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)
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.
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)
(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.
(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)
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)
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.
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