Account for the following:

(i) pK_b of aniline is more than that of methylamine.

(ii) Ethylamine is soluble in water whereas aniline is not.

(iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide.

(iv) Although amino group is o– and p– directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of mnitroaniline.

(v) Aniline does not undergo Friedel-Crafts reaction.

(vi) Diazonium salts of aromatic amines are more stable than those of aliphatic amines.

(vii) Gabriel phthalimide synthesis is preferred for synthesising primary amines?

(i) pK_b of aniline is more than that of methylamine.       

pk_b value is the negative logarithm of the basicity constant (K _b) .           i.e.,

pK_b = -log K_b

 

Evidently, smaller the value of pK_b, stronger is the base (strong tendency to donate electrons)  the structure of methyl amine is CH ₃NH₂  the structure of aniline is:

 

Aniline (C₆H₅NH₂) shows resonance:

 

As a result of resonance, the lone pair of electrons on the nitrogen atom gets delocalized over benzene ring. As a result, electron density on the nitrogen decreases and thus is less easily available to donate electrons making it less basic. In contrast, in methyl amine (CH ₃NH₂), delocalization of the lone pair of electrons on the nitrogen atom by resonance is not possible. Furthermore, CH ₃ being an electron- releasing group, due to which +I effect of CH ₃ increases the electron density on the N- atom and thus is more easily available to donate electrons making it more basic.

 

(+I effect) (Pushing electrons)

Therefore, aniline is weaker base than methylamine and hence its pK _b value is higher than that of methylamine.

(ii) Ethylamine is soluble in water whereas aniline is not.

The structure of ethylamine is CH ₃CH₂NH₂  the structure of aniline is:

 

Ethylamine form H-bonds with water. It dissolves in water due to intermolecular H-bonding as shown below:

 

Thus, ethylamine is soluble in water.

However, in aniline, due to the larger hydrophobic part, i.e., hydrocarbon part (C₆H₅ group), which tends to retard the formation of H-bonds. The extent of H-bonding decreases and hence aniline is insoluble in water.

(iii) Methylamine in water reacts with ferric chloride to precipitate

hydrated ferric oxide.

The structure of methylamine is CH₃NH₂

Methylamine is more basic than water due to the presence of CH ₃ (electron releasing group) and +I effect (pushing electrons). Being more basic, methylamine accepts a proton from water liberating OH ^- ions.

 

Dissociation of ferric chloride in water to give Fe³⁺ and Cl^-

FeCl₃→ Fe³⁺ + 3Cl^-

Now, the liberated OH ^- ions combine with Fe³⁺ ions present in H₂O to form brown precipitate of hydrated ferric oxide.

2Fe³⁺ + 6OH^- à 2Fe (OH) ₃ or Fe₂O₃.3H₂O

Hydrated Ferric Oxide (brown ppt)

(iv) Although amino group is o– and p– directing in aromatic electrophilic

substitution reactions, aniline on nitration gives a substantial amount of m-nitroaniline.

The below diagram shows the position of ortho (o), para (p) and meta (m) derivatives of amino group:

 

Electrophilic addition reaction of amines: In addition to the reaction of the amino group (NH₂ group), aromatic amines also undergo typical electrophilic substitution reactions of the aromatic ring. In all these reactions, the NH₂ group strongly activates the aromatic ring through delocalization of the lone pair of electrons of the N-atom over the aromatic ring.

As a result, electron density increases more at ortho and para positions. Therefore NH ₂ group directs the incoming group to ortho and para positions, i.e., NH ₂ are an o-, p-directing group. But it has been observed that on nitration of aniline gives a substantial amount of m-nitroanilne.

Explanation: Nitration is usually carried out in acidic medium in the presence of concentrated HNO₃ and concentrated H₂SO₄. As a result, most of the aniline is converted into anilinium ion (gets protonated) and since - +NH₃ is m-directing group, therefore, an unexpected large amount of m-nitroaniline is obtained.

