Page 19 - Mechanism and Theory in Organic Chemistry

3. Wheretw~~cu-es with f~maL&@avt= tWe number of bmds
e
and appra-xhubely thesmluhrg~-s_e~aration? the structure with chag~~&
more electronegative atom will usually besomewhat lower in energy, but the_
difference will ordinarilv be small enough that both structures ___- be-
must
in -- the - composit~pi~ttllre. Thus in 17a t, 17b, 17a should be more stable, but the

C f-+ ,c,.;
H' \\C-H H C-H

chemistry of the ion can be understood only if it is described by the superposition
of both structures.
4. A a pair of atoms ; j o n d in
-
any structure must lie in thexme plane. For example, the structure 18b cannot

contribute, because the bridged ring prevents carbons 6 and 7 from lying in the
same plane as carbon 3 and the hydrogen on carbon 2. The imp~sibilitydstm-
aith do~_b_lehnn~~alL.hrldg~is~Br&s~
&.l Double bonds can occur at a bridgehead if the rings are sufficiently large.
Molecular Geometry
Lewis structures provide a simple method of estimating molecular shapes. The
geometry about any atom covalently bonded to two or more other atoms is found
by counting the number of electron groups around the atom. Each unshaared pair
counts as one group, and each bond, whethe~s~n~l~r multiple4 counts a$ one
group. The number of electron aroupamnd a ~ ~ q u to the a l
sum of the number of electron pairs on the atomand-the number of other atoms
bonded to it. The Peometry islinear if the number of electron goups is two, tri-
gonal if the number is thre~md-r.
The rule is based on the electron-pair repulsion model, which postulates that-
m e electron pairs repel each other, thev will try to stay as far apart as possible.
In trigonal and tetrahedral geometries, the shape will be exactly trigonal (120"
bond angles), or exactly tetrahedral (109.5" bond angles) if the electron groups
are all equivalent, as for example in BH, or CH, + (trigonal), or in CH, or NH, +
(tetrahedral). -

(a) F. S. Fawcett, Chem. Rev., 47,219 (1950); (b) J. R. Wiseman and W. A, Pletcher, J. Amer. Chem.
Sac., 92, 956 (1970); (c) C. B. Quinn and J. R. Wiseman, J. Amer. Chem. Sac., 95, 6120 (1973);
(d) C. B. Quinn, J. R. Wiseman, and J. C. Calabrese, J. Amr. Chem. Soc., 95, 6121 (1973).