Chapter 12 Aldehydes Ketones and Carboxylic Acids

Chapter12

AldehydesKetonesandCarboxylicAcids

1MarkQuestions

1.GiveoneuseofFormalin.

Ans.Formalinisusedasadisinfectant,preservativeforbiologicalspecimensandinleather

industry.

2.WhatisthechemicalnameofTollen’sreagentandFehling’ssolution.

Ans.Tollen’sreagent=AmmoniacalSilverNitrate

Fehlingssolution=SodiumPotassiumTartarate.

3.Writethestructureofalkenesthatonozonolysiswillgiveketoneonly.

Ans.

4.Whatisthefunctionof inrosenmundreaction?

Ans. actsasacatalyticpoisonwhichpreventsfurtherreductionofaldehydeto

alcohol.

5.Nametheisomerswithmolecularformula .Whichonewillhavehighboiling

point?

Ans.Thetwoisomersare and .Acetoneboilsathigher

temperatureduetopresenceoftwoelectrondonatingalkylgroups.

6.Writeachemicaltesttodistinguishbetweenaldehydeandketone.

Ans.AldehydesandketonescanbedistinguishedbyTollen’stest.Aldehydesgiveasilver

mirroronreactingwithTollen’sreagentwhereasketoneswillnotreact.

7.Whathappenswhenacetaldehydeiskeptwithatraceofsulphuricacid?Writethe

structureofproduct.

Ans.Atrimerofacetaldehyde,calledparaldchydeisformed.

8.WhatistheHofmannbromamidereaction?Illustratewithoneexample.

Ans.Hoffmanbromamidereactionisareactioninwhichamidesareconvertedtoaminesof

onecarbonlessthanthestartingamide.Itisaveryimportantstep–downreaction.

9.GiveIUPACnamesoffollowing

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

(ix)HOOC–CH=CH–COOH

(x)

Ans.(i).5-Chloro-3-ethylpentan-2-one.

(ii).2–(2-bromophenyl)ethanal

(iii).2-Phenylpropanal

(iv).5-Chloro-3-methylpentan-2-one

(v).4-Hydroxypentan-2-one

(vi).3-Methylbutan-2-one

(vii).2-Ethyl-2-methylbutanoicacid

(viii).2-(3-Bromophenyl)ethanal

(ix).But-2-en-1,4–dioicacid

(x).4-Methoxybenzaldehyde

10.Drawthestructureofthefollowing–

(i)4-Methoxybenzaldehyde

(ii)5-Bromo-3-Chloro-2-iodobenzoicacid

(iii)3,3–Dimethyl-1-Chlorobutane

(iv)2,3-Dihydroxy-4-methylpentanal

(v)3-Hydroxy-2-methyl-propanal

(vi)2,4–Dimethyl-3-pentanone

(vii)1,2–Ethaneodiocacid

(viii)3-Pentene-2-one

(ix)1,3–Propane–dioicacid

Ans.(i).

(ii).

(iii).

(iv).

(v).

(vi).

(vii).

(viii).

(ix).

2MarkQuestions

1.Ethanoicacidhasmolarmassof120invapourstate.

Ans.Carboxylicacidondissociationformcarboxylateionwhichisstabilizedbytwo

equivalentresonancestructureinwhichnegativechargeisatthemoreelectronegative

oxygenatom,whereastheconjugatebaseofphenol,phenoxideion,hasnon–equivalent

resonancestructuresinwhichnegativechargeisatthelesselectronegativecarbonatom.

Thereforeresonanceisnotasimportantasitisincarboxylateion.Moreoverthenegative

chargeisdelocalizedovertwomoreelectronegativeoxygenatomsincarboxylateion

whereasitislesseffectivelydelocalizedoveroneoxygenatomandonecarbonatomin

phenoxideion.Thereforethecarboxylateionismorestabilizedthanphenoxideionand

carboxylicacidsarestrongeracidsthanphenol.

2.CarboxylicacidsdonotgivecharacteristicreactionsofCarboxylicacidisstronger

acidthanphenol.

Ans.EthanolcanformintermolecularHydrogenbondingwithwatermolecules,ethyl

chloridecannot.Thereforeethanolissolubleinwaterandethylchlorideisnot.

3.Ethanolismoresolubleinwaterthanethylchloride

Ans.AldehydesaremorereactivethanKetonesduetostericandelectronicreasons.In

Ketonesduetopresenceoftworelativelylargealkylgroups,theapproachofnucleophileis

morehinderedthaninaldehydeshavingonlyonesuchsubstitute.Moreoverthe+Ieffectof

alkylgroupsreducestheelectophilicityofcarbonylgroupmoreinKetonethaninaldehydes.

4.AldehydesaremorereactivethanKetonestowardsnucleophilicadditions.

