Chemistry Reference
In-Depth Information
4. Which cation from Problem 3 is more stable,(g) or (h)? Explain using partial charges.
Of the 16 resonance forms of the triphenylmethyl cation shown in the solution for
Problem 3(g), no two resonance forms place the positive charge on adjacent atoms.
However, when looking at the 16 resonance forms of the phenylfluorenyl cation
shown in the solution for Problem 3(h), there are multiple pairs of resonance forms
(one of which is shown below) where the positive charge may be placed on adjacent
atoms. This is a disfavored electronic relationship and is destabilizing to the cation
itself. Thus, through charge distribution and delocalization, because the phenylfluorenyl
cation possesses partial positive charges on two adjacent atoms, the triphenylmethyl
cation [Problem 3(g)] is more stable.
Note: There is another explanation relating to the definitions of aromatic and
antiaromatic ring systems. See if you can explain the answer to this problem using
these definitions.
5. How will the following substituents affect the pK
a
of benzoic acid (raise, lower, or no
change)? Explain using partial charges to illustrate inductive effects. Remember,o
refers to ortho positions,mrefers to meta positions, and p refers to the para position.
In addressing these problems, assume that the acidity of the carboxylic acid is influ-
enced solely by the partial charges induced by additional ring substituents.
Note: It is important to realize that in addition to inductive effects, there are other
factors that influence acidity and pK
a
values. Therefore, while this problem asks for
expectations regarding how inductive effects influence pK
a
values, in actuality, the
measured values may be different than anticipated.
The pK
a
of benzoic acid is 4.19
a. o-NO
2
The structure of o-nitrobenzoic acid is shown below with partial charges assigned to
the ring system. Because the electron-withdrawing nitro group is located ortho to the
