Chapter10

HaloalkanesandHaloarenes

1MarksQuestions

1.GiveIUPACnamesoffollowingcompounds

(i).

(ii).

(iii).

(iv).

(v).

(vi).

(vii).

(viii).

(ix).

(x).

Ans.(i).1,3-Dibromobutane

(ii).1-Cholopropan-2-ol

(iii).2,3–Dibromo-1-chloro-3-methylpentane

(iv).2-Choloro-3-ethyl-2-methylpentane

(v).1-Chloro-2-phenylethane

(vi).1-Chloro-1-phenylethane

(vii).1,2,3,4,5,6-hexachlorocyclohexane

(viii).2,2-Dihexyl1,1,1-Trichloroethane

(ix).4,4-dibromobiphenyl

(x).1,3-Dibromo-3-methylbutane

(2).Givethestructuresoffollowing.:

(i).1,3-Dichloro-2-(bromomethyl)propane

(ii).Isobutylchloride

(iii).Orthobromotoluene

(iv).1–Bromo–4–chlorobutane

(v).3–Bromo–5–chloro–3,5–dimethyloctane

(vi).2,3–Dibromo–1–chloro-3-methylpentane

(vii).2–Chloro–3–ethyl-1,4-pentadieme

(viii).2,3–Dibromo–1–chloro-3-methylpentane

(ix).2–Chloro–1–phemylpropane

(x).Tert–butylchloride

Ans.(i)

(ii).

(iii).

(iv).

(v).

(vi).

(vii).

(viii).

(ix).

(x).

3.Convert

(i).1-Buteneto1—chlorobutane.

Ans.

(ii).Ethenetoethanol.

Ans.

(iii).Chlorobenzenetophenol.

Ans.

(iv).Methylbromidetoaceticacid.

Ans.

(v).2-chlorobutanetosec-butylethylether.

Ans.

(vi).Chlorobenzenetobenzylchloride.

Ans.

(vii).ChlorobenzenetoBenzene.

Ans.

(viii).MethanetoEthane.

Ans.

(ix).Benzenetoo-chlorotoluene.

Ans.

(x).1-chloropropaneto2-iodopropane.

Ans.

4.Whatislucasreagent?

Ans.AmixtureofHClandanhydrous isknownasLucasreagent.

5.Whichofthefollowingwillshowopticalisomerism

1–bromobutaneor2–bromobutane?

Ans.

2-bromobutanewillbeopticallyactiveasithasonechiralcarbon .

6.Arrange Br, Br, Brinorderof

increasingboilingpoints.

Ans.Theorderofincreasingboilingpointsis

Boilingpointdecreaseswithincreaseinbranchingasitreducesthesurfacearea.

7.Giveanexampleof

(a)Fittigreaction

(b)Finkelsteinreaction.

Ans.(a)Fittigreaction

(b)Finkelsteinreaction

2MarksQuestions

1.Thionylchlorideispreferredforconvertingalcoholtohaloalkane.

Ans.Thionylchlorideispreferredforconvertingalcoholtohaloalkanebecausethebi-

productsformedareallgaseswhichescapeintotheatmosphere.

2.PhenolcannotbeconvertedtochlorobenzenebyreactingwithHCl.

Ans.Inphenol,duetoresonance,thecarbon–oxygenbondhasapartialdoublebond

characterandisdifficulttobreakbeingstrongerthanasinglebond.Thereforeitcannotbe

convertedtochlorobenzenebyreactingwithHCl.

3. isaddedduringiodinationofbenzene.

Ans.Whenbenzeneisreactedwithiodine,thereactionisreversibleinnature.Itleadstothe

formationofreactantsback.Thereforeandoxidizingagentlike oxidizestheHI

formedinthereactionandkeepsthereactioninforwarddirection.

4.p-dichlorobenzenehashighermeltingpointthanmeta–dichlorobenzene.

Ans.

p-dichlorebenzeneishavingsymmetricalstructurethereforeitcanfitbetterintothecrystal

latticewhichincreasesitsmeltingpoint.

5.Theboilingpointsofisomerichaloalkenesdecreasewithincreaseinbranching.

Ans.Theboilingpointsofisomerichaloakanesdecreaseswithbranchingduetodecreasein

surfaceareaswithbranching.Asbranchingincreasingthestructurebecomesmorespherical

andthesurfaceareadecreases.e.g.theboilingpointsofisomersof Brfollowsthe

order.

6.Hydrolysisofopticallyactive2-bromobutaneformsopticallyinactivebutan-2-ol.

Ans.