Ans.Carboxylicacidshavemoreextensiveassociationofmoleculesthroughintermolecular

hydrogenbondingthanalcohols.Moreovertheirboilingpointsarehigherthanalcoholsof

samecarbonatoms.

5.Carboxylicacidshashigherboilingpointsthanalcoholsofsameno.ofcarbonatoms.

Ans.Ethanoicacidexistsasdimerinvapourstateinwhichtwomoleculesremaintogether

byhydrogenbonding.Thisincreasestheeffectivemolecularmassto120.

6.carbonylgroup.

Ans.Incarboxylicacidsduetopresenceofresonance,theC=Ogroupisnotapurecarbonyl

group&thereforetheydonotshowcharacteristicreactionsofcarbonylgroup.

7.Formaldehydedoesnotundergoaldolcondensation.

Ans.Formaldehydedoesnothaveany -hydrogenandthereforeitcannotshowaldol

condensation.

8.Floroaceticacidisastrongeracidthanaceticacid.

Ans.Influoroaceticacid,Fluorinebeingelectronwithdrawinggroupstabilizestheconjugate

basethroughdelocalizationofthenegativecharge

Thereforefluoroaceticacidisastrongeracidthanaceticacid.

3MarkQuestions

1.Toluenetobenzaldehyde

Ans.

2.AcetaldehydetoAcetamide

Ans.

3.Methanoltoaceticacid

Ans.

4.MethanoltoEthanol

Ans.

5.AceticacidtoPropionicacid

Ans.

6.Ethylalcoholtoacetone

Ans.

7.Acetonetotertbutylalcohol

Ans.

8.Toluenetom-nitrobenzoicacid

Ans.

9.PhenoltoacetophenoneAns.

10.AcetaldehydetoAcetone

Ans.

11.GivetheIUPACnamesofthefollowingcompounds:

(i)

(ii)

(iii)

(iv)

Ans.(i)3-Phenylpropanoicacid

(ii)3-Methylbut-2-enoicacid

(iii)2-Methylcyclopentanecarboxylicacid

(iv)2,4,6-Trinitrobenzoicacid

5MarkQuestions

1.Acompound‘A’withformula givesapositive2,4–DNPtestbutanegative

Tollen’stestItcanbeoxidizingtocarboxylicacid‘B’ofmolecularformula ,

whentreatedwithalk. undervigorousconditions.Thesaltof‘B’givesa

hydrocarbon‘C’onKolbes’electrolyticdecarboxylation.IdentifyA,B.C&write

chemicalequations.

Ans.

AsthecompoundAgivesapositive2,4-DNPtestbutnegativeTollen’stest,itisaketone.

Sinceonoxidation,itgivesanacidB,ofmolecularformula ,itis

andBis .AsCisobtainedbyKolbes

decarboxylationofB,Cis .

ThereforeA=Pentan-3one,

B=Propanoicacid

AndC=Butane

Thesequenceofreactionsis

2.AcompoundAwithmolecularformula onoxidationformscompoundBwith

molecularformula .ThecompoundBgivesiodoformtestbutdoesnotreduce

ammoniacalsilvernitrate.ThecompoundBonreductionwithZn–Hg/HClgives

compoundCwithmolecularformula .IdentifyA,B.C&givethechemical

reactionsinvolved.

Ans.

SinceBgivesanegativeTollen’stestbutpositiveIodoformtest,itismethylketone,i.e,

.AlsoitisformedbyoxidationofA.

ThereforeAissecondaryalcoholi.e, onreductionBgives

pentanewithZn–Hg/HCl.

ThereforeCis

Therefore

A=

B=

C=

Reactions:-

Ans.

4.

Ans.

5.

Themolecularformulaof(B)andcharacteristicodourof(A)suggeststhat(A)isanaromatic

aldehyde, and(B)isalcohol, .As(C)isasodiumsaltofanacid&

giveshydrocarbon(D)onheatingwithsodalime,(C)issodiumbenzoateand(D)isbenzene.

Therefore:-

Ans.(A)=

(B)=

(C)=

6.

Ans.(A)=

(B)=

(C)=

7.

Ans.(A)=

(B)=

(C)=

8.

Ans.X=

Y=

Z=

9.

Ans.A=

B=

C=

10.Twomolesofcompound(A)ontreatmentwithastrongbasegivestwocompounds

(B)and(C).Thecompound(B)ondehydrogenationwithCugives(A)whileacidification

of(C)givescarboxylicacid(D)havingmolecularformula .Identify(A)to(D).

Ans.

Since(D)isacarboxylicacidwithonecarbononly,itisHCOOH.Asitisobtainedfrom(C)

acidification,(C)COONaand(A)isHCHOwhichontreatmentwithstrongbase(NaOH)gives

&HCOONa(cannizaro’sreaction).

Thereactionsare:-

11.Writethestructuresofthefollowingcompounds.

