Amines (Carboxylic Acids), Class 12, Chemistry Detailed Chapter Notes PDF Download

 Basic Character and  Preparation of Amines (all methods) - Amines, Class 12, Chemistry

1. BASIC NATURE OF AMINES

1. Aniline is less basic than ammonia. The phyenyl group exerts- I (inductive effect, i.e., it withdraw electrons. This results to the lower availability of electrons on nitrogen for protonation.

2. Ethylamine and acetamide both contain an amino group but acetamide does not show basic nature. This is because lone pair of electrons on nitrogen is delocalized by resonance with the carbonyl group which makes it less available for protonation.

3. The compound with least `s' character (sp<sup>3</sup> hydrolized) is most basic and with more `s' character (sp-hydrilized) is least basic. Examples in decreasing order of basicity are :

(i) 

(ii) CH₃CH₂CH₂NH₂ > H₂C = CHCH₂NH₂ > HC  CCH₂NH₂

(iii) (CH₃)₂NH > CH₃NH₂>NH₃ > C₆H₅NH₂

Electron withdrawing (C₆H₅—) groups cause decrease in electron density on nitrogen atom and there by decreasing basicity.

(v) CH₃CH₂NH₂ > C₆H₅CONH₂ > CH₃CONH₂

2. METHODS OF PREPARATION

1. Hafmann's bromamide reaction : Amines (only primary) can also be prepared by Hoffmann degradation. In this method the amine will have one carbon atom less than the amide. The reaction proceeds via formation of nitrene.

Mechanism of above reaction has been proposed as given below :

Br₂  KOH  K O^—Br HBr

Mechanism

2NaOH Br₂  NaOBr NaBr H₂O

(a)  (b)

R — NH₂  CO₂  R — N = C = O (Isocyanate)

2. Curtius, Schmidt and Lossen Rearrangment : These reaction are basically rearragment reaction in which carbon migrates from carbon to nitrogen with the formation of an isocyanate. In these migration i.e., 1, 2 shift, migrating group is an alkyl or aryl group and leaving group may be

— Br in Hoffmann rearrangement

— N₂ in curtius and Schmidt rearrangement

The isocyanate formed on hydrolysis gives amine.

(a) Curtius Reaction : Acid chloride on treatment with sodium azide give acid azides which on pyrolysis gives isocyanates on hydrolysis gives corresponding amines.

RCOCl NaN₃  RCON₃  NaCl

(b) Schmidt Reaction : Carboxylic acid reacts with hydrozoic acid in presence of concentrated H₂SO₄ to give isocyanates.

(c) Lossen Reaction : Hydroxylamine on treatement with acid chloride gives acyl derivatives of hydroxyl amine the acyl derivatives exist in two tautomeric form keto form called hydroxamic form and enol form called hydroximic acid. The hydroxamic form.

The hydroxamic form (keto form) forms o-acyl derivatives of hydroxamic form which on heating with bases forms isocyanates and finally amines upon hydrolysis.

3. By reduction of nitrothane :

4. By reduction of methyl cyanide :

5. By reduction of acetamide : Ethylamine is obtained by reduction of acetamide with sodium and absolute alcohol or LiAlH₄ in ether or hydrogen in presence of nickel catalyst.

6. By reduction of aldoxime : Aldoxime on reduction with hydrogen and nickel catalyst or sodium and absolute alcohol or LiAlH₄ in ether yields ethylamine.

7. By the hydrolysis of ethyl isocyanate : Ethyl isocyanate on heating with caustic potash solution undergoes hydrolysis forming ethylamine.

8. By the acid hydrolysis of ethyl isocyanide : Ethyl isocyanide undergoes hydrolysis with a mineral acid and forms ethylamine.

9. By Schmidt reaction :

In this reaction the acyl azide (RCON)₂ and alkyl isocyanate (R —NCO) are formed as an intermediate.

10. By the action of chloramine on Grignard reagent : When chloramine reacts with ethyl magnesium iodide, the formation of ethylmine occurs.

11. By Gabriel's phthalimide reaction :

12. Laboratory preparation of ethylamine : Ethylamine is prepared in the laboratory by Hofmann's bromide reaction. Proprionamides is heat with bromine and pottassium hydroxide solution.

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Physical and Chemical Properties of Amines - Amines, Class 12, Chemistry

3. physical properties

(a) It is a colourless inflammable liquid. It's boiling point is 19ºC.

(b) It has fishy ammonical odour.

(c) It is highly soluble in water. Its aqueous solution is basic in nature and turns red limus blue.

The solubility in water is due to hydrogen bonding with water moelcules.

