Class 12 Chemistry Amines Structure-basicity relationship of amines

Structure-basicity relationship of amines

  • Basicity of amines to their structure.
  1. Alkanamines versus ammonia
    1. Reaction of alkanamines and ammonia in order to compare their basicity.

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Amines

    1.  Due to the electron releasing nature of alkyl group, it (R) pushes electrons towards nitrogen and thus making them available for the unshared electron pair for the proton of the acid.
    2. The substituted ammonium ion formed from the amine gets stabilised due to dispersal of the positive charge by the +I effect of the alkyl group.
    3. Hence, alkylamines are stronger bases than ammonia. Thus, the basic nature of aliphatic amines should increase with increase in the number of alkyl groups.
    4. The order of basicity of amines in the gaseous phase follows the expected order: tertiary amine > secondary amine > primary amine > NH3.
    5. But this not same in aqueous state, in that the substituted ammonium cations get stabilised not only by electron releasing effect of the alkyl group (+I) but also by solvation with water molecules.
    6. The greater the size of the ion, lesser will be the solvation and the less stabilised is the ion.
    7. The order of stability of ions are as follows:

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Amines1

    1. Thus, the order of basicity of aliphatic amines should be: primary > secondary > tertiary, which is opposite to the inductive effect based order.
    2. When the alkyl group is small, like –CH3 group, there is no steric hindrance to H-bonding.
    3. In case the alkyl group is bigger than CH3 group, there will be steric hindrance to H-bonding.
    4. Therefore, the change of nature of the alkyl group, e.g., from –CH3 to –C2H5 results in change of the order of basic strength.
    5. Thus, there is a subtle interplay of the inductive effect, solvation effect and steric hindrance of the alkyl group which decides the basic strength of alkyl amines in the aqueous state.
    6. The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows:
  • (C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3
  • (CH3)2NH > CH3NH2 > (CH3) 3N > NH3
  1. Arylamines versus ammonia
    1. pKb value of aniline is quite high because in aniline or other arylamines, the -NH2  group is attached directly to the benzene ring.
    2. As a result the unshared electron pair on nitrogen atom will be less available for protonation as it is in conjugation with the benzene ring.
    3. Aniline is resonance hybrid of 5 resonance structures.

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Amines2

    1. On the other hand, anilinium ion obtained by accepting a proton can have only two resonating structures (kekule).

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Amines3

    1. Aniline is more stable than anilinium ion as it has greater number of resonating structure.
    2. Therefore, the proton acceptability or the basic nature of aniline or other aromatic amines would be less than that of ammonia.
    3. In case of substituted aniline, it is observed that electron releasing groups like –OCH3 , –CH3  increase basic strength whereas electron withdrawing groups like –NO2 , –SO3H, –COOH, –X decrease it.
  1. Alkylation
  1. Amines undergo alkylation on reaction with alkyl halides.
  1. Acylation
  1. The reaction of aliphatic and aromatic primary and secondary amines with acid chlorides, anhydrides and esters by nucleophilic substitution reaction is known as Acylation.
  2. In this reaction there will be replacement of hydrogen atom of –NH2 or >N–H group by the acyl group.
  3. The products obtained by acylation reaction are known as amides. The reaction is carried out in the presence of a base stronger than the amine, like pyridine, which removes HCl so formed and shifts the equilibrium to the right hand side.

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Amines_Acylation

  1. Amines also react with benzoyl chloride (C6H5 COCl). This reaction is known as benzoylation.

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Amines_Benzoylation

  1. They form salts when they react with carboxylic acids at room temperature.
  1. Carbylamine reaction
  1. Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form isocyanides or carbylamines which are foul smelling substances.
  2. Secondary and tertiary amines do not show this reaction. This reaction is known as carbylamines reaction or isocyanide test and is used as a test or primary amines.

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Amines_Carbylamine_Reaction

  1. Reaction with nitrous acid
  1. Different types of amines react differently with nitrous acid which is prepared in situ from a mineral acid and sodium nitrite.

(i)  Primary Amines:-

  1. Primary aliphatic amines react with nitrous acid to form aliphatic diazonium salts which being unstable, liberate nitrogen gas quantitatively and alcohols.
  2. Quantitative evolution of nitrogen is used in estimation of amino acids and proteins.

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Primary_Amines_Reaction

   (ii)  Secondary Amines:-

    1.   Aromatic amines react with nitrous acid at low temperatures (273-278 K) to form diazonium salts, a very important class of compounds used for synthesis of a variety of aromatic compounds.

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Secondary_Amines_Reaction

  • Secondary and tertiary amines react with nitrous acid in a different manner.
  1. Reaction with aryl sulphonyl chloride
  • Reaction with Benzene sulphonyl Chloride: - Benzene sulphonyl chloride (C6H5SO2Cl), which is also known as Hinsberg’s reagent, reacts with primary and secondary amines to form sulphonamides.
  1. The reaction of benzenesulphonyl chloride with primary amine yields N-ethylbenzene sulphonyl amide.

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Secondary_Amines_Reaction_1

  1. The hydrogen attached to nitrogen in sulphonamide is strongly acidic due to the presence of strong electron withdrawing sulphonyl group.
  2. Hence, it is soluble in alkali.
  • Reaction of secondary Amine benzenesulphonyl chloride: - In this reaction, N, N-diethylbenzenesulphonamide is formed.
  1. Since N, N-diethyl benzene sulphonamide does not contain any hydrogen atom attached to nitrogen atom; it is not acidic and hence insoluble in alkali.

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Secondary_Amines_Reaction_2

  • Tertiary amines do not react with benzenesulphonyl chloride.
  • Note:-
    1. Amine reacting with benzenesulphonyl chloride in a different manner is used for the distinction of primary, secondary and tertiary amines and also for the separation of a mixture of amines.
    2. These days benzenesulphonyl chloride is replaced by p-toluenesulphonyl chloride.
  1. Electrophilic Substitution
  • Bromination :-
  • Aniline reacts with bromine water at room temperature to give a white precipitate of 2, 4, 6-tribromoaniline.
  • Because of high reactivity of aromatic amines problems occur during electrophilic substitution because it occurs at ortho- and para-positions.

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Bromination

  • In order to prepare monosubstituted aniline derivative activating effect of –NH2 group be controlled done by protecting the -NH2  group by acetylation with acetic anhydride, then carrying out the desired substitution followed by hydrolysis of the substituted amide to the substituted amine.

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Bromination1

  • The lone pair of electrons on nitrogen of acetanilide interacts with oxygen atom due to resonance as shown below:

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Bromination2

  • Hence, the lone pair of electrons on nitrogen is less available for donation to benzene ring by resonance.
  • Therefore, activating effect of –NHCOCH3 group is less than that of amino group.
  • Nitration:-
  • 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.

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Amines_Nitration

  • In order to protect –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.

Class_12_Chemistry_Amines_Structure-Basicity_Relationship_Of_Amines_Nitration1

  • Sulphonation:-
  • 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.
  • 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.

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