Page 105 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

84 the same component in the anti and eclipsed conformations, so other factors must
contribute to reducing the total energy difference.
CHAPTER 1
Another molecule that has received attention is fluoromethylamine. From MP2/6-
Chemical Bonding 31G ∗∗ calculations the anti arrangement of the nitrogen unshared pair and the C–F
and Molecular Structure
bond is found to be the most stable conformation by 7.5 kcal/mol. 114 Some of the
structural effects expected for hyperconjugation, such as lengthening of the C–F bond
are seen, but are not dramatic. 115

F F
H H H
N H N
H H H H
anti perpendicular

Experimental structural data (electron diffraction) is available for N-fluoromethyl-
N,N-dimethylamine. 116 The only conformation observed is the anti arrangement of the
unshared pair and C–F bond. MP2/6-311G(2d,p) calculations suggest that the gauche
alignment is about 5 kcal/mol less stable.

F F
CH 3 CH 3 CH 3
N N
H H H H
CH 3
anti gauche

Diaminomethane would also be expected to be stabilized by an anomeric effect.
Overall, there is a rather small preference for the gauche, gauche conformation, but
NPA analysis suggests that there is an n → component of 5–6 kcal/mol that is offset
∗
by other factors. 117

H N H

H N H

preferred gauche
conformation
for diaminomethane

We should conclude by emphasizing that the stabilization of these compounds
does not mean that they are unreactive. In fact, -halo ethers and -halo amines
are highly reactive toward solvolysis. The hyperconjugation that is manifested in net
stabilization weakens the carbon-halogen bond and the molecules dissociate readily
in polar solvents. Methoxymethyl chloride is at least 10 14 times as reactive a methyl

114
J. J. Irwin, T. K. Ha, and J. Dunitz, Helv. Chim. Acta, 73, 1805 (1990).
115 K. B. Wiberg and P. R. Rablen, J. Am. Chem. Soc., 115, 614 (1993).
116 D. Christen, H. G. Mack, S. Rudiger, and H. Oberhammer, J. Am. Chem. Soc., 118, 3720 (1996).
117
L. Carballeira and I. Perez-Juste, J. Comput. Chem. 22, 135 (2001).