(i)α-Methoxypropionaldehyde

(ii)3-Hydroxybutanal

(iii)2-Hydroxycyclopentanecarbaldehyde

(iv)4-Oxopentanal

(v)Di-sec-butylketone

(vi)4-Fluoroacetophenone

Ans.(i)

(ii)

(iii)

(iv)

(v)

(vi)

12.Writethestructuresofproductsofthefollowingreactions;

(i)

(ii)

(iii)

(iv)

Ans.

(iv)

13.Arrangethefollowingcompoundsinincreasingorderoftheirboilingpoints.

, , ,

Ans.Themolecularmassesofthegivencompoundsareintherange44to46.

undergoesextensiveintermolecularH-bonding,resultingintheassociationofmolecules.

Therefore,ithasthehighestboilingpoint.ismorepolarthan andso

hasstrongerintermoleculardipole-dipoleattractionthan andso

hasonlyweakvanderWaalsforce.Thus,thearrangementofthegiven

compoundsintheincreasingorderoftheirboilingpointsisgivenby:

< < <

14.Arrangethefollowingcompoundsinincreasingorderoftheirreactivityin

nucleophilicadditionreactions.

(i)Ethanal,Propanal,Propanone,Butanone.

(ii)Benzaldehyde,p-Tolualdehyde,p-Nitrobenzaldehyde,Acetophenone.

Hint:Considerstericeffectandelectroniceffect.

Ans.(i)

The+Ieffectofthealkylgroupincreasesintheorder:

Ethanal<Propanal<Propanone<Butanone

Theelectrondensityatthecarbonylcarbonincreaseswiththeincreaseinthe+Ieffect.Asa

result,thechancesofattackbyanucleophiledecrease.Hence,theincreasingorderofthe

reactivitiesofthegivencarbonylcompoundsinnucleophilicadditionreactionsis:

Butanone<Propanone<Propanal<Ethanal

(ii)

The+Ieffectismoreinketonethaninaldehyde.Hence,acetophenoneistheleastreactivein

nucleophilicadditionreactions.Amongaldehydes,the+Ieffectisthehighestinp-

tolualdehydebecauseofthepresenceoftheelectron-donating– groupandthelowestin

p-nitrobezaldehydebecauseofthepresenceoftheelectron-withdrawing- group.Hence,

theincreasingorderofthereactivitiesofthegivencompoundsis:

Acetophenone<p-tolualdehyde<Benzaldehyde

<p-Nitrobenzaldehyde

15.Predicttheproductsofthefollowingreactions:

(i)

(ii)

(iii)

(iv)

Ans.(i)

(ii)

(iii)

(iv)

16.Showhoweachofthefollowingcompoundscanbeconvertedtobenzoicacid.

(i)Ethylbenzene(ii)Acetophenone

(iii)Bromobenzene(iv)Phenylethene(Styrene)

Ans.(i)

(ii)

(iii)

(iv)

17.Whichacidofeachpairshownherewouldyouexpecttobestronger?

(i) or

(ii) or

(iii) or

(iv)

Ans.(i)

The+Ieffectof- groupincreasestheelectrondensityontheO-Hbond.Therefore,

releaseofprotonbecomesdifficult.Ontheotherhand,the-IeffectofFdecreasesthe

electrondensityontheO-Hbond.Therefore,protoncanbereleasedeasily.Hence,

isastrongeracidthan .

(ii)

Fhasstronger-IeffectthanCl.Therefore, canreleaseprotonmoreeasilythan

.Hence, isstrongeracidthan .

(iii)

Inductiveeffectdecreaseswithincreaseindistance.Hence,the+IeffectofFin

ismorethanitisin .Hence,

isstrongeracidthan .

(iv)

Duetothe-IeffectofF,itiseasiertoreleaseprotoninthecaseofcompound(A).However,in

thecaseofcompound(B),releaseofprotonisdifficultduetothe+Ieffectof group.

Hence,(A)isastrongeracidthan(B).

18.Whatismeantbythefollowingterms?Giveanexampleofthereactionineachcase.

(i)Cyanohydrin(ii)Acetal(iii)Semicarbazone

(iv)Aldol(v)Hemiacetal(vi)Oxime

(vii)Ketal

(viii)Imine(ix)2,4-DNP-derivative(x)Schiff'sbase

Ans.(i)Cyanohydrin:Cyanohydrinsareorganiccompoundshavingtheformula

,where canbealkylorarylgroups.

Aldehydesandketonesreactwithhydrogencyanide(HCN)inthepresenceofexcesssodium

cyanide(NaCN)asacatalysttofieldcyanohydrin.Thesereactionsareknownascyanohydrin

reactions.

Cyanohydrinsareusefulsyntheticintermediates.

