Amines are the derivatives of ammonia prepared by the replacement of one, two, or all three hydrogen atoms by alkyl and/or aryl groups.
Examples:
(i)CH3-NH2
StructureofAmines.
•Inamines, then nitrogen atom is trivalentand has an unshared pair of electrons. Hencethenitrogenorbitalsaresp3hybridisedwithpyramidalgeometry.
•The three-sp3 hybrid orbitals of nitrogen overlap with the orbitals of hydrogen or carbon dependingon the nature of the amines.
• The fourth orbital of nitrogen in all amines contains an unshared pair ofelectrons. It is due to the presence of a pair of electrons, the angle C-N-E is less than 109.5°.
Forexample: Thebondanglefortrimethylamineis108°.
ClassificationofAmines
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Nomenclature of Amines by IUPAC System
Common Names of Amines
An aliphatic amine is named by prefixing alkyl group to amine, i.e. alkylamine as 1 word.
We are considering primary amines.
In secondary and tertiary amines, when two or more groups are the same, the prefix di or tri is added to the before the name of alkyl group.
In case of mixed amine, alkyl group are written in alphabetical order.
IUPAC System
In IUPAC system, amines are named as alkanamines, derived by replacement by ‘e’ of alkane by the word amine.
For Example: – Methanamine (CH3NH2), Ethan amine (C2H5NH2).
For naming higher member hydrocarbon, longest chain containing amino group is selected. C atom to which amino group is attached is given to lower number.
In case, more than one amino group is present at different positions in the parent chain, their positions are specified by giving numbers to carbon atoms bearing –NH2 groups and suitable prefix such as di, tri etc. is attached to the amine. The letter ’e’ of the suffix of the hydrocarbon part is retained.
For Example:- (Ethane 1,2 diamine) H2N-CH2-CH2-NH2
Each alkyl group bonded to the N atom is named as N-alkyl group.
Aryl Amines
In aryl amines, -NH2 group is directly attached to the benzene ring.
C6H5NH2 is the simplest example of aryl amine. It is also known as aniline.
Naming aryl amines according to IUPAC system, suffix ‘e’ of arene is replaced by ‘amine’.
For Example: – C6H5-NH2 is named as benzenamine.
PreparationofAmines
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ReductionofNitroCompounds
Nitro compounds on reduction with hydrogen gas in the presence of finely divided nickel, palladium orplatinumandalsoonreductionwithmetalsinacidicmediumgiveamines.
Ammonolysis
Alkyl halides or benzyl halide on reaction with an ethanolic solution of ammonia undergoesnucleophilic substitution reaction in which halogen atom is replaced by an amino (-NH2) group.TheprocessofcleavageoftheC-Xbondbyammoniamoleculeisknownasammonolysis.
Loweraliphaticaminesaresolubleinwaterbecausetheycanformahydrogenbond with water.Solubility decreases with increase in molar mass of amines due to an increase in the size of thehydrophobicgroup.
The tertiary ammonium ion is less hydrated than the secondary ammonium ion, which is lesshydratedthantheprimaryamine.Thus,tertiaryamineshavelesstendencytoform ammoniumionandconsequentlyareleastbasic.
On the basis of the solvation effect, the order of basicity of aliphatic amines should be:Primaryamine> Secondaryamine>Tertiaryamine NH3
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Stericfactor
As the crowding of the alkyl group increases from primary to tertiary, amine hindrance tohydrogen bonding increases which eventually decreases the basic strength. Thus, there is asubtleinterplayoftheinductiveeffect, solvationeffectandsterichindranceofthealkylgroupwhichdecidesthebasicstrengthofalkylaminesintheaqueousstate.
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When the alkyl group is small like CH3, there is no steric hindrance to hydrogen bonding.Inthiscase, theorderofbasicityinaqueousmediumis:
In the above resonating structures, there is a positive charge on the nitrogen atom making thelonepairlessavailableforprotonation.Hence,anilineislessbasicthanethylamine whichhasnoresonatingstructures.
The reaction is carried out in the presence of a stronger base than the amine, such as pyridine,whichremovesHClformedandshiftstheequilibriumtotheproductside.
Primary aliphatic amines react with nitrous acid to form aliphatic diazonium salts. Being unstablediazonium salts liberate nitrogen gas quantitatively which is used in the estimation of amino acidsandproteins.
Aniline reacts with bromine water at room temperature to give a white precipitate of 2, 4,6-tribromoaniline.
Due to the high reactivity of aromatic amines, electrophilic substitution takes place at ortho- and para-positions.
Forpreparingmonosubstitutedanilinederivative,the-NH2groupisprotectedbyacetylationwithaceticanhydride then carrying out the desired substitution followed by the hydrolysis of the substitutedamidetothesubstitutedamine.
In strong acidic medium, aniline is protonated to form the anilinium ion which is meta directing.Hencebesidestheorthoandparaderivatives, significantamountofmetaderivativeisalsoformed.
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Howeverifweprotectthe –NH2groupbyacetylationreaction withaceticanhydride,thenitrationreaction can be controlled and the p-nitro derivative derivative can be prepared as the majorproduct.
AnilinedoesnotundergoFriedel-CraftsreactionduetosaltformationwithLewisacidaluminiumchloridewhich is used as a catalyst. As a result, nitrogen of aniline acquires positive charge and hence acts as astrongdeactivatinggroupforfurther reaction.
Alternatively, chlorine or bromine can also be introduced in the benzene ring by treating thediazonium saltsolutionwithcorrespondinghalogenacidinthepresenceofCupowder.Thisisreferredas Gattermanreaction.
Benzenediazoniumchloridereactswithphenol in whichthephenolatitsparapositioniscoupledwiththediazoniumsalttoformorangecolourdyecalledp-hydroxyazobenzene.
Theamineformedhasonecarbonatomlessthanthestartingamide.
Aniline reacts with bromine water at room temperature to give a white precipitate of 2, 4,6-tribromoaniline.
