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Basic Organic Chemistry
Homework 3
Acids and Bases
Answer Key
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Homework Sheet Three
Organic Acids and Bases
Basic Organic Chemistry
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Q1: a) i) Identify the strongest and weakest acids from Figure 1.
ii) What is the conjugate base of benzoic acid?
iii) Which compound has the strongest conjugate base?
iv) Using curly arrows, draw the resonance forms of the conjugate
base of the keto-ester. Don’t forget formal charges.
v) Why is pBBA a stronger acid than benzoic acid?
b) Hence decide which of these anions (Figure 2) is the weakest base?
Would this anion deprotonate i) HCN or ii) CF3COOH?
Draw the acid-base reaction mechanism (i.e. using curly arrows) for
the favourable of the two possible reactions
Figure 1
OH
O
O
O
O
keto-ester
pKa = 11
Figure 2
O
benzoic
acid
pKa = 4.2
_
_
O
O
ethyl acetylene
pKa = 25
p-bromobenzoic
acid (pBBA)
pKa = 3.8
cyclohexanone
pKa = 19
O
_
O
Table 1
pKa
phenol
9.89
dichloroacetic acid 1.48
MeOH
15.54
Q4: See Table 1 a) What is the Ka for each compound? Use a calculator.
b) Write out the chemical equation for the dissolution equilibrium of
each acid in water, labelling the acid, base, conjugate acid and conjugate base
c) i) Which compound is the most acidic?
ii) Which compound is least acidic?
iii) Which compound has the weakest conjugate base?
iv) Which compound has the strongest conjugate base?
v) Why is Cl2CHCOOH a stronger proton donor than acetic acid (pKa = 4.8)?
vi) Hence, estimate the pKa of the analogous dihaloacetic acids Br2CHCOOH and F2CHCOOH. Give
reasons.
_
O
C
Br
O
O
Q2: a) Classify the following as Lewis acids or Lewis bases:
i) ZnCl2
v) Et2S
ii) Me-O-Me
vi) BCl3
iii) CH3CH2NHCH3
vii) AlCl3
iv) Cr3+
viii) PMe3
b) i) Using skeletal structures, curly arrows and formal charges
where required, show how CH3CH2NHCH3 and AlCl3 react
together to form a complex.
ii) What is the hybridisation of Al before and after the reaction?
iii) Does Al have a complete octet before and after the reaction?
iv) Which species is an electrophile and which is the nucleophile?
OH
HC C CH2-CH3
Br
O
_
C CH 2-CH 3
O
Q3: All the following proton transfers are favourable.
For each reaction, draw both products and label the acid,
base, conjugate base and conjugate acid.
i)
ii)
iii)
b) Again, draw the products of each reaction. This time
use pKa tables in Appendix B of McMurry to decide if
the proton transfer is spontaneous (i.e. does the
equilibrium favour starting materials or products?)
i)
ii)
iii)
Total: 80 marks
1
Q1: Which of these organic compounds is most acidic and which is
least acidic.
a)
OH
O
O
O
OH
HC C CH2-CH3
Br
O
O
O
ethyl acetylene
keto-ester cyclohexanone p-bromobenzoic
pKa = 25
acid (pBBA)
pKa = 19
pKa = 11
pKa = 3.8
benzoic
acid
pKa = 4.2
i) Identify the strongest and weakest acids from the list.
Weakest = ethyl acetylene
Strongest = pBBA
_
O
ii) What is the conjugate base of benzoic acid?
(C6H5COO-)
O
iii) Which compound has the strongest conjugate base?
ethyl
acetylene
Q1: Which of these organic compounds is most acidic and which is
least acidic.
a)
OH
O
O
O
OH
HC C CH2-CH3
Br
O
O
O
ethyl acetylene
keto-ester cyclohexanone p-bromobenzoic
pKa = 25
acid
(pBBA)
pKa = 19
pKa = 11
pKa = 3.8
benzoic
acid
pKa = 4.2
iv) Using curly arrows, draw the resonance forms of the conjugate
base of the keto-ester. Don’t forget formal charges.
B
_
H
O
O
O
_ O O
v) Why is pBBA a stronger acid than benzoic acid?
O
O
O
O
O
O
O
_
Electron-withdrawing effect of Br atom stabilises
anion (delocalises negative charge more)
2
Q1: b) Hence decide which of these anions (shown below) is the
weakest base?