(ii)Acetal:Acetalsaregem-dialkoxyalkanesinwhichtwoalkoxygroupsarepresentonthe

terminalcarbonatom.Onebondisconnectedtoanalkylgroupwhiletheotherisconnected

toahydrogenatom.

WhenaldehydesaretreatedwithtwoeQuivalentsofamonohydricalcoholinthepresence

ofdryHClgas,hemiacetalsareproducedthatfurtherreactwithonemoremoleculeof

alcoholtoyieldacetal.

(iii)Semicarbarbazone:Semicarbazonesarederivativesofaldehydesandketonesproduced

bythecondensationreactionbetweenaketoneoraldehydeandsemicarbazide.

Semicarbazonesareusefulforidentificationandcharacterizationofaldehydesandketones.

(iv)Aldol:A -hydroxyaldehydeorketoneisknownasanaldol.Itisproducedbythe

condensationreactionoftwomoleculesofthesameoronemoleculeeachoftwodifferent

aldehydesorketonesinthepresenceofabase.

(v)Hemiacetal:Hemiacetalsareα-alkoxyalcohols

GeneralstructureofahemiacetalAldehydereactswithonemoleculeofamonohydric

alcoholinthepresenceofdryHClgas.

(vi)Oxime:Oximesareaclassoforganiccompoundshavingthegeneralformula

,whereRisanorganicsidechainandiseitherhydrogenoranorganicside

chain.If isH,thenitisknownasaldoximeand ifisanorganicsidechain,itis

knownasketoxime.

Ontreatmentwithhydroxylamineinaweaklyacidicmedium,aldehydesorketonesform

oximes.

(vii)Ketal:Ketalsaregem-dialkoxyalkanesinwhichtwoalkoxygroupsarepresentonthe

samecarbonatomwithinthechain.Theothertwobondsofthecarbonatomareconnected

totwoalkylgroups.

KetonesreactwithethyleneglycolinthepresenceofdryHClgastogiveacyclicproduct

knownasethyleneglycolketals.

(viii)Imine:Iminesarechemicalcompoundscontainingacarbonnitrogendoublebond.

Iminesareproducedwhenaldehydesandketonesreactwithammoniaanditsderivatives.

(ix)2,4-DNP-derivative:2,4-dinitrophenylhydragonesare2,4-DNP-derivatives,which

areproducedwhenaldehydesorketonesreactwith2,4-dinitrophenylhydrazineina

weaklyacidicmedium.

Toidentifyandcharacterizealdehydesandketones,2,4-DNPderivativesareused.

(x)Schiff'sbase:Schiff'sbase(orazomethine)isachemicalcompoundcontainingacarbon-

nitrogendoublebondwiththenitrogenatomconnectedtoanaryloralkylgroup-butnot

hydrogen.Theyhavethegeneralformula .Hence,itisanimine.

Itisnamedafterascientist,HugoSchiff.

Aldehydesandketonesontreatmentwithprimaryaliphaticoraromaticaminesinthe

presenceoftraceofanacidyieldsaSchiff'sbase.

19.NamethefollowingcompoundsaccordingtoIUPACsystemofnomenclature:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

Ans.(i)4-methylpentanal

(ii)6-Chloro-4-ethylhexan-3-one

(iii)But-2-en-1-al

(iv)Pentane-2,4-dione

(v)3,3,5-Trimethylhexan-2-one

(vi)3,3-Dimethylbutanoicacid

(vii)Benzene-1,4-dicarbaldehyde

20.Drawthestructuresofthefollowingcompounds.

(i)3-Methylbutanal

(ii)p-Nitropropiophenone

(iii)p-Methylbenzaldehyde

(iv)4-Methylpent-3-en-2-one

(v)4-Chloropentan-2-one

(vi)3-Bromo-4-phenylpentanoicacid

(vii)p,p'-Dihydroxybenzophenone

(viii)Hex-2-en-4-ynoicacid

Ans.(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

21.WritetheIUPACnamesofthefollowingketonesandaldehydes.Whereverpossible,

givealsocommonnames.

(i)

(ii)

(iii)

(iv)Ph-CH=CH-CHO

(v)

(vi)PhCOPh

Ans.(i)

IUPACname:Heptan-2-one

Commonname:Methyln-propylketone

(ii)

IUPACname:4-Bromo-2-methylhaxanal

Commonname:(Y-Bromo-α-methyl-caproaldehyde)

(iii)

IUPACname:Heptanal

(iv)Ph-CH=CH-CHO

IUPACname:3-phenylprop-2-enal

Commonname: -Pheynolacrolein

(v)

IUPACname:Cyclopentanecarbaldehyde

(vi)PhCOPh

IUPACname:Diphenylmethanone

Commonname:Benzophenone

22.Drawstructuresofthefollowingderivatives.