ThecompoundundergoeshydrolysisbySN1mechanismviatheformationofcarbocation

whichisplanar.

Theattackofnucleophilecanresultinproductwhichisamixtureofcompoundsbothwith

sameconfigurationandinvertedconfiguration.

Thereforeitresultsintheformationofracemicmixturewhichisopticallyinactive.

7.Chlorobenzeneislessreactivetowardsnucleophilicsubstitutionreaction.

Ans.Chlorobenzeneislessreactivetowardsnucleophillicsubstitutiondueto–

i.resonance,C-Clbondacquiresadoublebondcharacterandbecomesstrongerthana

singlebond.

ii. hybridisationinCofC-Xbond,thecarbonbecomesmoreelectronegativeandholds

theelectronpairofC-Xbondmoretightlydecreasingthebondlength.

iii.Instabilityofphenylcation.

iv.Repulsionforincomingnucleophilefromelectronrichring.

8.Chloroformisstoredindarkcolouredbottles.

Ans.Chloroformgetsoxidsedslowlybyairinthepresenceoflighttoanextremely

poisonousgasphosgene.Thereforetoavoidanyexposuretoairandsunlight,itiskeptin

darkcolouredbottles.

9.TheorderofboilingpointsisRCl<RBr<RI.

Ans.Theboilingpointsofalkylhalidesdependsondipoleandvan-der-waal’sinteraction.

Theseattractionsgetstrongerasthemoleculesgetbiggerinsizeandhavemoreelectrons.As

thesizeofhalogensincreasesintheorder–

Theboilingpointsalsofollowtheorder

RCl<RBr<RI

10.Vinylchlorideislessreactivethanallylchloride.

Ans.DuetoresonanceC-Clbondgetsdoublebondcharacterandbecomesstrongerthana

singlebond,makingvinylchloridelessreactivethanallylchloride.

11.Whathappenswhen

a)ThionylchlorideactsuponI-propanol.

b)EthanolreactswithPBr3

Ans.(a)Whenthionylchlorideactsupon1-propanolchloropropaneisformed.

(b)WhenethanolreactswithPBr3,bromoethaneisformed.

12.Howmanyaromaticisomersarepossiblefortheformula ?Writethe

structureandnames.

Ans. -Threearomaticisomersarepossible.

13.Howischlorobenzenepreparedby

(a)directchlorination

(b)diazotizationmethod?

Ans.(a)bydirectchlorination-

(b)bydiazotizationmethod.

14.Howcanwedistinguishbetweenanalkylhalideandarylhalide?

Ans.Alkylhalidee.g. etc.canbedistinguishedfromarylhalide,

,by test.

3MarksQuestions

1.

Ans.X=

Y=

Z=

2.Anorganiccompound‘A’havingmolecularformula ontreatmentwith

aqueous given’B’whichontreatmentwithLucasreagentgives‘C’.The

compound‘C’ontreatmentwithethanolicKOHgivesbackoncompound‘A’.IdentifyA,

B,&C.

Ans.

Thequestionare

3.Anorganiccompound‘A’onheatingwithNH3andcuprousoxideathighpressure

givescompound‘B’.Thecompound‘B’ontreatmentwithicecoldsolutionof

andHClgives‘C”,whichonheatingwithcopperturningandHClgives‘A’again.

IdentifyA,B&C.compound

Ans.

Thequestionare

4.

Ans.

5.

Ans.

6.

Ans.

7.

Ans.

8.Acompound‘A’containscarbonandhydrogenonlyandhasmolecularmassof72.Its

photochlorinationgivesamixturecontainingonlyonemonochloroandtwodichloro

hydrocarbons.DeducethestructureofAandchlorinatedproducts.

Ans.Ais (mol.Wt.72)Sinceitsgivesonemonochloroandtwodichloroderivatives

onphotochemicalchlorination,itis

Thereactionsare

9.WhyissulphuricacidnotusedduringthereactionofalcoholswithKI?

Ans.Inthepresenceofsulphuricacid ,KIproducesHI

Since isanoxidizingagent,itoxidizesHI(producedinthereactionto ).

Asaresult,thereactionbetweenalcoholandHItoproducealkyliodidecannotoccur.

Therefore,sulphuricacidisnotusedduringthereactionofalcoholswithKI.Instead,anon-

oxidizingacidsuchas isused.

10.Ahydrocarbon doesnotreactwithchlorineindarkbutgivesasingle

monochlorocompound inbrightsunlight.Identifythehydrocarbon.

