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Counting
Pi
Electrons
and
Electrons
Involved
in
Aromaticity

How to find atoms with p orbitals:
When looking at a molecule and deciding how many lone pairs are in p orbitals, it is first
important to decide which atoms have p orbitals
...

These atoms have to have p orbitals because the p orbital overlap is what is involved in
the pi bond
...
If the atom can become
involved in conjugation the answer is usually yes
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These nitrogens are pointed out below:

How to decide which electrons are in p orbitals:
After you find all of the p orbitals, you need to decide which electrons are in the p
orbitals
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Next look at lone pairs
...
We will have to decide which lone pairs are in sp2 orbitals and which are in
p orbitals
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It has three sigma bonds so you know that all three sp2
orbitals are involved in sigma bonds
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The same logic can be applied to the lowest nitrogen
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You know that one p orbital
is used in the double bond and two sp2 orbitals are involved in the sigma bond
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Pi
bond
with
p

orbital

Sigma
bond
with
sp2

Lone
pair

must
go
in
sp2


Sigma
bond
with
sp2


The oxygen is involved in one pi bond and one sigma bonds
...
This leaves
two sp2 orbitals for the lone pairs to occupy
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First, count up all double bonds that are between two atoms in the closed loop
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Involved


Not
Involved

Involved


To count a double bond’s electrons, both atoms involved in the bond must be part of the
ring
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You cannot even say that the one electron from the carbon is involved
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This
includes the lone pair on the bottom nitrogen
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It does not include the lone
pair on the middle nitrogen because that lone pair is in an sp2 orbital
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Practice Problems
1
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What are the hybridizations of all the atoms in these molecules (except
for hydrogens)?
b
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How many pi electrons are in these molecules?
d
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All atoms are sp2
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b
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There is only an sp2 orbital
left for the lone pair
c
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Do not count the lone pair because it is in an sp2 orbital
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It is aromatic
Pyrimidine
a
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You can tell this because all are involved in a pi bond
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For both nitrogens, the lone pair is in the sp2 orbitals because the p orbital is
involved in a pi bond and two sp2 orbitals are involved in the sigma bonds
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c
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Do not count the lone pairs because they are in sp2
orbitals
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It is aromatic
Thymine
a
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The carbon in the methyl group is sp3
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b
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Nitrogen lone pairs are in p orbitals
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6 from double bonds
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d
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All atoms except for hydrogens are sp2
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b
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The
nitrogens without a double bond have their lone pair in a p orbital
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12: 8 from the double bonds and 4 from the nitrogens with lone pairs in a p
orbital
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Aromatic
Cytosine
Answers can be found in text because that was my example molecule
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All sp2 except hydrogen for same reasons as mentioned before
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The nitrogens with a double bond have their lone pairs in an sp2 orbital
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c
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aromatic
Revision April 30:
Remember to decide whether the question is asking for electrons involved in aromaticity
or pi electrons in the whole molecule
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H if you found this tutorial helpful
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Good luck on your exams!
Works Cited
Images of molecules from wikipedia and the Interdisciplinary Education Group
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