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

File name : Chapter-23-Nitrogen-containing-compounds-Part-2.pdf

 

  • By decarboxylation of a-amino acids

RC HCOOH ¾¾Ba(¾OH¾)2  ® RCHNH 2 ;  CH 2 – COOH ¾¾Ba(¾OH¾)2  ® CHNH 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

2                                                                               3                                                         3              22

C  + 2H  O ¾¾(HC¾l) ® RNH   + HCOOH ;  CH    NC+ 2HOH ¾¾H¾+  ® CH    NH    + HCOOH

 

H      OH

Alkyl amine

Aceto isonitile

 

Alkyl isocyanide

 

H      OH CH3N = C

H      OH

= O + 2KOH ® CH3 – NH 2 + KCO3 ;

R NCO + 2KOH ® R NH 2 + KCO3

Alkyl isocyanate

 

Methyl isocyanate

 

  • By Schmidt reaction :

R COOH+

Acid

N 3 H

Hydrazoic acid

¾¾Con¾c.H¾2SO¾4  ® RNH 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 + NH ® 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

R                         SOCl2                                                   NaN3

||                                         ||                                     ||

–    – OH ¾¾¾® R –   – Cl ¾¾¾® R –   – N

 

C                                        C

Acyl chloride

 

O

C        3

Acyl azide

 

||

R CN 3

¾¾ N¾2  ® RN = C = O ¾¾2Na¾O¾H ® RNH 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

 

3                               2       4

Schmidt reaction converts R COOH to RNH2, 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

 

C
2

R – || – OH ¾¾NaN¾3 +¾H2S¾O(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)

 

 

 

R
ê 3

é     COH ¾¾H¾+  ® H

2O +

R3 C +

¾¾HC¾N ®

+

R3 C N

º CH ¾¾HO ®

 

3

ë                                          Tert-carboniumion

CHO R3

 

O

||

CNH ¾¾OH¾-  ® R

CNH 2

Pri-amine

  • HCOO ù

úû

 

  • Reductive amination of aldehydes and ketones :

RCH+ NH 3  + H 2  ¾¾Ni,1¾50°¾C ® RCH 2  – NH 2 + HO

 

 

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

CH

CH3

2              2 ú

úû

 

||

R CCH3

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

 

RCHO + R¢NH2  ¾¾H2  ¾Ni ® RCHNHR¢ ;

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 :

RX  + 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 CH + 2H2O + CO2 + NH3 formate

O

||

  • C = O + 2HCONH 2 ®> CHNH CH + 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 ® CHCHNH 2

 

Ethylene

450°C,20atm

 

  • Methods yielding secondary amines
  • Reaction of primary amines with alkyl halides

 

 

R NH

+ RX ¾¾D ® R  NH + HX  ®

+              –                          +

;

+ NaOH ®

R NH

+ H O + NaX

 

2                          2                        R2 NHX

dialkyl ammonium salt

R2 N H2 X

2                     2

Secondary amine

 

  • Reduction of isonitriles :

RNC + 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 : êCaNCN ¾¾2Na¾O¾H ® NaNCN ¾¾2R¾X ® RNCN ú

 

ê    Calcium

ëê cyanamide

Sodium cyanamide

Dialkyl     ú

cyanamide  úû

 

R   N CN + 2HOH ¾¾Hor ®

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.

CHCOCH 3  + HCHO + RNH 2  ¾¾he¾at ® CHCOCHCHNHR

Which can be reduced to alkyl amines.

RCONHR¢ + 4[H] ¾¾LiA¾lH¾4  ® RCHNHR¢+ HO

 

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  ® RCHNR¢2  + HO

 

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 HI + 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

+

¾¾-2C¾2HO¾H ®  CONHR

;  COOCH5 +  HNR2   ¾¾CH5O¾¾H ®

CONH2

 

CO OCH5

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).

CHSOCl + HNHR ® CHSONHR ¾¾NaO¾H ® CHSON(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.

CHSOCl + HNR2  ® CHSONR2  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

 

HN& – H

;  RN& – H ;  RN& – H

; RN& – 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)

 

2

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´105

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 – CNH2 « 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,

CHN& 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 HOH

– ⇌ [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 ® RNR¢  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  ;

RNH + 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

2RNH + 2Na ¾¾D ® 2[RN] Na + + H 2  ­

Sod.salt

 

 

  • Action of halogens

 

RNH 2

¾¾X¾2  ® RNHX ¾¾X¾2  ®

RNX;

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 ® ROH+ RNN  = 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

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

 

SH

2 NH

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

RCHNH2  ¾¾[¾O] ® RCH  = NH ¾¾HO ® RCHO+ NH 3

 

Pri. amine

KMnO4

Aldimine

Aldehyde

 

