This article is on the Amines Notes Class 12 of Chemistry. The notes on Amines of class 12 chemistry have been prepared with great care keeping in mind the effectiveness of it for the students. This article provides the revision notes of the Amines chapter of Class 12 for the students so that they can give a quick glance of the chapter.
Alkyl derivatives of NH3 are called Amines. If a hydrogen atom of NH3 is replaced by an alkyl group, then it is called a primary amine and possesses -NH2 (amino) group. When two hydrogen atoms of NH3 are replaced then it is called a secondary amine and it possesses -NH (Imino) group. If all hydrogen atoms of NH3 are replaced then it is called a tertiary amine and has a nitrile N group. N is in sp3 hybridisation and has tetrahedral geometry. The bond angle increases from ammonia to 3o amines. NH (107o) < RNH2 < R2NH < R3N
Amines can be classified as 1°, 2° or 3° amines depending on the number of alkyl/aryl groups present in ammonia molecule, RNH2(1° amine), R2N-H(2° amine) and R3N(3° amine).
In a common system, an aliphatic amine is named by prefixing alkyl group to an amine, i.e., alkylamine. Amines are named as alkanamines in the IUPAC system. In secondary and tertiary amines, when two or more groups are the same, the prefix di or tri is appended before the name of the alkyl group.
STRUCTURE AND BONDING
Nitrogen orbitals in amines are sp3 hybridised and the geometry of amines is pyramidal. Due to the presence of an unshared pair of electrons, the angle C — N— R, (where R is C or H) is less than 109.5°.
PREPARATION OF AMINES
(a) From Nitro Compounds
R—NO2 — reduce → R—NH2
(ii) (1) Fe, HCl (2) OH–
(iii) (1) Zn, or Sn, or SnCl2 + HCl (2) OH–
(b) Hofmann’s Ammonolysis Reactions:
(i) RX limited to methyl and 1o alkyl halides.
(ii) Complicated by polyalkylation.
(iii) Useful for preparing 1o amines and quaternary ammonium salts.
(c) From Nitriles
(d) From Amides
(e) Gabriel Phthalamide Synthesis:
(f) Hofmann’s Bromamide Reaction:
PHYSICAL PROPERTIES OF AMINES
(a) Unlike most other organic compounds, amines are far more soluble in water, this is because all amines form a stronger H–bond with water.
(b) Like ammonia, amines are polar compounds and apart from 3o amines, they can form intermolecular H– bonds that’s why they have higher b.p.s’.
(c) The boiling points of amines are lesser than of alcohols and acids of comparable mol. weight because H– bonding in amines is less pronounced in 1o and 2o than that in alcohols and carboxylic acids. This is because nitrogen is less electronegative than oxygen.
(d) The boiling point of 1o, 2o and 3o amines follow the order.
1o > 2o > 3o amine
(e) Solubility in water follows the order.
1o > 2o > 3o amine
This is all due to H–Bonding.
CHEMICAL REACTIONS OF AMINES
Basicity of Amines:
Due to basic nature, they react with acids to form salts.
Amine salts on treatment with bases such as NaOH regenerate the parent amine.
Relative Basicity of Amines: Compare basicities by using conjugate acid pKa values.
(a) An Amine and Ammonia
H-NH3+ pK 9.3 NH3 (weaker base)
CH3CH2-NH3+ pK 10.8 CH3CH2-NH2(strong base)
(i) Alkylamines are stronger bases than NH3
(ii) Electron-donating alkyl group increase the electron density on N.
(b) An Alkylamine and an Arylamine
Aniline is a weaker base that ethylamine. Why?
(i) Electron donors (D) increase basicity. D can be –NH2, -OH, -OR, -NHCOR, -R
(ii) Electron withdrawers (W) decrease basicity. W can be -X, -CHO, -COR, -COOR, -COOH, -CN, -SO2H, -NO2, -NR3
(c) An Alkylamine and an Amide
(i) Amides are less basic than alkyl amines.
(ii) The preferred site of protonation is on oxygen.
(b) Carbylamine Reaction (Test for 1º Amine):
It consists of heating a mixture of 1º aliphatic or aromatic amines and chloroform with alcoholic KOH solution. The reaction proceeds via carbene mechanism.
R – NH2 + CHCl3 + 3KOH (Alcoholic) –∆→ R – N+ ≡ C– + 3KCl + 3H2O
(c) Reaction with arylsulphonyl chloride
Hinsberg’s Test (for distinguishing 1º, 2º and 3º amines): Three classes of amines are distinguished by Hinsberg reagent test. The Hinsberg reagent is a benzene sulphonyl chloride (C6H5SO2Cl).
1. Reaction of Hinsberg Reagent with Primary Amines: A primary amine forms a precipitate of N-alkyl benzene sulphonamide with Hinsberg reagent. This precipitate is soluble in alkali.
