Amines and Benzene Diazonium Salt: Preparation and Properties
Amines are derivatives of ammonia in which one or more hydrogen atoms have been replaced by alkyl, cycloalkyl, allyl or aryl groups. Amines are classified as 1º, 2º and 3º amines according to the number of hydrogen atoms replaced by the alkyl groups. Thus, the general formula of primary, secondary and tertiary amines may be written as RNH2 , R2NH, and R3N, respectively. Nitrogen atom in Amines is sp3-hybridized so it has a tetrahedral structure.
Classification of Amines
Depending upon the number of hydrogen atoms that are replaced by an alkyl or aryl group in ammonia, amines are classified as primary (1°), secondary (2°) and tertiary (3°). If only one hydrogen atom is replaced then primary amines are obtained. If two hydrogen atoms are replaced by alkyl/aryl groups then secondary amines are formed. If all the three hydrogen atoms are replaced by alkyl/aryl group then tertiary amines are obtained.
How many the number of primary, secondary and tertiary amines in case of C4H11N?
In C4H11N amine, there are four primary amines, three secondary amines and one tertiary amines.
Primary Amines: n-butyl amine, Iso-butyl amine, Secondary butyl amine and tertiary butyl amine
Secondary Amines: n-methylpropan-1 amine, n-methyl isopropyl amine and N,N-diethyl amine
Trtiary Amines: N,N-dimethylethyl amine
Methods of Preparation of Amines
Amines are an organic compounds that are formed when one or more hydrogen atoms in an ammonia molecule are replaced with an alkyl group. Amines can be prepared by a number of methods, some of which are discussed below-
Reduction of Nitro Compounds
Amines can be prepared by reduction of nitro compounds with hydrogen gas in the presence of a metal catalyst, such as nickel, palladium, or platinum. The reduction can also be carried out with metals in an acidic medium.
IIT-JEE(Main) Shift-1 Date:22.1.2025
Find molecular weight of A
Solution:
The product (A) biphenyl have the molecular formula C12H10
So, its moleculare weight is (12 × 12) + 10 = 154
IIT-JEE(Main) Shift-1 Date:23.1.2025
Consider the following sequence of reactions and find the molecular mass of the final product (A) formed in g mol–1
Solution:
The product (A) o-bromo toluene have the molecular formula C7H7Br
So, its moleculare weight is (7 × 12) + 7 + 80 = 171
Hoffmann Bromamide Degradation Reaction
Hoffmann developed a method for preparation of primary amines by treating an amide with bromine in an aqueous or ethanolic solution of sodium hydroxide. In this degradation reaction, migration of an alkyl or aryl group takes place from carbonyl carbon of the amide to the nitrogen atom. The amine so formed contains one carbon less than that present in the amide.
Gabriel Synthesis
Gabriel synthesis is used for the preparation of primary amines. Phthalimide on treatment with ethanolic potassium hydroxide forms potassium salt of phthalimide which on heating with alkyl halide followed by alkaline hydrolysis produces the corresponding primary amine. Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.
Exercise:9.12: Why cannot aromatic primary amines be prepared by Gabriel phthalimide synthesis?
In Gabriel's phthalimide synthesis, phthalimide reacts with a base to form the salt and that salt reacts with alkyl halide and forms product.
The attack of salt on alkyl halide is SN2 process. As we know, SN2 process cannot occur in aromatic halides that is why we cannot prepare aromatic primary amines.
Ammonolysis of Alkyl Halides
When alkyl halide is heated with an alcoholic solution of excess ammonia, it undergoes a nucleophilic substitution reaction in which the halogen atom is replaced by an amino (–NH2) group to form primary amine. This process of breaking of the C – X bond by ammonia is known as ammonolysis. The reaction is carried out in a sealed tube at 373 K. The primary amine thus obtained behaves as a nucleophile and can further react with alkyl halide to form secondary and tertiary amines, and finally quaternary ammonium salt.
The free amine can be obtained from the ammonium salt by treatment with a strong base:
Reduction of Amides
Amines can be prepared by reducing amides. Reaction involves the nucleophilic addition of a hydride ion to the amide carbonyl group.
Reduction of Nitriles
Primary amines can be prepared by reducing nitriles by lithium aluminum hydride.
