[Chemistry Class Notes] on Amines Pdf for Exam

Structure of Amine

Consider that nitrogen has five valence electrons and is trivalent with a lone pair.

According to the VSEPR theory, the nitrogen found in amines is sp3 hybridized. Due to the existence of a lone pair, it is pyramidal in form rather than tetrahedral in nature, which is the general configuration of most sp3 hybridized molecules.

Each of the three sp3 hybridized nitrogen orbitals overlaps with hydrogen or carbon orbitals, depending on amine’s configuration. Owing to the existence of a lone pair, the angle of C-N-H in amines is less than 109 degrees, a characteristic angle of tetrahedral geometry. The angle of the amines is near to 108 degrees.

Types of Amines

Amines can be classified into four types based on how the hydrogen atoms are replaced by an ammonia molecule.

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1. Primary Amines

If one of the hydrogen atoms in an ammonia molecule is substituted by an alkyl or aryl group, it is a primary amine.

Example: Aniline C₆H₅NH₂, Methylamine CH₃NH₂

2. Secondary Amines

Two organic substitutes are used to remove the hydrogen atoms in the ammonia molecule that forms an amine.

Example: Diphenylamine  (C₆H₅)2NH, Dimethylamine (CH₃)2NH

3. Tertiary Amines

When all the three hydrogen atoms are substituted by an organic substitution, it may be an aryl or an aromatic group.

Example: Ethylenediaminetetraacetic acid (EDTA), Trimethylamine N(CH₃)₃

4. Cyclic Amines

Cyclic amines are the ones in which the nitrogen has been incorporated into a ring structure by making it either a secondary or a tertiary amine effectively.

Example: A three membered ring aziridine, six membered ring piperidine

Basicity of Amines

Like ammonia, the primary and secondary amines have protic hydrogens and hence display a degree of acidity. While tertiary amines do not have protein hydrogen and thus do not have a degree of acidity. 

The pKa value for both primary & secondary amines is around 38, which makes them a very weak acid. Whereas if we take the pKb, it’s around 4. It makes amines even more essential than acidic ones. It makes amines even more essential than acidic ones. Thus an aqueous solution of amine is strongly alkaline.

What are Aliphatic Amines?

An amine in which there are no aromatic rings directly attached on the nitrogen atom is referred to as Aliphatic amine.

One of the examples of the Aliphatic amine type is listed below.

Preparation of Amines

Now, let us look at the preparation of amines from halogenoalkanes (which can also be called either haloalkanes or alkyl halides) and from nitriles.

It deals with only amines where the functional group is not connected directly to the benzene ring. Aroma amines, such as phenylamine (aniline), are usually developed differently.

Preparation of Amines From Halogenoalkanes

Firstly, the halogenoalkane is heated with a concentrated solution of ammonia in ethanol. This entire reaction is carried out in a sealed tube. We may not heat this mixture under reflux, because the ammonia simply would escape up the condenser as a gas.

We also can think about the reaction using 1-bromoethane as a typical halogenoalkane.

Now, we get a mixture of amines formed together with their salts. These reactions occur one after another.

Preparation of a Primary Amine

The reaction is going to happen in two steps. A salt is formed in the first stage which is known as ethylammoniam bromide. It’s just like an ammonium bromide, only that one of the hydrogens in the ammonium ion is substituted by an ethyl group. 

CH₃CH₂Br + NH₃ ⟶ CH₃CH₂NH₃ + Br⁻

There is also a possibility of a reversible reaction in the mixture of this salt and excess ammonia.

CH₃CH₂NH₃ + Br⁻ + NH₃  ⇋  CH₃CH₂NH₂ + NH₄ + Br⁻

Ammonia removes a hydrogen ion from the ethylammonium ion, to leave a primary amine (ethylamine).

The more amount of ammonia there in the mixture, the more the forward reaction highly favours.

Preparation of a Secondary Amine

The reaction will not end at the primary amine. Also, ethylamine reacts with bromoethane in the same two stages like before.

At the first stage, you get a salt formed, a diethylammonium bromide. Consider this as ammonium bromide with two hydrogens that have been substituted by ethyl groups.

Again, there is the possibility of a reversible reaction between this salt and surplus ammonia in the mixture. 

Ammonia removes the hydrogen ion from the dimethyl ammonium ion, to leave a secondary amine, which is diethylamine. A secondary amine is one that has two groups of alkyl attached to the nitrogen.

Preparation of  a Tertiary Amine

Still, it’s yet to finish! Diethylamine also reacts with bromoethane in the same two steps as before. 

You get triethylammonium bromide salt in the first stage. 

Again, there is the risk of a reversible reaction between the excess ammonia and this salt in the mixture.

Ammonia removes a hydrogen ion from the triethylammonium ion, to leave a tertiary amine, called triethylamine. Tertiary amine is the one having three alkyl groups attached to nitrogen.

Preparation of a Quaternary Ammonia Salt

This is the final stage! Triethylamine reacts with bromoethane creating tetraethylammonium bromide, a quaternary ammonium salt (one of all four hydrogens have been substituted by alkyl groups).

There is no hydrogen remaining on the nitrogen to be added this time. Here, the reaction stops.

Important Facts or Uses of Amines

Amines perform a significant part in the survival of life – they are actively involved in the formation of amino acids, the proteins building blocks of human creatures. Many vitamins are also made from amino acids.

Serotonin is an important amine functioning as one of the primary brain’s neurotransmitters. This regulates symptoms of hunger and is vital to the level at which generally the brain works. Also, it affects the state of happiness and helps to regulate the sleep and walk cycles of the brain.