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Cyclic Aliphatic Hydrocarbons

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Cyclic aliphatic hydrocarbons have the general formula CnH2n. Some examples of cyclic hydrocarbons are cyclopropane, compound I; cyclobutane, compound II; cyclopentane, compound III; cyclohexane, compound IV; bicylo[3.2.1]octane, compound V; cubane, pentacyclo[4.2.0.02,5.03,8. 04,7 ]octane, compound VI; and adamantane, tricyclo[3.3.1.13.7]nonane, compound VII.

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Compound VI, cubane, and compound VII, adamantane, have interesting structural features. Innovative and challenging syntheses have been devised to prepare cubane and adamantane. The synthesis of cubane is described in the following website:

http://en.wikipedia.org/wiki/Cubane

Another synthesis of cubane is represented in the following schema, and the synthesis is described in the Journal of the American Chemical Society (J. C. Barborak, L. Watts and R. Pettit, J. American Chemical Society, 88, 1328 (1966). The precursor for the reaction is 3,4-dichlorocyclobutene as a precursor.

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The nomenclature of cubane, as well as other multiple-fused ring saturated cyclo systems, is obtained using the following procedure.

(1) Select the portion of the ring system that gives the highest number of carbon atoms in a bicylco arrangement. For example, applying this rule to cubane would give the following carbon atom arrangement:

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not

or any other arrangement

(2) Number the bicylco system beginning at the bridge in such a manner that the portion of the bicycle system that has the greater number of carbon atoms receives the lowest numbers, i.e.,

not

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(3) Connect the appropriate carbon atoms, as indicated by the dotted lines, to form the bridges that will result in the formation of the desired molecule

(1) Indicate the number of atoms in the bicyclo structure formed in rule 1 proceeding from the highest number to the lowest number, such as 4.2.0. Indicate the other bridge connections by the number of carbon atoms in the bridge with a superscript number indicating the position of the carbon atoms connecting the bridge in ascending order:

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Therefore, the IUPAC name for cubane would be pentacyclo[4.2.0.02,5.03,8. 04,7 ]octane. The IUPAC name for adamantane can be obtained using the procedure outlined in steps 1-4.

(1) Select the portion of the ring system that gives the highest number of carbon atoms in a bicylco arrangement.

(2) Number the bicylco system beginning at the bridge in such a manner that the portion of the bicycle system that has the greater number of carbon atoms receives the lowest numbers, i.e.,

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(3) Connect the appropriate carbon atoms, as indicated by the dotted lines, to form the bridges that will result in the formation of the desired molecule.

(4) Indicate the number of atoms in the bicycle structure formed in rule 1 proceeding from the highest number to the lowest number, such as 4.2.0. Indicate the other bridge connections by the number of carbon atoms in the bridge with a superscript number indicating the position of the carbon atoms connecting the bridge in ascending order:

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Therefore, the IUPAC name for adamantane would be tricyclo[3.3.1.13,7 ]decane.

What is the IUPAC name for prismane, compound VIII?

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Physical Properties

Cyclic hydrocarbons have physical properties similar to straight chain hydrocarbons; however, the boiling points and densities of straight chain hydrocarbons are higher than cyclic hydrocarbons. Cyclic hydrocarbons are non-polar; consequently, they dissolve in nonpolar solvents like carbon tetrachloride, CCl4 , and petroleum ether. The following website provides information about petroleum ether:

http://en.wikipedia.org/wiki/Petroleum_ether

Cyclic hydrocarbons exhibit similar reactivity as straight chain hydrocarbons; however, small ring cycloalkanes are under considerable strain and will undergo ring-opening reactions rather than substitution reactions. For example, cyclopropane, unlike straight chain hydrocarbons or large ring compounds, will undergo the following reactions:

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Cyclobutane will undergo hydrogenation to produce n-butane. Unlike cyclopropane, cyclobutane will not react with Br2 in CCl44

; HBr; or H2SO4 to form addition reaction products, because it doesn’t have the same ring strain that cyclopropane possesses.

