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4.E: Organic Compounds: Cycloalkanes and their Stereochemistry (Exercises)

  • Page ID
    61691
  • These are the homework exercises to accompany the Textmap for McMurry's Organic Chemistry textbook.

    4.1 Naming Cycloalkanes

    4.1 Exercises

    Questions

    Q4.1.1

    Name the following compounds:

    Q4.1.2

    Draw the following structures

    1 = 1,2-dimethylcyclohexane

    2 = 2-cyclopropyl butane

    3 = 1,2,3-trimethyl cyclopentane

    Solutions

    S4.1.1

    1 = 1-cyclopropyl cyclopropane

    2 = 1-isopropyl cyclohexane

    3 = 2-propenyl cyclopentane

    4.2 Cis-Trans Isomerism in Cycloalkanes

    4.2 Exercises

    Questions

    Q4.2.1

    Draw the following molecules:

    trans-1,3-dimethylcyclohexane

    trans-1,2-dibromocyclopentane

    cis-1,3-dichlorocyclobutane

    Solutions

    S4.2.1

    4.3 Stability of Cycloalkanes: Ring Strain

    4.3 Exercises

    Questions

    Q4.3.1

    trans-1,2-Dimethylcyclobutane is more stable than cis-1,2-dimethylcyclobutane. Explain this observation.

    Solutions

    S4.3.1

    The trans form does not have eclipsing methyl groups, therefore lowering the energy within the molecule. It does however have hydrogen-methyl interactions, but are not as high in energy than methyl-methyl interactions.

    4.4 Conformations of Cycloalkanes

    4.4 Exercises

    Questions

    Q4.4.1

    If cyclobutane were to be planar how many H-H eclipsing interactions would there be, and assuming 4 kJ/mol per H-H eclipsing interaction what is the strain on this “planar” molecule?

    Q4.4.2

    In the two conformations of trans-cyclopentane one is more stable than the other. Explain why this is.

    Solutions

    S4.4.1

    There are 8 eclipsing interactions (two per C-C bond). The extra strain on this molecule would be 32 kJ/mol (4 kJ/mol x 8).

    S4.4.2

    The first conformation is more stable. Even though the methyl groups are trans in both models, in the second structure they are eclipsing one another, therefore increasing the strain within the molecule compared to the first structure where the larger methyl groups are anti to one another.

    4.5 Conformations of Cyclohexane

    4.5 Exercises

    Questions

    Q4.5.1

    Consider the conformations of cyclohexane, chair, boat, twist boat. Order them in increasing strain in the molecule.

    Solutions

    S4.5.1

    Chair < Twist Boat < Boat (most strain)

    4.6 Axial and Equatiorial Bonds in Cyclohexane

    4.6 Exercises

    Questions

    Q4.6.1

    Draw two conformations of cyclohexyl amine (C6H11NH2). Indicate axial and equatorial positions.

    Q4.6.2

    Draw the two isomers of 1,4-dihydroxylcyclohexane, identify which are equatorial and axial.

    Q4.6.3

    In the following molecule, label which are equatorial and which are axial, then draw the chair flip (showing labels 1,2,3).

    Solutions

    S4.6.1

    S4.6.2

    S4.6.3

    Original conformation: 1 = axial, 2 = equatorial, 3 = axial

    Flipped chair now looks like this.

    4.7 Conformations of Monosubstituted Cyclohexanes

    4.7 Exercises

    Questions

    Q4.7.1

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

    Solutions

    S4.7.1

    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.

    4.8 Conformations of Disubstituted Cyclohexanes

    4.8 Exercises

    Questions

    Q4.8.1

    For the following molecules draw the most stable chair conformation and explain why you chose this as an answer

    1 = trans-1,2-dimethylcyclohexane

    2 = cis-1,3-dimethylcyclohexane

    Solutions

    S4.8.1

    1 – The most stable conformation would be to have the methyl groups equatorial reducing steric interaction

    2 – The most stable conformation would be to have the groups equatorial this would reduce the strain if they were axial

    4.8.png

     

    4.9 Conformations of Polycyclic Molecules

    4.9 Exercises

    Questions

    Q4.9.1

    Someone stated that trans-decalin is more stable than cis-decalin. Explain why this is incorrect.

    Solutions

    S4.9.1

    Cis-decalin has fewer steric interactions than trans-decalin.