogen bonding between amine and water molecules
Solubility decreases with increase of molecular mass.
- 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).
- Basic nature of aliphatic amines : Amines like ammonia are basic in nature. The basic nature is due to the presence of an unshared pair (lone pair) of electrons on nitrogen atom. This lone pair of electrons is available for the formation of a new bond with a proton or Lewis
H – N& – H
; R – N& – H ; R – N& – H
; R – N& – R
|
H
Ammonia
|
H
Primary amine
|
R
Secondary amine
|
R
Tertiary amine
Amines are weak bases as they combine partially with the water to form hydroxyl ions.
R – NH 2
- H 2
+
O ⇌ R – N H3
- OH –
Alkyl ammonium ion
Applying law of mass action.
|
+
K = [R – N H3 ][OH ]
(where K
is dissociation constant of the base)
|
b [R – NH ] b
[Concentration of water is considered constant as it is present in large amounts.]
The value of Kb describes the relative strength of the bases. Strong bases have higher value of Kb while weak bases have low values.
NH 3 ; CH3 NH 2 ;(CH3 )2 NH;(CH3 )3 N
Kb ; 1.8´10–5
37´10–5
54´10–5
6.7´10–5
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 or 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.
The order of basic nature of various amines has been found to vary with nature of alkyl groups. Alkyl group Relative strength
CH3 – R2NH > RNH2 > R3N > NH3
C2H5 – R2NH > RNH2 > NH3 > R3N (CH3)2CH – RNH2 > NH3 > R2NH > R3N (CH3)3C – NH3 > RNH2 > R2NH > R3N
- Basic nature of aromatic amines : In aniline or other aromatic amines, the non-bonding electron pair is delocalized into benzene ring by resonance.
:NH2 +NH2
+NH2
+NH2
- · d+
- –·
But anilinium ion is less resonance stabilized than aniline.
N – H
|
H
Resonance hybrid
N – H
|
H
+
NH3
+
NH3
No other resonating structure possible
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 (– NO2, – CN, – SO3H, – COOHT – Cl, C6H5, 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 (– NH2, – OR, R –, etc.) increases basicity of aniline. Toluidine is more basic than aniline as – CH3 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,
R2 NH > RNH 2 > C6 H5 CH 2 NH 2 > NH 3 > C6 H5 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,
C6 H5 N(C2 H5 )2 > C6 H5 NHC2 H5 > C6 H5 N(CH3 )2 > C6 H5 NHCH3 > C6 H5 NH 2 NH 3 > C6 H5 NHC2 H5
- C6 H5 NHCH3 > C6 H5 NH 2 > C6 H5 NHC6 H5 In Toluidines –p-isomer > m– > o- Chloroanilines–p-isomer>m-> o-
Phenylene diamines –p-isomer > m- > o-
Nitroanilines–m-isomer > p– > o-
Note : ® 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 This is because lone pair of electrons on nitrogen is delocalised by resonance with the carbonyl group which makes it less available for protonation.
Not available due to delocalization
O–
& | +
CH3 – C– NH2 « CH3 – C = N H2
- The compounds with least ‘s’ character (sp3-hybridized) is most basic and with more ‘s’ character (sp- hybridized) is least Examples in decreasing order of basicity are,
CH3 N& H 2 > CH3 – N& = CHC H3 > CH3 – C º N&
(sp3 )
(sp2 )
(sp)
CH3 CH 2 CH 2 NH 2 > H 2 C = CHCH 2 NH 2 > HC º CCH 2 NH 2
(CH3 )2 NH > CH3 NH 2 > NH 3 > C6 H5 NH 2
- Electron withdrawing (C6H5 –) groups decrease electron density on nitrogen atom and thereby decreasing
(CH3 )2 NH > CH3 NH 2 > C6 H5 NHCH3 > C6 H5 NH 2
CH3 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 This effect diminishes with distance from the amino group.