Adolf von Baeyer (http://www.answers.com/topic/adolf-von-baeyer) predicted that small ring compounds and large ring compounds should be unstable. Baeyer’s premise of the instability of small-ring compounds and large-ring compounds was based on the assumption that cycloalkane molecules are flat like their polygonal counterparts with bond angles of 60o for cyclopropane; 90o for cyclobutane; 108o for cyclopentane; 120o for cyclohexane; 128.6o for cycloheptane; and 135o for cyclooctane. The structure of saturated hydrocarbons with carbon atoms exhibiting the maximum overlap of 2sp3 hybridized orbitals to form molecular orbitals results in the formation of tetrahedral structures with 109.5o bond angles. Baeyer assumed that small and large ring compounds cannot assume the tetrahedral angle required to alleviate angle strain (either too small or too large as compared to 109.5o). For example, Baeyer assumed that the angle strain in cyclopropane would be 49.5o (109.5o – 60o).

Ring strain can be determined by measuring the heat of combustion, quantity of heat evolved when one mole of the cycloalkane is oxidized to CO2 and H2O. Table 8.1 lists the heat of combustion (ΔHcombustion) per methylene group for C3 to C15 cycloalkanes.

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ring size1 2ΔH

combustion

kJ mol-1

3Strain kJ mol-1

Ring Strain kJ mol-1

3 697.1 38.5 116 4 686.2 27.6 110 5 663.6 5.0 25 6 658.6 0 0 7 663.6 5.0 25 8 663.6 5.0 40 9 664.4 5.8 52

10 663.6 5.0 50 11 662.8 4.2 46 12 659.4 0.8 9.6 13 660.3 1.7 22 14 658.6 0 0 15 659.0 0.4 6.0

1 cycloalkane ring size, i.e., the number of carbon atoms forming the ring 2 Enthalpy (ΔH) of Combustion per methylene group 3 strain per methylene (-CH2-) group

Table 8.1 strain, in kJ/mol, per methylene group for C3 to C15 cycloalkanes

Table 8.1 indicates that the ring strain in small ring cycloalkanes agrees with Baeyer’s theory; however, unlike Baeyer’s hypothesis, large ring cycloalkanes exhibit remarkable stability. These data conflict with Baeyer’s assumption that cycloalkanes are flat. The experimental data demonstrate that Baeyer’s assumption is incorrect. Cycloalkanes are not flat. For example, cyclohexane, which exhibits 0 strain per methylene group, is not flat. The puckered, not flat, conformation in cyclohexane accounts for the bond angles of 109.5o expected in tetrahedral molecules with 2sp3

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hybridized atomic orbitals. Following is a pictorial representation of cyclohexane:

Notice that each carbon atom in cyclohexane assumes a tetrahedral geometry.

An identical argument exists for larger ring compounds. For example, the following is a representation for the puckered cyclodecane molecule:

Again, notice that each carbon atom in cyclodecane assumes a tetrahedral geometry.

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Syntheses of Cycloalkanes

Recall that alkanes can be prepared by a variety of methods, a couple of which involve organometallic compounds coupling with alkyl halides. It is reasonable to visualize an intramolecular cyclization of small straight chain compounds to form small ring compounds.

However, long chain compounds have difficulty getting the two ends of the molecules in position to cyclize; therefore, these compounds tend to undergo polymerization rather than cyclization.

The Diels Alder Reaction can be used to prepare bicyclo compounds. The reaction involves a (4 + 2) cycloaddition reaction of a dienophile with a diene.

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The physics of cycloaddition reactions will be discussed in a future paper.

Cyclopropane can be prepared from 1,3-dihalopropane.

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Cyclopropane can be prepared by methylene insertion reactions with diazomethane or ketene.

The carbene can have either a singlet state or a triplet state. The singlet state carbene has an unshared pair of electrons, i.e., electrons with opposite spins:

The C-H bond length is 1.12Ǻ

The pairing of the electrons can be

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Therefore, the term singlet is applied to this carbene.

The triplet state carbene has unshared electrons which are not paired, i.e., electrons with or without opposite spins.

Therefore, the possible spin arrangements for the two electrons could be:

1:2:1 hence the term triplet is applied to this carbene system

In the triplet state, the C-H bond length is 1.03 Ǻ.

Singlet state carbenes add to alkenes stereospecifically to form cyclopropanes:

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Carbenes in the triplet state add to alkenes in a nonstereospecific manner to form cyclopropane

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Carbenes can also be generated by treating chloroform with a strong base:

(CH3)3CO- K+ + H:CCI3 → K+ -:CCI3 + (CH3)3COH

K+ - :CCl3 → KCl + :CCl2

The Simmons-Smith reaction, http://www.organic-chemistry.org/namedreactions/simmons-smith-reaction.shtm, uses an organozinc carbenoid to insert a methylene group

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stereospecifically into the double bond:

Diagram 8.1 shows several conformations of cyclohexane. The chair conformation of cyclohexane is the most stable arrangement for cyclohexane. In the chair conformation of cyclohexane, six hydrogen atoms labeled Ha are perpendicular to the plane containing the six carbon atoms. These hydrogen atoms are identified as axial “a” hydrogen atoms. Six hydrogen atoms labeled He are in the plane containing the six carbon atoms. These hydrogen atoms are identified as equatorial “e” hydrogen atoms.