Ans.Ahydrocarbonwiththemolecularformula, belongstothegroupwithageneral

molecularformula .Therefore,itmayeitherbeanalkeneoracycloalkane.

Sincehydrocarbondoesnotreactwithchlorineinthedark,itcannotbeanalkene.Thus,it

shouldbeacycloalkane.

Further,thehydrocarbongivesasinglemonochlorocompound, byreactingwith

chlorineinbrightsunlight.Sinceasinglemonochlorocompoundisformed,thehydrocarbon

mustcontainH-atomsthatareallequivalent.Also,asallH-atomsofacycloalkaneare

equivalent,thehydrocarbonmustbeacycloalkane.Hence,thesaidcompoundis

cyclopentane.

Cyclopentane( )

Thereactionsinvolvedinthequestionare:

11.Writetheequationsforthepreparationof1-iodobutanefrom

(i)1-butanol

(ii)1-chlorobutane

(iii)but-1-ene.

Ans.(i)

(ii)

(iii)

12.Whatareambidentnucleophiles?Explainwithanexample.

Ans.Ambidentnucleophilesarenucleophileshavingtwonucleophilicsites.Thus,ambident

nucleophileshavetwositesthroughwhichtheycanattack.

Forexample,nitriteionisanambidentnucleophile.

Nitriteioncanattackthroughoxygenresultingintheformationofalkylnitrites.Also,itcan

attackthroughnitrogenresultingintheformationofnitroalkanes.

13.Whichcompoundineachofthefollowingpairswillreactfasterin reactionwith

?

(i)

(ii)

Ans.(i)Inthe mechanism,thereactivityofhalidesforthesamealkylgroupincreasesin

theorder.Thishappensbecauseasthesizeincreases,thehalideionbecomesabetterleaving

group.

R-F<<R-Cl<R-Br<R-I

Therefore,CH3Iwillreactfasterthan in reactionswithOH-.

(ii)

The mechanisminvolvestheattackofthenucleophileattheatombearingtheleaving

group.But,incaseof ,theattackofthenucleophileatthecarbonatomis

hinderedbecauseofthepresenceofbulkysubstituentsonthatcarbonatombearingthe

leavinggroup.Ontheotherhand,therearenobulkysubstituentsonthecarbonatom

bearingtheleavinggroupin .Hence, reactsfasterthan in

reactionwith .

14.Writethemechanismofthefollowingreaction:

Ans.

Thegivenreactionisan reaction.Inthisreaction,CN-actsasthenucleophileandattacks

thecarbonatomtowhichBrisattached.CN-ionisanambidentnucleophileandcanattack

throughbothCandN.Inthiscase,itattacksthroughtheC-atom.

15.Outof and ,whichismoreeasilyhydrolysedby

aqueousKOH?

Ans.

HydrolysisbyaqueousKOHproceedsthroughtheformationofcarbocation.Ifcarbocationis

stable,thenthecompoundiseasilyhydrolyzedbyaqueousKOH.Now ,forms

carbocation,while forms carbocation,whichismorestable

than carbocation.Hence ,ishydrolyzedmoreeasilythan

byaqueousKOH.

16.p-Dichlorobenzenehashigherm.p.andlowersolubilitythanthoseofo-and

m-isomers.Discuss.

Ans.

p-Dichlorobenzeneismoresymmetricalthano-andm-isomers.Forthisreason,itfitsmore

closelythano-andm-isomersinthecrystallattice.Therefore,moreenergyisrequiredto

breakthecrystallatticeofp-dichlorobenzene.Asaresult,p-dichlorobenzenehasahigher

meltingpointandlowersolubilitythano-andm-isomers.

5MarksQuestions

1.Writestructuresofthefollowingcompounds:

(i)2-Chloro-3-methylpentane

(ii)1-Chloro-4-ethylcyclohexane

(iii)4-tert.Butyl-3-iodoheptane

(iv)1,4-Dibromobut-2-ene

(v)1-Bromo-4-sec.butyl-2-methylbenzene

Ans.(i)

2-Chloro-3-methylpentane

(ii)

1-Chloro-4-ethylcyclohexane

(iii)

4-tert-Butyl-3-iodoheptane

(iv)

1,4-Dibromobut-2-ene

(v)

1-Bromo-4-sec-butyl-2-methylbenzene

2.Writestructuresofdifferentdihalogenderivativesofpropane.

Ans.Therearefourdifferentdihalogenderivativesofpropane.Thestructuresofthese

derivativesareshownbelow.

