AMINES
Indtroduction
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
Forexample: Thebondanglefortrimethylamineis108°.
Classification of Amines
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
ReductionofNitroCompounds
Nitro compounds on reduction with hydrogen gas in the presence of finely divided nickel, palladium orplatinumandalsoonreductionwithmetals inacidicmediumgiveamines.
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.
Theprimaryaminepreparedbehavesasanucleophileandreactswith furtheralkylhalidetoformsecondary, tertiaryamines,andfinallyquaternaryammoniumsalt.
Thefreeaminecanbeobtainedfromtheammoniumsaltbytreatmentwithastrongbase.
Inthismethod, amixtureof primary, secondaryandtertiaryandalsoaquaternaryammoniumsalt.Howeveraprimaryamineispreparedbytakinglargeexcessofammonia.
TheorderofreactivityofhalideswithaminesisRI>RBr>RCl
ReductionofNitriles
Nitrilesonreducing withLiAlH4orcatalytichydrogenationproduceprimaryamines.
Reduction of Amides
AmidesonreducingwithLiAlH4yieldamines.
Gabrielpthalimidesynthesis
PthalimideonreactingwithethanolicsolutionofKOHformspotassiumsaltofpthalimidewhichonheating withalkylhalidefollowedbyalkalinehydrolysisyieldsthecorrespondingprimaryamine.
Hoffmannbromamidedegradationreaction
Inthismethod,primaryaminesarepreparedbytreatinganamidewith bromineinanaqueousorethanolicsolutionofNaOH.
Theamineformedhasonecarbonatomlessthanthestartingamide.
PhysicalPropertiesof Amines
Loweraliphaticaminesaresolublein waterbecausetheycanform ahydrogenbond with water.Solubility decreases with increase in molar mass of amines due to an increase in the size of thehydrophobicgroup.
Amongtheisomericamines,primaryandsecondaryamineshaveahighboilingpointbecausetheycanformhydrogenbonds.
Tertiaryaminescannotformhydrogenbondsduetotheabsenceofahydrogenatomforhydrogenbondformation.
Hence,theorderofboilingpointsofisomericaminesisPrimary> Secondary> Tertiary
ChemicalPropertiesof Amines
Aliphaticaminesarestrongerbasesthanammoniaduetothe+Ieffectofalkylgroups,leading tohighelectrondensityonthenitrogenatom.
Tertiaryamine> Secondaryamine> Primaryamine>NH3
(CH3)2NH>CH3NH2> (CH3)3N>NH3
Example: Ethylamineismorebasic thananiline.
Reactionof Amines
Acylationreaction
ImportantNote: Becausetertiaryaminesdonot containareplaceablehydrogenatom,theydonotundergoacylation.
Carbylaminereaction
ReactionwithNitrousacid
Reactionwitharylsulphonylchloride
Hinsberg’sreagentorbenzenesulphonylchloride(C6H5SO2Cl)reactswithprimaryaminesandsecondaryamines toform sulphonamides.
The hydrogen bonded to nitrogen is strongly acidic due to the presence of strong electronwithdrawingsulphonylgroupandis hencesolubleinalkali.
Electrophilicsubstitution
Ortho-andpara–positionstothe -NH2groupbecomecentresof highelectrondensity.So-NH2 groupis orthoandparadirectingandapowerfulactivatinggroup.
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.
Anilineonreacting withsulphuricacidformsaniliniumhydrogensulphate whichonheatingwithsulphuricacidat 453-473K givesp-aminobenzenesulphonicacidasthemajorproduct.
AnilinedoesnotundergoFriedel-CraftsreactionduetosaltformationwithLewisacidaluminiumchloridewhich is used as a catalyst. As a result, nitrogen of aniline acquires positive charge and hence acts as astrongdeactivatinggroupforfurther reaction.
DiazoniumSalts
PreparationofDiazoniumSalts
PhysicalProperties
ChemicalReactions
ReactionsinvolvingdisplacementofNitrogen
Thisreaction iscalledSandmeyer reactioninwhichnucleophiles likeCl–,Br–andCN– canbeeasilyintroducedinthebenzeneringinthepresenceofCu(I)ion.
Alternatively, chlorine or bromine can also be introduced in the benzene ring by treating thediazonium saltsolutionwithcorrespondinghalogenacidinthepresenceofCupowder.Thisisreferredas Gattermanreaction.
Iodobenzeneisformedontreating diazoniumsaltsolutionwithpotassium iodide.
Arenediazoniumchlorideontreating withfluoboricacidgivesaprecipitateofarenediazoniumfluoroboratewhichonheatingdecomposes togivearyl fluoride.
Hypophosphorusacidorethanolaremildreducingagentsandreduce diazonium saltstoarenesandthemselves get oxidisedtophosphorus acidandethanalrespectively.
Diazonium saltsolutiongetshydrolysedtophenol whenthetemperatureisallowedtoriseupto283K.
Onheatingdiazoniumfluoroboratewithaqueoussodiumnitritesolutioninthepresenceof copper,thediazoniumgroupis replacedby–NO2 group. 
Reactionsinvolvingretentionofdiazogroupcouplingreactions
Importanceof DiazoniumSaltsin Synthesisof OrganicCompounds
