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4.7: Conformations of Monosubstituted Cyclohexanes

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    67083
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    Objectives

    After completing this section, you should be able to

    1. account for the greater stability of the equatorial conformers of monosubstituted cyclohexanes compared to their axial counterparts, using the concept of 1,3‑diaxial interaction.
    2. compare the gauche interactions in butane with the 1,3‑diaxial interactions in the axial conformer of methylcyclohexane.
    3. arrange a given list of substituents in increasing or decreasing order of 1,3‑diaxial interactions.
    Key Terms

    Make certain that you can define, and use in context, the key term below.

    • 1,3‑diaxial interaction
    Study Notes

    1,3-Diaxial interactions are steric interactions between an axial substituent located on carbon atom 1 of a cyclohexane ring and the hydrogen atoms (or other substituents) located on carbon atoms 3 and 5.

    Be prepared to draw Newman-type projections for cyclohexane derivatives as the one shown for methylcyclohexane. Note that this is similar to the Newman projections from chapter 3 such as n-butane.

    Newman projections of methylcyclohexane and n-butane

    Newman projections of methylcyclohexane and n‑butane

    Because axial bonds are parallel to each other, substituents larger than hydrogen generally suffer greater steric crowding when they are oriented axial rather than equatorial. Consequently, substituted cyclohexanes will preferentially adopt conformations in which the larger substituents assume equatorial orientation.

    When the methyl group in the structure above occupies an axial position it suffers steric crowding by the two axial hydrogens located on the same side of the ring.

    The conformation in which the methyl group is equatorial is more stable, and thus the equilibrium lies in this direction.

    The relative steric hindrance experienced by different substituent groups oriented in an axial versus equatorial location on cyclohexane may be determined by the conformational equilibrium of the compound. The corresponding equilibrium constant is related to the energy difference between the conformers, and collecting such data allows us to evaluate the relative tendency of substituents to exist in an equatorial or axial location.A table of these free energy values (sometimes referred to as A values) may be examined by clicking here.

    Looking at the energy values in this table, it is clear that the apparent "size" of a substituent (in terms of its preference for equatorial over axial orientation) is influenced by its width and bond length to cyclohexane, as evidenced by the fact that an axial vinyl group is less hindered than ethyl, and iodine slightly less than chlorine.

    We noted earlier that cycloalkanes having two or more substituents on different ring carbon atoms exist as a pair (sometimes more) of configurational stereoisomers. Now we must examine the way in which favorable ring conformations influence the properties of the configurational isomers. Remember, configurational stereoisomers are stable and do not easily interconvert, whereas, conformational isomers normally interconvert rapidly. In examining possible structures for substituted cyclohexanes, it is useful to follow two principles:

    1. Chair conformations are generally more stable than other possibilities.
    2. Substituents on chair conformers prefer to occupy equatorial positions due to the increased steric hindrance of axial locations.

    Strain values for other cyclohexane substituents can also be considered. The relative steric hindrance experienced by different substituent groups oriented in an axial versus equatorial location on cyclohexane determined the amount of strain generated. The strain generated can be used to evaluate the relative tendency of substituents to exist in an equatorial or axial location. Looking at the energy values in this table, it is clear that as the size of the substituent increases, the 1,3-diaxial energy tends to increase, also. Note that it is the size and not the molecular weight of the group that is important. Table 4.7.1 summarizes some of these strain values values.

    Table 4.7.1: A Selection of ΔG° Values for the Change from Axial to Equatorial Orientation of Substituents for Monosubstituted Cyclohexanes
    Substituent -ΔG° (kJ/mol) Substituent -ΔG° (kJ/mol)
    \(\ce{CH_3\bond{-}}\) 7.4 \(\ce{HO\bond{-}}\) 3.9
    \(\ce{CH_2H_5\bond{-}}\) 7.6 \(\ce{N#C\bond{-}}\) 0.8
    \(\ce{(CH_3)_2CH\bond{-}}\) 9.2 \(\ce{CH_3O\bond{-}}\) 2.1
    \(\ce{(CH_3)_3C\bond{-}}\) 20.0 \(\ce{HO_2C\bond{-}}\) 5.9
    \(\ce{F\bond{-}}\) 1.4 \(\ce{H_2C=CH\bond{-}}\) 2.0
    \(\ce{Cl\bond{-}}\) 2.4 \(\ce{C_6H_5\bond{-}}\) 12.0
    \(\ce{Br\bond{-}}\) 2.4    
    \(\ce{I\bond{-}}\) 2.4    

     

    Exercises

    Exercise \(\PageIndex{1}\)

    In the molecule, cyclohexyl ethyne there is little steric strain, why?

    Bond line drawing of cyclohexyl ethyne.

    Answer

    The ethyne group is linear and therefore does not affect the hydrogens in the 1,3 positions to say to the extent as a bulkier or a bent group (e.g. ethene group) would. This leads to less of a strain on the molecule.

    4.7.png

    Exercise \(\PageIndex{2}\)

    Calculate the energy difference between the axial and equatorial conformations of bromocyclohexane?

    Answer

    Bromocyclohexane will have two 1,3 diaxial interactions. The table above states that the total strain between axial and equatorial bromocyclohexane will be 2.4 kJ/mol.

    Exercise \(\PageIndex{3}\)

    Using your answer from the previous question estimate the percentages of axial and equatorial conformations of bromocyclohexane at 25o C.

    Two chair conformations one where the bromo group is axial and another where the bromo group is equatorial.

    Answer

    Remembering that the axial conformation is higher in energy, the energy difference between the two conformations is ΔE = (E equatorial - E axial) = (0 - 2.4 kJ/mol) = -2.4 kJ/mol. After converting oC to Kelvin and kJ/mol to J/mol we can use the equation ΔE = -RT lnK to find that -ΔE/RT = lnK or (2.4 x 103 J/mol) / (8.313 kJ/mol K • 298 K) = lnK. From this we calculate that K = 2.6. Because the ring flip reaction is an equilibrium we can say that K = [Equatorial] / [Axial]. If assumption is made that [Equatorial] = X then [Axial] must be 1-X. Plugging these values into the equilibrium expression produces K = [X] / [1-X]. After plugging in the calculated value for K, X can be solved algebraically. 2.6 = [X] / [1-X] → 2.6 - 2.6X = X → 2.6 = 3.6X → 2.6/3.6 = X = 0.72. This means that bromocyclohexane is in the equatorial position 72% of the time and in the axial position 28% of the time.

    Exercise \(\PageIndex{4}\)

    There very little in 1,3-diaxial strain when going from a methyl substituent (3.8 kJ/mol) to an ethyl substituent (4.0 kJ/mol), why? It may help to use molecular model to answer this question.

    Answer

    The fact that C-C sigma bonds can freely rotate allows the ethyl subsistent to obtain a conformation which places the bulky CH3 group away from the cyclohexane ring. This forces the ethyl substituent to have only have 1,3- diaxial interactions between hydrogens, which only provides a slight difference to a methyl group.

    Chair conformations of methylcyclohexane and ethylcyclohexane.

     

    Contributors and Attributions

    >Robert Bruner (http://bbruner.org)


    4.7: Conformations of Monosubstituted Cyclohexanes is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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