(i)

1,1-Dibromopropane

(ii)

2,2-Dibromopropane

(iii)

1,2-Dibromopropane

(iv)

1,3-Dibromopropane

3.Amongtheisomericalkanesofmolecularformula ,identifytheonethaton

photochemicalchlorinationyields

(i)Asinglemonochloride.

(ii)Threeisomericmonochlorides.

(iii)Fourisomericmonochlorides.

Ans.(i)Tohaveasinglemonochloride,thereshouldbeonlyonetypeofH-atominthe

isomerofthealkaneofthemolecularformula .Thisisbecause,replacementofany

H-atomleadstotheformationofthesameproduct.Theisomerisneopentane.

Neopentane

(ii)Tohavethreeisomericmonochlorides,theisomerofthealkaneofthemolecularformula

shouldcontainthreedifferenttypesofH-atoms.

Therefore,theisomerisn-pentane.ItcanbeobservedthattherearethreetypesofHatoms

labelledasa,bandcinn-pentane.

(iii)Tohavefourisomericmonochlorides,theisomerofthealkaneofthemolecularformula

C5H12shouldcontainfourdifferenttypesofH-atoms.Therefore,theisomeris2-

methylbutane.ItcanbeobservedthattherearefourtypesofH-atomslabelled

asa,b,c,anddin2-methylbutane.

4.Drawthestructuresofmajormonohaloproductsineachofthefollowingreactions:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

Ans.(i)

(ii)

(iii)

(iv)

(v)

(vi)

5.Arrangeeachsetofcompoundsinorderofincreasingboilingpoints.

(i)Bromomethane,Bromoform,Chloromethane,Dibromomethane.

(ii)1-Chloropropane,Isopropylchloride,1-Chlorobutane.

Ans.(i)

Foralkylhalidescontainingthesamealkylgroup,theboilingpointincreaseswithan

increaseintheatomicmassofthehalogenatom.

SincetheatomicmassofBrisgreaterthanthatofCl,theboilingpointofbromomethaneis

higherthanthatofchloromethane.

Further,foralkylhalidescontainingthesamealkylgroup,theboilingpointincreaseswith

anincreaseinthenumberofhalides.Therefore,theboilingpointofDibromomethaneis

higherthanthatofchloromethaneandbromomethane,butlowerthanthatofbromoform.

Hence,thegivensetofcompoundscanbearrangedintheorderoftheirincreasingboiling

pointsas:

Chloromethane<Bromomethane<Dibromomethane<Bromoform.

(ii)

Foralkylhalidescontainingthesamehalide,theboilingpointincreaseswithanincreasein

thesizeofthealkylgroup.Thus,theboilingpointof1-chlorobutaneishigherthanthatof

isopropylchlorideand1-chloropropane.

Further,theboilingpointdecreaseswithanincreaseinbranchinginthechain.Thus,the

boilingpointofisopropylalcoholislowerthanthatof1-chloropropane.

Hence,thegivensetofcompoundscanbearrangedintheincreasingorderoftheirboiling

pointsas:

Isopropylchloride<1-Chloropropane<1-Chlorobutane

6.Whichalkylhalidefromthefollowingpairswouldyouexpecttoreactmorerapidly

byan mechanism?Explainyouranswer.

(i)

(ii)

(iii)

Ans.(i)

2-bromobutaneisa alkylhalidewhereas1-bromobutaneisa alkylhalide.The

approachingofnucleophileismorehinderedin2-bromobutanethanin1-bromobutane.

Therefore,1-bromobutanereactsmorerapidlythan2-bromobutanebyan mechanism.

(ii)

2-Bromobutaneis alkylhalidewhereas2-bromo-2-methylpropaneis alkylhalide.

Therefore,greaternumbersofsubstituentsarepresentin alkylhalidethanin alkyl

halidetohindertheapproachingnucleophile.Hence,2-bromobutanereactsmorerapidly

than2-bromo-2-methylpropanebyan mechanism.

(iii)

Boththealkylhalidesareprimary.However,thesubstituent isatagreaterdistance

tothecarbonatomlinkedtoBrin1-bromo-3-methylbutanethanin1-bromo-2-methylbutane.

Therefore,theapproachingnucleophileislesshinderedincaseoftheformerthanincaseof

thelatter.Hence,theformerreactsfasterthanthelatterby mechanism.

7.Inthefollowingpairsofhalogencompounds,whichcompoundundergoesfaster

reaction?