(i)The2,4-dinitrophenylhydrazoneofbenzaldehyde

(ii)Cyclopropanoneoxime

(iii)Acetaldehydedimethylacetal

(iv)Thesemicarbazoneofcyclobutanone

(v)Theethyleneketalofhexan-3-one

(vi)Themethylhemiacetalofformaldehyde

Ans.(i)

(ii)

(iii)

(iv)

(v)

(vi)

23.Predicttheproductsformedwhencyclohexanecarbaldehydereactswithfollowing

reagents.

(i)PhMgBrandthen

(ii)Tollens'reagent

(iii)Semicarbazideandweakacid

(iv)Excessethanolandacid

(v)Zincamalgamanddilutehydrochloricacid

Ans.(i)

(ii)

(iii)

(iv)

(v)

24.Whichofthefollowingcompoundswouldundergoaldolcondensation,whichthe

Cannizzaroreactionandwhichneither?Writethestructuresoftheexpectedproducts

ofaldolcondensationandCannizzaroreaction.

(i)Methanal

(ii)2-Methylpentanal

(iii)Benzaldehyde

(iv)Benzophenone

(v)Cyclohexanone

(vi)1-Phenylpropanone

(vii)Phenylacetaldehyde

(viii)Butan-1-ol

(ix)2,2-Dimethylbutanal

Ans.Aldehydesandketoneshavingatleastoneα-hydrogenundergoaldolcondensation.The

compounds(ii)2-methylpentanal,(v)cyclohexanone,(vi)1-phenylpropanone,and(vii)

phenylacetaldehydecontainoneormoreα-hydrogenatoms.Therefore,theseundergoaldol

condensation.

Aldehydeshavingnoα-hydrogenatomsundergoCannizzaroreactions.Thecompounds(i)

Methanal,(iii)Benzaldehyde,and(ix)2,2-dimethylbutanaldonothaveanyα-hydrogen.

Therefore,theseundergocannizzaroreactions.

Compound(iv)Benzophenoneisaketonehavingnoα-hydrogenatomandcompound(viii)

Butan-1-olisanalcohol.Hence,thesecompoundsdonotundergoeitheraldolcondensation

orcannizzaroreactions.

Aldolcondensation

(ii)

(v)

(vi)

(vii)

Cannizzaroreaction

(i)

(iii)

(ix)

25.Howwillyouconvertethanalintothefollowingcompounds?

(i)Butane-1,3-diol(ii)But-2-enal(iii)But-2-enoicacid

Ans.(i)Ontreatmentwithdilutealkali,ethanalproduces3-hydroxybutanalgivesbutane-1,

3-diolonreduction.

(ii)Ontreatmentwithdilutealkali,ethanalgives3-hydroxybutanalwhichonheating

producesbut-2-enal.

(iii)WhentreatedwithTollen'sreagent,But-2-enalproducedintheabovereactionproduces

but-2-enoicacid.

26.Writestructuralformulasandnamesoffourpossiblealdolcondensationproducts

frompropanalandbutanal.Ineachcase,indicatewhichaldehydeactsasnucleophile

andwhichaselectrophile.

Ans.(i)Takingtwomoleculesofpropanal,onewhichactsasanucleophileandtheotheras

anelectrophile.

(ii)Takingtwomoleculesofbutanal,onewhichactsasanucleophileandtheotherasan

electrophile.

(iii)Takingonemoleculeeachofpropanalandbutanalinwhichpropanalactsasa

nucleophileandbutanalactsasanelectrophile.

(iv)Takingonemoleculeeachofpropanalandbutanalinwhichpropanalactsasan

electrophileandbutanalactsasanucleophile.

27.Anorganiccompoundwiththemolecularformula forms2,4-DNP

derivative,reducesTollens'reagentandundergoesCannizzaroreaction.Onvigorous

oxidation,itgives1,2-benzenedicarboxylicacid.Identifythecompound.

Ans.Itisgiventhatthecompound(withmolecularformula )forms2,4-DNP

derivativeandreducesTollen'sreagent.Therefore,thegivencompoundmustbean

aldehyde.

Again,thecompoundundergoescannizzaroreactionandonoxidationgives1,2-

benzenedicarboxylicacid.Therefore,the-CHOgroupisdirectlyattachedtoabenzenering

andthisbenzaldehydeisortho-substituted.Hence,thecompoundis2-ethylbenzaldehyde.

Thegivenreactionscanbeexplainedbythefollowingequations.

28.Anorganiccompound(A)(molecularformula )washydrolysedwithdilute

sulphuricacidtogiveacarboxylicacid(B)andanalcohol(C).Oxidationof(C)with

chromicacidproduced(B).(C)ondehydrationgivesbut-1-ene.WriteeQuationsforthe

reactionsinvolved.

