Amines (-NH₂) are one of the functional groups, derivatives of Ammonia (NH₃) in which one or more hydrogen atoms have been replaced by Alkyl groups
Many low lower molecular weight amines have very foul orders. Trimethylamine(CH3)3N, formed when enzymes break down certain fish proteins, has the characteristics odor of rotting fish. Putrescine (NH₂CH₂CH₂CH₂CH₂NH₂) and Cadaverine (NH₂CH₂CH₂CH₂CH₂CH₂CH₂NH₂) are both poisonous diamines with putrid odors. They, too present in the rotting of fish, and partly responsible for the odor of semen, urine and bad breath.
Classifications Of Amines ;
In general, Amines are classified into three major types,
- Primary (1′) Amines
- Secondary (2′) Amines
- Tertiary(3′) Amines
The classification is depend on the Number of Alkyl Groups Replaced in terms of the hydrogen Atoms in the Ammonia molecule.
So, Primary (1′) Amines Contains One Alkyl group in it
Secondary (2′) Amines contains two Alkyl groups in it
Tertiary(3′) Amines contains three Alkyl groups in it
The following rules are applied while naming amines,
1. Common System :
Amines are named by naming the Alkyl groups attached to the nitrogen atom followed by the ending –amine.
The names should be written as one word.
| Molecular Formula
When two or three identical alkyl groups are attached with the nitrogen atoms,then
ADD PREFIX – di– or tri– to the name of the amine
2. Substitute System :
In this system, an amine is named in much the same way as the analogous alcohol,except that the suffix amine is used.
In diamine nomenclature, as in diol nomenclature, the final ‘e’ of the hydrocarbon name is retained.
The Priority order of citation of amine groups as principal groups is just below that of alcohols
Carboxylic Acid > Aldehyde > Ketone > Alcohol > Amine
When cited as a substituent, the -NH₂ group is called as Amino group.
Salts of amines :
- All amines are basic in nature.
- They react with acids to form salts.The salts can be considered as being related to Ammonium (NH₄⁺) salts where the hydrogens of ammonium ion have been replaced by Alkyl groups.
- When the four Hydrogen atom of Ammonim ion have been replaced by Alkyl groups,that compound is called as Quaternary Ammonium Salt
The amine salts are named as substituted ammonium salts.The alkyl groups are named first, and are followed by ending with -ammonium. The name of the anion present in the salts is then written as a second word.
Structure of Amines :
Let us consider,
Methylamine for illustrating the orbital make up of amines.In Methylamine both nitrogen and carbon are sp3 hybridised.
one of the sp³ orbital of nitrogen is completely filled and cannot take part in bond formation.
- The C-N(sigma) bond in methylamine is formed by overlap of an sp³ orbital of carbon and an sp³ orbital of nitrogen.
- Each N-H (sigma) bond is formed by the overlap of an sp³ orbital of nitrogen and s orbital of hydrogen.
- Each C-H (sigma) bond id formed by the overlap of an sp³ orbital of carbon and s orbital of hydrogen.
Molecular Shape ; TETRAHEDRAL
The bonding in an amine is directly analogous to that in ammonia : an sp³-hybridised nitrogen atom bonded to three other atoms or groups (H or R) and with a pair of unshared valence electrons in the remaining sp³ orbitals
Basicity Of Amines :
first of all, you have to know about
What is Lewis base ?
Lewis Base : If a species donates an electron pair to form a new bond to a hydrogen, we can simply say it is acting as Base
Generally, when we speak about the basicity of amine, we are referring to its ability to form a new bond to hydrogen using its unshared valence electron pair. In order to consider the relative basicities of two or more amines, we need to specify a standard reaction system for comparison,
The Equilibrium established upon reactions of the amine with water is most commonly used,
By the position of the Equilibria of the reaction, we can compare the Basicities of various Amines
Basicities Of Some Amines :
|| pKa of the conjugate acid
Structural Relationship Of Amine Basicity :
If an amine is protonated, the alkylammonium ion that forms has a charge of 1⁺. Due to this,a reasonable expectation of relative basicities of amines would seem to be :
Indeed,proton transfer to amines in the gas phase is in accord with these predictions.However,we are usually concerned with reaction in solutions,and as we have seen basicities are usually compared in terms of equilibria established in aqueous solution.
Interestingly, the order of the basicities follows no simple trend under these conditions, you can see in following image
Importance of Hybridization :
Other factors being equal, the basicities of amines and other compounds containing an unshared valence electron pair on the Nitrogen correlate with the hybridization of the nitrogen atom. The unshared electron pair can be considered as occupying one of the hybrid orbitals, and for each type of hybridization, there is different fractions of p character.
