RESONANCE, USING CURVED ARROWS, AND ACID-BASE REACTIONS
A STUDENT SHOULD BE ABLE TO:
1.
Properly use curved arrows to draw resonance structures: the tail and the head of every
arrow must be drawn in exactly the proper location. The tail shows where the electrons
are coming from, and the head shows where the electrons are going. Recognize “bad
arrows.” Identify insignificant resonance structures.
When drawing resonance structures: avoid breaking a sigma bond; never exceed an octet
for second-row elements; recognize and properly notate formal charges.
2.
Define terms “resonance”, “resonance stabilization”, “delocalization” and “resonance
hybrid”. Be able to identify allylic and vinylic positions in a molecule. Also identify
delocalized lone pairs, including allylic, and differentiate them from localized lone pairs.
Identify lone pairs that appear to be allylic, but are not delocalized because of the
geometry of their orbitals.
3.
Define Bronsted-Lowry acids (proton donors) and bases (proton acceptors); conjugate
acids and conjugate bases. Bronsted-Lowry acids are all protic (have an ionizable
proton). Be able to draw a conjugate base for any acid and a conjugate acid for any base.
Use arrows to show how an acid loses a proton with the help of a base. Note that this
process, called proton transfer, is shown with at least two arrows. Proton transfers play an
integral role in reaction mechanisms.
Acidity increases
Carbon based "acids"
R
R
pKa ~50
H
R
R
pKa ~44
C
C
pKa ~25
H
N
O
Halogen based
acids
F
H
pKa 3.2
H
O
H
pKa 15.7
H
O
pKa ~38
C
C
R
R
H
R
R
Oxygen based acids
pKa 16-18
C
R
Nitrogen based acids
H
H
Br
pKa -8
pKa 9.9
R
R
C
H
O
O
H
Cl
pKa -7
H
pKa 4-5
Bond polarity and acid strength increase
H
I
pKa -9
Acidity increases
Define pKa, and be able to predict the acidity of different hydrogens, using pKa values.
4. Write acid-base reactions, keeping in mind that reactions will proceed in the direction
from a stronger to a weaker base formation. Remember that the strongest acid would
form the weakest conjugate base, and the weakest acid would form the strongest
conjugate base. Consider factors affecting the stability of negative charges: ARIO. (1)
which Atom the charge is on: more electronegative and larger atoms stabilize negative
charge better; (2) Resonance effect: resonance-stabilized bases are more stable and
therefore weaker; (3) Inductive effect: electron withdrawing atoms and groups stabilize
negative charge better, and (4) hybridization: Orbitals with more s-character stabilize
negative charge better (that is, negative charge is most stable on sp-orbital, then sp2, then
sp3).
Predict the position of an equilibrium based on comparative strength of acids or bases, or
by using pKa values. Also be able to predict which base would be suitable for removal of
a particular proton (deprotonation). Keep in mind: bases that are stronger than hydroxide
cannot be used when the solvent is water. Counterions (cations) always accompany
negatively charged bases and can be ignored: NaOH, LiOH, KOH are all the same base –
hydroxide.
Use the following series for comparative strength of bases:
Weakest
base
Strongest
base
O
RSO3 < Hal < RCO < PhO < HO < RO < RC
C < R2N < R2C CR < R3C
sp2
sp
sp3
Base strength increases
5.
Define Lewis bases (electron pair donors) and Lewis acids (electron pair acceptors).
Lewis acids may be protic (have an ionizable proton) or aprotic (do not have an ionizable
proton). Be able to show Lewis acid-base reactions with curved arrows. Different from
Bronsted acid-base reactions, Lewis acid-base reactions can often be shown with one
arrow and lead to one product:
To best prepare for this module, please work Chapter 2 (2.7-2.12) and Chapter 3 Skill Builder
problems in the textbook.
A STUDENT WHO HAS MASTERED THE OBJECTIVES FOR THIS UNIT SHOULD BE
ABLE TO SOLVE THE FOLLOWING PROBLEMS AND RELATED ONES:
1.1
Using curved arrows, draw resonance structures for the following:
1.2
Identify bad arrows in the following drawings:
1.3
Identify insignificant resonance structures in the following pairs:
1.4
In each pair, indicate if the structures are isomers or resonance structures.
2.1
Identify all allylic positions in the following molecules:
2.2
How many vinylic hydrogens do the structures above contain?
2.3
Identify delocalized lone pairs of electrons, including allylic, in the following structures.
2.4
Draw resonance structures for each of the molecules in 2.3 that show delocalization of
the lone pairs identified above.
3.1
Use arrows to show how each of the following acids loses its proton with the help of the
base in the following acid-base reactions:
3.1
3.2
Draw conjugate bases for the following compounds:
a) H2O
b) NH3
c) HCl
d) H2SO4
e) C2H5OH
3.3
Draw conjugate acids for the following compounds:
3.4
Using Table 3.1 in your textbook, identify the most acidic hydrogen in each of the
following compounds:
CH2O-H
O
a)
b)
C
H-O
C
O-H
H
H
O
c) H
O
S
H
O
O
H
O
d)
O
H
H
4.1
For each of the following reactions, write the acid/base reaction product or indicate no
reaction. (Hint: use base strength series).
4.1
b)
CH3CO2-Na+ + F3CCO2H
O-Na+ + H2O
c)
d)
CH3CH2-Li+ + H2O
e)
CH3O-Na+ + CH3S-H
f)
H3CC
C-Na+ + CH4
g)
H3CC
CH
h)
CH3SO3-Na+ + CH3O-H
i)
CH3SO3H + NaOH
+
Na+NH2-
4.2
Predict the position of the equilibrium for each of the following reactions.