Ans.AnorganiccompoundAwithmolecularformula givesacarboxylicacid(B)

andanalcohol(C)onhydrolysiswithdilutesulphuricacid.Thus,compoundAmustbean

ester.Further,alcoholCgivesacidBonoxidationwithchromicacid.Thus,BandCmust

containeQualnumberofcarbonatoms.

SincecompoundAcontainsatotalof8carbonatoms,eachofBandCcontain4carbon

atoms.

Again,ondehydration,alcoholCgivesbut-1-ene.Therefore,Cisofstraightchainandhence,

itisbutan-1-ol.

Onoxidation,Butan-1-olgivesbutanoicacid.Hence,acidBisbutanoicacid.

Hence,theesterwithmolecularformula isbutylbutanoate.

AllthegivenreactionscanbeexplainedbythefollowingeQuations.

29.Arrangethefollowingcompoundsinincreasingorderoftheirpropertyasindicated:

(i)Acetaldehyde,Acetone,Di-tert-butylketone,Methyltert-butylketone(reactivity

towardsHCN)

(ii) , , (acid

strength)

(iii)Benzoicacid,4-Nitrobenzoicacid,3,4-Dinitrobenzoicacid,4-Methoxybenzoicacid

(acidstrength)

Ans.(i)WhenHCNreactswithacompound,theattackingspeciesisanucleophile,CN-.

Therefore,asthenegativechargeonthecompoundincreases,itsreactivitywithHCN

decreases.Inthegivencompounds,the+Ieffectincreasesasshownbelow.Itcanbe

observedthatsterichindrancealsoincreasesinthesame

Hence,thegivencompoundscanbearrangedaccordingtotheirincreasingreactivities

towardHCNas:

Di-tert-butylketone<Methyltert-butylketone<Acetone<Acetaldehyde

(ii)Afterlosingaproton,carboxylicacidsgainanegativechargeasshown:

Now,anygroupthatwillhelpstabilisethenegativechargewillincreasethestabilityofthe

carboxylionandasaresult,willincreasethestrengthoftheacid.Thus,groupshaving+I

effectwilldecreasethestrengthoftheacidsandgroupshaving-Ieffectwillincreasethe

strengthoftheacids.Inthegivencompounds, grouphas+IeffectandBr-grouphas

-Ieffect.Thus,acidscontainingBr-arestronger.

Now,the+Ieffectofisopropylgroupismorethanthatofn-propylgroup.Hence,

isaweakeracidthan .

Also,the-Ieffectgrowsweakerasdistanceincreases.Hence,

isaweakeracidthan .

Hence,thestrengthsofthegivenacidsincreaseas:

< < <

(iii)Aswehaveseeninthepreviouscase,electron-donatinggroupsdecreasethestrengthsof

acids,whileelectron-withdrawinggroupsincreasethestrengthsofacids.Asmethoxygroup

isanelectron-donatinggroup,4-methoxybenzoicacidisaweakeracidthanbenzoicacid.

Nitrogroupisanelectron-withdrawinggroupandwillincreasethestrengthsofacids.As3,4-

dinitrobenzoicacidcontainstwonitrogroups,itisaslightlystrongeracidthan4-

nitrobenzoicacid.Hence,thestrengthsofthegivenacidsincreaseas:

4-Methoxybenzoicacid<Benzoicacid<4-Nitrobenzoicacid

<3,4-Dinitrobenzoicacid

30.Givesimplechemicalteststodistinguishbetweenthefollowingpairsofcompounds.

(i)PropanalandPropanone

(ii)AcetophenoneandBenzophenone

(iii)PhenolandBenzoicacid

(iv)BenzoicacidandEthylbenzoate

(v)Pentan-2-oneandPentan-3-one

(vi)BenzaldehydeandAcetophenone

(vii)EthanalandPropanal

Ans.(i)Propanalandpropanonecanbedistinguishedbythefollowingtests.

(a)Tollen'stest

Propanalisanaldehyde.Thus,itreducesTollen'sreagent.But,propanonebeingaketone

doesnotreduceTollen'sreagent.

(b)Fehling'stest

AldehydesrespondtoFehling'stest,butketonesdonot.

PropanalbeinganaldehydereducesFehling'ssolutiontoared-brownprecipitateof ,

butpropanonebeingaketonedoesnot.

(c)Iodoformtest:

Aldehydesandketoneshavingatleastonemethylgrouplinkedtothecarbonylcarbonatom

respondtoiodoformtest.Theyareoxidizedbysodiumhypoiodite(NaOI)togiveiodoforms.

Propanonebeingamethylketonerespondstothistest,butpropanaldoesnot.

(ii)AcetophenoneandBenzophenonecanbedistinguishedusingtheiodoformtest.

Iodoformtest:

Methylketonesareoxidizedbysodiumhypoioditetogiveyellowppt.ofiodoform.

Acetophenonebeingamethylketonerespondstothistest,butbenzophenonedoesnot.