CH3 CH 2 NH 2 > C6 H5 CONH 2 > CH3 CONH 2
- Salt formation : Amines being basic in nature, combine with mineral acids to form
R – NH
+
+ HCl ®
; 2R – NH
+
+ H SO ® –
2 RNH3Cl
Alkylammonium chloride
2 2 4
(RNH3 )2 SO4
Alkylammonium sulphate
- Nature of aqueous solution : Solutions of amines are alkaline in
RNH 2 + HOH ⇌
+
R N H3 OH
– ⇌ [RNH 3
]+ + OH –
R2 NH + HOH ⇌
+
R2 N H 2 OH
– ⇌ [R2
NH 2
]+ + OH –
R3 N + HOH
+
⇌ R3 N HOH
– ⇌ [R3
NH]+ + OH –
The aqueous solutions of amines behaves like NH4OH and give ferric hydroxide precipitate with ferric chloride and blue solution with copper sulphate.
3RNH 3 OH + FeCl3 ® Fe(OH)3 + 3RNH 3 Cl
- Reaction with alkyl halides (Alkylation)
RNH
¾¾R¾¢X ® RNHR¢ ¾¾R¾¢X ® R – NR¢ ¾¾R¾¢X ®
+ ¢ –
2
Pri.amine
–HX
Sec.amine
–HX
2
Tert. amine
(R – N R3 )X
Quaternary salt
- Reaction with acetyl chloride (Acylation
RNH 2 + ClOCCH3 ¾¾– H¾Cl ® RNHOCCH3 ;
R2 NH + ClOCCH3 ¾¾– H¾Cl ®
R2 NOCCH3
Pri. amine
N – Alkyl acetamide
Sec. amine
N,N -Dialkyl acetamide
Tertiary amines do not react since they do not have replaceable hydrogen on nitrogen. Therefore, all these above reactions are used to distinguish between P,S and T-amines.
- Action of sodium
2RNH2 + 2Na ¾¾D ® 2[RNH]– Na+ + H2 ;
Sod. salt
2R2 NH + 2Na ¾¾D ® 2[R2 N]– Na + + H 2
Sod.salt
- Action of halogens
RNH 2
¾¾X¾2 ® RNHX ¾¾X¾2 ®
RNX 2 ;
R2 NH
¾¾X¾2 ®
R2 NX
Alkyl amine
NaOH
NaOH
Dihalo-alkyl amine
Dialkyl amine
NaOH
Halo-dialkyl amine
- Reaction with Grignard reagent
RNH2
- Mg
CH3 ® CH I 4
- RNH – Mg – I ;
R2 NH + CH3
– Mg – I ® CH4
- R2
N – Mg – I
- 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
RNH2 + CHCl3 + 3KOH ®
(Alc.)
RNC
Alkyl isocyanide (carbyl amine)
- 3KCl + 3H2O
Isocyanides are bad smelling compounds and can be easily detected.
- Reaction with nitrous acid
- Primary amines form alcohols with nitrous acid (NaNO2+ HCl). Nitrogen is
RNH2 + HONO ® ROH+ N 2 + H2O
Pri. amine
Alcohol
Methyl amine is an exception to this reaction, i.e.,
CH3 NH2 + 2HONO ® CH3 – O – N = O + N 2 + 2H2O
Methyl nitrite
2CH3 NH2 + 2HONO ® CH3 – O – CH3 + 2N 2 + 3H2O
Dimethyl ether
- Secondary amines form nitrosoamines which are water insoluble yellow oily
R2 NH + HONO ® R2 NNO + H2O
Sec. amine
Dialkyl nitrosoamine
Nitrosoamine on warming with phenol and conc. H2SO4 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.
- Tertiary amines react nitrous acid to form nitrite salts which are soluble in water. These salts on heating give alcohols and
R3 N + HONO ®
[R3 NH]+ NO–
¾¾he¾at ® R – OH+ R2 N – N = O
Tert.amine
2
Trialkyl ammoniumnitrite
Alcohol
Nitrosoamine
This reaction (nitrous acid test) is used to make distinction between primary, secondary and tertiary amines.