(i)

(ii)

Ans.(i)

SN1reactionproceedsviatheformationofcarbocation.Thealkylhalide(I)is while(II)is

.Therefore,(I)forms carbocationwhile(II)forms carbocation.Greaterthe

stabilityofthecarbocation,fasteristherateofSN1reaction.Since carbocationismore

stablethan carbocation.(I),i.e.2-chloro-2-methylpropane,undergoesfasterSN1reaction

than(II)i.e.,3-chloropentane.

(ii)

Thealkylhalide(I)is while(II)is . carbocationismorestablethan carbocation.

Therefore,(I),2-chloroheptane,undergoesfaster reactionthan(II),1-chlorohexane.

8.IdentifyA,B,C,D,E,Rand inthefollowing:

Ans.

SinceDof getsattachedtothecarbonatomtowhichMgBrisattached,Cis

Therefore,thecompoundR-Bris

WhenanalkylhalideistreatedwithNainthepresenceofether,ahydrocarboncontaining

doublethenumberofcarbonatomsaspresentintheoriginalhalideisobtainedasproduct.

ThisisknownasWurtzreaction.Therefore,thehalide, ,is

Therefore,compoundDis

And,compoundEis

9.NamethefollowinghalidesaccordingtoIUPACsystemandclassifythemasalkyl,

allyl,benzyl(primary,secondary,tertiary),vinylorarylhalides:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

(ix)

(x)

(xi)

(xii)

Ans.(i)

2-Chloro-3-methylbutane

(Secondaryalkylhalide)

(ii)

3-Chloro-4-methyhexane

(Secondaryalkylhalide)

(iii)

1-Iodo-2,2-dimethylbutane

(Primaryalkylhalide)

(iv)

1-Bromo-3,3-dimethyl-1-phenylbutane

(Secondarybenzylhalide)

(v)

2-Bromo-3-methylbutane

(Secondaryalkylhalide)

(vi)

1-Bromo-2-ethyl-2-methylbutane

(Primaryalkylhalide)

(vii)

3-Chloro-3-methylpentane

(Tertiaryalkylhalide)

(viii)

3-Chloro-5-methylhex-2-ene

(Vinylhalide)

(ix)

4-Bromo-4-methylpent-2-ene

(Allylhalide)

(x)

1-Chloro-4-(2-methylpropyl)benzene

(Arylhalide)

(xi)

1-Chloromethyl-3-(2,2-dimethylpropyl)benzene

(Primarybenzylhalide)

(xii)

1-Bromo-2-(1-methylpropyl)benzene

(Arylhalide)

10.GivetheIUPACnamesofthefollowingcompounds:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

Ans.(i)

2-Bromo-3-chlorobutane

(ii)

1-Bromo-1-chloro-1,2,2-trifluoroethane

(iii)

1-Bromo-4-chlorobut-2-yne

(iv)

2-(Trichloromethyl)-1,1,1,2,3,3,3-heptachloropropane

(v)

2-Bromo-3,3-bis(4-chlorophenyl)butane

(vi)

1-chloro-1-(4-iodophenyl)-3,3-dimethylbut-1-ene

11.Writethestructuresofthefollowingorganichalogencompounds.

(i)2-Chloro-3-methylpentane

(ii)p-Bromochlorobenzene

(iii)1-Chloro-4-ethylcyclohexane

(iv)2-(2-Chlorophenyl)-1-iodooctane

(v)Perfluorobenzene

(vi)4-tert-Butyl-3-iodoheptane

(vii)1-Bromo-4-sec-butyl-2-methylbenzene

(viii)1,4-Dibromobut-2-ene

Ans.(i)

2-Chloro-3-methylpentane

(ii)

p-Bromochlorobenzene

(iii)

1-Chloro-4-ethylcyclohexane

(iv)

2-(2-Chlorophenyl)-1-iodooctane

(v)

Perfluorobenzene

(vi)

4-Tert-Butyl-3-iodoheptane

(vii)

1-Bromo-4-sec-butyl-2-methylbenzene

(viii)

1,4-Dibromobut-2-ene

12.Whichoneofthefollowinghasthehighestdipolemoment?

(i) (ii) (iii)

Ans.(i)

Dichlormethane( )

=1.60D

(ii)

Chloroform( )

=1.08D

(iii)

Carbontetrachloride( )

=0D

isasymmetricalmolecule.Therefore,thedipolemomentsofallfourC-Clbondscancel

eachother.Hence,itsresultantdipolemomentiszero.

Asshownintheabovefigure,in ,theresultantofdipolemomentsoftwoC-Clbonds

isopposedbytheresultantofdipolemomentsofoneC-HbondandoneC-Clbond.Sincethe

resultantofoneC-HbondandoneC-ClbonddipolemomentsissmallerthantwoC-Clbonds,

theoppositionistoasmallextent.Asaresult, hasasmalldipolemomentof1.08D.

