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Topic10.3:StructuralElucidationinOrganicChemistry
1. HCI/2009/P3/Q3e
2. DHS/2009/P3/Q2b
ThecarbontohydrogenratioofPis1:2.Pdoesnotcontainbenzenering,butcouldcontain alkenegroup.P undergoes electrophilic addition with hydrogen chloride gas to form QandRinunequalproportion.Pisanunsymmetricalalkene.QandRarehalogenoalkanes.Q and R undergo nucleophilic substitution with sodium hydroxide to form S and Trespectively.SandTarealcohols.Sisnotoxidizedbyacidifiedpotassiummanganate(VII).
Sisatertiaryalcohol.Tisoxidizedbyacidifiedpotassiummanganate(VII)toform U.T is a primary or secondary alcohol. U could be carboxylic acid orketone.
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U undergoes neutralisation / acid base reaction with sodium carbonatetoliberatecarbondioxidegas.Uisacarboxylicacid,henceTisaprimaryalcohol.P undergoes (strong) oxidation with acidified potassium manganate (VII) to
formV,C5H10O,whichhasonlyoneoxygenatom,andcarbondioxide.Phasaterminalalkenegroup.Visaketone.V undergoes condensation with 2,4-dinitrophenylhydrazine to form anorangeprecipitate.Visaketone/carbonylcompound.V undergoes reduction with lithium aluminium hydride to form W, whichdoesnotexhibitopticalactivity.Wisasecondaryalcoholandisasymmetricalmolecule.
PQR
STUVW
C C
H
H
CH2CH3
CH2CH3
C CH
H CH2CH3
CH2CH3H
ClC CCl
H CH2CH3
CH2CH3H
H
C CH
H CH2CH3
CH2CH3H
OH C COH
H CH2CH3
CH2CH3H
H C COH
O CH2CH3
CH2CH3
H
O C
CH2CH3
CH2CH3
OH C
CH2CH3
CH2CH3
H
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3. IJC/2009/P3/Q1f
4. ACJC/2009/P2/Q4b
5. JJC/2009/P2/Q6c
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(ii)ExcessNH3,heatunderrefluxinethanol
6. MJC/2009/P3/Q4c
7. NJC/2009/P3/Q2e
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8. NYJC/2009/P3/Q5c
STRUCTURES
[1] Mis
C
CH3
CH CH
OH
CH CH CH3
[1] Nis
C
CH3
O
[1] Ris
C
O-
O
Na+
[1] Pis
COH
O
CC
OH
O
O
[1] Qis
C CH3
OH
O EXPLANATIONOFTHEREACTIONSDESCRIBED[1] Mdecolourisesaqueousbromineduetoelectrophilicadditionacrossthealkene(C=C)functionalgroup.[1] Mproduceswhitefumes(ofHCl)withPCl5duetoasubstitutionreactionofthe(secondary)alcoholfunctionalgroup.
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[1] Mproduced3compounds,N,PandQwithhotacidifiedpotassiummanganate(VII)duetooxidativecleavageofthe(2)alkenefunctionalgroupspresent.[1] NdoesnotundergomildoxidationwithFehlingsreagentasitisnotanaldehyde.
[1] Ngivesayellowppt(ofCHI3)andsaltRwithaqalkalineiodineduetothe(mild)oxidation(andcleavage)oftheCH3CORgroup.[1] Pgivesaneffervescence(ofCO2)withNa2CO3(s)duetoaneutralizationreactionwiththecarboxylicacidfunctionalgroup.[1] Pgivesanorangeprecipitatewith2,4-DNPHduetocondensationwiththeketonefunctionalgroup.[1] Qgivesaneffervescence(ofCO2)withNa2CO3(s)duetoaneutralizationreaction
withthecarboxylicacidfunctionalgroup.[1] Qdoesnotgiveanorangeprecipitatewith2,4-DNPHduetotheabsenceofacarbonylfunctionalgroupforcondensation.
