unit 2 org. 《Amines 》.pdf

Learn this organic amines.

1. UNIT-2
Amines
1

2. ❑Introduction
▪Amines
➢areorganicderivativesofammonia
➢containanitrogenatomwithalonepairofelectrons
✓Mostofthechemistryofaminesdependsonthepresenceofthislonepairof
electrons.
✓Whichismakingaminesbothbasicandnucleophilic.
•Aminescanbeeither
✓Alkyl-substituted(Alkylamines)or
✓Aryl-substituted(Arylamines)
•Dependingonthenumberoforganicsubstituentsattachedtonitrogen,aminesare
classifiedas
•Primary(RNH2)amine,
•Secondary(R2NH)amine,or
•Tertiary(R3N)amine
•Compoundscontaininganitrogenatomwithfourattachedgroupsalsoexist,butthe
nitrogenmustcarryaformalpositivecharge,suchcompoundsarecalledquaternary
ammoniumsalts..
2N R
R
R
R X
A quaternary ammonium salt

3. ❑Naming Amines
•Forsimpleprimaryamines
➢Byaddingthesuffix–aminetothenameofalkylsubstituent.
E.g.
➢Byusingthesuffix–amineinplaceofthefinal–einthenameoftheparent
compound.
E.g.
•Forprimaryamineswithmorethanonefunctionalgroup
➢Byconsideringthe–NH2asanaminosubstituentontheparentmolecule.
E.g.
3

4. .
•For symmetrically substituted secondary and tertiary amines,
➢By adding the prefixdi-or tri-to the alkyl group.
E.g.
•For unsymmetricallysubstituted secondary and tertiary amines,
➢Named as N-substituted primary amines
✓the largest alkyl group is chosen as the parent name
✓the other alkyl groups are N-substituentson the parent (N because they’re
attached to nitrogen)
E.g.
4

5. •For heterocyclic amines
-Each different heterocyclic ring system has its own parent name.
-The heterocyclic nitrogen is always numbered as position 1.
5

6. ❑Properties of Amines
•Alkylamineshastetrahedralgeometry
➢ThenitrogenatomisSP3-hybridized.
➢Thethreesubstituentsoccupyingthreecornersofatetrahedron.
➢Thelonepairofelectronsoccupyingthefourthcorner.
➢TheC-N-Cbondanglesareclosetothe109º.
Note:Thebondinginalkylaminesissimilartothebondonginammonia.
6

7. •Amineswiththreedifferentsubstituentsonnitrogenischiral
•Unlikechiralcarboncompounds,chiralaminescan’tusuallyberesolved.
✓becausethetwoenantiomericformsrapidlyinterconvertbyapyramidal
inversion.
•Amineswithfewerthanfivecarbonatomsaregenerallywater-soluble.
•AmineshavehigherboilingpointsthanalkanesofsimilarMWt.
✓becauseoftheirabilitytoformhydrogenbonds.
•LowMWt.amineshaveadistinctivefish-likearoma(odor).

8. 8

9. ❑Basicityof Amines
•Thechemistryofaminesisdominatedbythe
➢lonepairofelectronsonnitrogen
✓whichmakesaminesbothbasicandnucleophilic.
•Aminesreactwithacidstoformacid-basesalts.
•Aminesreactwithelectrophilesinmanyofthepolarreaction
•Aminesaremuchstrongerbasesthanalcoholsandethers.
9

10. ➢Whenanamineisdissolvedinwater,anequilibriumisestablishedinwhichwateractsasanacidandtransfers
aprotontotheamine.

11. .
•Arylaminesaregenerallylessbasicthanalkylamines.
✓becausethenitrogenlone-pairelectronsaredelocalizedbyinteractionwiththe
aromaticringπelectronsystemandarelessavailableforbondingtoH+.
Note
•Inresonanceterms,arylaminesarestabilizedrelativetoalkylamines,
✓becauseoftheirfiveresonanceforms.
11

12. .
▪Amides (RCONH2)
•Amides
➢arenonbasic.
➢don’tundergosubstantialprotonationbyaqueousacids.
➢arepoornucleophiles.
•Themainreasonforthisdifferenceinbasicitybetweenaminesandamidesisthat
➢Amideisstabilizedbydelocalizationofthenitrogenlone-pairelectronsthroughorbitaloverlap
withthecarbonylgroup.
➢Inresonanceterms,amidesaremorestableandlessreactivethanamines.
✓becausetheyarehybridofthetworesonanceforms.
•Theamideresonancestabilizationislostwhenthenitrogenatomisprotonated,so
protonationisdisfavored.
12

13. .
13

14. ❑Acidity of Amines
▪Primaryandsecondaryamines
➢Inadditiontotheirbehaviorasbases,theycanalsoactasveryweakacids.
✓BecauseanN-Hprotoncanberemovedbyasufficientlystrongbase.
Forexample,
•Diisopropylaminereactswithbutyllithiumtoyieldlithiumdiisopropylamide(LDA).
14

15. ❑Preparation of Amines
➢Thesimplestmethodofalkylaminesynthesisis
✓BySN2alkylationofammoniaoralkylaminewithanalkylhalide.
➢AmmoniaandotheraminesaregoodnucleophilesinSN2reactions
•SN2alkylationofammoniawithanalkylhalideresultsaprimaryamine.
•SN2alkylationofprimaryalkylaminewithanalkylhalideresultsasecondary
amine.
•SN2alkylationofsecondaryalkylaminewithanalkylhalideresultsatertiary
amine.
15

