Chem 21
Homework set 4
Practice problems on acid-base chem, and one page you can turn in for credit on Fri 10/9.
End-of-chapter problems from Hornback: Ch 4: 23, 24, 29 - 32, 34 (the pKa of acetylene is 25;
the others can be estimated — no huge tables necessary), 35, 36, 37 (here you may need to find a
few pKas in the table), 39, 40, 41 (no tables needed!), 42 - 46.
1. (a) Draw acetamide, CH3CONH2 (pKa = 17) and its conjugate base (Note: always consider
loss of the most acidic H, unless stated otherwise). Explain why acetamide is so much more
acidic than ethyl amine, CH3CH2NH2. (b) How much more favorable is deprotonation of
acetamide in kcal/mol?
(c) Draw acetone CH3COCH3 (pKa = 20) and its conjugate base. Why is it so much more acidic
than a simple alkane? (d) What's the difference in kcal/mol?
(e) How about a carboxylic acid vs and alcohol? What's the difference in kcal/mol?
(f) Why does the difference in acidity change so much on going from the O–H acids to the N–H
acids to the C–H acids?
2. For each series below, state whether each compound is a stronger acid, a weaker acid, or
about the same acidity as the reference compound in the box.
(a)
OH
SH
OH
OH
O
H2O
O
(b)
O
H3N
NH2
NH2
NH
NH2
N
(c)
NHCH3
H4N
F3C
NH3
NH2
PH2
N(CH3)2
For the next two series, state whether each compound is a stronger base, a weaker base, or
about the same basicity as the reference compound in the box.
(d) (basicity)
H
SH
OH
NH2
O
O
H2O
(e) (basicity)
H3N
Cl
Cl
Cl
N
NH2
NH2
NH2
NH
3. (a) Complete the following acid-base reactions. Draw in the lone pairs first. Use curved
arrows to show shifts of electrons (you may need to draw Hs or redraw the reactants to do this
properly). Be sure to include any important resonance structures. Label the acids with their
pKas. The pKa of acetylene (HCCH) is 25, the pKa of hydrogen sulfide (H2S) is 7, the pKa of
phenol (Ph-OH) is 10, and the pKa of acrylamide is about the same as that of acetamide (prob 1).
You should be able to estimate the other pKas you need. Label the stronger and weaker acids
and bases, and state whether the equilibrium lies to the left or the right in each case.
(a)
CH3
C
C
(b)
NaSH
+
H–O–CH3
+
OH
(c)
O
NH2
+
+
LiH
KOH
(d)
OH
For each equilibrium above, use the pKas to determine the equilibrium constant, and then
estimate the ΔG°. Check that the magnitude of your Keq (>1 vs <1) and the sign of your ΔG° are
consistent with the direction of the equilibrium that you predicted.
4.
(a)
Which of the compounds below are strong enough bases to deprotonate
trimethylammonium ion, (CH3)3NH+ (i.e. to completely deprotonate it — so that the
equilibrium lies well to the right)? Please circle your choices.
NH
CH3CH2NH
CO2
CH3OH
H:
HO
(b) Which are strong enough acids to completely protonate ethylamine (CH3CH2NH2)?
OH
OH
O
CH3OH2
(c)
H:
PH3
H2
N
O
H
Which are strong enough bases to completely deprotonate CH3SH (pKa= 11)?
O
CH3
S O
O
OH
NH2
NH2
NaOH
O
CO2
Chem 21
Fall 2009
HW set 4
15 points; due Fri, Oct 9
Name _________________________________________
1. (a) Would you expect acetamide, CH3CONH2, to be protonated on N or O? Draw the two
conjugate acids that would result, and decide which is the more stable.
(b) Based on your answer to part a, which is the more basic site in acetamide, the N or the O?
2. Which of the Ns of imidazole is the most basic? Draw the two possible conjugate acids and
decide which process is the more favorable. Briefly explain your choice.
H
N
N