11Fall, 2009Fall, 2009

Organic Chemistry IOrganic Chemistry ICycloalkanesCycloalkanes

Dr. Ralph C. GatroneDr. Ralph C. GatroneDepartment of Chemistry and PhysicsDepartment of Chemistry and Physics

Virginia State UniversityVirginia State University

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ObjectivesObjectives

• NomenclatureNomenclature

• Ring StrainRing Strain

• Conformational AnalysisConformational Analysis

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CycloalkanesCycloalkanes

• Hydrocarbons where carbons join in a Hydrocarbons where carbons join in a ringring

• General formula is CGeneral formula is CnnHH2n2n

• Referred to as alicyclic compoundsReferred to as alicyclic compounds

• Chemistry is the same as straight-chain Chemistry is the same as straight-chain alkanesalkanes

• They burnThey burn

• They react with halogen in lightThey react with halogen in light

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ExamplesExamples

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NomenclatureNomenclature

• Determine number Determine number of carbons in ringof carbons in ring

• Name the alkane Name the alkane with cyclo in frontwith cyclo in front

• 12 carbons12 carbons

• DodecaneDodecane

• cyclododecanecyclododecane

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NomenclatureNomenclature

• Sustituted cycloalkanesSustituted cycloalkanes

• If substituent has less than or equal to the If substituent has less than or equal to the number of carbons in the ring number of carbons in the ring

• name as an alkyl substituted ringname as an alkyl substituted ring

sec-butylcyclopentane

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NomenclatureNomenclature

• If the substituent has more carbons than If the substituent has more carbons than the ringthe ring

• Name as a cycloalkyl substituted alkaneName as a cycloalkyl substituted alkane

6-cyclohexyl-2,3-dimethylheptane

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NomenclatureNomenclature

• Number substituentsNumber substituents

• If two groups can receive the same If two groups can receive the same number, prioritize using alphabetnumber, prioritize using alphabet

1-sec-butyl-3-isobutylcyclohexane

not

1-isobutyl-3-sec-butylcyclohexane

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NomenclatureNomenclature

• Halogens are treated like alkyl Halogens are treated like alkyl groupsgroups

• Fluorine becomes fluoroFluorine becomes fluoro

• Chlorine becomes chloroChlorine becomes chloro

• Bromine becomes bromoBromine becomes bromo

• Iodine becomes iodoIodine becomes iodo

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NomenclatureNomenclature

• Three substituentsThree substituents

• Number substituents such that sum of the Number substituents such that sum of the numbers chosen is as low as possiblenumbers chosen is as low as possible

Br

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NomenclatureNomenclature

Br

Possilbe numbering

Br Ethyl Methyl

1 3 4 = 81 6 5 = 123 1 7 = 116 1 2 = 95 7 1 = 134 2 1 = 7

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NomenclatureNomenclature

Br

Possilbe numbering

Br Ethyl Methyl

1 3 4 = 81 6 5 = 123 1 7 = 116 1 2 = 95 7 1 = 134 2 1 = 7

correct name is 4-bromo-2-ethyl-1-methylcycloheptane

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Isomerism in CycloalkanesIsomerism in Cycloalkanes

• Rotation about C-C bonds in cycloalkanes is limited Rotation about C-C bonds in cycloalkanes is limited • Rings have two “faces” and substituents are labeled Rings have two “faces” and substituents are labeled

as to their relative facial positionsas to their relative facial positions• There are two different 1,2-dimethyl-cyclopropane There are two different 1,2-dimethyl-cyclopropane

isomers, one with the two methyls on the same side isomers, one with the two methyls on the same side (cis) of the ring and one with the methyls on opposite (cis) of the ring and one with the methyls on opposite sides (trans)sides (trans)

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IsomerismIsomerism

• Atoms are connected the sameAtoms are connected the same

• Differ only in spatial arrangementDiffer only in spatial arrangement

• StereoisomerismStereoisomerism

• Constitutional isomerismConstitutional isomerism

• Differ in arrangement of atomsDiffer in arrangement of atoms

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Constitutional and Constitutional and StereoisomersStereoisomers

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Stability of Cycloalkanes:Stability of Cycloalkanes:Baeyer Ring StrainBaeyer Ring Strain• Baeyer (1885): Baeyer (1885): since carbon prefers to have bond since carbon prefers to have bond

angles of approximately 109°, ring sizes other than angles of approximately 109°, ring sizes other than five and six may be too strained to existfive and six may be too strained to exist