16. ❑Reactions of Amines
▪Alkylation and Acylation
➢1º, 2º, and 3º amines can be alkylatedby reaction with a primary alkyl halide.
➢Alkylation of 1º and 2º amines are difficult to control and often give a mixture
of products.
➢3º amines are clearly alkylatedto give quaternary ammonium salts.
.
16

17. .
•1ºand2º(butnot3º)aminescanalsobeacylatedby
➢Nucleophilicacylsubstitutionreactionwithanacidchlorideoranacidanhydrideto
yieldanamide.
Note
•Overacylationofthenitrogendoesnotoccur
✓becausetheamideproductismuchlessnucliophilicandlessreactivethanthe
startingamine.
17

18. ▪Diazotization Reactions
➢Primaryarylaminesreactwithnitrousacid(HNO2)toyieldstablearenediazoniumsalts(Ar-
N+≡NX¯),aprocessiscalleddiazotizationreaction.
Note
•Alkylaminesalsoreactwithnitrousacid,butthealkanediazoniumproductsaresoreactive
theycan’tbeisolated.Instead,theylossnitrogeninstantlytoyieldcarbocations.
•TheanalogouslossofN2fromarenediazoniumiontoyieldanarylcationisdisfavoredbythe
instabilityofthecation.
18

19. ▪SandmeyerReaction
➢Itisareactionofanarenediazoniumsaltwiththecorrespondingcopper(I)halide(CuX).
➢Isaprocessusedtopreparearylchlorideandbromides.
➢AryliodidescanbepreparedbydirectreactionwithNaIwithoutusingacopper(I)salt.
19

20. •Mechanistically, these diazonioreplacement reactions occur through radical rather than polar
pathways.
In the presence of copper (I) cpd, the arenediazoniumion is first converted to an aryl radical plus
copper (II), followed by subsequent reaction to give product plus regenerated copper (I) catalyst.
20

21. •Similar treatment of arenediazoniumsalt with CuCNyields the nitrile(ArCN), which can then be
further converted into other functional groups such as carboxyl.
For example,
Sandmeyerreaction of o-methylbenzenediazoniumbisulfate with CuCNyields o-methylbenzonitrile,
which can be hydrolyzed to give o-methylbenzoicacid.
21

22. ▪Reduction of DiazoniumSalt to Arene
➢Occursontreatmentwithhypophosphorousacid(H3PO2).
➢Thisreactionisusedprimarily
✓whenthereisaneedfortemporarilyintroducinganaminosubstituentontoaringtotake
advantageofitsdirectingeffect.
Forexample,
•Tomake3,5-dibromotolunefromp-methylaniline.
✓Theproductcan’tbemadebydirectbrominationoftolunebecausereactionwouldoccur
atposition2and4.
✓However,dibrominationoccursorthotothestronglydirectingaminosubstituent,and
diazotizationfollowedbytreatmentwithH3PO2yieldsthedesiredproduct.
22

23. .
23

24. ▪DiazoniumCoupling Reactions
➢Arenediazoniumsaltsundergoacouplingreactionwithactivatedaromatic
ringssuchasphenolandarylaminestoyieldazocompounds(Ar-N=N-Ar´).
➢aretypicallyelectrophilicaromaticsubstitutionreactioninwhich
✓thepositivelychargeddiazoniumionistheelectrophilethatreactswiththe
electron-richringofaphenolorarylamine.
➢usuallyoccursattheparaposition,
•Butorthoreactioncantakeplaceiftheparapositionisblocked.
24

25. .
25

26. ▪Hofmann Elimination
•Likealcohols,aminescanbeconvertedintoalkenesbyaneliminationreaction.
➢Forthisreaction,anamideion(NH2¯)mustfirstbeconvertedintoabetterleavinggroup.
•IntheHofmanneliminationreaction,
➢anamineismethylatedbyreactionwithexcessiodomethanetoproduceaquaternaryammonium
salt,whichthenundergoeseliminationtogiveanalkeneonheatingwithabase(typicallysilver
oxide(Ag2O)).
➢TheactualeliminationstepisanE2reaction,
✓inwhichhydroxideionremovesaprotonatthesametimethepositivelychargednitrogenatom
leaves.
•Silveroxideactsbyexchanginghydroxideionforiodideioninthequaternarysalt,
➢Thus,providingthebasenecessarytocauseelimination.
26

27. For example, 1-methylpentylamine is converted into 1-hexene.
27

28. .
•AninterestingfeatureoftheHofmanneliminationisthat
➢ItgivesproductsdifferentfromthoseofmostotherE2reactions.
✓i.e.ThemorehighlysubstitutedalkeneproductgenerallypredominatesintheE2reaction
ofalkylhalide(Zaitsev’srule).However,intheHofmanneliminationthelesshighly
substitutedalkenepredominates.
•Thereasonforthisselectivityisprobablysteric.
✓becauseofthelargesizeofthetrialkylamineleavinggroup,thebasemust
abstractahydrogenfromthemoststericallyaccessible,leasthinderedposition.
28