Chemical Properties of Amines

Basic Nature of Amines

Like ammonia, amines are converted into their salts by aqueous mineral acids and are liberated from their salts by aqueous hydroxides. Thus , like ammonia, amines are more basic than water and less basic than hydroxide ions.

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

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.

RNH₂ +PtCl₄+2HCl --> (RNH₃)₂+PtCl₆^2-

       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.
 

Alkylation of Amines

?Like ammonia, an amine can react with alkyl halide to form next higher class of amine. Here, again it is the presence of electron pair on nitrogen which makes amine to behave as nuclephile and alkyl halide thus undergo nucleophilic substitution reaction.

RNH₂RNHCH₃RN(CH₃)₂RN⁺(CH₃)₃I

1⁰ amine      2⁰ amine      3⁰ amine     Quaternary ammonium iodide   

Quarternary ammonium salts are useful in synthetic organic chemistry as phase-transfer catalysts and in the preparation of alkenes1
 

Halogenation of Amines

In order to introduce only one halogen atom, the activating effect of the –NH₂ group must be lowered using acetylation.  

Nitration of Amines

Direct nitration of aniline with nitric acid gives a complex mixture of mono – di and trinitro compounds and oxidation products. If however, NH₂ group is protected by acetylation, main product of nitration is p – nitro derivative.  

Sulphonation of Amines

Aniline reacts with conc. H₂SO₄ to form the salt anilinium hydrogen sulphate which on heating at 455 – 475 K gives sulphanilic acid (p – amino benzene sulphonic acid).  

Sulphanilic acid exists as Zwitter ion i.e. a dipolar ion which exists in the form of internal salt structure. Such ion has positive as well as negative charge within same molecular structure.
 

Acylation of Amines

Reaction refers tro the reaction of amines 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 of Amines (Schotten Baumann Reaction)

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

R-NH₂ + Cl – COC₆H₅ + NaOH →  R-NH-COC₆H₅ + HCl

                           ^{(Benzoyl chloride)                 Benzoyl alkyl amine}
 

Carbylamine Reaction

This reaction is 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

Primary amines add as nucleophile to the carbonyl group of an aldehyde or a ketone to form carbinoamines, which are then dehydrated to form imines which are also known as Schiff’s bases as the final product.

C₂H₅NH₂ + CH₃CHO → C₂H₅NH-CH(CH₃)-OH →  C₂H₅N = CHCH₃ + H₂O

Ethylamine   Acetaldehyde                                 Ethylidene ethylamine

                                                                           (Schiff’s base)

Both the addition and elimination   phase of the reaction are accelerated by acid catalysis.

Secondary amines add to aldehydes and ketones to form carbinoamines which can be dehydrated to a stable product leading to a carbon-carbon double bond.

(C₂H₅)₂NH + CH₃CHO → (C₂H₅)₂N-CH(CH₃)-OH →  C₂H₅N-CH=CH₂ + H₂O

                                                                                 Enamine
 

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.

Secondary amines also react with CS2 to form dithiocarbamic acids, but the latter do not react with mercuric chloride

R₂NH + S = C = S → S=C(SH)NR₂ No. Reaction

Tertiory amines do not react with carbon disulphide.
 

Reaction of Amindes with  Carbonyl Chloride

This reaction is given only by primary amines. 
C₂H₅ – NH₂ + COCl₂ →  C₂H₅NCO + 2HCl
                                Ethylisocyanate
 

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.

 Diazonium salts of Amines

What are 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: RN₂⁺X^-

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.)

Mechanism for Diazotization 

Step 1:

Step 2:

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

Mechanism  

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

Chain initiation

ArN₂⁺Cl^– + H₃PO₂ = HCl + ArN₂PH(O)OH

ArN₂PH(O)OH →  ArH + N₂ + ^.PH(O)OH

Chain propagation

Ar× + H₂PO₂ →  ArH + PH(O)OH

ArN₂⁺ + ^.PH(O)OH →  ArN₂^· + P⁺H(O)OH

ArN₂^· →  Ar^. + N₂                P⁺H(O)OH + H₂O →  H₃PO₃ + H⁺
 

Reaction of Tertiary  and Secondary 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.

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

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.

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 –NHCOCH₃, 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.

Oxidation of Amines

?Amines are usually oxidised at N, rather than at C. Primary and secondary aliphatic amines are although oxidisable, in most cases useful products are not obtained Complicated side reaction often occurs, causing the formation of complex mixtures.

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.

Separation of a Mixture of Amines

a) Hinsberg’s Method

Treating a mixture of 3 amines with Hinsbergs reagent (benzene sulfonyl chloride) and finally treating the product formed with NaOH can separate the 3 class of amines.

Primary amine:RNH₂ + C₆H₅SO₂Cl →  C₆H₅– SO₂ – NH – R + HCl 

C₆H₅– SO₂ – NH – R : N-alkyl benzene sulfonamides

Dissolves in NaOH due to acidic H-attached to Nitrogen)                                                

Secondary amine

Tertiary amine : Tertiary amines do not react with Hinsberg’s reagent.

After reacting with NaOH the aqueous layer  and the second layer [Secondary and Tertiary) can be separated by ether. Aqueous layer Hydrolysed with conc. HCl gives primary amine. The ether layer is distilled and tertiary amine is distilled over. Residue hydrolysed with conc. HCl to recover secondary amine

a) Hofmann’s Method:

The mixture of amines is treated with diethyloxalate, which forms a solid oxamide with primary amine, a liquid oxime ester with secondary amine. The tertiary amine does not react.

Test for Amines

Primary amine is treated with a strong base in presence in chloroform, an isocyanide is formed and this isocyanide thus formed has a very foul smell.

Here attacking electrophile is the dichlorocarbene  (:CCl₂). The primary amine can be identified with its foul smell.

Secondary amine is converted into nitrosoamine by treating the amine with nitrous acid. The resultant solutions warmed with phenol and concentrated H₂SO₄, a brown or red colour is formed at first soon it changes to blue and then to green. The colour changes to red on dilution and further changes to greenish blue on treating with alkali.

Tertiary arylamines react with nitrous acid to form o-nitroso aromatic compound

You can also refer   JEE Organic Chemistry Syllabus & Reference books of Organic Chemistry

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