Advices For Studying Organic Chemistry

Table 4-1 Energy costs for interactions in alkane conformers

INTERACTION CAUSE (^) (kcalE/molNERGY COST) (kJ/mol^ )
H–H eclipsed Torsional strain 1.0 4
H–CH 3 eclipsed Mostly torsional strain 1.4 6
CH 3 –CH 3 eclipsed Torsional plus steric strain 2.5 11
CH 3 –CH 3 gauche Steric strain 0.9 4
Sir Derek H. R. Barton (1918-1998, formerly Distinguished Professor of Chemistry
at Texas A&M University) and Odd Hassell (1897-1981, formerly Chair of Physical
Chemistry of Oslo University) shared the Nobel prize in 1969 “for developing and
applying the principles of conformation in chemistry.” Their work led to
fundamental understanding of not only the conformations of cyclohexane rings, but
also the structures of steroids (Section 23.4) and other compounds containing
cyclohexane rings.

4.12A CONFORMATION OF HIGHER CYCLOALKANES

1. Cycloheptane, cyclooctane, and cyclononane and other higher cycloalkanes exist
   in nonplanar conformations.


2. Torsional strain and van der Waals repulsions between hydrogen atoms across
   rings (transannular strain) cause the small instabilities of these higher
   cycloalkanes.


3. The most stable conformation of cyclodecane has a C−C−C bond angles of 117°.
   
   
   1. It has some angle strain.
   
   
   2. It allows the molecules to expand and thereby minimize unfavorable repulsions
      between hydrogen atoms across the ring.