- Reaction with carbon di sulphide : This Hofmann’s mustard oil reaction and is used as a test for primary amines.
RNH2
1°
¾¾S=C¾=¾S ® S = C
heat
NHR
SH
¾¾HgC¾l2 ®
RNC = S
Alkyl isothiocyanate
- HgS + 2HCl
Black ppt.
Alkyl dithiocarbamic acid
(Mustard oil smell)
R ¾¾S=C¾=¾S ®
S = C
NR2
¾¾HgC¾l2 ® No reaction
|
2 NH
2°
Dialkyl dithiocarbamic acid
- Oxidation : All the three types of amines undergo oxidation. The product depends upon the nature of oxidising agent, class of amine and the nature of the alkyl
- Oxidation of primary amines
RCH2 NH2 ¾¾[¾O] ® RCH = NH ¾¾H2¾O ® RCHO+ NH 3
Pri. amine
KMnO4
Aldimine
Aldehyde
R2CHNH2 ¾¾[¾O] ® R2C = NH ¾¾H2¾O ® R2CO+ NH 3
KMnO4
Ketimine
Ketone
- Oxidation of secondary amines :
R2 NH ¾¾[¾O] ®
R2 N – NR2
; R2 NH ¾¾[¾O] ®
R2 NOH
Sec. amine
KMnO4
Tetra-alkyl hydrazine
H2SO5
Dialkyl hydroxylamine
- Oxidation of tertiary amines : Tertiary amines are not oxidised by potassium permanganate but are oxidised by Caro’s acid or Fenton’s reagent to amine
R3 N + [O] ® [R3 N ® O]
Tert. amine Amine oxide
- Reaction with other electrophilic regents
O O
|| ||
RNH2 + O = HCR¢ ® RN = HC R¢ ; 2RNH2 + Cl – C– Cl ® RNH – C– NHR+ 2HCl
Pri. amine
Aldehyde
Schiff’s base
O
||
Carbonyl chloride
Dialkyl urea (Symmetrical)
S
||
RNHH + O = C = N – R¢ ® RNH – C– HNR¢ ; RNHH + S = C = N – R¢ ® RNH – C– NHR¢
Isocyanate
Dialkyl urea (Unsymmetrical)
Isothiocyanate
Dialkyl thiourea
- Ring substitution in aromatic amines : Aniline is more reactive than The presence of amino group activates the aromatic ring and directs the incoming group preferably to ortho and para positions.
- Halogenation
NH2
NH2
Br Br
+ 3Br2 + 3HBr
This reaction is used as a test for aniline.
Br
2, 4, 6-Tri Bromoaniline (white ppt.)
However, if monosubstituted derivative is desired, aniline is first acetylated with acetic anhydride and then halogenation is carried out. After halogenation, the acetyl group is removed by hydrolysis and only monosubstituted halogen derivative is obtained.
It may be noted that – NH2 group directs the attacking group at o- and p-positions and therefore, both o- and
p-derivatives are obtained.
NH2
(CH3CO)2O
–CH3COOH
NHCOCH3
Br2
NHCOCH3
Br
+
NHCOCH3
H2O, H+, –CH3COOH
NH2
Br
+
NH2
Aniline
Acetanilide
p-Bromoacetanilide (minor)
Br
p-Bromoacetanilide (major)
o-Bromoaaniline (minor)
Br
p-Bromoaniline (major)
Acetylation deactivates the ring and controls the reaction to monosubstitution stage only because acetyl group is electron withdrawing group and therefore, the electron pair of N-atom is withdrawn towards the carbonyl group.
- Nitration : Aromatic amines cannot be nitrated directly because they are readily This is because,
HNO3 is a strong oxidising agent and results in partial oxidation of the ring to form a black mass.
