Chemical Reaction

Basic Nature Of Chemical Reaction

Amines turn red litmus blue and also combine with water and mineral acids to form corresponding salts.

R – NH₂ + HCl → R –

R–NH₂ + H₂SO₄ →

When the amine salts are treated with strong bases like NaOH, the parent amines are regenerated.

RN+H₃Cl– + OH– → RNH₂ + H₂O + Cl–

Amine salt Amine

(Soluble is water) (insoluble in water)

Further, due to basic character amines react with auric and platinic chlorides in presence of HCl to form double salts.

Chloroplatinic acid

These double salts decompose on ignition to pure metal, therefore, the formation and decomposition of the double salts is used for determining the molecular weight of amines.

Acylation (Reaction with Acyl Chlorides or Acid Anhydrides)

Primary and secondary amines can react with acid chlorides or acid anhydrides to form substituted amides.

RNH₂ + R′COCl → R′CO NHR an N-substituted amide

R2NH + R′COCl → R′CO.NR₂ an N,N disubstituted amide

Benzoylation (Schotten Baumann Reaction)

Primary amine reacts with benzoyl chloride to give the acylated product.

(Benzoyl chloride) Benzoyl alkyl amine

Carbylamine Reaction (Given Only by Primary Amines)

Primary amines when heated with chloroform and alcoholic caustic potash give isocynaides (carbylamines) having very unpleasant smell, which can be easily detected

C₂H₅NH₂ + CHCl₃ + 3KOH → C₂H₅NC + 3KCl + 3H₂O

Ethylamine Ethyl isocyanide

C₆H₅ NH₂ + CHCl₃ + 3KOH → C₆H₅NC + 3KCl + 3H₂O

Aniline Phenyl isocyanide

Action with Aldehyde and Ketone

Both primary aliphatic and aromatic amines react with aldehydes and ketones to form schiff’s bases also called anils.

C₂H₅NH₂ + CH₃CHO → C₂H₅N = CHCH₃ + H₂O

Ethylamine Acetaldehyde Ethylidene ethylamine

(Schiff’s base)

Hofmann Mustard Oil Reaction

Primary amines when warmed with alcoholic carbon disulphide followed by heating with excess of mercuric chloride form isothiocyanates having pungent smell similar to mustard oil.

C₆H₅NH₂ + S = C = S C₆H₅NCS + 2HCl + HgS

Reaction with Carbonyl Chloride

This reaction is given only by primary amines.

C₂H₅ – NH₂ + COCl₂ → C₂H₅NCO + 2HCl

Ethylisocyanate

Reaction of Quaternary Ammonium Salts

Hofmann Elimination

When a quaternary ammonium hydroxide is heated strongly (125° or higher) it decomposes to yield water, a tertiary amine and an alkene

This reaction is called as the Hofmann elimination. The formation of quaternary ammonium salts followed by an elimination of the kind just described and identification of the alkene and tertiary amine formed was once used in the determination of the structure of complicated amines.

Reaction of Amines with Nitrous Acid

The diazonium salts of amines

What are these diazonium salts? Let us look at the name. The name suggests that, the compound has two nitrogen atoms (diazo) and the whole group has a positive charge (ium). There is also an anion to balance it (It is a salt)

So, the possible structure can be

How to prepare them? The preparation is quite simple if we adhere to the experimental conditions.

These diazonium salts are prepared by treating a primary amine with NaNO₂ in presence of con. HCl; the temperature being 0°C. (Here the temperature has to be taken care of and if the temperature exceeds 5°C, the reaction will not take place.)

Let us take a case of aliphatic amines,

CH₃CH₂CH₂NH₂ CH₃CH₂CH₂N⁺₂Cl–

Mechanism for Diazotization is as follows

The diazonium salts of aliphatic amines are generally unstable and they decompose to give different products.

Thus we can have a wide range of products. Let us now see some thing about aromatic amines

These aryl diazonium salts undergo a variety of displacement reaction. The reactions are simple and are summarised below.

Mechanism of ArN₂+ Cl– + H₃PO₂ reaction takes place through free radical pathway.

Chain initiation

ArN₂+Cl– + H₃PO₂ = HCl + ArN₂PH(O)OH

ArN₂PH(O)OH → Ar⋅ + N₂ + (O)OH

Chain propagation

Ar⋅ + H₂PO₂ → ArH + (O)OH

ArN₂⁺ + (O)OH → ArN₂∙ + P+H(O)OH

ArN₂∙ → Ar⋅ + N₂  P+H(O)OH + H₂O → H₃PO₃ + H+

Reaction of secondary Amines with Nitrous acid

Secondary amines both aryl and alkyl react with nitrous acid to yield N-nitrosoamines. N-nitrosoamines usually separate from the reaction mixture as oily yellow liquid.

N-nitrosoamines are very powerful carcinogens (cancer causing substances)

Reaction of Tertiary amines with Nitrous acid

When a tertiary aliphatic amine is mixed with nitrous acid, an equilibrium is established among the tertiary amine, its salt, and an N-Nitrosoammonium compound.

Tertiary arylamines react with nitrous acid to form C-nitroso aromatic compound. Nitrosation takes place almost exclusively at the para position if it is open and if not, at the ortho position. The reaction is another example of electrophilic aromatic substitution.

Coupling Reactions of Arene Diazonium Salts

Arenediazonium ions are weak electrophiles; they react with highly reactive aromatic compounds with phenols and tertiary arylamines to yield azo compound. This electrophilic aromatic substitution is called a diazo coupling reaction occurring mainly at p-position.

Couplings between arenediazonium cations and phenols take place most rapidly in slightly alkaline solution. If the solution is too alkaline (pH • 10), however, the arenediazonium salt itself reacts with hydroxide ion to form a relatively unreactive diazohydroxide or diazotate ion.

Hydrazo compounds are also made as follows:

Ph—NO₂ Ph—NH—NH—Ph

Diaryl hydrazo compounds undero the benzidine rearrangement

Mechanism:

Ring Substitution in Aromatic Amines

The –NH₂, – NHR and –NR₂ are benzene activating groups through resonance effect of nitrogen where the lone one pair of electron of nitrogen is shifted to the benzene ring making ortho and para, position available for electrophilic attack.

The carbocation formed as intermediate are

The group – NHCOCH₃ is less powerful ortho and para director because of the electron-withdrawing character of oxygen makes nitrogen a poor source of electrons. This fact is made use in preparing mono substituted aniline. The –NH₂ group is such a powerful activator, that substitution occurs at all available ortho and para positions of aniline. If, however, –NH2 group is converted to –NHCOCH_3, the molecule becomes less powerful activator. Hence only mono substitution products are obtained. Finally – NHCOCH₃ is converted back to –NH₂ by hydrolyzing with acid. This technique is especially used while nitrating aniline as strong oxidizing agent destroys the highly reactive ring.

Aniline -X rearrangement

Such compounds are not much stable so the group X migrates mainly at p-position.

1. Fisher-Hepp rearrangement

2. Phenylhydroxylamine - p-aminophenol rearrangement.

Mechanism

Nucleophilic attack by H₂O at p - position.