(iii)Phenolandbenzoicacidcanbedistinguishedbyferricchloridetest.

Ferricchloridetest:

Phenolreactswithneutral toformaniron-phenolcomplexgivingvioletcolouration.

Butbenzoicacidreactswithneutral togiveabuffcolouredppt.offerricbenzoate.

(iv)BenzoicacidandEthylbenzoatecanbedistinguishedbysodiumbicarbonatetest.

Sodiumbicarbonatetest:

Acidsreactwith toproducebriskeffervescenceduetotheevolutionof gas.

Benzoicacidbeinganacidrespondstothistest,butethylbenzoatedoesnot.

(v)Pentan-2-oneandpentan-3-onecanbedistinguishedbyiodoformtest.

Iodoformtest:

Pentan-2-oneisamethylketone.Thus,itrespondstothistest.Butpentan-3-onenotbeinga

methylketonedoesnotrespondtothistest.

(vi)Benzaldehydeandacetophenonecanbedistinguishedbythefollowingtests.

(a)Tollen'sTest

AldehydesrespondtoTollen'stest.BenzaldehydebeinganaldehydereducesTollen'sreagent

togiveared-brownprecipitateof ,butacetophenonebeingaketonedoesnot.

(b)Iodoformtest

Acetophenonebeingamethylketoneundergoesoxidationbysodiumhypoiodite(NaOI)to

giveayellowppt.ofiodoform.Butbenzaldehydedoesnotrespondtothistest.

(vii)Ethanalandpropanalcanbedistinguishedbyiodoformtest.

Iodoformtest

Aldehydesandketoneshavingatleastonemethylgrouplinkedtothecarbonylcarbonatom

respondstotheiodoformtest.Ethanalhavingonemethylgrouplinkedtothecarbonyl

carbonatomrespondstothistest.Butpropanaldoesnothaveamethylgrouplinkedtothe

carbonylcarbonatomandthus,itdoesnotrespondtothisstate.

31.Howwillyoupreparethefollowingcompoundsfrombenzene?Youmayuseany

inorganicreagentandanyorganicreagenthavingnotmorethanonecarbonatom

(i)Methylbenzoate

(ii)m-Nitrobenzoicacid

(iii)p-Nitrobenzoicacid

(iv)Phenylaceticacid

(v)p-Nitrobenzaldehyde.

Ans.(i)

(ii)

(iii)

(iv)

(v)

32.Howwillyoubringaboutthefollowingconversionsinnotmorethantwosteps?

(i)PropanonetoPropene

(ii)BenzoicacidtoBenzaldehyde

(iii)Ethanolto3-Hydroxybutanal

(iv)Benzenetom-Nitroacetophenone

(v)BenzaldehydetoBenzophenone

(vi)Bromobenzeneto1-Phenylethanol

(vii)Benzaldehydeto3-Phenylpropan-1-ol

(viii)Benazaldehydetoα-Hydroxyphenylaceticacid

(ix)Benzoicacidtom-Nitrobenzylalcohol

Ans.

33.Describethefollowing:

(i)Acetylation

(ii)Cannizzaroreaction

(iii)Crossaldolcondensation

(iv)Decarboxylation

Ans.(i)Acetylation:Theintroductionofanacetylfunctionalgroupintoanorganic

compoundisknownasacetylation.Itisusuallycarriedoutinthepresenceofabasesuchas

pyridine,dirnethylaniline,etc.Thisprocessinvolvesthesubstitutionofanacetylgroupfor

anactivehydrogenatom.Acetylchlorideandaceticanhydridearecommonlyusedas

acetylatingagents.

Forexample,acetylationofethanolproducesethylacetate.

(ii)Cannizzaroreaction:Theselfoxidation-reduction(disproportionation)reactionof

aldehydeshavingnoα-hydrogensontreatmentwithconcentratedalkalisisknownasthe

Cannizzaroreaction.Inthisreaction,twomoleculesofaldehydesparticipatewhereoneis

reducedtoalcoholandtheotherisoxidizedtocarboxylicacid.

Forexample,whenethanolistreatedwithconcentratedpotassiumhydroxide,ethanoland

potassiumethanoateareproduced.

(iii)Cross-aldolcondensation:Whenaldolcondensationiscarriedoutbetweentwo

differentaldehydes,ortwodifferentketones,oranaldehydeandaketone,thenthereaction

iscalledacross-aldolcondensation.Ifboththereactantscontainα-hydrogens,four

compoundsareobtainedasproducts.

Forexample,ethanalandpropanalreacttogivefourproducts.

(iv)Decarboxylation:Decarboxylationreferstothereactioninwhichcarboxylicacidslose

carbondioxidetoformhydrocarbonswhentheirsodiumsaltsareheatedwithsoda-lime.