Therefore, to solve this problem, nitration is carried out by protecting the –NH2 group by acetylation. The acetylation deactivates the ring and therefore, controls the reaction.
The hydrolysis of nitroacetanilides removes the protecting acyl group and gives back amines.
NH2
O
||
+ Cl – C – CH3
Acetyl chloride
NHCOCH3
HNO3, H2SO4
288 K
NHCOCH3
NO2
+
NHCOCH3
–CH3COOH H2O, H+
NH2
NO2
+
NH2
Aniline
- Sulphonation
NH2
Acetaniline
o-Nitroacetanilide
NH3+ HSO4–
NO2
p-Nitroacetanilide
NH2
o-Nitroaniline (minor)
NH3+
NO2
p-Nitroaniline (major)
+ H2SO4
Heat 453-473 K
Aniline
Anilinium hydrogen sulphate
SO3H
Sulphanic acid (I)
SO3–
Zwitter ion structure (II)
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.
(8) Uses :
- Ethylamine is used in solvent extraction processes in petroleum refining and as a stabiliser for rubber latex.
- The quaternary ammonium salts derived from long chain aliphatic tertiary amines are widely used as
- Aliphatic amines of low molecular mass are used as
Distinction between primary, secondary and tertiary amines
| Test | Primary amine | Secondary amine | Tertiary amine |
| Action of CHCl3 and alcoholic KOH. | Bad smelling carbylamine (Isocyanide) is formed. | No action. | No action. |
| (Carbylamine test) | |||
| Action of CS2 and HgCl2. (Mustard oil test) | Alkyl isothiocyanate is formed which has pungent | No action. | No action |
| smell like mustard oil. | |||
| Action of nitrous acid. | Alcohol is formed with | Forms nitrosoamine | Forms nitrite in cold |
| evolution of nitogen. | which gives green colour | which on heating gives | |
| with phenol and conc. | nitrosoa- mine which | ||
| H2SO4 (Liebermann’s | responds to | ||
| test). | Liebermann’s test. |
| Action | of | acetyl | Acetyl | derivative | is | Acetyl derivative is | No action. |
| chloride. | formed. | formed. | |||||
| Action of Hinsberg’s Monoalkyl sulphonamide Dialkyl sulphonamide is No action. reagent. is formed which is soluble formed which is insoluble
in KOH. in KOH. Action of methyl iodide. 3 molecules (moles) of 2 moles of CH3I to form One mole of CH3I to CH3I to form quaternary quaternary salt with one form quaternary salt with salt with one mole of mole of secondary amine. one mole of tertiary primary amine. amine. |
|||||||
Note : ® Aniline does not form alcohol with nitrous acid but it forms benzene diazonium chloride which
shows dye test.
Aniline was first prepared by Unverdorben (1826) by dry distillation of indigo. In the laboratory, it can be prepared by the reduction of nitrobenzene with tin and hydrochloric acid.
C6 H 5 NO2 + 6H ¾¾Sn,¾H¾Cl ® C6 H 5 NH 2 + 2H 2 O
Nitrobenzene Aniline
Aniline produced combines with
H 2 SnCl6 (SnCl4 + 2HCl) to form a double salt.
2C6 H 5 NH 2 + SnCl4 + 2HCl ® (C6 H 5 NH 3 )2 SnCl6
Double salt
From double salt, aniline is obtained by treating with conc. caustic soda solution.
(C6 H 5 NH 3 )2 SnCl6 + 8 NaOH ® 2C6 H 5 NH 2 + 6 NaCl + Na 2 SnO3 + 5H 2 O
On a commerical scale, aniline is obtained by reducing nitrobenzene with iron filings and hydrochloric acid.
NO2
NH3+Cl–
NH2
Fe3/HCl 30% Na2CO3
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.
2C6 H5 Cl + 2NH 3 + Cu2 O ¾¾200¾°¾C ® 2C6 H5 NH 2 + Cu2 Cl2 + H 2 O
300-400 atm
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.