• Angle StrainAngle Strain

• Based upon geometryBased upon geometry

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Angle StrainAngle Strainangle = 60o

angle = 90o

angle = 108o

angle = 120o

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Angle StrainAngle Strain

• Experimental Data for Strain in RingsExperimental Data for Strain in Rings

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Angle StrainAngle Strain

• Cyclopropane and cyclobutane – strainedCyclopropane and cyclobutane – strained

• Cyclopentane more strain than predictedCyclopentane more strain than predicted

• Cyclohexane – no strainCyclohexane – no strain

• Baeyer assumed rings are flatBaeyer assumed rings are flat

• Rings adopt 3-dimensional shapeRings adopt 3-dimensional shape

• Reduces angle strainReduces angle strain

• Angle strain present in small rings which Angle strain present in small rings which have little flexibilityhave little flexibility

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Strain in CycloalkanesStrain in Cycloalkanes

• Torsional StrainTorsional Strain • due to eclipsing H’s on adjacent carbon due to eclipsing H’s on adjacent carbon

atomsatoms

• Steric StrainSteric Strain • due to repulsion between nonbonded atoms due to repulsion between nonbonded atoms

that get too closethat get too close

• Angle StrainAngle Strain • present in small nonflexible ringspresent in small nonflexible rings

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CyclopropaneCyclopropane • 3-membered ring must have planar structure3-membered ring must have planar structure• C–C–C bond angles of 60°C–C–C bond angles of 60°• Requires that spRequires that sp33

based bonds are bent (and based bonds are bent (and weakened)weakened)

• All C-H bonds are eclipsedAll C-H bonds are eclipsed

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Bonding in CyclopropaneBonding in Cyclopropane

• Structural analysis of cyclopropane shows Structural analysis of cyclopropane shows that electron density of C-C bond is that electron density of C-C bond is displaced outward from inter-nuclear axisdisplaced outward from inter-nuclear axis

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Bent Bonds in CyclopropaneBent Bonds in Cyclopropane

Br

Br

Br2

Bond reacts with bromine leading Bond reacts with bromine leading to ring opening and addition to ring opening and addition productsproducts

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CyclobutaneCyclobutane

Cyclobutane - less angle strain than Cyclobutane - less angle strain than cyclopropane cyclopropane Greater torsional strain Greater torsional strain Because of its larger number of ring hydrogensBecause of its larger number of ring hydrogensCyclobutane is slightly bent out of plane Cyclobutane is slightly bent out of plane The bending The bending increasesincreases angle strain angle strain

But But decreasesdecreases torsional torsional strain strain

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CyclopentaneCyclopentane• Planar cyclopentane would have no angle strain but Planar cyclopentane would have no angle strain but

very high torsional strainvery high torsional strain• Actual conformations of cyclopentane are nonplanarActual conformations of cyclopentane are nonplanar• Reducing torsional strainReducing torsional strain• Four carbon atoms are in a planeFour carbon atoms are in a plane• The fifth carbon atom is above or below the planeThe fifth carbon atom is above or below the plane• EnvelopeEnvelope

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CyclohexaneCyclohexane

• Strain free molecule?Strain free molecule?

• Why?Why?

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CyclohexaneCyclohexane

• No eclipsing H’s – no torsional strainNo eclipsing H’s – no torsional strain

• No angle strainNo angle strain

• No steric strainNo steric strain

• No strain energyNo strain energy

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CyclohexaneCyclohexane

• Strain free moleculeStrain free molecule

• Adopts a chair conformationAdopts a chair conformation

• Important in carbohydrate chemistryImportant in carbohydrate chemistry

O

OH

HOH

H

OH

OH

H CH2OH

glucose

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Axial and Equatorial Axial and Equatorial PositionsPositions

• 3 hydrogens up and down (red) – axial3 hydrogens up and down (red) – axial

• 6 hydrogens in plane – equatorial6 hydrogens in plane – equatorial

• Each C atom has one axial, one Each C atom has one axial, one equatorialequatorial

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Axial and Equatorial Axial and Equatorial PositionsPositions

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Conformational MobilityConformational Mobility • Chair conformations readily interconvert, Chair conformations readily interconvert,

resulting in the exchange of axial and resulting in the exchange of axial and equatorial positions by a equatorial positions by a ring-flipring-flip

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Ring FlipRing Flip• Conformational mobility is fastConformational mobility is fast

• One methylcyclohexaneOne methylcyclohexane

• One cyclohexanolOne cyclohexanol

• One bromocyclohexaneOne bromocyclohexane

CH3OH Br

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Ring FlipRing Flip

CH3

H

CH3

H

OH

H

OH

H

Br

H

Br

H

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Ring FlipRing Flip

• Equatorial methyl becomes axial methylEquatorial methyl becomes axial methyl

• Equatorial bromo becomes axial bromoEquatorial bromo becomes axial bromo

• Equatorial hydroxy becomes axial Equatorial hydroxy becomes axial hydroxyhydroxy

• Barrier to ring flip is 45kJ/moleBarrier to ring flip is 45kJ/mole

• Rapid process at room temperatureRapid process at room temperature

• See only a single structureSee only a single structure

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MethylcyclohexaneMethylcyclohexane

• C1 to C4 = butaneC1 to C4 = butane

• When equatorial – no interaction with ringWhen equatorial – no interaction with ring

• When axial – gauche interaction (3.8kJ/mole)When axial – gauche interaction (3.8kJ/mole)

H

CH3

H

H

CH3

H2C

1

1

2

2

334

4

ring

H

CH3

H

Hring

CH3

H

H

H

H2C

1

2

4

1

24

gauche

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Axial Methyl InteractionsAxial Methyl Interactions

H

CH3H

H

H

CH3

H

H

CH3H

H

CH3

1

1

2

2

334 4

1 2 3

23

axial methyl interacts with H three C away1,3-diaxial interactioneach interaction = 3.8kJ/mole

equatorial methylno interactions

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EquilibriumEquilibrium

• Ring flip occursRing flip occurs

• Equilibrium processEquilibrium process

• Calculate KCalculate K

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Determination of K for Ring Determination of K for Ring FlipFlip

ΔE = -RTlnK ΔE = lnK -RT K = e-ΔE/RT

where ΔE = difference in energy of the conformers in J/mole; R = gas constant (8.315 J/Kmole) T = temperature in K

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Determination of K for Determination of K for MethylcyclohexaneMethylcyclohexane

CH3

H

CH3

H

no 1,3 diaxial interactions two 1,3-diaxial interactions

K = e-ΔE/RT

E for no interactions = 0KJ/mole

E for two interactions is 3.8 KJ/mole x 2 = 7.6KJ/mole = 7,600J/mole

ΔE = 0 – 7600 = -7600KJ/mole

K = e-ΔE/RT

K = e-(-7600/8.315x298)

K = e3.07

K = 21.5

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Determination of % of Determination of % of IsomersIsomers

CH3

H

CH3

H

no 1,3 diaxial interactions two 1,3-diaxial interactions

K = 21.5

K = 21.5 = Most Stable

1 Least Stable

% Most Stable = Most Stable Total % Most Stable = 21.5 = 95.5% 21.5 + 1

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Disubstituted CyclohexanesDisubstituted Cyclohexanes

• Steric effects of both substituents Steric effects of both substituents must be taken into account in both must be taken into account in both conformationsconformations

• There are two isomers of 1,2-There are two isomers of 1,2-dimethylcyclohexane. dimethylcyclohexane. ciscis and and transtrans

• Consider the sum of all interactionsConsider the sum of all interactions

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cis-1,2-dimethylcyclohexanecis-1,2-dimethylcyclohexane

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cis-1,2-dimethylcyclohexanecis-1,2-dimethylcyclohexane

• In the cis isomer, both methyl groups In the cis isomer, both methyl groups same face of the ring, and compound can same face of the ring, and compound can exist in two chair conformationsexist in two chair conformations

• Interactions for both ring-flip Interactions for both ring-flip conformations are the sameconformations are the same

• Energy is the sameEnergy is the same• K = 1K = 1• 50% of each conformer is present50% of each conformer is present

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trans-1,2-trans-1,2-dimethylcyclohexanedimethylcyclohexane

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trans-1,2-trans-1,2-dimethylcyclohexanedimethylcyclohexane• Methyl groups are on opposite faces of the ringMethyl groups are on opposite faces of the ring• One trans conformation has both methyl groups One trans conformation has both methyl groups

equatorial with only a gauche butane interaction equatorial with only a gauche butane interaction between methyls (3.8 kJ/mol) and no 1,3-diaxial between methyls (3.8 kJ/mol) and no 1,3-diaxial interactionsinteractions

• The ring-flipped conformation has both methyl The ring-flipped conformation has both methyl groups axial with four 1,3-diaxial interactionsgroups axial with four 1,3-diaxial interactions

• Steric strain of 4 Steric strain of 4 3.8 kJ/mol = 15.2 kJ/mol 3.8 kJ/mol = 15.2 kJ/mol makes the diaxial conformation 11.4 kJ/mol less makes the diaxial conformation 11.4 kJ/mol less favorable than the diequatorial conformationfavorable than the diequatorial conformation

• transtrans-1,2-dimethylcyclohexane will exist almost -1,2-dimethylcyclohexane will exist almost exclusively (>99%) in the diequatorial exclusively (>99%) in the diequatorial conformationconformation

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SummarySummary

• All organic molecules face the same All organic molecules face the same strainsstrains– AngleAngle– TorsionalTorsional– StericSteric

• Molecule will adopt the structure that Molecule will adopt the structure that reduces the total strain in moleculereduces the total strain in molecule– A minimum energy structureA minimum energy structure– Trade-offs among the strains is necessaryTrade-offs among the strains is necessary