Chemical Reactions of Amines

Have you ever wondered how different clothes get different colours? There is various type of dyes that colour your clothes beautifully. However, do you know what these are? These are Amino acids. You must have heard that proteins are made up of amino acids. In Organic chemistry, you will often see the Amino group in many compounds. The chemical reactions of amines are super awesome and at times super useful also. The nitrogen derivatives of hydrocarbons make up many crucial compounds. These include dyes and drugs.  Let us find out more!

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Amines

There are organic compounds and functional groups containing a nitrogen atom with a lone pair of electrons. Amines are one of them. Amines are the derivatives of ammonia. They have one or more hydrogen atoms. We can replace these hydrogen atoms with an alkyl or aryl group.

Here, we will discuss the chemical reactions of amines. So we will know some interesting facts which are associated with chemical reactions of amines.

Classification of Amines

On the basis of the presence of replaceable hydrogen atoms, we can classify amines into three types:

• Primary Amines: The amine is primary amine when an alkyl or Aryl group replaces one of three hydrogen atoms.
• Secondary Amines: It is a secondary amine when the alkyl or aryl group replaces two hydrogen atoms.
• Tertiary Amines: The amine is a tertiary amine when total three hydrogen atoms are replaced by alkyl or aryl group.

Normally, secondary or tertiary amines form a cyclic structure.

Chemical Reactions of Amines

Amines can act as both bases and nucleophiles due to the unshared electron pair.

Basic Nature of Amines

In this section, we will look at the basic nature of some common amines.

• Aniline is less basic than ammonia. The phenyl group exerts an inductive effect, i.e., it withdraws electrons. This results in the lower availability of electrons on nitrogen for protonation.
• Ethylamine and acetamide both contain an amino group. However, acetamide does not show its 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.

• The compound with least s’ character (sp3 hydrolized) is most basic and with more s’ character (sp-hydrolyzed) is least basic. Examples in decreasing order of basicity are :

CH3CH2CH2NH2 > H2C = CHCH2NH2 > HC = CHCH2NH2

(CH3)2NH > CH3NH2>NH3 > C6H5NH2

Electron withdrawing (C6H5—) groups cause a decrease in the electron density on the nitrogen atom. Thus, they result in a decrease in the basicity.

CH3CH2NH2 > C6H5CONH> CH3CONH2

These are all about the basic nature of amines. Now, we move on to the important segment: Preparation of amines.

Preparation of Amines

1) Alkylation

Alkylation of 1o amine generates 2o amine, 3o amine and finally the quaternary salts.

2) Acylation

Acylation is the reaction with acid chlorides, anhydrides and esters. These occur by nucleophilic substitution reactions. The reaction is proceeded by the replacement of hydrogen atom of –NH2 or >N–H group by the acyl group (RCOX).

We cannot acylate Tertiary (3o) amine. This is because there is no Hydrogen bonded to nitrogen.

3) Benzoylation

Here, methenamine reacts with benzoyl chloride. The products are hydrochloric acid and N Methylbenzamide.

Chemical Reactions of Amines

1) Reaction with Nitrous Acid, HNO2

Three classes of amines react differently with nitrous acid as follows:

2) Carbylamine Reaction

Primary amine reacts with CHCl3/KOH to form isocyanides (carbylamines). You get a very unpleasant smell with the products. Both aliphatic and aromatic amines take part in this reaction. This reaction is as follows:

3) Reaction with Aldehyde and Ketones

1o amines react with aldehydes and ketones to form Schiff’s bases. The reaction is given below.

4) Electrophilic Substitution Reactions

Aniline is ortho and para directing towards electrophilic substitution reaction. This is because of high electron density at ortho and para-positions. Some of the electrophilic substitution reactions of aniline are as follows:

(a) Bromination

To prepare monosubstituted aniline derivative:

• You have to carry out the first acetylation of aniline with acetic anhydride.
• You would have to follow this by the desired substitution of the substituted amide.
• This is then hydrolyzed. We thus, obtain the monosubstituted amine.

(b) Nitration

To get p-nitro derivative as the major product, you have to carry the acetylation of aniline with acetic anhydride. Then, you should carry out the desired substitution of anilide. After this, you must carry out hydrolysis of the substituted anilide to the substituted amine.

Solved Example for You

Q: How can we convert secondary amine into nitrosoamine?

Ans: We can convert secondary amine into nitrosoamine by treating the amine with the nitrous acid.

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