Amine Nomenclature and Properties

• Amines are organic compounds derived from ammonia through the replacement of one or more hydrogen atoms with alkyl or aryl groups.
• Amines are found in nature, including in proteins, vitamins, alkaloids, and hormones.
• Examples of synthetic amines include polymers, dyes, and drugs.
• Adrenaline and ephedrine are two biologically active compounds containing a secondary amino group, used to increase blood pressure.
• Novocain is a synthetic amino compound used as an anesthetic in dentistry.
• Benadryl is an antihistaminic drug containing a tertiary amino group.
• Quaternary ammonium salts are used as surfactants, and diazonium salts are intermediates in the synthesis of aromatic compounds, including dyes.

Structure of Amines

• Amines are derivatives of ammonia, where one, two, or all three hydrogen atoms are replaced by alkyl and/or aryl groups.
• The nitrogen atom in amines is trivalent with an unshared pair of electrons and _sp_3 hybridized.
• The geometry of amines is pyramidal.
• The bond angle C-N-E, where E is C or H, is less than 109.5 degrees due to the presence of an unshared pair of electrons and is 108 degrees in trimethylamine.

Classification

• Based on the number of hydrogen atoms in ammonia replaced by alkyl or aryl groups, amines are classified as primary (1°), secondary (2°), and tertiary (3°).
• RNH2 or ArNH2 represents a primary amine (1°).
• R-NHR' represents a secondary amine.
• Replacement of another hydrogen in secondary amine forms a tertiary amine.
• Amines are considered 'simple' when all alkyl or aryl groups are the same and 'mixed' when they are different.

Nomenclature of Amines

• In the common system, aliphatic amines are named by prefixing the alkyl group to amine (e.g., methylamine).
• With two or more identical groups in secondary and tertiary amines, di- or tri- prefixes are used.
• In the IUPAC system, primary amines are named as alkanamines, where the 'e' of alkane is replaced by 'amine'.
• The positions of multiple amino groups on the parent chain are specified with numbers, retaining the 'e' in the hydrocarbon suffix (e.g., ethane-1,2-diamine).
• Locant N is used to designate substituents attached to a nitrogen atom in secondary and tertiary amines such as N-methylethanamine.
• –NH2 group is directly attached to the benzene ring in arylamines.
• The simplest arylamine, C6H5NH2, is known as aniline. In the IUPAC system, it is named as benzenamine.

Preparation of Amines

• Amines are prepared by these methods:
• Reduction of nitro compounds
• Ammonolysis of alkyl halides
• Reduction of nitriles
• Reduction of amides
• Gabriel phthalimide synthesis
• Hoffmann bromamide degradation reaction

Reduction of Nitro Compounds

• Nitro compounds are reduced to amines by hydrogen gas with finely divided nickel, palladium, or platinum.
• Reduction can also be achieved with metals in an acidic medium.
• Nitroalkanes can be reduced to alkanamines.
• Reduction with iron scrap and hydrochloric acid is preferred, as FeCl2 hydrolyzes to release hydrochloric acid, requiring only a small amount to initiate the reaction.

Ammonolysis of Alkyl Halides

• The carbon-halogen bond in alkyl or benzyl halides can be cleaved by a nucleophile (-NH2 group).
• An ethanolic solution of ammonia reacts with an alkyl or benzyl halide via nucleophilic substitution, replacing the halogen atom with an amino group.
• This process is called ammonolysis and is conducted in a sealed tube at 373K.
• The primary amine obtained can further react with alkyl halide to form secondary and tertiary amines, and finally, a quaternary ammonium salt.
• Using a large excess of ammonia helps to make the primary amine the major product.

Reduction of Nitriles

• Nitriles are reduced to primary amines by lithium aluminum hydride (LiAlH4) or catalytic hydrogenation.
• This reaction is useful for increasing the amine series, preparing amines with one more carbon atom than the starting compound.

Reduction of Amides

• Amides are reduced to amines using lithium aluminum hydride.

Gabriel Phthalimide Synthesis

• This is used for preparing primary amines.
• Phthalimide is treated with ethanolic potassium hydroxide to form potassium salt of phthalimide.
• Heating the potassium salt with alkyl halide, followed by alkaline hydrolysis, produces the corresponding primary amine.
• Aromatic primary amines cannot be prepared using this method because aryl halides do not undergo nucleophilic substitution with the phthalimide anion.

Hoffmann Bromamide Degradation Reaction

• Reacting an amide with bromine in aqueous or ethanolic sodium hydroxide.
• Involves migration of an alkyl or aryl group from the carbonyl carbon to the nitrogen atom.
• The resulting amine has one less carbon atom than the original amide.

