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Chapter 8 279
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b.
+
This is the only one that is aromatic; it is cyclic, it is planar, every ring atom has a
p
orbital,
and it has three pairs of
p
electrons.
The first compound is not aromatic, because one of the atoms is
sp
3
hybridized and, there-
fore, does not have a
p
orbital.
The third compound is not aromatic, because it has four pairs of
p
electrons.
51.
Solved in the text.
52.
a.
Cyclopentadiene has a lower p
K
a
value. That is, it is a stronger acid. When cyclopentadiene loses a
proton, a relatively stable aromatic compound is formed. When cycloheptatriene loses a proton, an
unstable antiaromatic compound is formed (Section 8.19). Recall that the more stable the base, the
stronger its conjugate acid.
H H
H
H
H H
−
aromatic
antiaromatic
+
H
+
+
H
+
−
b.
Cyclopropane has a lower p
K
a
value because a very unstable antiaromatic compound is formed when
cyclopropene loses a proton.
H H
H
−
+
H
+
antiaromatic
H H
H
−
+
H
+
53.
3-Bromocyclopropene is more soluble in water because it is more apt to ionize since an aromatic com-
pound is formed when its carbon–bromine bond breaks.
+
+
Br
−
aromatic
+
+
Br
−
Br
Br