- Anils (Schiff’s bases from aniline) are used as antioxidants in rubber
etc.
- It is used for the manufacture of its some derivatives such as acetamide, sulphanilic acid and sulpha drugs,
- It is used as an accelerator in vulcanizing
(1) Conversion of methylamine to ethylamine (Ascent)
CH3 NH 2 ¾¾HN¾O¾2 ® CH3 OH
¾¾P¾I3 ®
CH3 I
¾¾NaC¾N ® CH3 CN
¾¾LiA¾lH¾4 ® CH3 CH 2 NH 2
Methylamine
Methyl alcohol
Methyl iodide
Methyl cyanide
Ethylamine
(2) Conversion of ethylamine to methylamine. (Descent)
CH3CH2 NH2 ¾¾HN¾O2 ® CH3CH2OH
¾¾[¾O] ®
CH3CHO ¾¾[¾O] ® CH3COOH ¾¾SOC¾l2 ® CH3COCl
Ethylamine
Ethanol
K2Cr2O7 H2SO4
Acetaldehyde
Acetic acid
Acetyl chloride
¾¾NH¾3 ® CH3CONH2 ¾¾B¾r2 ® CH3 NH2
Acetamide
KOH
Methylamine
- Conversion of ethylamine to acetone
C2 H5 NH2 ¾¾HN¾O2 ® C2 H5OH ¾¾K2C¾r2O¾7 ® CH3CHO ¾¾K2C¾r2O¾7 ® CH3COOH ¾¾Ca(¾OH¾)2 ®(CH3COO)2 Ca
Ethylamine
Ethyl alcohol
H2SO4
Acetaldehyde
H2SO4
Acetic acid
Calcium acetate
¾¾he¾at ® CH 3 COCH 3
Acetone
(4) Conversion of propionic acid to :
- Ethylamine, (ii) n-Butylamine.
- CH3CH2COOH ¾¾SOC¾l2 ® CH3CH2COCl ¾¾NH¾3 ® CH3CH2CONH2 ¾¾B¾r2 ® CH3CH2 NH2
Propionic aicd
Propionyl chloride
Propionamide
KOH
Ethylamine
or C2 H 5 COOH ¾¾N3¾H ® C2 H 5 NH 2
H2SO4 (conc.)
- CH3CH2COOH ¾¾LiA¾lH¾4 ® CH3CH2CH2OH ¾¾PB¾r5 ® CH3CH2CH2 Br ¾¾KC¾N ® CH3CH2CH2CN
Propionic acid
Ether
n– Propyl alcohol
Propyl bromide
Propyl cyanide
¾¾Na+¾C2¾H5 O¾H ® CH 3 CH 2 CH 2 CH 2 NH 2
(5) Conversion of ethylene to 1,4-diaminobutane :
or LiAlH4
n-Butylamine
CH 2 = CH 2 ¾¾B¾r2 ® CH 2 Br.CH 2 Br ¾¾NaC¾N ® NCCH 2 CH 2 CN ¾¾LiA¾lH¾4 ® NH 2 CH 2 CH 2 CH 2 CH 2 NH 2
Ethylene
CCl4
Ethylene bromide
Ethylene cyanide
1,4-Diaminobutane
|
The diazonium salts have the general formula ArN + X – , where X– may be an anion like Cl–, Br– etc. and the
|
group N + (- N º N + ) is called diazonium ion group.
- 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 For example,
N+ º NCl–
CH3
Cl HO
N+ º NCl– N+ º NCl–
N+ º NBr–
Benzenediazonium chloride
p-Toluenediazonium chloride
o-chlorobenzenediazonium chloride
m-Hydroxybenzenediazonium bromide
The diazonium salt may contain other anions also such as
NO – , HSO – , BF
etc.