RNH2 + C6H5SO2Cl → R-NH-SO2-C6H5 — NaOH→ R-N-Na+– SO2-C6H5 (soluble)
2. Reaction of Hinsberg Reagent with a Secondary Amine: Secondary amine reacts with the Hinsberg reagent to form a precipitate N, N-dialkyl benzene sulphonamide. But this precipitate is insoluble in alkalis.
R2NH + C6H5SO2Cl → R2N-SO2-C6H5 — NaOH→ Insoluble (no reaction)
3.Reaction of Hinsberg Reagent with a Tertiary Amine: Tertiary amines do not react with Hinsberg reagent.
R3N + C6H5SO2Cl → No reaction
(d) Reaction with Nitrous acid (For Distinguishing 1o, 2o and 3o amines):
Nitrous acid (HNO2 or HONO) reacts with aliphatic amines in a fashion that provides a useful test for distinguishing, primary, secondary and tertiary amines.
1 o -Amines + HONO (cold acidic solution) → Nitrogen Gas Evolution from a Clear Solution
2 o -Amines + HONO (cold acidic solution) → An Insoluble Oil (N-Nitrosamine)
3 o -Amines + HONO (cold acidic solution) → A Clear Solution (Ammonium Salt Formation)
Electrophilic Substitution Reaction for Aniline
Electrophilic substitution: Due to +M eﬀect of –NH2 generate electron density at ortho and para position hence, aniline active toward electrophilic substitution
(a) Bromination: Aniline reacts with bromine water at room temperature to give a white precipitate of 2,4,6-tribromoaniline.
If we have to prepare a monosubstituted aniline derivative. This can be done by protecting the –NH2 group by acetylation with an acetic anhydride, then carrying out the desired substitution followed by the hydrolysis of the substituted amide to the substituted amine.
The lone pair of electrons on the nitrogen of acetanilide interacts with oxygen atom due to resonance as shown below:
Hence, the lone pair of electrons on nitrogen is less available for donation to the benzene ring by resonance. Therefore, activating the eﬀect of –NHCOCH3 group is less than that of amino group.
(b) Nitration: Direct nitration of aniline is not possible because in the strongly acidic medium, aniline is protonated to form the anilinium ion which is meta directing.
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.
The diazonium salts have the general formula RN2X where R stands for an aryl group and X ion may be Cl, Br, HSO4, BF3 etc. Resonance of benzene diazonium ion is
Chemical Reactions Involving Displacement of Nitrogen
(a) Replacement by ‒OH group: ArN2Cl + HOH — Boiling→ ArOH + N2 + HCl
(b) Replacement by H: ON reduction with sodium stannite (Na2SnO2) (alkaline SnCl2) or hypophosphorous acid (H3PO2) or on warming with C2H5OH, ArN2Cl gives benzene.
ArN2Cl+H3PO2+H2O → ArH+N2+2CuCl2
ArN2Cl+Na2SnO2+H2O → ArH+N2+ Na2SnO3+HCl
(c) Sandmeyer reaction (replacement by halogen):
ArN2Cl+Cu2Cl2+HCl → ArCl+N2+2CuCl2
ArN2Br+Cu2Br2+HBr → ArBr+N2+2CuBr2
(d) Gattermann reaction:
ArN2Cl —divided Cu/HCl→ ArCl + N2
ArN2Br — Finely divided Cu/HBr → ArBr + N2
(e) Iodo compounds:
ArN2Cl + KI (aq.) — Boiling → Ar–I + N2 + KCl
(f) Balz-Schiemann reaction:
(g) Cyano compounds:
ArN2Cl + KCN — or Cu powder CuCN → ArCN + N2 + KCl
ArN2Cl + KCN — K3[Cu(CN)4]→ ArCN + N2 + KCl
(h) Nitro compounds:
ArN2Cl + HONO — Cu2O → ArNO2 + N2 + HCl
ArN2Cl + CuNO2 — ∆→ ArNO2 + N2 + CuCl
ArN2⊕BF4⊖ + NaNO2 —∆/Cu powder → Ar–NO2 + NaBF4 + N2
(i) Gomberg reaction (replacement by an aryl group)
Whatever is the nature of the substituent in the second component, o-and p-substitutions occur and the reaction takes place by the free radical mechanism.
Coupling Reactions (Retention of Diazo Group)
(d) With an excess of diazonium salts, the bis-azo (o- and p-) and the tris-azo compounds may be produced.
The introduction of a second azo group is facilitated by the presence of an alkyl group in the para-position to the hydroxyl group or by two (–OH) groups in the m-position.
(e) Dye test (test for 1o aromatic amine)
Coupling with α -or 1-naphthol takes places at the position shown by an arrow.
This article has tried to highlight all the important points of Amines in the form of notes for class 12 students in order to understand the basic concepts of the chapter has also been designed for the different competitive exams such as iit jee, neet, etc.