ChemActivity
9
Part A: The Core Charge Effect
(Which atoms have lower potential energy electron pairs?)
Model 1: Core Charge Revisited
= electron
Shell #2
Together, the nucleus
and core electrons have
a net charge of +4
nucleus
Shell #1
+6
=
+4
Each valence electron
"feels" a charge of +4
carbon
Critical Thinking Questions
1. Draw a core charge representation (like the one above right) for nitrogen, oxygen
and fluorine.
Each valence electron
"feels" a charge of +7
Each valence electron
"feels" a charge of +4
+4
carbon
+5
nitrogen
carbon holds its electrons least
tightly of the four since it has the
smallest core charge holding onto
those electrons.
+6
+7
oxygen
fluorine
fluorine holds its electrons most
tightly of the four since it has the
highest core charge holding onto
those electrons.
2. Based on the core charge model, explain why fluorine holds its valence electrons
more tightly than oxygen (and O holds electrons more tightly than N, and N holds
electrons more tightly than C). see above
Model 2: Core Charge Representations of Ions
+1
+5
H
N
+1
+6
H
O
+1
+7
H
F
The valence electrons of a fluoride ion (F–) see a core charge of +7. This is used to
explain the fact that electrons on this ion are “happier” than electrons on the other ions
shown in Model 2.
(“happier” = less reactive, lower in potential energy, less likely to combine with H+.)
Critical Thinking Questions
3. Based on the information in Model 2, which do you expect to be “happier” an
electron on hydroxide (HO–) or an electron on H2N–? Explain your reasoning.
Based on our definition of “happy,” the electrons on hydroxide should be much more happy since they
are in proximity to a higher + core charge (+6 vs. +5 for NH2).
4. Rank the four bases below from strongest to weakest.
H
C
H
H
N
H
O
H
F
H
Highest P.E. (1)
(2)
(3)
Lowest P.E. (4)
They all have a negative charge. So the charge effect can't help us decide. The core charge of F is
higher than the core charge of C so an electron on a negatively charged F is held more tightly than an
electron on a negatively charged C. This means a lone pair on F– is lower in potential energy than a
lone pair on C–.
5. Are the pKa data below consistent with your answer to CTQ 4? Yes.
H
H
C
H
H
H
H
V.E.
N
H
H
O
H
H
CH4
pKa = ~50
H
F
N
H
H
H O
pKa = 35
F— H
pKa = 15.7
pKa = 3.2
Model 3A: Electronegativity
Previously, we discussed why a fluorine atom holds electrons more tightly than a
carbon atom–and that the tighter an electron pair is held, the lower its potential energy.
This capacity for certain atoms to lower the potential energy of electrons is so important
that chemists give it a name: electronegativity.
Table 3: Electronegativities of First Row Elements
C
N
O
F
2.5
3.1
3.5
4.1
• The higher the electronegativity number, the tighter that atom holds its electrons.
Model 3B: Electronegativity Effect vs. Formal Charge Effect
• Formal Charge Effect = lone pair electrons on an atom with formal charge = zero
are much happier than lone pair electrons on an atom with formal charge = –1.
• Electronegativity Effect (a.k.a the core charge effect) = an electron pair is happier
on an atom with higher core charge.
(most happy) F > O > N > C (least happy).
• In most cases the formal charge effect outweighs the atom identiy effect.
For example: a lone pair on NH3 is happier than a lone pair on HO—
The important exceptions to this rule are that NH3 is a stronger base (higher P.E. lone
pair) than any of the halide ions (F— , Cl— , Br— or I— ) and H2O is a stronger base than
than three of the halide ions (Cl— , Br— or I— ). See Model 3 in ChemActivity 8
Critical Thinking Questions
6. Which of the following TRUE statements illustrates that the charge effect is
stronger than the electronegativity effect?
H
F– higher in P.E. than H2O;
H2O higher in P.E. than Cl–;
H2N– higher in P.E. than NH3
Part B: The Inductive Effect
(Which is stronger the electronegativity effect or the inductive effect?.)