Ontheotherhand,incaseof ,theresultantofthedipolemomentsoftwoC-Clbonds

isstrengthenedbytheresultantofthedipolemomentsoftwoC-Hbonds.Asaresult,

hasahigherdipolemomentof1.60Dthan l3i.e.,CH2Cl2hasthehighest

dipolemoment.

Hence,thegivencompoundscanbearrangedintheincreasingorderoftheirdipole

momentsas:

< <

13.Writetheisomersofthecompoundhavingformula .

Ans.Therearefourisomersofthecompoundhavingtheformula .Theseisomers

aregivenbelow.

(a)

1-Bromobutane

(b)

2-Bromobutane

(c)

1-Bromo-2-methylpropane

(d)

2-Bromo-2-methylpropane

14.Predictallthealkenesthatwouldbeformedbydehydrohalogenationofthe

followinghalideswithsodiumethoxideinethanolandidentifythemajoralkene:

(i)1-Bromo-1-methylcyclohexane

(ii)2-Chloro-2-methylbutane

(iii)2,2,3-Trimethyl-3-bromopentane.

Ans.(i)

Inthegivencompound,therearetwotypesof-hydrogenatomsarepresent.Thus,

dehydrohalogenationofthiscompoundgivesonlyonealkenes.

(ii)

Inthegivencompound,therearetwodifferentsetsofequivalent-hydrogenatomslabelled

asaandb.Thus,dehydrohalogenationofthecompoundyieldstwoalkenes.

Saytzeff'sruleimpliesthatindehydrohalogenationreactions,thealkenehavingagreater

numberofalkylgroupsattachedtoadoublybondedcarbonatomsispreferablyproduced.

Therefore,alkene(I)i.e.,2-methylbut-2-eneisthemajorproductinthisreaction.

(iii)

2,2,3-Trimethyl-3-bromopentane

Inthegivencompound,therearetwodifferentsetsofequivalent-hydrogenatomslabelled

asaandb.Thus,dehydrohalogenationofthecompoundyieldstwoalkenes.

AccordingtoSaytzeff'srule,indehydrohalogenationreactions,thealkenehavingagreater

numberofalkylgroupsattachedtothedoublybondedcarbonatomispreferablyformed.

Hence,alkene(I)i.e.,3,4,4-trimethylpent-2-eneisthemajorproductinthisreaction.

15.Howwillyoubringaboutthefollowingconversions?

(i)Ethanoltobut-1-yne

(ii)Ethanetobromoethene

(iii)Propeneto1-nitropropane

(iv)Toluenetobenzylalcohol

(v)Propenetopropyne

(vi)Ethanoltoethylfluoride

(vii)Bromomethanetopropanone

(viii)But-1-enetobut-2-ene

(ix)1-Chlorobutaneton-octane

(x)Benzenetobiphenyl.

Ans.(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

(ix)

(x)

16.Explainwhy

(i)thedipolemomentofchlorobenzeneislowerthanthatofcyclohexylchloride?

(ii)alkylhalides,thoughpolar,areimmisciblewithwater?

(iii)Grignardreagentsshouldbepreparedunderanhydrousconditions?

Ans.(i)

Inchlorobenzene,theCl-atomislinkedtoa hybridizedcarbonatom.Incyclohexyl

chloride,theCl-atomislinkedtoa hybridizedcarbonatom.Now, hybridized

carbonhasmores-characterthan hybridizedcarbonatom.Therefore,theformeris

moreelectronegativethanthelatter.Therefore,thedensityofelectronsofC-Clbondnear

theCl-atomislessinchlorobenzenethanincydohexylchloride.

Moreover,the-Reffectofthebenzeneringofchlorobenzenedecreasestheelectrondensity

oftheC-ClbondneartheCl-atom.Asaresult,thepolarityoftheC-Clbondin

chlorobenzenedecreases.Hence,thedipolemomentofchlorobenzeneislowerthanthatof

cyclohexylchloride.

(ii)Tobemisciblewithwater,thesolute-waterforceofattractionmustbestrongerthanthe

solute-soluteandwater-waterforcesofattraction.Alkylhalidesarepolarmoleculesandso

heldtogetherbydipole-dipoleinteractions.Similarly,strongH-bondsexistbetweenthe

watermolecules.Thenewforceofattractionbetweenthealkylhalidesandwatermolecules

isweakerthanthealkylhalide-alkylhalideandwater-waterforcesofattraction.Hence,alkyl

halides(thoughpolar)areimmisciblewithwater.

