conformational isomers
dihedral angles - definition by four atoms
ethane conformations - eclipsed, staggered
butane conformations - gauche, anti
general conformations:
syn-periplanar (0°)
syn-clinal (60°)
anti-clinal (120°)
anti-periplanar (180°)
Figure 3.3 - a complete 360° range of descriptors
reference substituents for the dihedral angle:
the unique or highest priority group on each carbon
planar conformations: s-cis and s-trans (for dienes)
conformational analysis
ethane - torsional strain ( ~3 kcal barrier to rotation - eclipsed vs staggered))
butane - van der Waals strain (~0.8 kcal - anti vs gauche)
small rings - angle strain (cyclopropane ~ 27.6 kcal, cyclobutane ~ 26.2
kcal)
cyclohexane - axial vs equatorial
A value - equatorial preference in kcal (e.g., for methyl 1.7 kcal)
Table 3.2 - explain methyl, ethyl, isopropyl, t-butyl
calculation of equilibria: G = H - TS = - RT lnK
molecular mechanics
calculation of steric energies due to mechanical deformations
methods: MM2, MM3, AMBER, CHARMM, others
E(steric) = E(r) + E(q) + E(ø) + E(d)
total strain: bond stretch, bond angle, torsion (dihedral angle), van
der Waals
plus some combinations of stretch & bend
typical MM2 calculation:
input atom locations
calculate strain energies
minimize total strain by systematic variation of atom locations
repeat until improvement is minimal (below some threshold)
e.g., for gauche butane, total steric energy is 3.0348 kcal
output is structure parameters and steric energy contributions
local vs global minima - e.g., for anti butane, steric energy is 2.1714 kcal
for cyclohexane, steric energy is ~ 6 kcal (2 for gauche, 4 for van der Waals 1,4)
special conformational effects
gauche effect: 1,2-difluoroethane prefers gauche rather than anti
anomeric effect: in a 6-membered ring with heteroatom, an electronegative
group prefers to be anti to a lone pair (i.e., axial)
caused by stereoelectronic effect of the MOs (or curved bonds)
strained compounds
small rings, polycyclics
Bredt's rule - no double bonds at bridgeheads
arynes