Alicyclic compounds

Alicyclic compounds

1. Alicyclic Hydrocarbons

2. Open-chainandcycliccompounds
Inthecompoundsthatwehavestudiedinpreviouschapters,thecarbonatomsare
attachedtooneanothertoformchains;thesearecalledopen-chaincompounds,In
manycompounds,however,thecarbonatomsarearrangedtoformrings;theseare
calledcycliccompounds.
Inthischapterweshalltakeupthealicyclichydrocarbons(aliphaticcyclic
hydrocarbons).Muchofthechemistryofcycloalkanesandcycloalkeneswealready
know,sinceitisessentiallythechemistryofopen-chainalkanesandalkenes.
Nomenclature
Cyclicaliphatichydrocarbonsarenamedbyprefixingcyclo-tothenameofthe
correspondingopen-chainhydrocarbonhavingthesamenumberofcarbonatomsas
thering.Forexample:
Substituentsontheringarenamed,andtheirpositionsareindicatedbynumbers,the
lowestcombinationofnumbersbeingused,insimplecycloalkenesand
cycloalkynesthedoubly-andtriply-bondedcarbonsareconsideredtooccupy
positions1and2.Forexample:

3. Forconvenience,aliphaticringsareoftenrepresentedbysimplegeometricfigures:
atriangleforcyclopropane,asquareforcyclobutane,apentagonforcyclopentane,
ahexagonforcyclohexane,andsoon.Itisunderstoodthattwohydrogensare
locatedateachcornerofthefigureunlesssomeothergroupisindicated.For
example:
Polycycliccompoundsinavarietyofstrangeandwonderfulshapeshavebeen
made,andtheirpropertieshaverevealedunexpectedfacetsoforganicchemistry.
Underlyingmuchofthisresearchtherehasalwaysbeenthechallenge;cansucha
compoundbemodel

4. Industrialsource
Cycloalkanesareconvertedbycatalyticreformingintoaromatichydrocarbons,and
thusprovideoneofthemajorsourcesoftheseimportantcompounds.Forexample:
soadditionofhydrogentoaromaticcompoundsyieldscyclicaliphaticcompounds,
specificallycyclohexanederivatives.Animportantexampleofthisisthe
hydrogenationofbenzenetoyieldpurecyclohexane.

5. Aswemightexpect,hydrogenationofsubstitutedbenzenesyieldssubstituted
cyclohexanes.Forexamplecyclohexanol.
Fromcyclohexanolmanyothercycliccompoundscontainingasix-memberedring
canbemade.
Preparation
Preparationofalicyclichydrocarbonsfromotheraliphaticcompoundsgenerally
involvestwostages:(a)conversionofsomeopen-chaincompoundintoacompound
thatcontainsaring,aprocesscalledcyclization;(b)conversionofthecyclic
compoundthusobtainedintothekindofcompoundthatwewant:forexample,
conversionofacyclicalcoholintoacyclicalkene,orofacyclicalkeneintoacyclic
alkane.Veryoften,cycliccompoundsaremadebytheadaptingofastandard
methodofpreparationtothejobofclosingaring.Forexample,thatthealkyl
groupsoftwoalkylhalidescanbecoupledtogetherthroughconversionofone
halideintoanorganometalliccompound(alithiumdialkylcopper):

6. Thesamemethodappliedtoatf/halidecanbringaboutcouplingbetweentwoalkyl
groupsthatarepartofthesamemolecule:
Problem.1Startingwithcyclohexanol,howwouldyouprepare:
(a)cyclohexene,(b)3-bromocyclohexene,(c)1,3-cyclohexadiene?
Problem.2Bromocyclobutanecanbeobtainedfromopen-chaincompounds.How
wouldyoupreparecyclobutanefromit?

7. Reactions
Alicyclichydrocarbonsundergothesamereactionsastheiropen-chainanalogs.
Cycloalkanesundergochieflyfree-radicalsubstitutionForexample:
Cycloalkenesundergochieflyadditionreactions,bothelectrophilicandfree
radical;likeotheralkenes,theycanalsoundergocleavageandallylicsubstitution.
Forexample:

8. Reactionsofsmall-ringcompounds.Cyclopropaneandcyclobutane
Thetwosmallestcycloalkanes,cyclopropaneandcyclobutane,showcertain
chemicalpropertiesthatareentirelydifferentfromthoseoftheothermembersof
theirfamily.
Ineachofthesereactionsacarbon-carbonbondisbroken,andthetwoatomsofthe
reagentappearattheendsofthepropanechain:
Cyclobutanedoesnotundergomostofthering-openingreactionsofcyclopropane;itis
hydrogenated,butonlyundermotevigorousconditionsthanthoserequiredforcyclopropane,thus
cyclobutaneundergoesadditionlessreadilythancyclopropaneand,withsomeexceptions,
cyclopropanelessreadilythananalkene.Theremarkablethingisthatthesecycloalkanesundergo
additionatall.

9. Baeyer strain theory
ThusBaeyerconsideredthattingssmallerorlargerthancyclopentaneorcyclohexanewere
unstable;itwasbecauseofthisinstabilitythatthethree-andfourmemberedrings
underwentring-openingreactions;itwasbecauseofthisinstabilitythatgreatdifficultyhad
beenencounteredinthesynthesisofthelargerrings.
Conformationsofcyclohexanethatarefreeofanglestrain
Equatorialandaxialbondsincyclohexane
Letusreturntothemodelofthechairconformationofcyclohexanefigureabove.Althoughthe
cyclohexaneringisnotflat,wecanconsiderthatthecarbonatomslieroughlyinaplane.Ifwe
lookatthemoleculeinthisway,weseethatthehydrogenatomsoccupytwokindsofposition:
sixhydrogenslieintheplane,whilesixhydrogenslieaboveorbelowtheplane.

10. Aromaticcompoundsarebenzeneandcompoundsthatresemblebenzeneinchemicalbehavior.
Aromaticpropertiesarethosepropertiesofbenzenethatdistinguishitfromaliphatic
hydrocarbons.

11. Besidesthecompoundsthatcontainbenzenerings,therearemanyothersubstances
thatarecalledaromatic;yetsomeofthesesuperficiallybearlittleresemblanceto
benzene.
Benzenemolecule:shapeandsize
Fromtheexperimentalstandpoint,aromaticcompoundsarecompoundswhose
molecularformulaswouldleadustoexpectahighdegreeofunsaturation,andyet
whichareresistanttotheadditionreactionsgenerallycharacteristicofunsaturated
compounds.