NCERT Solutions for Class 12 Chemistry Chapter 13 - Organic Compounds containing Nitrogen (Amines and Diazonium Salts)

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(i) Since propanamine contains 3 carbon atoms, the amide molecule must contain four carbon atoms.

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The decreasing order of basic strength is:
(C₂H₅)₂ NH > C₂H₅NH₂ > NH₃> C₆H₅NH₂

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1. Primary (1$°$)
2. Tertiary(3$°$)
3. Primary (1$°$)
4. Secondary (2$°$)

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(i), (ii)
Primary amines

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Four structural isomers are possible

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1. Propan-2-amine (1$°$)
2. Propan-1-amine (1$°$)
3. N-methylpropan-2-amine (2$°$)
4. 2-Methylpropan-2-amine (1$°$)
5. N-Methylbenzenamine(2$°$)
6. N-Ethyl-N-methylethanamine (3$°$)
7. 3-Bromobenzenamine or 3-Bromoaniline (1$°$)

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(i) These can be distinguished by carbylamine test. When heated with alcoholic solution of KOH and chloroform, methylamine gives foul smell of methyl isocyanide.

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(i) In aniline, the lone pair of electrons on N atom is delocalized over the benzene ring. As a result, electron density on the nitrogen decreases. On the other hand, in CH₃NH₂, +I effect of CH₃ group increases the electron density on N atom. Therefore, aniline is less basic than methylamine and hence pKb of aniline is higher than that of methylamine.

(ii) Ethylamine dissolves in water due to intermolecular hydrogen bonding as shown below:

However, because of large hydrophobic part (i.e., hydrocarbon part) of aniline, the extent of hydrogen bonding is less and therefore, aniline is insoluble in water.

(iii) Methylamine is more basic than water and therefore, accepts a proton from water forming OH– ions

(iv) Under strongly acidic conditions of nitration (in the presence of a mixture of conc. HNO₃ + H₂SO₄), aniline gets protonated and is converted into anilinium ion having –NH₃⁺ group. This group is deactivating group and is m-directing. So, the nitration of aniline gives o, p-nitroaniline (mainly p-product) while the nitration of anilinium ion gives m-nitroaniline.

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The primary, secondary and tertiary amines can be distinguished by the following tests :

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(i) Carbylamine reaction. When a primary amine is warmed with chloroform and alcoholic potash, an alkyl isocyanide (carbylamine) is formed which gives very foul smell.

This reaction is not given by secondary and tertiary amines. Therefore, this reaction provides an excellent test for the identification of a primary amine.

(ii) Diazotisation. The reaction of aniline or other aromatic amines with nitrous acid at 0–5$°$C to form diazonium salts is called diazotisation. Nitrous acid needed for this reaction is prepared in situ by the action of dil. HCl on NaNO₂.

(iii) From amides by Hoffmann’s degradation method. Primary amines can be prepared from amides by treatment with Br₂ and KOH solution. The amine formed contains one carbon atom less than the parent amide.

(iv) Coupling reaction. Aromatic amines react with diazonium salts to form azo compounds in acidic medium called dyes. The reaction is known as coupling or diazo reaction. For example, aniline couples with benzene diazonium chloride to form diazo amino benzene which ultimately changes to p-amino azo benzene on warming with a small quantity of hydrochloric acid.

(v) Ammonolysis of alkyl halides. The hydrogen atoms in ammonia can be substituted by alkyl groups by heating alkyl halides with ammonia.

The primary amine so formed reacts further with alkyl halides to give secondary and tertiary amines and quaternary salts.

(vi) Acylation. Primary and secondary amines (which contain replaceable hydrogen atoms) react with acid chlorides to form substituted amides. For example,

Since tertiary amines do not contain replaceable hydrogen atom, they donot react with acetyl chloride.

(vii) Gabriel’s phthalimide synthesis. This method is used for preparing only primary amines. In this method, phthalimide is treated with alcoholic KOH to give potassium phthalimide, which is treated with alkyl halide or benzyl halide to form N-alkyl or aryl phthalimide. The hydrolysis of N-alkyl phthalimide with 20% HCl under pressure or refluxing with NaOH gives primary amine.

Phthalic acid can again be converted into phthalimide and is used again and again. This method is very useful because it gives pure amines. Aryl halides cannot be converted to aryl amines by Gabriel synthesis because they do not undergo nucleophilic substitution with potassium phthalimide.

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1. Since the compound C of molecular formula C₆H₇N is formed from B on treatment with Br₂ and KOH (Hoffmann bromamide reaction), therefore, the compound ‘B’ must be an amide and ‘C’ must be an amine. The only aromatic amine having molecular formula C₆H₇N is C₆H₅NH₂ (aniline).
2. Since ‘C’ is aniline, the amide from which is formed by must be benzamide (C₆H₅CONH₂).

   

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Aromatic amines such as aniline cannot be prepared by Gabriel phthalimide reaction because it requires the treatment of potassium phthalimide with C₆H₅Cl or C_6H5Br. Since aryl halides donot undergo nucleophilic substitution reaction under ordinary conditions, therefore, the reaction does not occur. Hence, aniline cannot be prepared by this method.

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Aromatic primary amines react with HNO₂ at 273–278 K to form aromatic diazonium salts.

Aliphatic primary amines also react with HNO2 at 273–278 K to form aliphatic diazonium salts. However, these are unstable even at low temperature and therefore, decompose readily to form alcohols (generally predominate) and N₂ is evolved.

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1. Loss of proton from amines give amide ion whereas loss of a proton from alcohol gives an alkoxide ion.

   

   Since O is more electronegative than N, therefore, RO_– can accommodate the –ve charge more easily than RNH_–. Consequently, RO_– is more stable than RNH_–. Thus, alcohols are more acidic than amines.
2. Primary amines (RNH₂) have two hydrogen atoms on the N atom and therefore, form intermolecular hydrogen bonding.

   

   Tertiary amines (R₃N) donot have hydrogen atoms on the N atom and therefore, these donot form hydrogen bonds. As a result of hydrogen bonding in primary amines, they have higher boiling points than tertiary amines of comparable molecular mass. For example, b.p. of n-butylamine is 351 K while that of tert-butylamine is 319 K.
3. Both arylamines and alkylamines are basic in nature due to the presence of lone pair on N-atom. But arylamines are less basic than alkylamines. For example, aniline is less basic than ethylamine as shown by K_b values :

Ethylamine : K_b = 4.7 × 10^–4 Aniline : K_b = 4.2 × 10^–10
The less basic character of aniline can be explained on the basis of aromatic ring present in aniline. Aniline can have the following resonating structures :

It is clear from the above resonating structures that three of these (III, IV and V) acquire some positive charge on N atom. As a result, the pair of electrons become less available for protonation. Hence, aniline is less basic than ethyl amine in which there is no such resonance.