4. chemical properties

(i) Basic nature : It is more basic than ammonia. Following reactions prove its basic nature

(a) It forms ethyl ammonium hydroxide when dissolved in water.

Ethyl ammonium hydroxide ionises to give OH^_ ions

C₂H₅NH₃OH  C₂H₅ 

(b) It reacts with acids to form salts.

C₂H₅NH₂  HCl  C₂H₅NH₃Cl or C₂H₅NH₂.HCl

Ethyl ammonium chloride

or

Ethylamine hydrochloride

C₂H₅NH₂  H₂SO₄  (C₂H₅NH₃)₂ SO₄

Ethyl ammonium sulphate

(c) Its aqueous solution behaves like ammonium hdyroxide. The aqueous solution of ethylamine precipitates iron, chlromium and aluminium as hydroxides when salts are treated with it.

(d) It's hydrochloride, like ammonium chloride, forms double salts with PtCl₄ and AuCl₃

2C₂H₅NH₃Cl PtCl₄  

C₂H₅NH₃Cl AuCl₃ 

These double salts decomposes on heating to pure metal and this method is used to determine the molecular mass of amines.

(ii) Reaction with alkyl halides (Alkylation) :

Ethylamine reacts with alkyl halides and form secondary, tertiary amines and quaternary ammonium salt.

(iii) Reaction with acetyl chloride or acetic anhydride : Acetylation takes place when ethylamine combines with acetyl chloride or acetic anhydride.

(iv) Carbylamine Reaction

Nucleophilic RNH₂ attacks electrophilic intermediater [:CCl₂] dichlorocarbene.

This reaction is used for the detection of primary amines.

Mechanism

(v) Reaction with sodium : Hydrogen is evolved when ethylmine is heated with sodium.

2C₂H₅NH₂  2Na  2C₂H₅NHNa H₂

Sodium derivative

of ethylamine

(vi) Reaction with Grignard reagents : Ethylamine reacts with Gragnard reagents to form alkanes.

(vii) Hofmann's mustard oil reaction : Carbon disulphide reacts with ethylamine in presence HgCl₂ to form ethyl isothiocyanate which has a mustard oil like smell.

C₂H₅NH₂  CS₂  HgCl₂  C₂H₅N = C = S 2HCl

Ethyl isothiocyanate

(viii) Reaction with aldehydes : Ethylamine reacts additively with aldehydes to form a-hydroxyl amines which are changed to Schiff bases with the elimination of water molecule.

DISTINCTION BETWEEN PRIMARY, SECONDARY AND TERTIARY AMINES

5. general chemical properties of aromatic amines

5.1 reaction with nitrous acid

1. Primary Amines : Primary amines react with nitrous acid to produce diazonium ion as follows.

ArNH₂  HNO₂  Ar — N N :

R — NH₂  HNO₂  R — N N

But the diazonium ions of aliphatic amines are very unstable and produces carbocation immediately, which can produce different products.

R — N  N  R  (Carbocation)

6. Ring reactions of aromatic amines

NH₂ — NHR and —NR₂ strongly activate the benene ring toward electrophilic substitution

1. Haogenation : For monohalogenation, —NH₂ is first acetylated, because

is only moderately activating and is o and p-director in nature

2. Sulfonation :

The dipolar ion structure of sulfanilic acid account for its (a) high melting point, (b) insolubility in H₂O and organic solvents, (c) solubility in aqueous NaOH, (d) insolubility in aqeous HCl.

will not exists as a dipolar ino since, _COOH is too weakly acidic to transfer an H to the weakly basic _NH₂ attached to the electron withdrawing benzene ring. When attached to an aliphatic C, the NH₂ is sufficiently basic to accept H _ from COOH.

(i) The Hinsberg Test

Ths test can be used to demonstrate whether an amine is primary, secondary or tertiary. Primary amines react with benzenesulfonyl chloride to form N-substituted benzenesulfonamides. These, in turn, undergo acid-base reactions with the excess potassium hydroxide to form water - soluble potassium salt. Acidification of this solution will, cause the water-soluble in the next stage, cause the water-insoluble N-substituted sulfonamide to precipitate.

Secondary amies react with benzenesulfonyl chloride in aqueous potassium hydryoxide to form insoluble, N - N-disubtituted sulfonamides that precipitate after the first stage. N₃N_

Disubstituted sulfonamides do not dissolved in aqueous potassium hydroxide.

If the amine is a tertiary amine and if it is water insoluble, no apprated change will take place in the mixture as we shake it with benzenesulfonyl choride and aqueous KOH. When we acidify the mixture, the tertiary amine dissolves because it forms a water soluble salt.