Amines are one of the most important classes of organic compounds which can be derived when we replace one or more hydrogen atoms of ammonia molecules with an alkyl group.
An amine is generally a functional group with a nitrogen atom having a lone pair. Amines resemble ammonia structurally where nitrogen can bond up to 3 hydrogen atoms. It is also characterized by various properties that are based on carbon connectivity.
Compounds of nitrogen connected to a carbonyl group are called as amides, they have a structure R–CO–NR′R″ and varies in properties with amines.
Amines are organic compounds that contain nitrogen atoms with a lone pair. Basically, they are derived from ammonia (NH3) in which one or more hydrogen atom is replaced by an alkyl or aryl group and so they are known as alkylamines and arylamines respectively.
Nitrogen has 5 valence electrons and so is trivalent with a lone pair. As per VSEPR theory, nitrogen present in amines is sp3 hybridized and due to the presence of lone pair, it is pyramidal in shape instead of tetrahedral shape which is a general structure for most sp3 hybridized molecules. Each of the three sp3 hybridized orbitals of nitrogen overlap with orbitals of hydrogen or carbon depending upon the configuration of amines. Due to the presence of lone pair, the C-N-H angle in amines is less than 109 degrees which is a characteristic angle of tetrahedral geometry. The angle of amines is near about 108 degrees.
Naturally, amines occur in proteins, vitamins, hormones, etc. and they are also prepared synthetically to make polymers, drugs, and dyes.
Amines have widespread application in our daily lives. Some of the uses of amines are listed below:
On the basis of how the hydrogen atoms are replaced by an ammonia molecule, amines can be divided into 4 types.
When one of the hydrogen atoms of the ammonia molecule is replaced by an alkyl or aryl group.
Eg: Methylamine CH3NH2, Aniline C6H5NH2
Two organic substituents replace the hydrogen atoms of the ammonia molecule forming an amine.
Eg: Dimethylamine (CH3)2NH, Diphenylamine (C6H5)2NH
When all 3 of the hydrogen atoms are replaced by an organic substituent, it could be an aryl or aromatic group.
Eg: Trimethylamine N(CH3)3, Ethylenediaminetetraacetic acid (EDTA)
These are secondary or tertiary amines in an aromatic ring structure. Eg: Piperidine (CH2)5NH, Aziridines C2H5N
Similar to ammonia, primary & secondary amines have protic hydrogens and thus they showcase a degree of acidity. Whereas teritary amines have no protic hydrogen and thus do not possess a degree of acidity.
pKa value for primary & secondary amines is about 38, which makes them a really week acid. Whereas if we take the pKb, it is about 4. This makes the amines much more basic than acidic. Thus an aqueous solutions of an amine strongly alkaline.
This process will be carried out in a sealed tube. Here halogenoalkanes will be heated with the concentrated solution of ammonia in ethanol. The mixture cannot be heated under the reflux as ammonia would move out in the form of gas from a container.
Now coming to the preparation of primary amine from halogen alkanes the reaction takes place in two stages. Salt will be formed at the first stage. Here ethyl ammonium bromide is the salt. It is similar to ammonium bromide expect the fact that one of the hydrogens in the ammonium atom is replaced by an ethyl group.
A reverse reaction can occur between ammonia and the salt.
We can get primary amines when nitriles are reduced with lithium aluminum hydride. This method is mainly used for the preparation of amines which contains one carbon atom more than the starting amine.
We can get primary amines easily by Gabriel synthesis. In this process, on the treatment of phthalamide with ethanoic potassium hydroxide, we get potassium salts of phthalamide. When this is further heated with alkyl halide followed by alkaline hydrolysis then primary amine is produced. We cannot prepare aromatic primary amines because aryl halides do not undergo nucleophilic substitution with the anion which is formed by pthalimide.