Strain of Cycloalkanes

Introduction

  • The structures and energies of cyclic alkanes are highly dependent on the size of their rings
  • Small ring strain: a strain associated with ring sizes below six that arises from nonoptimal bond angles (optimal angle is 109.5)
  • Cyclic alkanes of 4 carbons or more have rapidly interconverting conformations with varying degrees of torsional strain along their C-C single bonds   * The bonds can only rotate so far without breaking the ring (rotations limited to certain angles)

Cyclopropane

  • Observed bond angles of cyclopropane: 60 (much less than 109.5)   * This compression causes considerable angle strain
  • Since cyclopropane is planar, there are 6 pairs of C-H bonds that are fully eclipsed and introduce torsional strain
  • Because of their extreme degree of intramolecular strain, cyclopropane and its derivatives undergo several ring opening reactions not seen with larger cycloalkanes

Cyclobutane

  • Nonplanar or puckered conformations are favored in all cycloalkanes bigger than cyclopropane
  • If cyclobutane were planar, it's C-C-C bond angles would be 90 and there would be 8 pairs of eclipsed C-H bonds, which would maximize torsional strain
  • Rotations along the C-C bonds can slightly relieve strain puckering of the ring alters strain energy in 2 ways   * It decreases the torsional strain associated with eclipsed interactions   * It further increases the angle strain caused by the compression of C-C-C bond angles
  • Because the decrease in torsional strain is greater than the increase in angle strain, the puckered conformation is more stable than the planar conformation of cyclobutane
  • Not static but undergoes interconversions between puckered conformations

Cyclopentane

  • If cyclopentane were planar, all C-C-C bond angles would be 108
  • Little angle strain but there are 10 pairs of fully eclipsed C-H bonds, creating a lot of torsional strain
  • To relieve part of this torsional strain, the ring twists by rotations along the C-C bonds into the envelope conformation
  • Envelope conformation: 4 carbons are in the same plane and the fifth one is bent upward (like a flap on an envelope)
  • Exists as a dynamic equilibrium of five envelope conformations in which each carbon atoms alternates as the out of plane carbon
  • In the envelope conformation, the average C-C-C bond angle is reduced to 105   * Increases angle strain   * The number of C-H interactions is reduced which reduces torsional strain

Cyclohexane

  • Cyclohexane adopts a number of puckered conformations that interconvert via C-C bonds   * Most stable is chair conformation     * all C-C-C bond angles are 110.9 (minimizing angle strain) and all hydrogens on adjacent carbons are staggered with respect to one another (minimizing torsional strain)     * No 2 atoms are close enough to each other for nonbonded interaction strain to exist     * Very little strain     * C-H bonds are orients in 2 ways       * 6 bonds are axial bonds: a bond to a chair conformation of cyclohexane that extends from the ring parallel to the imaginary axis through the center of the ring; a bond that lies roughly perpendicular to the equator of the ring         * 3 axial bonds point straight up and the other 3 point straight down         * Axial bonds alternate first up and then down as you move from one carbon to the next       * 6 bonds are equatorial bonds: a bond to a chair conformation of cyclohexane that extends from the ring roughly perpendicular to the imaginary axis through the center of the ring; a bond that lies roughly along the equator of the ring         * Equatorial bonds alternate first slightly up and then slightly down as you move from one carbon to the next     * If the axial bond on a carbon points upward, the equatorial bond points slightly downward (vice versa)   * Boat conformation: a nonplanar conformation of a cyclohexane ring in which carbons 1 and 4 of the ring are bent toward each other     * Considerably less stable than a chair conformation because of the torsional strain associated with 4 pairs of eclipsed C-H interactions and the steric strain between the 2 flagpole hydrogens   * Twist-boat conformation: a nonplanar conformation of a cyclohexane ring that is twisted from and is slightly more stable than a boat conformation     * Twisting from a boat conformation to this one relieves some of the strain   * The 2 chair conformations can be interconverted by first twisting into a boat and then into an alternative chair     * When this occurs, there's a change in the relative orientations in space of the hydrogen atoms bonded to each carbon       * All hydrogens axial in one chair become equatorial in another (and vice versa)   * Diaxial interaction: the steric strain arising from interaction between an axial substituent and an axial hydrogen (or another group) on the same side of a chair conformation of a cyclohexane ring

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