(iii)Grignardreagentsareveryreactive.Inthepresenceofmoisture,theyreacttogive

alkanes.

Therefore,Grignardreagentsshouldbepreparedunderanhydrousconditions.

17.Givetheusesoffreon12,DDT,carbontetrachlorideandiodoform.

Ans.UsesofFreon-12

Freon-12(dichlorodifluoromethane, )iscommonlyknownasCFC.Itisusedasa

refrigerantinrefrigeratorsandairconditioners.Itisalsousedinaerosolspraypropellants

suchasbodysprays,hairsprays,etc.However,itdamagestheozonelayer.Hence,its

manufacturewasbannedintheUnitedStatesandmanyothercountriesin1994.

UsesofDDT

DDT(p,p-dichlorodiphenyltrichloroethane)isoneofthebestknowninsecticides.Itisvery

effectiveagainstmosquitoesandlice.Butdueitsharmfuleffects,itwasbannedintheUnited

Statesin1973.

Usesofcarbontetrachloride

(i)Itisusedformanufacturingrefrigerantsandpropellantsforaerosolcans.

(ii)Itisusedasfeedstockinthesynthesisofchlorofluorocarbonsandotherchemicals.

(iii)Itisusedasasolventinthemanufactureofpharmaceuticalproducts.

(iv)Untilthemid1960's,carbontetrachloridewaswidelyusedasacleaningfluid,a

degreasingagentinindustries,aspotreamerinhomes,andafireextinguisher.

Usesofiodoform

Iodoformwasusedearlierasanantiseptic,butnowithasbeenreplacedbyother

formulations-containingiodine-duetoitsobjectionablesmell.Theantisepticpropertyof

iodoformisonlyduetotheliberationoffreeiodinewhenitcomesincontactwiththeskin.

18.Writethestructureofthemajororganicproductineachofthefollowingreactions:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

Ans.

(ii)

(viii)

19.Arrangethecompoundsofeachsetinorderofreactivitytowards displacement:

(i)2-Bromo-2-methylbutane,1-Bromopentane,2-Bromopentane

(ii)1-Bromo-3-methylbutane,2-Bromo-2-methylbutane,3-Bromo-2-methylbutane

(iii)1-Bromobutane,1-Bromo-2,2-dimethylpropane,1-Bromo-2-methylbutane,1-Bromo-

3-methylbutane.

Ans.(i)

An reactioninvolvestheapproachingofthenucleophiletothecarbonatomtowhichthe

leavinggroupisattached.Whenthenucleophileisstericallyhindered,thenthereactivity

towards displacementdecreases.Duetothepresenceofsubstituents,hindrancetothe

approachingnucleophileincreasesinthefollowingorder.

1-Bromopentane<2-bromopentane<2-Bromo-2-methylbutane

Hence,theincreasingorderofreactivitytowards displacementis:

2-Bromo-2-methylbutane<2-Bromopentane<1-Bromopentane

(ii)

Sincesterichindranceinalkylhalidesincreasesintheorderof ,the

increasingorderofreactivitytowards displacementis

.

Hence,thegivensetofcompoundscanbearrangedintheincreasingorderoftheirreactivity

towards displacementas:

2-Bromo-2-methylbutane<2-Bromo-3-methylbutane<1-Bromo-3-methylbutane

[2-Bromo-3-methylbutaneisincorrectlygiveninNCERT]

(iii)

Thesterichindrancetothenucleophileinthe mechanismincreaseswithadecreasein

thedistanceofthesubstituentsfromtheatomcontainingtheleavinggroup.Further,the

sterichindranceincreaseswithanincreaseinthenumberofsubstituents.Therefore,the

increasingorderofsterichindrancesinthegivencompoundsisasbelow:

1-Bromobutane<1-Bromo-3-methylbutane<1-Bromo-2-methylbutane

<1-Bromo-2,2-dimethylpropane

Hence,theincreasingorderofreactivityofthegivencompoundstowards displacement

is:

1-Bromo-2,2-dimethylpropane<1-Bromo-2-methylbutane<1-Bromo-3-methylbutane<1-

Bromobutane

20.Howthefollowingconversionscanbecarriedout?