Leuckart Reaction
Aldehydes or ketones are converted into amines by treating with formamide or formic acid and ammonia at high temperature.
Read more Leuckart Reaction
Physical Properties of Amines
The lower aliphatic amines are gaseous with a fishy smell. Primary amines with three or higher carbon atoms are liquids at room temperature, whereas higher ones are solids. Among arylamines, the lower members are liquid while the higher are solids.
Solubility
Lower aliphatic amines are soluble in water because they form hydrogen bonds with water molecules. An increasing the size of the hydrophobic alkyl group and molar mass decreases its solubility in water. Higher amines are naturally insoluble in water. In organic solvents such as, alcohol, benzene, and ether, amines are readily dissolve.
Boiling Point
Primary and secondary amines are in intermolecular association due to polar hydrogen bonding between the nitrogen of one molecule and hydrogen of the other molecule.
In case of primary amines, the intermolecular association is more prominent as compared to secondary due to the presence and availability of two hydrogen atoms. There is no intermolecular association in tertiary amines due to the absence of a free hydrogen atom for bonding. The order of boiling point of amines is as follows:
Primary > Secondary > Tertiary.
Chemical Properties of Amines
Carbylamine Reaction
Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form isocyanides or carbylamines which are foul smelling substances. Secondary and tertiary amines do not show this reaction. This reaction is known as carbylamine reaction or isocyanide test and is used as a test for primary amines.
R-NH2 + CHCl3 + 3KOH → R-NC + 3KCl + 3H2O
Reaction with nitrous acid (HNO2)
A primary amine on reaction with nitrous acid gives an alcohol, while a secondary amine on reacting with it gives a nitroso amine. Trialkyl ammonium nitrate is formed by tertiary amine.
R–NH2 + HNO2 → R–OH + N2↑ + H2O
R2NH + HNO2 → R2N–OH + H2O
R3NH + HNO2 → R3N+ NO2– + H2O
Reaction with NOCl
A primary amine on reaction with NOCl gives an alkyl halide whereas a secondary amine on reacting with it gives an oily product nitroso amine and a tertiary amine does not react with NOCl. This reaction is called Nitrosation.
R–NH2 + NOCl → R–Cl + N2↑ + H2O
R2NH + NOCl → R2N–NO + HCl
Reaction with COCl2
This reaction is given by primary and secondary amines only.
2R–NH2 + COCl2 → R–NH–CO–NH–R + 2HCl
2R2NH + COCl2 → R2N–CO–NR2 + 2HCl
Reaction with CH3COCl or Acetylation
Acetylation of amines introduces an acetyl group into an amine. The presence of NH2 group is confirmed by using this reaction.
R–NH2 + CH3COCl → CH3–CO–NH–R + HCl
Oxidation
Primary amines on oxidation gives aldimine, which on hydrolysis gives an aldehyde while secondary amine on oxidation gives ketamine which on hydrolysis gives ketone.
Secondary amine on oxidation by Caro's acid (H2SO5) or H2O2 gives dialkyl hydroxylamine
R2NH → R2N–OH
When secondary amine is oxidised by KMnO4, tetraalkyl hydrazine is formed.
R2NH → R2N–NR2 + H2O
Tertiary amine on oxidation by Fenton's reagent or Caro's-acid gives tertiary amine oxide.
R3N → R3NO
Hoffmann Mustard Oil Reaction
Hofmann mustard oil reaction is a test for primary amines in which primary amines are warmed with alcoholic carbon disulphide and then heated with excess of mercuric chloride leading to the formation of isothiocyanates.
Electrophilic substitution Reaction of Aniline
–NH2 group is ortho and para directing and a powerful activating group.
Bromination of Aniline
Aniline reacts with bromine water at room temperature to give a white precipitate of 2,4,6-tribromoaniline.
Nitration of Aniline
Direct nitration of aniline yields tarry oxidation products in addition to the nitro derivatives. Moreover, in the strongly acidic medium, aniline is protonated to form the anilinium ion which is meta directing. That is why besides the ortho and para derivatives, significant amount of meta derivative is also formed.
However, by protecting the –NH2 group by acetylation reaction with acetic anhydride, the nitration reaction can be controlled and the p-nitro derivative can be obtained as the major product.
