Amines

Amines are functional groups and compounds holding a basic nitrogen atom with a lone pair. In formal terms, amines are the derivatives of ammonia (NH₃), wherein one or more hydrogen atoms have been replaced with a substituent like the groups of alkyl or aryl. These may respectively be called as alkylamines and arylamines. Amines in which all forms of substitutes are bound to one nitrogen atom can be referred to as alkyl aryl amines.


Structurally, amines resemble ammonia, where the nitrogen can be in bond up to 3 hydrogen atoms. Also, it is characterized by different properties based on carbon connectivity.


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.


(image will be uploaded soon)


Types of Amines

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


(images will be uploaded soon)


  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

  1. 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

  1. 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

  1. 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.


(image will be uploaded soon)


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.


(image will be uploaded soon)


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


(image will be uploaded soon)


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. 


(image will be uploaded soon)


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


(image will be uploaded soon)


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).


(image will be uploaded soon)


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.

FAQ (Frequently Asked Questions)

What is the Basicity of Amines?

Let us look at the basicity of amines - Basicity constants and Relative Basicities. Amines are basic in nature because they possess an unshared pair (lone pair) of electrons on nitrogen. This electron lone pair is accessible for a new bond formation either with proton or Lewis acid. Thus amines react with acids forming salts.


(image will be uploaded soon)


Basicity Constants

Strong bases (e.g., KOH or NaOH) ionize completely in aqueous solutions. Amines are a weak base. They are only partially ionized in aqueous solutions, and there is an equilibrium between ionized and unionized forms.

(image will be uploaded soon)


The ionization degree is defined as the Kb constant of equilibrium, known as the Basicity Constant. It is defined as the concentration of ionization products in moles per liter divided by the concentration of the unionized basis.


Order of Basicity

Suppose that 1, 2, 3 indicates Primary, Secondary, and Tertiary amines respectively.

The Group attached to amine is ‘R’.


In aqueous phase

  • If R is a methyl group, then order of basicity is 2>1>3

  • Is R is other than methyl group, the basicity order is 2>3>1


In gas phase

  • Order or basicity for gas is 3>2>1


If it is not mentioned in any of the questions, we can assume the default as aqueous phase.

Aliphatic Amines are More Basic than Aromatic Amines. Why?

The amine group -NH₂ in aliphatic amines is attached to an alkyl group called as an electron donating group. It means they have a ‘+I’ effect. Because of this they increase the availability of a pair of electrons on the Nitrogen Atom and resultantly Aliphatic amines are always more basic than Ammonia and aromatic amines.


Whereas in aromatic amines, the -NH₂ Group is attached to PHENYL Group (-C₆H₅ ) which is electron attracting. So, the availability of lone pair of electrons on the Nitrogen atom will be less and therefore will be less basic than Aliphatic amines.