_
_
_
O
O
O
_
O
O
C C CH2-CH3
Br
O
O
_
O
Would this anion deprotonate i) HCN pKa = 9
or ii) CF3COOH pKa = 0
_
O
Br
O
is the weakest base; it would deprotonate stronger acid
CF3COOH but not weak acid (HCN). Or: pBBA- is
weaker base than CN- but is stronger than CF3COO-
Draw the acid-base reaction mechanism (i.e. using curly arrows) for
the favourable of the two possible reactions
O
_
OH
CF3
O
Br
H
O
O
O
Br
O
_
CF3
O
Q2:
a) Classify the following as Lewis acids or Lewis bases:
i) ZnCl2 Lewis Acid
ii) Me-O-Me Lewis Base
iii) CH3CH2NHCH3 Lewis Base
iv) Cr3+ Lewis Acid
v) Et2S Lewis Base
vi) BCl3 Lewis Acid
vii) AlCl3 Lewis Acid
viii) PMe3 Lewis Base
b) i) Using skeletal structures, curly arrows and formal charges where
required, show how CH3CH2NHCH3 and AlCl3 react together to form
a complex. (see below)
ii) What is the hybridisation of Al before and after the reaction?
iii) Does Al have a complete octet before and after the reaction?
iv) Which species is an electrophile and which is the nucleophile?
3
Q2: b) i)
vacant 3p orbital
Et
Cl
N
Cl
Al
Cl
Lewis Acid
(electrophile)
Et
H
H
N
_
Me
Lewis Base
(nucleophile)
+
Al
Cl
Me
Cl
Cl
sp3 hybridised Al
complete octet
sp2 hybridised Al
incomplete octet
Q3: a) All the following proton transfers are favourable. For each
reaction, draw both products and label the acid, base, conjugate base
and conjugate acid.
i)
_
pKa = 15.5
H3C O
NH3
pKa = 38
_
O
ii)
iii)
4.8
O
C C
25
H3C
_
O
15.5 H
H2
35
4
b) Again, draw the products of each reaction. This time use pKa tables in
Appendix B of McMurryto decide if the proton transfer is spontaneous
(i.e. does the equilibrium favour starting materials or products?)
O_ K
i)
+
pKa = 16
+
ii)
NH2
pKa = -1
pKa = 35
_
iii)
pKa = -9
+
O H
LHS
favoured
pKa =
H2O 15.7
HSO4
SO
_ 3 RHS
fav.
RHS
fav.
pKa = -7.3
Q4: a) What is the Ka for each compound? Use a calculator when
necessary.
i) phenol (pKa = 9.89)
15.54)
Ka = 10-9.89
= 1.3 x 10-10
ii) dichloroacetic acid (pKa = 1.48)
Ka = 10-1.48
= 0.033
iii) MeOH (pKa =
Ka = 10-15.54
= 2.9 x 10-16
Ka = 10-pKa (since pKa = -logKa)
b) Write out the chemical equation for the dissolution equilibrium of
each acid in water, labelling the acid, base, conjugate acid and
conjugate base
C6H5OH (aq)
H3O+ (aq)
H2 O
Cl2CHCOOH (aq)
+ H2 O
H3O+ (aq) +
Cl2CHCOO- (aq)
H2 O
H3O+ (aq) +
CH3O- (aq)
base
conjugate acid
CH3OH (aq)
acid
+
+
C6H5O- (aq)
+
conjugate base
5
Q4:
c)
i) Which compound is the most acidic?
dichloroacetic acid
ii) Which compound is least acidic?
methanol
iii) Which compound has the weakest conjugate base?
dichloroacetic acid
iv) Which compound has the strongest conjugate base? methanol
v) Why is Cl2CHCOOH a stronger proton donor than acetic acid
Electron-withdrawing effect of Cl atoms
(pKa = 4.8)?
stabilises carboxylate anion (conjugate base)
vi) Hence, estimate the pKa of the analogous dihaloacetic acids
Br2CHCOOH and F2CHCOOH. Give reasons.
pKa Br2CHCOOH = 2.0
F2CHCOOH = 1.0
Electron-withdrawing effect of F and
Br depends on their electronegativity
6