Model 4: Bond Polarization
• An electronegative atom such as F attracts electrons toward its high core charge. This
means it “hogs” more than its share of nearby electrons.
• This creates polarized bonds with more negative charge at one end (δ–) and less
negative charge at the other end (δ+).
• δ+ means “a partial charge between 0 and +1.” (The Greek letter δmeans “small
amount.”)
• A polarized bond is often represented by a dipole arrow with a + sign at the positive
end of the bond (see examples below).
δ
H
H
H
H
C
C
F
δ
δ
H
δ
H
H
O
δ
C
Example 2
H
Example 3
Critical Thinking Questions
7. Place a δ+ next to any atom in Model 4 that you expect will have a partial positive
charge, and a δ– next to any atom in Model 4 that you expect will have a partial
negative charge.
8. Why is the arrow in Example 2 smaller than the one in Example 1?
Since F has a larger core charge than O, it pulls the electrons from C more than O. Also, O is pulling
on electrons from two different bonds.
Model 5: Inductive Effect (= Long Range Electronegativity Effect)
The effects of an electronegative atom are felt on neighboring atoms, AND can be
felt more than one atom away. This is called an inductive effect. It is like the ripple
effect that is caused when you throw a brick into a still pond. The waves are intense near
the impact site, but get smaller farther away.
Figure 5a: Graphical Illustration of the Inductive Effect
H2
C
H3C
H
H
H
Example 1
non-polar
F
C
H2
Example 4
9. Draw the three possible conjugate bases of 1-fluoropropane (shown above).
Hint: they result from removal of an H+ from C1, C2, and C3, respectively.
See Figure 5b, next page.
Figure 5b: Energy Required for H+ Removal from 1-Fluoropropane
F
C
H
C
H
C
H
H
anion #2
F
H
H
H
+40 pKa
units
H
+36 pKa
units
+29 pKa
units
H
F
H
H
C
C
C
H
H
H
anion #3
F
H
H
C
C
C
H
H
H
H
anion #1
H C1 C2 C3 H
H
H
H
1-fluoropropane
Information
There are two good ways of determining which is the most acidic H on a molecule such
as 1-fluoropropane. (The second is generally more reliable.)
• Determine which H-C bond is most polarized toward C. That is, which H-C bond is
already partially broken the most.
• Draw all possible conjugate bases (in this case there are three-see above), and
determine which one is lowest in potential energy = happiest. The happiest one is
most likely to form.
Critical Thinking Questions
10. There are three different kinds of H’s on 1-fluoropropane. In the drawing below,
they are labeled a, b and c. Explain why all the H’s with a given letter are
chemically equivalent and therefore indistinguishable from each other.
Ha
F
Hb
Hc
C1
C2
C3
Ha
Hb
Hc
Hc
It looks like one of the Ha's is closer to F, but that's because the drawing does not show the 3D nature
of the molecule. If you were to build a model, you would see that the positions of all the H's labeled
with a given letter can be interchanged via single bond rotations that are happening at room
temperature.
a)
b)
Add polarization arrows to 1-fluoropropane so as to show the pull of
electron density by the electronegative fluorine atom.
Construct an explanation for why it is easiest (least energy required) to
remove an H+ from C1 of 1-fluoropropane.
It requires lots of potential energy to generate anion #3 because ripping an H off C3 leaves a very high
potential energy lone pair of electrons. This lone pair is so high in energy compared to the others
because it is hardly stabilized at all by the electronegative F atom. The F atom is so far away that its
effects are not felt very much. Compare this to ripping an H off C1. The resulting lone pair and excess
negative charge is very effectively stabilized by the next door neighbor F atom. This effect of stealing
(or helping out with) electron density on neighboring atoms is called an inductive effect.
c)
Circle the most acidic H (or H’s if more than one equivalent H) on 1fluoropropane.
d)
Which of the three anions shown in Figure 5b is the strongest base?
(Strongest base = will release the most energy when combined with an H+.)
Anion #3 is the strongest base since it would release the most energy when combined with an H+. Note
that anion #3 would actually be unstable because it would probably rip an H+ off of C1, thereby
changing itself into the lower energy Anion #1.