R2CHNH2  ¾¾[¾O] ® R2C = NH ¾¾HO ® R2CO+ NH 3

 

KMnO4

Ketimine

Ketone

 

  • Oxidation of secondary amines :

RNH  ¾¾[¾O] ®

R2 N NR2

;  RNH 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 CCl ® RNH CNHR+ 2HCl

 

Pri. amine

Aldehyde

Schiff’s base

 

O

||

Carbonyl chloride

Dialkyl urea (Symmetrical)

S

||

 

RNHH + O = C = N R¢ ® RNH CHNR¢ ; RNHH + S = C = N R¢ ® RNH CNHR¢

 

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

 

TestPrimary amineSecondary amineTertiary 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 pungentNo action.No action
 smell like mustard oil.  
Action of nitrous acid.Alcohol is formed withForms nitrosoamineForms nitrite in cold
 evolution of nitogen.which gives green colourwhich on heating gives
  with phenol and conc.nitrosoa- mine which
  H2SO4 (Liebermann’sresponds to
  test).Liebermann’s test.

 

 

 

ActionofacetylAcetylderivativeisAcetyl           derivative          isNo 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.

CHNO2 + 6H ¾¾Sn,¾H¾Cl ® CHNH 2 + 2HO

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.

2CHCl + 2NH 3  + CuO ¾¾200¾°¾C  ® 2CHNH 2  + CuCl2  + HO

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)

 

CHNH 2  ¾¾HN¾O¾2  ® CHOH

¾¾P¾I3  ®

CH3 I

¾¾NaC¾N ®  CHCN

¾¾LiA¾lH¾4  ® CHCHNH 2

 

Methylamine

Methyl alcohol

Methyl iodide

Methyl cyanide

Ethylamine

 

(2)  Conversion of ethylamine to methylamine. (Descent)

 

CH3CHNH2  ¾¾HN¾O2  ® CH3CH2OH

¾¾[¾O] ®

CH3CHO ¾¾[¾O] ® CH3COOH ¾¾SOC¾l2  ® CH3COCl

 

Ethylamine

Ethanol

K2Cr2O7  H2SO4

Acetaldehyde

Acetic acid

Acetyl chloride

 

¾¾NH¾3  ® CH3CONH2  ¾¾B¾r2  ® CHNH2

 

Acetamide

KOH

Methylamine

 

  • Conversion of ethylamine to acetone

CHNH2  ¾¾HN¾O2  ® CH5OH ¾¾K2C¾r2O¾7  ® CH3CHO ¾¾K2C¾r2O¾7  ® CH3COOH ¾¾Ca(¾OH¾)2  ®(CH3COO)2 Ca

 

Ethylamine

Ethyl alcohol

H2SO4

Acetaldehyde

H2SO4

Acetic acid

Calcium acetate

 

¾¾he¾at ® CHCOCH 3

Acetone

 

(4)  Conversion of propionic acid to :

  • Ethylamine, (ii) n-Butylamine.
  • CH3CH2COOH ¾¾SOC¾l2 ® CH3CH2COCl ¾¾NH¾3  ® CH3CH2CONH2  ¾¾B¾r2  ® CH3CHNH2

 

Propionic aicd

Propionyl chloride

Propionamide

KOH

Ethylamine

 

or  CHCOOH  ¾¾NH ® CHNH 2

H2SO4 (conc.)

 

  • CH3CH2COOH ¾¾LiA¾lH¾4 ® CH3CH2CH2OH ¾¾PB¾r5  ® CH3CH2CHBr ¾¾KC¾N ® CH3CH2CH2CN

 

Propionic acid

Ether

n– Propyl alcohol

Propyl bromide

Propyl cyanide

 

¾¾Na+¾CHO¾H ® CHCHCHCHNH 2

 

 

(5) Conversion of ethylene to 1,4-diaminobutane :

or LiAlH4

n-Butylamine

 

CH 2  = CH 2  ¾¾B¾r2  ® CHBr.CHBr ¾¾NaC¾N ® NCCHCHCN ¾¾LiA¾lH¾4  ® NHCHCHCHCHNH 2

 

Ethylene

CCl4

Ethylene bromide

Ethylene cyanide

1,4-Diaminobutane

 

 

2

The diazonium salts have the general formula ArN + X , where X may be an anion like Cl, Br etc. and the

2

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

4

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

N = N
OH

(pH » 9-10)                         

273-278 K

p-Hydroxyazobenzene (yellow)

 

 

 

 

N+ º NCl +                  NH2

H+(pH » 4.5)                       273-278 K

N = N

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+O3S

NH2

NaNO2, HCl

273-278 K

Na+O3S

N º NCl

 

Sod. Salt of sulphanilic acid

 

 

Na+O3S

N º N

N(CH3)2

N, N-Dimethylaniline

OH

273-278 K

Na+O3S

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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