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Axial hydrogen atoms and equatorial hydrogen atoms are convertible one to the other by the ability of the chair conformation to convert into its superimposable mirror image. When this conversion process occurs, the hydrogen atoms labeled equatorial hydrogen atoms become axial hydrogen atoms, and the hydrogen atoms labeled axial hydrogen atoms become equatorial hydrogen atoms.

These two chair conformations are identical; however, if two of the hydrogen atoms are replaced with alkyl groups then the two conformations would not be identical. Alkyl groups in the 1,2-axial positions encounter 1,3 –axial interaction with hydrogen atoms. Alkyl groups in the equatorial position are more stable than if they are in the axial position.

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Diagram 8.1 conformational analysis of cyclohexane

Following are three dimensional pictures of the chair, half chair, twist boat, and boat conformations of cyclohexane with potential energy values for the various stages for the conversion of one chair conformation to the other chair conformation.

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chair conformation conformation

half chair conformation

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twist boat conformation

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boat conformation

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twist boat conformation

half chair conformation

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chair conformation

Stereoisomerism of Cyclohexane

Bulky alkyl groups would prefer to assume the equatorial position in order to eliminate the 1,3-axial interactions with hydrogen atoms.

Disubstituted cyclohexanes exhibit interesting stereochemical

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properties. For instance, compound XIX, trans-1,2-dimethylcyclohexane has both methyl groups in the equatorial position. If compound XIX converts to the other chair conformation, compound XX, the 1,3-axial interactions between the methyl groups and the hydrogen atoms would reduce the stability of the compound; therefore, the trans-1,2-dimethylcyclohexane would be the preferred conformation with the methyl groups in the diequatorial positions, compound XIX, rather than the diaxial conformation, compound XX.

The mirror image of trans-1,2-dimethylcyclohexane is nonsuperimposable, and the less stable diaxial conformations would also be nonsuperimposable. Therefore, the pure enantiomers are optically active.

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The diequatorial trans-(1S,2S)-1,2-dimethylcyclohexane can convert to the less stable diaxial trans-(1S,2S)-1,2-dimethylcyclohexane. These two compounds are nonsuperimposable.

trans-(1S,2S)-(e,e)-1,2-dimethylcyclohexane trans-(1S,2S)-(a,a)-1,2- dimethylcyclohexane

The diequatorial trans-(1R,2R)-1,2-dimethylcyclohexane (the mirror image of trans -(1S, 2S)-dimethylcyclohexane) can convert to the less stable diaxial trans-(1S,2S)-1,2-dimethylcyclohexane. These isomers are nonsuperimposable.

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The trans-(1R,2R)-(e,e)-1,2-dimethylcyclohexane and the trans-(1S,2S)-(e,e)-1,2-dimethylcyclohexane isomer are nonsuperimposable. In addition, the trans-(1R,2R)-(a,a)-1,2-dimethylcyclohexane and the trans-(1S,2S)-(a,a)-1,2-dimethylcyclohexane isomers are nonsuperimposable.

Picture 1 is a three dimensional illustration of the chair conformation of trans-(1R,2R)-(e,e)-1,2-dimethylcyclohexane and its mirror image, trans-(1S,2S)-(e,e)-1,2-dimethylcyclohexane.

Picture 1

The trans (1R, 2R) diequatorial isomer and the trans (1S,2S) diequatorial isomer convert to their diaxial isomers.

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trans-(1S,2S)-(e,e)-1,2-dimethylcyclohexane trans-(1S,2S)-(a,a)-1,2- dimethylcyclohexane

Picture 2 is a three dimensional image of the trans-(1S,2S)-(a,a)-1,2-dimethylcyclohexane and its mirror image, trans-(1R,2R)-(a,a)-1,2-dimethylcyclohexane.

Picture 2

Therefore, the trans (1S,2S)-(e,e)-1,2-dimethylcyclohexane and the trans (1R,2R)-(e,e)-1,2-dimethylcyclohexane are resolvable enantiomers. The pure enantiomers would exhibit optically activity, i.e., one would be levorotatory and the other would be dextrorotatory.