Model 6: Hierarchy of Electron Effects
Strongest = Formal charge effect (= neutral happier than negative)
Very Strong = Electronegativity effect (= e– pair on F happier than e– pair on O>N>C)
Weak = Inductive effect (= long range electronegativity effect)
Critical Thinking Questions
11. For each compound below, circle the most acidic H (or H’s if there is a tie).
H
HO
H2
C
F
C
H
H
C
H2C
C
H2
H
O
N
H
I
OH
H
OH
Cl
II
III
a) Explain how compound I demonstrates that the electronegativity effect is
stronger than the inductive effect?
The H directly attached to the oxygen is more acidic than the H attached to a C, even though the latter
is very near an F and subject to an inductive effect.
b) Explain how compound II demonstrates that the formal charge effect is
stronger than the electronegativity effect?
The H’s attached to the + charged nitrogen are more acidic than the H attached to O, even though O
is more electronegative than N.
c) Draw an isomer of compound III with the Cl in a position such that it has an
equivalent inductive effect on both of the two OH groups. (This compound
will have two equivalent most acidic H’s.)
There are two possibilities.
HO
HO
Cl
OH
OH
Cl
Exercises for Part A
1. Choose which species is more likely to act as the base and use the arrow formalism
to illustrate the most likely acid-base reaction. Also draw the resulting products.
H
N
H
O
H
a)
b)
c)
d)
H
H
Which species is more likely to act as a base? Explain your choice.
Predict whether the reaction is up hill or down hill. Explain your reasoning.
What species would you look for on the table if you were looking for a pKa
value that would tell you the potential energy of the lone pair of electrons on
NH3?
Draw an energy diagram for the reaction, including an estimated value for
∆H in pKa units.
Exercises for Part B
2. For each molecule below, draw in the most acidic hydrogen/s. If there is a tie,
between 2 or more equivalent hydrogens— draw in all these H’s.) Then circle the
molecule in each row that is more acidic. The first one is done for you
most acidic H's (2 equiv. ones)
example:
H
most acidic H
H
H
more acidic
molecule
F
F
F
O
O
a)
F
F
O
b)
F
F
F
c)
F
3. Rank the nitrogen bases below according to how much energy will be released by
an acid-base reaction with H3O+. (1 = most energy released...3 = least energy
released.) Assume no energy is released or consumed when breaking the O–H bond
on H3O+.
H
F
H
O
H
NH2
F
H
F
H
O
H
NH2
H
NH2
H
F
a)
b)
c)
O
H
Use the arrow formalism to illustrate each reaction.
Draw the products of the reaction.
Draw the energy diagrams for the three reactions. Do not attempt to
calculate exact ∆Hrxn, but your reaction diagrams should indicate which
reaction is most (and least) downhill in terms of energy.
4. Circle the most acidic H on each structure below, then rank them from strongest
acid (1) to weakest acid (3).
H
F
H
C
C
H
H
H
O
H
H
H
C
C
H
H
H
O
H
H
F
C
C
H
H
O
H
5. It is easier to remove an H+ from A than B (see below). In general it is difficult to
remove a 2nd H+ to make a –2 anion. One explanation is that the anionic oxygen on
B actually donates electron density, as shown by the squiggly arrows. Table 3 says
that O is more electronegative than C. Explain why this oxygen on B donates
electron density instead of withdrawing it from the carbon.
CH3
O
H
C
CH3
O
O
H
H
H
A
C
O
H
B
Hint: Consider a very absorbent sponge. The ability of this sponge to attract and absorb
more water disappears as it becomes more water logged. In fact, it can eventually
become a SOURCE of water.
6. Consider the following bases:
O
OH
O
O
O
NH2
OH
O
NH2
O
F
a) For each base above, circle the atom with the highest P.E. (most unhappy)
lone pair of electrons. (Note that for one of them, there is a tie between two
atoms.)
b) Put a 1 and the words highest P.E. next to the strongest base. Put a 7 and the
words lowest P.E. next to the weakest base.
c) Number the others 2-6 according to base strength.
7. Read the assigned pages in the text and do the assigned problems.
O
O