Problem Set 3: Acidity and Basicity-ANSWER KEY
Chemistry 260
1.
Organic Chemistry
Ethanol in water is in equilibrium with ethoxide and hydronium ion. K for
this reaction is 1.19 x 10-16 at room temperature. Is ethanol or ethoxide
favoured? What is the pKa of ethanol?
Ethanol is favoured (ΔG˚ is positive).
To calculate pKa:
2.
Ka
=
1.19 · 10-16
pKa
pKa
=
=
- log Ka
15.9
In acid-base reactions, the weaker (more stable, less acidic, less reactive)
acid-base pair will be thermodynamically favoured. For the following
reaction, circle the weaker acid-base pair. Should reactants or products be
favoured?
The products will be favoured; this is the weaker acid-base pair. In other
words, HO- is a weaker base than H2N- and NH3 is a weaker acid than H2O.
3.
Circle the stronger (less stable, more acidic, more reactive) acid in each of
the following pairs of molecules. Consider the acidity of only the indicated
hydrogen atom.
(a)
1
(b)
(c)
2
4.
Circle the stronger (less stable, more basic, more reactive) base in each of
the following pairs of molecules. Consider the basicity of only the indicated
atom.
(a)
Recall from question 3 that the thiol is the stronger acid (lower pKa).
(b)
Oxygen is more electronegative than nitrogen. This means that a species
with a negative charge on oxygen is more stable (and thus less basic) than
a species with a negative charge on nitrogen. This analysis is more
complicated if the atoms of interest have different hybridization. For this
course, we will compare atoms of the same hybridization.
(c)
Oxygen is more electronegative than carbon. The same analysis applies in
this situation as in part (b).
5.
For each of the following acid-base reactions, indicate the position of the
equilibrium (K > 1, K = 1 or K < 1).
(a)
K < 1. Hydroxide is a much stronger base than chloride; HCl is a much
stronger acid than water.
3
(b)
K < 1. Hydroxide is a much stronger base than ethanol; protonated ethanol
is a stronger acid than water.
(c)
K > 1. Acetic acid (pKa ~ 5) is a stronger acid than ammonium (pKa ~ 10).
(d)
K > 1. Water is a much, much stronger acid than methane.
6.
The equilibrium in the following acid-base reaction lies far to the right. Draw
the products of the reaction.
Carboxylic acids are more acidic (not surprisingly) than alcohols. Thus, with
one equivalent of hydroxide, only the carboxylic acid is deprotonated.
7.
Which side does the equilibrium rest for the following reactions?
Equilibrum lies to the right. Carboxylic acid is a much stronger acid than an
alcohol.
4
Equilibrium lies to the left. Carboxylic acid is a weaker acid then hydronium
8.
Given the pKa of H2 is 36 and the pKa of CH3OH is 16, explain why bubbles
form when NaH (sodium hydride) is added to a flask containing methanol
maintained at room temperature. Provide a balanced equation and indicate
which way the position of the equilibrium lies to help illustrate your
explanation.
The equilibrium lies to the right, methanol is a much stronger acid then H2,
and the balanced equation is:
The bubbles occur when the H- from NaH deprotonates the CH3OH generating
H2 gas
10.
Classify the indicated compounds as either a Lewis acid or Lewis base.
(a)
Lewis Base (b) Lewis Base (c) Lewis Acid (d) Lewis Acid
5
11.
For each of the following reactions, predict what side of the equilibrium will
be favoured.
(a) right (b) left (c) right (d) left
12.
Circle the stronger (less stable, more acidic, more reactive) acid in each of
the following pairs of molecules. Consider the acidity of the indicated
hydrogen atom.
6
7