Page 165 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

two methyl-hydrogen eclipsing interactions occur, as in 2-methylpropane, the barrier 145
is raised to 3.9 kcal/mol. The increase in going to 2,2-dimethylpropane, in which the
barrier is 4.7 kcal/mol, is 1.8 kcal/mol for the total of three methyl-hydrogen eclipsing SECTION 2.2
interactions. For 2,2,3,3-tetramethylbutane, in which there are three methyl-methyl Conformation
interactions, the barrier is 8.4 kcal/mol. Rotational barriers in kcal/mol are shown
below.

H CH 3 CH 3 CH 3 CH 3
H H H H H H H H CH 3 CH 3

H H H H H CH 3 CH CH 3 CH 3 CH 3
H H H 3 H CH 3
2.8 3.4 3.9 4.7 8.4

The magnitudes of the barriers to rotation of many small organic molecules have been
measured. 18 The experimental techniques used to study rotational processes include
microwave spectroscopy, electron diffraction, ultrasonic absorption, and infrared
spectroscopy. 19 Some representative barriers are listed in Table 2.1. As with ethane,
the barriers in methylamine and methanol appear to be dominated by hyperconjugative
stabilization of the anti conformation. The barrier decreases (2 9 → 2 0 → 1 1) in
proportion to the number of anti H–H arrangements (3 → 2 → 1). (See Topic 1.1 for
further discussion.) 20

: H : : H H
O N
H H H H
H H

The conformation of simple alkenes can be considered by beginning with propene.
There are two families of conformations available to terminal alkenes: eclipsed and
bisected conformations, as shown below for propene. The eclipsed conformation
is preferred by about 2 kcal/mol and represents a barrier to rotation of the methyl
group. 21 22 A simple way to relate the propene rotational barrier to that of ethane is to
regard the bond as a “banana bond” (see p. 7). The bisected conformation of propene
is then seen to correspond to the eclipsed conformation of ethane, while the more
stable eclipsed conformation corresponds to the staggered conformation of ethane. 23

18
For reviews, see (a) J. P. Lowe, Prog. Phys. Org. Chem., 6, 1 (1968); (b) J. E. Andersen, in The
Chemistry of Alkenes and Cycloalkens, S. Patai and Z. Rappoport, eds., Wiley, Chichester, 1992,
Chap. 3II. D.
19 Methods for determination of rotational barriers are discussed in Ref. 18a and by E. Wyn-Jones and
R. A. Pethrick, Top. Stereochem., 5, 205 (1969).
20 J. K. Badenhoop and F. Weinhold, Int. J. Quantum Chem., 72, 269 (1999); V. Pophristic and
L. Goodman, J. Phys. Chem. A., 106, 1642 (2002).
21
J. R. Durig, G. A. Guirgis, and S. Bell, J. Phys. Chem., 93, 3487 (1989).
22 Detailed analysis of the rotation shows that it is coupled with vibrational processes. L. Goodman,
T. Kundu, and J. Leszczynski, J. Phys. Chem., 100, 2770 (1996).
23
K.-T. Lu, F. Weinhold, and J. C. Weisshaar, J. Chem. Phys., 102, 6787 (1995).