9. RJC/2009/P2/Q4(a),(b)
(a) Structural/positionalisomerism(b) Geometricisomerism
C CH H
CH3
HO
OCH3
C C
H
CH3H
HO
CH3O
cis trans
10. SAJC/2009/P3/Q4c
Information Deduction
StereoisomerA - contains either an alkene with non-
identicalgroupsonthesamecarbonor
chiralcarbon
A reacts with hot acidified potassium
manganate(VII)solution
-Aundergoesoxidation
-AcontainsC=C
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B forms a yellow precipitate with hot
alkalineiodinesolution.
-BcontainsCH3CO-
(donotacceptCH3CHOH-)
Cold alkaline hydrogen cyanide was
added to B and the mixture was
reducedtoformcompoundC
-Bisaketonewhichundergoes
nucleophilicadditionwithHCN
-Ccontainsanaminefunctionalgroup
CompoundDformedwhiteprecipitate
withethanolicsilvernitratesolution.
-Dcontainschloroalkanewhichforms
AgCl
CompoundDwasreactedwith
phosphorouspentachloridetoform
compoundE.
-Disanalcoholorcarboxylicacid
(donotacceptDcontainsOHgroup)
whichundergoesnucleophilic
substitution/displacementreactionto
formEwhichisaanacidchloride.
CompoundsCandEwerethenreacted
toformtwocompoundsFandG,with
identical molecular formulaC7H14O2NCl.
-condensationreactiontookplace
CompoundFisneutral. -FisanamideorFdoesnotcontain
phenol/carboxylicacid/amine.
Compound Structure Compound Structure
A CH2ClC
H
CCH2CH3
CH3
D CH2Cl
C O
HO
B
C
CH2CH3
CH3
O
E
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C
C
CH2CH3
CH3
HO CH2NH2
F
C
CH2CH3
CH3
HO CH2NHCOCH2Cl
G
C
CH2CH3
CH3
CH2ClCOO CH2NH2
11. SAJC/2009/P3/Q5c
Information Deduction
Both A and B do not react with
Na2CO3
-AandBdonothaveCOOH
AandBdonotgivepositivetest A and B do not contain any methyl
ketoneCH3CO-orCH3CHOH-group
0.370 g of A reacts with sodium to
form a flammable gas that takes up
125.8cm3ofspaceat32.5
oCand101
kPa.
FlammablegasH2
NoofmolesofA=0.370/74=0.005
mol
101000 x 125.8 x 10-6 =n x 8.31 x
(273+32.5)
NoofmolesofH2=0.005mol
-moleratioofA:H2=1:1
-Acontains2alcoholgroups
BdoesnotreactwithNa -BdoesnotcontainOHgroup.
*Alternatively,studentscangiveequationstoillustratethereactionsundergoneby AandB
Structure Structure
A B
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or any structure with two OH
groups (not CH3CHOH-) and
C=C.
*
*[notstable]
or
12. SRJC/2009/P3/Q3e
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2+
2+
&+ &+&+
(
)CH2Br
13. TJC/2009/P3/Q1b
HighC:HratioinfersthatPisanaromaticcompound.
P
WithanhydrousaluminiumchloridePundergoeselectrophilicsubstitutionreaction
toformQ.AstheOHisanactivatinggroup,thesubstituentwillbedirected
totheorthoorparaposition.
Q
OR
Pundergoeselectrophilicsubstituitionasitdecolourisesaqueousbromine toform
whitepptR.Ris2,4,6-tribromophenolas-OHisahighlyactivatinggroup.
Q reactwithaqueousbrominetoformwhiteppt S, withthesamenoofbromine
atomspermoleculeofR. AsQhasa C-Cdoublebond,electrophilicaddition
takes place as well. This implies that 1 Br will be attached to one of the
carbonsoftheC-Cdoublebonds.Thus,only2Brwillbeattacheddirectlyto
thebenzene.
S
2+
CH2CH=CH2
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Sisopticallyactivebecauseofitschiralcarbon.ThemirrorimageofSisnon-superimposable.