O2N
|
N+ º NHSO –
3 4 4
p-Nitrobenzenediazonium hydrogen sulphate
- Preparation of diazonium salts : NaNO2 + HCl ® NaCl + HONO
NH2 N2+Cl–
NaNO2 HCl, 273 K
+ NaCl + H2O
Aniline
Benzene diazonium chloride
The reaction of converting aromatic primary amine to diazonium salt is called diazotisation.
(3) Physical properties of diazonium salts
- Diazonium salts are generally colourless, crystalline
- These are readily soluble in water but less soluble in
- They are unstable and explode in dry Therefore, they are generally used in solution state.
- Their aqueous solutions are neutral to litmus and conduct electricity due to the presence of
(4) Chemical properties of diazonium salts
- Substitution reaction : In substitution or replacement reactions, nitrogen of diazonium salts is lost as N2
and different groups are introduced in its place.
- Replacement by –OH group N2+Cl– OH
+ H2O
Warm
+ N2 + HCl
- Replacement by hydrogen
Benzene diazonium chloride
N2+Cl–
Phenol
+ H3PO2 + H2O
Hypophosphoric acid
Benzene diazonium chloride
Benzene
+ N2 + H3PO3 + HCl
- Replacement by–Cl group
N2+Cl– Cl
Cu2Cl2
+ N2
This reaction is called Sandmeyer reaction.
Chlorobenzene
When the diazonium salt solution is warmed with copper powder and the corresponding halogen acid, the respective halogen is introduced. The reaction is a modified form of Sandmeyer reaction and is known as
Gattermann reaction.
N2+Cl– Cl
Cu HCl
+ N2
- Replacement by iodo (–I) group
N2+Cl– I
+ KI
Heat
+ N2 + KCl
- Replacement by – F group
N2+Cl–
+ HBF4
Fluoroboric acid
This reaction is called Schiemann reaction.
- Replacement by Cyano (– CN) group
N2+Cl–
Iodobenzene
N2+BF4–
Heat
CN
F
Fluorobenzene
CuCN
+ N2
The nitrites can be hydrolysed to acids.
CN
Cyanobenzene
COOH
Hydrolysis
Benzoic acid
This method of preparing carboxylic acids is more useful than carbonation of Grignard reagents.
- Replacement by – NO2 group
N2+Cl–
HBF4
N2+BF4–
NaNO2
Cu
NO2
+ NaBF4 + N2
Diazonium fluoro borate
- Replacement by thio (–SH) group
Nitrobenzene
N2+Cl–
+ KSH
Potassium hydro sulphide
SH
+ N2 + KCl
Thiophenol
- Coupling reactions : The diazonum 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 – NH2), 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 The azo compounds contain –N = N– bond and the reaction is called coupling reaction.
N+ º NCl– + OH
Phenol
Base
|
|
(pH » 9-10)
273-278 K
p-Hydroxyazobenzene (yellow)
N+ º NCl– + NH2
H+(pH » 4.5) 273-278 K
|
p-Aminoazobenzene (orange)
NH2
N+ º NCl– +
CH3
N
CH3
H+(pH » 4.5) 273-278 K
(orange)
CH3
N
CH3
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.
Na+O3–S
NH2
NaNO2, HCl
273-278 K
Na+O3–S
N º NCl
Sod. Salt of sulphanilic acid
Na+O3–S
N º N
N(CH3)2
N, N-Dimethylaniline
OH–
273-278 K
Na+O3–S
N = N
Methyl orange
N(CH3)2
Note : ® 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 :
NH2
Br
Br2
NH2
Br Br
Diazotisation
N2+Cl–
Br
Br
CuBr
Br
Br
Sn, HCl
NO2
p-Nitroaniline
NO2
NO2
Br
Br
Br Br
Diazotisation
NO2
Br
Br
Br Br Br
H3PO2
(5) Uses of diazonium salts
- For the manufacture of azo
NH2
N2+Cl–
1, 2, 3-Tribromo benzene
- For the industrial preparation of important organic compounds like m-bromotoluene, m-bromophenol,
- For the preparation of a variety of useful halogen substituted
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