Electron distribution in molecules
Major concepts
 The structure of a molecule determines its electron distribution.
 A more accurate electronic structure of a molecule can be determined with knowledge of
induction, hybridization, resonance, and delocalization.
 We can predict the relative stability of electrons in a molecule by weighing a series of principles.
 Based on the stability of electrons in a molecule, we can make educated predictions about the
molecule’s properties and reactivity.
Vocabulary
 Electron distribution
Students should be able to:
 Given a Lewis structure, determine its electron distribution in terms of partial charges and
localized/delocalized electrons
 Apply principles to determine the relative stability of electrons within the same molecule
 Given a structure, predict which atoms will be sites of reactivity
Daily Problems
1. What concepts do you need to apply to a Lewis structure to get a better picture of its real electronic
structure?
Resonance, hybridization, delocalization, induction
2. List stability principles used to determine relative stability of electrons.
1. identity of atom (more electronegative atom stabilizes lone pairs better)
2. resonance- if lone pair is delocalized, it is relatively more stable
3. induction- if electron density is pulled from an electron rich site, it is stabilized. If it is pulled
from an electron poor site, it is destabilized.
4. hybridization- sp orbitals are closer to the nucleus and stabilize lone pairs better.
3. Label the indicated electrons as “localized” (L) or “delocalized” (D). Indicate what type of orbital or
hybridized orbital they are in.
D (p)
D (p)
Pi bond is in
conjugation
L (sp2)
D (p)
L (sp3)
L (sp)
L (p)
This is an
isolated pi bond
Cumulative problems Most problems in this section are cumulative. All that we have learned up to
this point is for the purpose of getting a relatively accurate electron distribution picture of a molecule.
4. Which atom(s) hold the most reactive lone pair in these molecules? Explain your process. Which
principle(s) did you use?
NH ₂
NH ₂
N
N
difference in hybridization
sp³ is less stable and more reactive
O
localized
O
H
N
OH
O
charged lone pairs are more reactive
based on resonance the lone pairs are
delocalized over two oxygens
nitrogen is the less electronegative
atom, so the lone pair is less stabilized
Due to resonance, all three
of these atoms "hold" the lone
pair
5. Label these atoms as “Lewis acid”(LA), “Lewis base”(LB), or “Neither”(N). Be sure to consider the real
structure.
LA
LB
LB
LB
LB
LA
LA
LA
N
LB
N
LB
LA
LB
N
N
LA
N
LA
6. Which of these compounds is a more reactive Lewis acid?
b
a
The most reactive is a. Both have partial positive
charges on the carbonyl, but the amide stabilizes
this charge
The more reactive is c. Compound c has a
resonance stabilized (less reactive) carbocation.
d
c
The more reactive is f because it has a greater
partial positive charge due to induction.
e
f
7. Which ion is more stable? Which is less reactive?
The first compound is
more stable and less
reactive than the second.
Both are charged and
delocalized, but the
difference is the identity
of the atom. Oxygen
(electronegative) is a
better stabilizer.
O
The first compound is
more stable and less
reactive due to resonance
stabilization. Likewise,
the second compound is
destabilized by resonance
that puts a positive
charge next to a partial
positive charge.
Extension problems We will take a break from extension problems—Go study for exam 1!