(i)Propenetopropan-1-ol

(ii)Ethanoltobut-1-yne

(iii)1-Bromopropaneto2-bromopropane

(iv)Toluenetobenzylalcohol

(v)Benzeneto4-bromonitrobenzene

(vi)Benzylalcoholto2-phenylethanoicacid

(vii)Ethanoltopropanenitrile

(viii)Anilinetochlorobenzene

(ix)2-Chlorobutaneto3,4-dimethylhexane

(x)2-Methyl-1-propeneto2-chloro-2-methylpropane

(xi)Ethylchloridetopropanoicacid

(xii)But-1-eneton-butyliodide

(xiii)2-Chloropropaneto1-propanol

(xiv)Isopropylalcoholtoiodoform

(xv)Chlorobenzenetop-nitrophenol

(xvi)2-Bromopropaneto1-bromopropane

(xvii)Chloroethanetobutane

(xviii)Benzenetodiphenyl

(xix)tert-Butylbromidetoisobutylbromide

(xx)Anilinetophenylisocyanide

Ans.(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

(ix)

(x)

(xi)

(xii)

(xiii)

(xiv)

(xv)

(xvi)

(xvii)

(xviii)

(xix)

(xx)

21.ThetreatmentofalkylchlorideswithaqueousKOHleadstotheformationof

alcoholsbutinthepresenceofalcoholicKOH,alkenesaremajorproducts.Explain.

Ans.Inanaqueoussolution,KOHalmostcompletelyionizestogiveOH-ions.OH-ionisa

strongnucleophile,whichleadsthealkylchloridetoundergoasubstitutionreactiontoform

alcohol.

Ontheotherhand,analcoholicsolutionofKOHcontainsalkoxide ion,whichisa

strongbase.Thus,itcanabstractahydrogenfromthe -carbonofthealkylchlorideand

formanalkenebyeliminatingamoleculeofHCl.

ionisamuchweakerbasethanRO-ion.Also, ionishighlysolvatedinan

aqueoussolutionandasaresult,thebasiccharacterof iondecreases.Therefore,it

cannotabstractahydrogenfromthe -carbon.

ConceptInsight: isastrongnucleophilebutweakerbasethan

22.Primaryalkylhalide (a)reactedwithalcoholicKOHtogivecompound

(b).Compound(b)isreactedwithHBrtogive(c)whichisanisomerof(a).When(a)is

reactedwithsodiummetalitgivescompound(d), whichisdifferentfromthe

compoundformedwhenn-butylbromideisreactedwithsodium.Givethestructural

formulaof(a)andwritetheequationsforallthereactions.

Ans.Therearetwoprimaryalkylhalideshavingtheformula, .Theyaren-bulyl

bromideandisobutylbromide.

Therefore,compound(a)iseithern-butylbromideorisobutylbromide.

Now,compound(a)reactswithNametaltogivecompound(b)ofmolecularformula,

,whichisdifferentfromthecompoundformedwhenn-butylbromidereactswithNametal.

Hence,compound(a)mustbeisobutylbromide.

Thus,compound(d)is2,5-dimethylhexane.

Itisgiventhatcompound(a)reactswithalcoholicKOHtogivecompound(b).Hence,

compound(b)is2-methylpropene.

Also,compound(b)reactswithHBrtogivecompound(c)whichisanisomerof(a).Hence,

compound(c)is2-bromo-2-methylpropane.

23.Whathappenswhen

(i)n-butylchlorideistreatedwithalcoholicKOH,

(ii)bromobenzeneistreatedwithMginthepresenceofdryether,

(iii)chlorobenzeneissubjectedtohydrolysis,

(iv)ethylchlorideistreatedwithaqueousKOH,

(v)methylbromideistreatedwithsodiuminthepresenceofdryether,

(vi)methylchlorideistreatedwithKCN.

Ans.(i)Whenn-butylchlorideistreatedwithalcoholicKOH,theformationofbut-l-ene

takesplace.Thisreactionisadehydrohalogenationreaction.

(ii)WhenbromobenzeneistreatedwithMginthepresenceofdryether,phenylmagnesium

bromideisformed.

(iii)Chlorobenzenedoesnotundergohydrolysisundernormalconditions.However,it

undergoeshydrolysiswhenheatedinanaqueoussodiumhydroxidesolutionata

temperatureof623Kandapressureof300atmtoformphenol.

(iv)WhenethylchlorideistreatedwithaqueousKOH,itundergoeshydrolysistoform

ethanol.

(v)Whenmethylbromideistreatedwithsodiuminthepresenceofdryether,ethaneis

formed.ThisreactionisknownastheWurtzreaction.

(vi)WhenmethylchlorideistreatedwithKCN,itundergoesasubstitutionreactiontogive

methylcyanide.