Sulphonation of Aniline
Aniline reacts with concentrated sulphuric acid to form anilinium hydrogensulphate which on heating with sulphuric acid at 453-473K produces p-aminobenzene sulphonic acid, commonly known as sulphanilic acid, as the major product.
Why aniline does not undergo Friedel-Crafts reaction?
Aniline does not undergo Friedel-Crafts reaction (alkylation and acetylation) due to salt formation with aluminium chloride, the Lewis acid, which is used as a catalyst. Due to this, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction.
Schotten Bauman Reaction
The Schotten-Baumann reaction synthesize amides from an amine and an acyl chloride. It involves the reaction of an amine with an acyl chloride in the presence of a base such as pyridine or sodium hydroxide. The reaction proceeds via a nucleophilic acyl substitution mechanism.
R/Ar–NH2 + R'–CO–Cl → R/Ar–NH–CO–R' + H–Cl
Arylation
Diphenyl aniline is formed when aniline reacts with chlorobenzene or phenol at high temperature and pressure.
Ph–NH2 + Ph–Cl → Ph–NH–Ph + HCl
Reaction with HNO2
Aniline reacts with nitrous acid with HCl in cold condition (0-5°) forms, benzene diazonium chloride.
Ph–NH2 + HNO2 → Ph–N=N–Cl + NaCl + 2H2O
Reduction of Aniline
Aniline redduction to aminocyclohexane.
Important Reactions of Aniline
Exercise:9.10: An aromatic compound 'A' on treatment with aqueous ammonia and heating forms compound 'B' which on heating with Br2 and KOH forms a compound 'C' of molecular formula C6H7N. Write the structures and IUPAC names of compounds A, B and C.
Solution:
Compound 'C' having the molecular formula, C6H7N is formed by heating compound 'B' with Br2 and KOH. This is a Hoffmann bromamide degradation reaction. Therefore, compound 'B' is an amide and compound 'C' is an amine. The only amine having the molecular formula, C6H7N is aniline, (C6H5NH2).
Benzamide 'B' is formed by heating compound 'A' with aqueous ammonia. Therefore, compound 'A' must be benzoic acid.
Test to distinguish among Primary, Secondary and Tertiary Amines
Hinsberg's Test
Primary amine gives N-methyl benzene sulphonamide (soluble in alkali) when reacts with hinsberg's reagent (C6H5SO2Cl).
RNH2 + C6H5SO2Cl ⟶ RNH—SO2C6H5
Secondary amine gives N,N-dimethyl benzene sulphonamide (not soluble in alkali) when reacts with hinsberg's reagent (C6H5SO2Cl).
R2NH + C6H5SO2Cl ⟶ R2N—SO2C6H5
Tertiary amine does not react with hinsberg's reagent (C6H5SO2Cl).
R3N + C6H5SO2Cl ⟶ No Reaction
IIT-JEE(Main) Shift-1 Date:23.1.2025
Which of the following react with Hinsberg reagent?
(A) Aniline
(B) N, N-Dimethyl aniline
(C) Methyl amine
(D) C6H5NHC6H5
A. A only
B. A and C only
C. A, C and D
D. A and B only
Solution:
Option C is correct answer.
Hinsberg reagent (C6H5SO2Cl) reacts with primary and secondary amines only, but not tertiary amines. N, N-Dimethyl aniline is a tertiary amine so it cannot react with Hinsberg reagent.
Read Hinsberg's Test
Read Tests for Amines.
Exercise:9.4: Why are aliphatic amines stronger bases than aromatic amines?
Solution:
Aliphatic amines are stronger bases than aromatic amines because the lone pair of electrons on the nitrogen atom in aromatic amines is less available for donation because of the resonance effect (-R effect), which involves the delocalization of the lone pair of electrons with the aromatic nucleus.
The alkyl group in aliphatic amines has an electron donating (+I) effect, which increases the electron density on the nitrogen atom. This makes aliphatic amines stronger bases.
Benzene Diazonium Salts
The diazonium salts have the general Formula R–N2–X where R represents an aryl group & X– ion is the Cl–, Br–, HSO4–, BF4–, etc. The N2 group is a nitrogen atom connected to another Nitrogen by a triple bond (N≡N) called the diazonium.
Read Benzene Diazonium Chloride in details.