 

Nitration of aniline gives mainly p-nitroaniline

 

Nitration of anilinium ions gives m-nitroanilne (due to protonation)

 

Hence, nitration of aniline gives a mixture of p-nitroaniline and mnitroaniline in approx. 1:1 ratio.

 

(v) Aniline does not undergo Friedel-Crafts reaction.

 Friedel- Crafts reaction: When any benzene or its derivative is treated with alkyl halide (R-X, X=Cl) or acetyl chloride (CH₃-COCl) in the presence of anhydrous aluminium chloride (AlCl ₃) to form alkyl or acetyl substituted benzene or its derivative, this reaction is called Friedel-Crafts reaction.

 

 Friedel-Crafts alkylation: When any benzene or its derivative is treated with alkyl halides (R-X, X=Cl, Br) in the presence of anhydrous aluminium chloride (AlCl₃) to form alkyl substituted benzene or its derivatives, this reaction is called Friedel-Crafts alkylation. For example:

 

Friedel-Crafts acylation: When any benzene or its derivative is treated with acetyl chloride (R-COCl) in the presence of anhydrous aluminium chloride (AlCl ₃) to form acetyl substituted benzene or its derivatives, this reaction is called Friedel-Crafts acylation. For example:

 

Aniline does not undergo Friedel-Crafts reaction.

Explanation: Aniline is a Lewis base (electron-pair acceptor) while AlCl₃ is Lewis acid (electron-pair donor). They combine with each to form a salt.

 

Due to presence of positive charge on nitrogen (N) atom in the salt, the group N⁺H₂AlCl₃^- acts as a strong electron withdrawing group (strong deactivating group). As a result, it reduces the electron density in the benzene ring and hence aniline does not undergo Friedel-Crafts (alkylation or acetylation) reaction

(vi) Diazonium salts of aromatic amines are more stable than those of

aliphatic amines.

Diazonium salts: Diazonium salts have the general formula

(R/Ar—N₂⁺Cl^-) where R stands for the alkyl group and Ar stands for the aryl group; the structure is given below:

 

Formation of Diazonium salts:

 

Diazonium salt is obtained by treating aromatic amine (aniline) dissolved in dil. HCl with HNO₂ at 273-278K (0° -5° C)

Diazonium salts of aromatic amines are more stable than those of aliphatic amines.

Explanation:

Aromatic amine form arenediazonium salts, which are stable for a short time in solution at low temperature (273-278K). The diazonium salts of aromatic amines are more stable due to the dispersal of the positive charge on the benzene ring (resonance) as shown below:

 

The aliphatic amines, on the other hand, form highly unstable alkane diazonium salt (R—N₂⁺Cl^-) they rapidly decompose even at low temperature (<272-278K) forming carbocation and nitrogen gas.

 

Hence, diazonium salts of aromatic amine are much more stable than aliphatic diazonium salts.

  

Note: Carbocation is an ion in which carbon atom consists of positive charge (vii) Gabriel phthalimide synthesis is preferred for synthesising primary amines

Gabriel phthalimide synthesis is a very convenient method for the preparation of pure aliphatic amines ( especially primary amines) Phthalimide on treatment with ethanolic KOH gives potassium phthalimide which on heating with a suitable alkyl halide gives N-substituted phthalimides. These upon subsequent hydrolysis with dil.HCl under pressure or with alkali give primary amines. Step 1: Phthalimide is treated with KOH to form potassium phthalimide

 

 

Step 2: Potassium phthalimide is treated with suitable alkyl halide to form N substituted phthalimides.

 

Step 3: N-substituted phthalimides undergoes hydrolysis in the presence of dilute HCl or with alkali (NaOH) to give primary amines.

 

Therefore, Gabriel phthalimide synthesis results in the formation of primary(1° amine) only. Secondary or tertiary amines are not formed through this synthesis. Hence, Gabriel phthalimide synthesis preferred for the formation of primary amines only.