Physical Properties

• Lower aliphatic amines are gases and have a fishy odor.
• Primary amines with three or more carbon atoms are liquids, while higher ones are solids.
• Aniline and other arylamines are colorless but become colored when stored due to atmospheric oxidation.
• Lower aliphatic amines are water-soluble due to hydrogen bonding, but solubility decreases with increasing molar mass.
• Amines are soluble in organic solvents like alcohol, ether, and benzene.
• Primary and secondary amines engage in intermolecular association through hydrogen bonding.
• Primary amines exhibit greater intermolecular association than secondary amines, as they have two hydrogen atoms available for bonding.
• Tertiary amines lack intermolecular association due to the absence of a hydrogen atom available for hydrogen bond formation.
• The boiling points of isomeric amines decrease in this order: primary > secondary > tertiary

Chemical properties

• Amines are reactive due to differences in electronegativity between nitrogen and hydrogen atoms and the presence of an unshared electron pair.
• Amines behave as nucleophiles because of their unshared electron pair.

Basic Character of Amines

• Act as Lewis bases due to an unshared pair of electrons on the nitrogen atom.
• They react with acids to form salts.
• Amine salts, when treated with a base like NaOH, regenerate the parent amine.
• Larger the value of Kb or smaller the value of pKb, stronger is the base
• Aliphatic amines are stronger bases than ammonia because of the +I effect of alkyl groups, which increases electron density on the nitrogen atom.
• Aromatic amines are weaker bases than ammonia due to the electron-withdrawing nature of the aryl group.
• The order of basicity of amines in the gaseous phase is tertiary amine > secondary amine > primary amine > NH3

Alkylation of Amines

• Amines undergo alkylation when they react with alkyl halides.

Acylation of Amines

• Aliphatic and aromatic primary and secondary amines react with acid chlorides, anhydrides, and esters through a nucleophilic substitution reaction.
• H atom of the –NH2 or >N-H group is replaced by an acyl group.
• This reaction is carried out in the presence of a base (like pyridine) to remove the HCl formed, shifting the equilibrium to the right.

Carbylamine Reaction

• Aliphatic and aromatic primary amines when heated with chloroform and ethanolic KOH will form isocyanides or carbylamines (foul smelling substances).
• Secondary and tertiary amines do not show this reaction.
• Used as a test for primary amines.

Reaction with Nitrous Acid

• Three classes of amines react differently with nitrous acid.

Primary Amines

• React with nitrous acid to form diazonium salts (unstable) that liberate nitrogen gas quantitatively & alcohols.
• The quantitative evolution of nitrogen gas is used in the estimation of amino acids and proteins.

Aromatic Amines

• React with nitrous acid at low temperatures to form diazonium salts
• Class of compounds used for the synthesis of variety of aromatic compounds

Secondary Amines

• React with nitrous acid in a different manner.

Reaction with Arylsulphonyl Chloride

• Benzenesulphonyl chloride (C6H5SO2Cl) - also known as Hinsberg’s reagent, reacts with primary & secondary amines to form sulphonamides.

Primary Amine

• Yields N-ethylbenzesulphonyl amide.

Secondary amine

• N,N-diethyl-benzenesulphonamide is formed
• Tertiary amines do not react with benzenesulphonyl chloride.

Electrophilic Substitution

Aniline is a resonance hybrid.

Ortho- & para-

-NH2 group increase electron density in the product

• The –NH2 group are highly activating groups

-Bromination

• Aniline + bromine water → white precipitate of 2,4,6 tribromoaniline.

Diazonium Salts

• Have the general formula RN2+X−, where R is an aryl group, and X− is an anion.
• named by adding diazonium with the name of the parent hydrocarbon + the name of the anion
• The diazonium salts is named benzendiazonium.
• Primary aliphatic amines produce highly unstable alkyldiazonium salts.

Preparation of Diazonium Salts

Arenediazonium salts.

• Arenediazonium salts stay stable at low temperatures for a short time.

Chemical Reactions Involving Diazonium Salts

• Replacement by halide or cyanide ion.
• Copper (I) ion catalyzed reaction is called Sandmeyer Reaction.

Involving Retention of Diazo Group: Coupling Reactions

• Coupling reaction - benzene diazonium chloride will react with phenol to form p-hydroxyazobenzene
• the two aromatic rings are bonded through the -N=N-bond

Explore IUPAC nomenclature rules for amines, including primary, secondary, and tertiary amines. Learn about the classification, structure, and properties of amines, including bond angles and hybridization. Includes arylamines and common names.