The More p character in the hybrid orbitals, the higher will be the energy
The predicted order of the orbital energy are sp³ > sp² > sp.
So,the amine containing the unshared electron pair in the higher orbital will be acting as a more basic character.
The Bascities order of the Amines, imines, and Nitriles are
Methods Of Preparation Of Amines :
1. Reaction of alkyl halides with ammonium or amines :
- Ammonia or amines carries an unshared pair of electrons and can act as a nucleophile in substitution reactions with Alkyl Halide.The product of this reaction with Ammonia or amine is an amine salt.
- The free amine can be prepared by treating the amine salt with bases like NaOH.
- It follows an SN² reaction.
2. Gabriel Phthalimide Method :
- This involves the treatment of Phthalimide with potassium hydroxide to form the potassium salt.
- The salt is then heated with an alkyl halide to give N-alkylphthalimide,which in turns react with an potassium hydroxide to form potassium Phthalimide salt and a pure primary amine
3. Reduction Of Nitroalkanes :
- Primary amines can be obtained by reduction of nitroalkanes with H₂ + Pt (or Ni) or Lithium aluminium hydride
4. Reduction of Nitriles :
- The primary amines can be prepared by reduction of nitriles (alkyl cyanide) with H₂+Pt (or Ni) or Lithium aluminium hydride
5. Reduction of amides :
- Primary amines can be obtained by reduction of of the simple amides with Lithium aluminium hydride.
- Notice that the product contains the same number of carbons as the reactant Amide
6. Hoffmann’s Degradation of amides :
- This is good laboratory method for the preparation of the amides to a pure primary amine.
- The amide is warmed with the Bromine and concentrated Aqueous NaOH solutions
7. Curtius Rearrangement :
The reaction converts acyl azides to 1′ amines.Following step are involved
Step 1 : Conversion of acid halides to acyl azides by treatment with NaN₃
Step 2 : Hydrolysis of acyl azides gives 1′ amine.The amines product has one less carbon atom.
Physical Properties Of Amines :
- lower amines are gases or low-boiling liquids and possess characteristic ammonia like smell
- Aniline and other arylamines are generally colorless and transparent
- Secondary amines are more basic when compared with Primary and tertiary amines. Because the two alkyl groups present in the Secondary amine has better stabilized the conjugate acid (inductive effect)
- Primary and secondary amines are capable of intermolecular hydrogen bondng, because the contain N-N bonds.Because Nitrogen is more electronegative than Oxygen, however intermolecular hydrogen bonds between N and H are weaker than those O and H
- Amines are soluble in organic solvents. All amines having < 5Cs are water-soluble because they can hydrogen bond with H₂O. Amines having > 5C’s are water-insoluble because the nonpolar alkyl portion is too large to dissolve in the polar H₂O solvent
- Tertiary (3′) amines have a lower boiling points than 1′ and 2′ amines of comparable molecular weight because they have no N-H bonds and are incapable of hydrogen bonding.
Chemical Properties Of Amines :
The Main reactions of amines are due to the lone pair electrons on nitrogen. This lone pair of electrons is available for donation to electron-deficient (seeking) reagents. The amines generally come under Nucleophilic Reagents.
1. Salt Formation :
Amines are basic in nature. They react with acids to form ammonium salts
When these salts are treated with NaOH, parent amines are librated.
2. Reaction with Alkyl Halide :
When an amine reacts with an alkyl halide, Hydrogen atoms of the N atom are successively replaced by alkyl groups. In the end, the tertiary amines add one molecule of the Alkyl halide to yield quaternary ammonium salts.
3. Reaction with Acid Chlorides (Acylation) :
- Primary amines react with acid chlorides or acid anhydrides to form N-substituted amides
- Secondary amines react with acid chlorides to form N, N-disubstituted amides
- Tertiary amines do not react with acid chlorides, since they do not have any replaceable hydrogen on the nitrogen.
4. Carbylamine Reaction :
Primary amines react with chloroform and a solution of KOH in ethanol to form isocyanides (carbylamine). Secondary and Tertiary amines do not give this reaction.
5. Reaction with Carbon disulfide :
Primary amines react with carbon disulfide to form a dithiocarbamic acid. Dithiocarbamic acids undergo decomposition with mercuric chloride to give corresponding allyl isothiocyanate.
To know In-depth details about Amines – Organic Chemistry
Reference: Bahl & Arun Bahl ( Advanced Organic Chemistry)