14. TPJC/2009/P3/Q1d
(i)
O O
OHOH Br Br
Br
BrA B C D E AundergoeselectrophilicadditionwithHBrtoform DandE.Aundergoesvigorousoxidation/oxidativecleavagewithhotacidifiedKMnO 4toformC.
Ccanundergoneutralizationoracid-basereactionwithaqueousNa 2CO3toliberateCO2gas.Bundergoesvigorousoxidation/oxidativecleavagewithhotacidifiedKMnO 4toformCO2BundergoeselectrophilicadditionwithHBrfollowedbynucleophilicsubstitutionwithaqueousNaOHtoformhexan-2,3,5-triol. (ii)Acannotformcis-transisomersasitisacyclicalkeneandthetransisomerwillresultinbondanglestrain.Bcanformapairofcis-transisomers asithasanC=Cbondwitheachcarbonbondedtotwodifferentgroupsofatoms.
H
H HH
trans cis hexan-2,3,5-triolhas3chiralcarbons, henceno.ofopticalisomers=2 3=8
YJC/2009/P3/Q5c15.
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ACJC/2009/P3/Q4b16. A has amine group being basic. No reaction with aq Br2 shows absence of
phenylamine.Aisaketone..Ahasabenzenering,andis2,4directing.Aundergoes
nucleophilic substitution with aq OH- forming C. C has a primary alcohol forming
COOHconfirmingbyreactionwithcarbonate.Aundergoeselimination. A
CH2NH2
COCH2Cl
BCH
2NH
2
CO N N
H
NO2
CH2Cl
NO2
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C
CH2NH2
CO
CH2OH
DCH2NH2
CO
CH2OH
E
CH2
NH
C
O
IJC/2009/P3/Q3a17. (i)
(ii)
IJC/2009/P3/Q5d18.
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SRJC/2009/P3/Q3e19.
CH3 C C
Cl Cl
CH2Cl
CH3H A
CH3 C C
OH OH
CH2OH
CH3H B
CHI3 C
C COH
CH2OH
CH3
O
Na+-O D
HOOC C
OH
COOH
CH3 E
NYJC/2009/P3/Q5c20.
STRUCTURES
[1] Mis
C
CH3
CH CH
OH
CH CH CH3
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[1] Nis
C
CH3
O
[1] Ris
C
O-
O
Na+
[1] Pis
COH
O
CC
OH
O
O
[1] Qis
C CH3
OH
O
EXPLANATIONOFTHEREACTIONSDESCRIBED[1] Mdecolourisesaqueousbromineduetoelectrophilicadditionacrossthealkene(C=C)functionalgroup.
[1] Mproduceswhitefumes(ofHCl)withPCl5duetoasubstitutionreactionofthe(secondary)alcoholfunctionalgroup.[1] Mproduced3compounds,N,PandQwithhotacidifiedpotassiummanganate(VII)duetooxidativecleavageofthe(2)alkenefunctionalgroupspresent.[1] NdoesnotundergomildoxidationwithFehlingsreagentasitisnotanaldehyde.[1] Ngivesayellowppt(ofCHI3)andsaltRwithaqalkalineiodineduetothe(mild)oxidation(andcleavage)oftheCH 3CORgroup.
[1] Pgivesaneffervescence(ofCO2)withNa2CO3(s)duetoaneutralizationreactionwiththecarboxylicacidfunctionalgroup.[1] Pgivesanorangeprecipitatewith2,4-DNPHduetocondensationwiththeketonefunctionalgroup.[1] Qgivesaneffervescence(ofCO2)withNa2CO3(s)duetoaneutralizationreactionwiththecarboxylicacidfunctionalgroup.
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[1]Qdoesnotgiveanorangeprecipitatewith2,4-DNPHduetotheabsenceofacarbonyl functionalgroupforcondensation.
RI/2009/P3/Q1c-e
21.
(c) CompoundDis
DdoesnotcontainOHnorCOOHsinceitdoesnotreactwithsodium.
