Knights content lecture 6-10

strain is the distortion of bond lengths and angles from their ideal values

the strain present in the structure

Etot = Er + Eθ + EΦ + Es

strain due to bond stretching/compression. This strain produces only very small structural changes because even small distortions in bond lengths result in a large increase in energy

angle strain due to bond bending. A distortion of +/- 10 degrees from the tetrahedrally angle of 109 degrees causes an energy increase of about 4 KJmol-1

torsional strain (resistance to bond twisting). This occurs whenever two groups are force to eclipse one another

steric strain, this is caused whenever two atoms are forced closer together than the sum of their van der walls radii

can be determined by measuring its heat of combustion. The lowest energy system, cyclohexane is arbitrarily assigned a strain energy of 0 and all other cycloalkenes are measured relative to this

3&4 membered rings are referred to as small rings and suffer high angle strain (bond angles < 109 degrees) and torsional strain (eclipsing)

5-7 membered rings are referred to as normal rings and are relatively untrained since the bond angles are equal to 109 degrees. 5 and 7 membered rings suffer torsional eclipsing strain

8-13 membered rings are referred to as medium rings and suffer torsional strain and steric strain (transangular interactions worst for 9 and 10 membered rings)

14+ membered rings and larger are referred to as large, and are unstrained since a ring of this size is flexible enough to avoid angle, torsional or steric strain

the activation energy

ΔH≠ the enthalpy of activation - which will be strongly dependent on the total strain energy of the cyclic product. ΔS≠ the entropy of activation - which will be related to the likelihood of the two reacting atoms meeting each other

small rings are highly strained the enthalpy of activation is large and cyclisation is slow

formation of the 10 membered ring is particularly slow since this medium size rings suffers from both a large enthalpy of activation and a very large entropy of activation

Is flat and has a triangular shape with all carbon atoms in the same plane. The bond angles are 60 degrees therefor it has high angle strain and all of the CH2 groups eclipse on another so torsional strain is also large

Cyclobutane exists in a puckered form. Puckering actually reduces the bond angles to <90 degrees which increases angle strain. Puckering reduces the eclipsing interactions reducing the torsional strain

4 carbon atoms are coplanar, lowest energy conformation of cyclopentane

planar structure is too high in energy. Angle strain ~ 0 but has a very high torsional eclipsing strain, to reduce the eclipsing interactions cyclopentant can be drawn as an envelope or a half chair

three carbons in the same plane and the remaining two, alternately above and below the plane. No torsional strain

they are both fluxional i.e the non-coplanar and coplanar atoms are constantly interchanging

a planar 6 membered ring would have bond angles of 120 degrees so would have large angle and torsional strain due to eclipsing interactions

angle strain ~ 0 all c-c-c bond angles 109 degrees. Torsional strain ~ 0 all adjacent CH2 groups are staggered (synclinal) to one another Steric strain ~ 0 there are no close interactions