Verify your answers with pKa values where possible.
4.3
Identify whether the reagent shown would be suitable to accomplish the task described.
Explain why or why not.
4.3
5.
In each case identify the Lewis acid and the Lewis base. Show with curved arrows how
the base attacks the acid, and draw the reaction products.
SOLUTIONS TO SAMPLE PROBLEMS:
1.1
1.1
b)
c)
1.2
1.3
1.4
Arrows a) and c) are good. Arrows b), d), e) and f) are bad.
Resonance structure on the right is insignificant in each pair.
a) resonance structures: only electrons shift b) isomers: different arrangement of atoms
2.1
Circled positions are allylic:
2.2
Structure a) has two vinylic hydrogens; structure b) has two vinylic hydrogens.
2.3
Delocalized lone pairs of electrons are circled.
Note that in structure (d) the lone pair on the second nitrogen appears to be allylic.
However, that nitrogen (sp2 hybridization) already has a double bond, so the lone pair
occupies a perpendicular orbital. Therefore, it cannot be delocalized and participate in
resonance, as the orbital of the -bond already does.
2.4
a) HO
HO
NH2
NH2
b)
N
N
N
N
3.1
3.2
3.3
3.4
a) HO-
b) -NH2
c) Cl-
d) HSO4-
e) C2H5O-
4.1
a)
CH3-Li+ + H2C=CH2
b)
CH3CO2-Na+ + F3CCO2H
c)
4.2
O-Na+ + H2O
CH4 + H2C=CH- Li+
CH3CO2H + CF3CO2-Na+
no reaction
d)
CH3CH2-Li+ + H2O
CH3CH3 + Li+ -OH
e)
CH3O-Na+ + CH3S-H
CH3O-H + CH3S-Na+
Equilibrium always favors a weaker acid (greater pKa) and weaker base. There is no need
to check both acids and bases. Checking only acids or only bases is enough.
The pKa of the alcohol is similar to water, so both acids and both bases will be present.
4.3
a) No, because hydroxide is a weaker base than H2C=CH (ARIO)
b) Yes, because hydroxide is a weaker base than amide (ARIO)
c) Yes, because hydroxide is a stronger base than phenolate (PhO-) (ARIO)
d) No, because stronger base would be produced (ARIO)
5.
Ph
a)
Cl
Ph
Cl
Cl
Al
Al
+
P
P
Ph
Ph
Cl
Ph
Cl
Ph
Cl
Note: Ph is
b)
H
O
H
B
O
+
BH3
H
+
O
B
F
F
F
F
F
c)
O
B
F
Name ________________________________________________ Third Drill Test (Sample A)
Organic Chemistry 2210D
Answer All Questions
1.
Using curved arrows, draw all resonance structures for each of the following. Where
possible, indicate which of the structures is least significant and explain why. (Hint: you
may want to label each atom first and keep all atoms in the same place in each structure).
2.
The electron pair on nitrogen in pyridine appears to be allylic. However it is not
delocalized. Explain why.
3.
Amines contain single C-N bonds, and imines contain C=N double bonds. Based on
factors that affect the stability of a negative charge, predict which conjugate base will be
stronger. Explain your prediction. Hint: first draw both conjugate bases, then analyze the
difference between them.
4.
Predict the position of the equilibrium in the following reactions. Show with curved
arrows how the base deprotonates the acid to form products according to your conclusion
about the equilibrium position.
O-H
a)
H3CC
b)
c)
O-Na+
+
O-H
C-H + Na+NH2-
H2C=CH- Li+ +
N -H
O
d)
C
H3C
H3CC
C-Na+
N Li
+ H2C=CH2
O
+ CCl3CO2-Na+
O-H
C
Cl3C
O-Na+
+
+ NH3
O
+
O-H H3C
C
O-Na+
5.
You are assigned to deprotonate a compound A using one of the available bases:
sodium phenolate (PhO-Na+), sodium hydroxide (NaOH), or sodium acetate
(CH3COO-Na+). Based on the following pKa data, identify which of the bases you could
use to complete the task. (Hint: the weakest acid produces the strongest conjugate base).
Write the corresponding acid-base reaction(s) and show with curved arrows how the
base(s) would deprotonate compound A.
6.
Use curved arrows to derive products of the following Lewis acid-base reactions. Draw
the products and make sure to show charges where they belong.
Name _______________________________________________ Third Drill Test (Sample B)
Organic Chemistry 2210D
Answer All Questions
1.
Using curved arrows, draw all resonance structures for each of the following. For part c,
indicate which of the structures is the least significant and explain why. (Hint: you may
want to label each atom first and keep all atoms in the same place in each structure).
a)
b)
c)
NH2
O
2.
How many allylic hydrogens do the following structures have? Label them on the
structures.
How many vinylic hydrogens are there in the following molecules? Label them on the
structures.
3.
Find delocalized electron pairs in the following structures, and show with arrows how
each one can be delocalized. Draw corresponding resonance structure(s):
4.
Draw conjugate acids and conjugate bases for the following compounds:
5.
Identify the most acidic hydrogen in each of the following molecules:
6.
Use curved arrows to derive products of the following acid-base reactions. Indicate
where there will be a partial reaction or no reaction.
N
a)
b)
Na
+ H
O-H
+ NaOH
O
c)
CH3CH2S-Na+ + CH3CH2O-H
7.
Label Lewis acids and Lewis bases among the following compounds:
8.
In the molecule shown below, the C-H is more acidic than the O-H, in contradiction to
the general periodic trends. Explain.