ItisadiesterformedwhenOHandCOOHgroupsofonelacticacidmoleculereactswiththeCOOHandOHgroupsrespectivelyofanotherlacticacidmolecule.
(d)(i) Ehasachiralcentreasitrotatesplanepolarisedlight. E isan aldehyde sinceit reactswithFehlings solution to give areddish brown
precipitate.
AmountofhydrogenevolvedwithsodiumpermolofE=0.24
0.0124
=1mol
HencetwodisplaceableHarepresenttherearetwoOHgroup.
Eis (ii) Eisanalcohol. The stability ofits anion is lowerthan thestabilityof the anionof
lacticacidastheanionisdestabilisedbytheelectrondonatingCH2CH(OH)CHOgroup.
Lacticacidis amuchstronger acidasits anionforms twoequivalent resonancestructureswiththenegativechargeonOdelocalisedovertwoelectronegativeOatoms.
(e)(i) maleicacidfumaricacid
(ii) Intramolecular hydrogenbonding occurs in maleic acid and thus it has lower
meltingpointastherearelesssitesforintermolecularhydrogenbonding.
& &
& &
222
2
+ +
+
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RI/2009/P3/Q5b22.
(b) Information Deductionofstructure
FormulaofG=C8H11O3N Indexofunsaturation
=(2x8+211+1)=4
(orhighCtoHratio)
Gislikelytocontainabenzenering/aromatic.
Grotatesplanepolarisedlight. Gislikelytocontainatleastonechiralcarbon.
Ggivesavioletcolourationwith
neutralFeCl3solution
G is a phenol with violet colourtion formed by
complexformation.
Greactswith2molofaqNaOH Thereare2phenolicOHgroupinG,reactingwith
OHviaacid-basereaction.
G is insoluble in water but
dissolvesindilH2SO4.
G is likely to contain an amine group, withNH2
beingprotonatedbyH+.
G is oxidised by K2Cr2O7 to
form a product which reacts
with 2,4DNPH to give an
orangeppt.
G is a secondary alcohol and is oxidised to a
ketonebyK2Cr2O7.
H is formed by reaction of G
withAl2O3
H is formed from G by dehydration, it is likely an
alkene.
H reacts with aq Br2 to form
C8H7O3NBr4.
The product is likely to be a bromohydrin formed
by electrophilic addition and electrophilic
substitutionintoactivatedbenzenering.
G reacts with ClCOCOCl to The phenol, secondary alcohol and amine reacts
intramolecularhydrogenbond
+
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form J which has 3 hexagonal
rings.
withClCOCOClvianucleophilicsubstitution.
3 hexagonal ring cyclic esters and amides
formed with the phenolicOHs andalcoholicOH
andamineonadjacentcarbonatoms.
SRJC/2009/P3/Q5a
TPJC/2009/P3/Q4b24.
23. (a) GundergoesoxidationwithacidifiedKMnO4.
HundergoesnucleophilicsubstitutionwithPCl5.
o Hisanalcoholorcarboxylicacid
o Jisahalogenoalkaneoracylchloride
Jundergoesreductionwithtininconcentratedhydrochloricacid.
o Jcontainsnitrobenzeneo Kcontainsphenylamine
K reacts with (C2H5)2NCH2CH2OH through esterification/nucleophilic
substitution/acylation
o Jmustcontainacylchloride
o Hisacarboxylicacid
Kis
COClH2N
or
COOHH2N
Jis
COClNO2
His
COOHNO2
GisNO2 CH3
or
NO2 C OH
H
H orCNO2
O
H
(7outof8marks)
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TPJC/2009/P3/Q5c25.
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F
CH3
CH2C
HO H
O
GCH
3
CH2C
HO H
OBr
HCOOH
COOH
HO
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CH3
CH2
CHO
HO Cu2+ - -
CH3
CH2COO-
-OCu
2O H
2O
+ 2 + 5 OH
+ + 3
CH3
CH2CHO
HO
COOH
COOH
HO CO2 H2O
+ 7O
+ + 2