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Cis-1,2-dimethylcyclohexane, compound XXI, has one methyl group in the equatorial position and one methyl group is the axial position:

Conversion of compound XXI to the other chair conformation would produce an isomer with identical specifications, i.e., one methyl group in the axial position and the other in the equatorial position.

cis-(1S,2R)-(e,a)-1,2-dimethylcyclohexane cis-(1S,2R)-(a,e)-dimethylcyclohexane

XXI

The mirror image of the (1S,2R) conformation is nonsuperimposable on its enantiomer and the mirror image of the (1S,2R) conformation is nonsuperimposable on its enantiomer.

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The mirror image of cis-(1S,2R)-(e,a)-1,2-dimethylcyclohexane, cis- (1R, 2S)-(e,a)-1,2-dimethylcyclohexane, can flip into the cis-(1S, 2R)-(e,a)-1,2-dimethylcyclohexane chair conformation; therefore, the molecule would be nonresolvable and optically inactive. Likewise, the mirror image of cis-(1R,2S)-1,2-dimethylcyclohexane, cis-(1S.2R)-(e,a)-1,2-dimethylcyclohexane, can flip into the cis-(1R,2S)-1,2-dimethylcyclohexane; therefore, the molecule would be nonesolvable and optically inactive. In summary, unlike the (e,e) or (a,a)-1,2-dimethylcyclohexanes, the e,a-1,2-dimethylcyclohexane molecules are not resolvable.

Three dimensional images of the cis isomers of 1,2-dimethylcyclohexane and their mirror images can be represented by the Pictures 3 and 4. The isomers interconvert from one chair conformation to the other chair conformation; therefore, the interconversion allows the isomers to be superimposable and nonresolvable even though they do not have planes of symmetry.

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Picture 3

Picture 4

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1,2-Dimethylcyclopentane

1,2-Dimethylcyclopentane is a simpler model than 1,2-dimethylcyclohexane. Trans-1,2-dimethylcyclopentane, compound XXII, and cis-1,2-bimethylcyclopentane, compound XXIII, can be treated as if they are flat.

A and A’ are nonsuperimposable; therefore, they are resolvable enantiomers. Consequently, pure A and A’ are optically active.

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B and B’ are superimposable; therefore they are nonresolvable. Also, B or B’ has a plane of symmetry. B and B’ are identical,and; therefore, are optically inactive. A and A’ are enantiomers; A and B are diastereoisomers; and A’ and B are diastereoisomers.

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Problems

Cyclic Aliphatic Hydrocarbons

1. Cyclopropane has a much greater reactivity than propane.

Explain this observation.

2. What characteristic of cyclohexane contributes to its large

stability?

3. Small ring cyclic aliphatic hydrocarbons can be prepared from the

intermolecular cyclization of certain dihaloalkanes. This synthetic

procedure does not work for large ring cyclic aliphatic

hydrocarbons. Provide a rationale for this observation.

1. Suggest possible syntheses for the following compounds:

5. What are three ways that can be used to generate carbenes?

(a) (b) (c)

H3CCH3

CH3

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6. Show the mechanisms for the following reactions:

(a) Carbene in the singlet state with 1,2-dimethylcyclopentene.

(b) Carbene in the triplet state with 1,2-dimethylcyclopentene.

7. Write syntheses for the following using the addition of carbenes to

alkenes:

8. Complete the following reactions:

(a)

(b)

(a) (b)(c)

CH3 CH3

H H

CH3

CH3H

H

Cl

ClCH3

CH3

CH3

H

+ HBr

CH3

H

O3(1)

(2) H+

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(c)

(d)

(9) The heat of combustion per methylene unit in alkanes and large

cyclic rings is approximately 659.0 kJ/mol. The heat of

combustion per methylene unit for cyclopropane is about 697.1

kJ/mol. Please provide a rational for this experimental

observation.

10. Draw all possible chair conformations for the following:

(a) cis and trans 1,2-dimethylcyclohexane

(b) cis and trans 1,3-dimethylcyclohexane

(c) cis and trans 1,4-dimethylcyclohexane

11. Which of the conformational isomers in problem 10 are chiral?

+ Br2

dil KMnO4

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12. Referencing problem 10:

(a) Which structure is the most stable of 10(a)?

(b) Which structure is the most stable of 10(b)?

(c) Which structure is the most stable of 10(c)?