DecarboxylationalsotakesplacewhenaQueoussolutionsofalkalimetalsaltsofcarboxylic

acidsareelectrolyzed.ThiselectrolyticprocessisknownasKolbe'selectrolysis.

34.Completeeachsynthesisbygivingmissingstartingmaterial,reagentorproducts

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

(ix)

(x)

(xi)

Ans.(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

(ix)

(x)

(xi)

35.Giveplausibleexplanationforeachofthefollowing:

(i)Cyclohexanoneformscyanohydriningoodyieldbut2,2,6trimethylcyclohexanone

doesnot.

(ii)Therearetwo- groupsinsemicarbazide.However,onlyoneisinvolvedinthe

formationofsemicarbazones.

(iii)Duringthepreparationofestersfromacarboxylicacidandanalcoholinthe

presenceofanacidcatalyst,thewaterortheestershouldberemovedassoonasitis

formed.

Ans.(i)CyclohexanonesformcyanohydrinsaccordingtothefollowingeQuation.

Inthiscase,thenucleophileCN-caneasilyattackwithoutanysterichindrance.However,in

thecaseof2,2,6trimethylcydohexanone,methylgroupsatα-positionsoffersteric

hindrancesandasaresult,CN-cannotattackeffectively.

Forthisreason,itdoesnotformacyanohydrin.

(ii)Semicarbazideundergoesresonanceinvolvingonlyoneofthetwo- groups,which

isattacheddirectlytothecarbonyl-carbonatom.

Therefore,theelectrondensityon- groupinvolvedintheresonancealsodecreases.As

aresult,itcannotactasanucleophile.Sincetheother- groupisnotinvolvedin

resonance;itcanactasanucleophileandcanattackcarbonyl-carbonatomsofaldehydes

andketonestoproducesemicarbazones.

(iii)Esteralongwithwaterisformedreversiblyfromacarboxylicacidandanalcoholin

presenceofanacid.

Ifeitherwateroresterisnotremovedassoonasitisformed,thenitreactstogivebackthe

reactantsasthereactionisreversible.Therefore,toshifttheeQuilibriumintheforward

directioni.e.,toproducemoreester,eitherofthetwoshouldberemoved.

37.Anorganiccompoundcontains69.77%carbon,11.63%hydrogenandrestoxygen.

Themolecularmassofthecompoundis86.ItdoesnotreduceTollens'reagentbut

formsanadditioncompoundwithsodiumhydrogensulphiteandgivepositiveiodoform

test.Onvigorousoxidationitgivesethanoicandpropanoicacid.Writethepossible

structureofthecompound.

Ans.%ofcarbon=69.77%

%ofhydrogen=11.63%

%ofoxygen={100-(69.77+11.63)}%

=18.6%

Thus,theratioofthenumberofcarbon,hydrogen,andoxygenatomsintheorganic

compoundcanbegivenas:

=5.81:11.63:1.16

=5:10:1

Therefore,theempiricalformulaofthecompoundis .Now,theempiricalformula

massofthecompoundcanbegivenas:

=86

Molecularmassofthecompound=86

Therefore,themolecularformulaofthecompoundisgiven .

SincethegivencompounddoesnotreduceTollen'sreagent,itisnotanaldehyde.Again,the

compoundformssodiumhydrogensulphateadditionproductsandgivesapositiveiodoform

test.Sincethecompoundisnotanaldehyde,itmustbeamethylketone.

Thegivencompoundalsogivesamixtureofethanoicacidandpropanoicacid.

Hence,thegivencompoundisPentan-2-one.

ThegivenreactionscanbeexplainedbythefollowingeQuations:

38.Althoughphenoxideionhasmorenumberofresonatingstructuresthan

carboxylateion,carboxylicacidisastrongeracidthanphenol.Why?

Ans.Resonancestructuresofphenoxideionare:

ItcanbeobservedfromtheresonancestructuresofphenoxideionthatinII,IIIandIV,less

electronegativecarbonatomscarryanegativecharge.Therefore,thesethreestructures

contributenegligiblytowardstheresonancestabilityofthephenoxideion.Hence,these

structurescanbeeliminated.OnlystructuresIandVcarryanegativechargeonthemore

electronegativeoxygenatom.

Inthecaseofcarboxylateion,resonatingstructures containachargecarried

byamoreelectronegativeoxygenatom.

Resonancestructuresofcarboxylateionare:

Further,inresonatingstructures ,thenegativechargeisdelocalizedovertwo

oxygenatoms.ButinresonatingstructuresIandVofthephexoxideion,thenegativecharge

islocalizedonthesameoxygenatom.Therefore,theresonatingstructuresofcarboxylateion

contributemoretowardsitsstabilitythanthoseofphenoxideion.Asaresult,carboxylate

ionismoreresonance-stabilizedthanphenoxideion.Hence,carboxylicacidisastronger

acidthanphenol.