Chemistry 181
Professor S. Alex Kandel
Fall 2015
Due 11/18/2015
Chem 10181, Fall 2011
Problem Set 5
September 28, 2011
Problem Set 11
Practice Problems: 11.47, 11.49, 11.50, 11.52, 11.55, 11.56.
Problems to discuss:
Discussion problems:
(1)
Triple play. Rank each of the following triples of Brønsted acids in order of increasing
acidity,
and briefly
rationalize
the order.
(When therethe
is some
the protons
in
We will
discuss
these in
class (3:30)
on Wednesday
18th.ambiguity,
Be prepared
to present
question
are
circled.)
answers to and to discuss any of these up at the blackboard.
(a)
HI, HCl, HF.
1.(b)
RankHthe
following molecules in terms of acidity (of the circled proton), and
4SiO4, H3AsO4, H2SeO4.
explain the reasoning for this ranking.
(c)
NH4+, NH3, NH2–.
(a)
H
H
C
(d)
H
O
C
C
H
C
C
H
S
C
H
C
H
H
H
(2)
H
S
H
H
H
The
middle
compound
is Hthe weakest
acid—the conjugate
C
O
H
C base
O only has
H
C
O
C
C
H
C
H
C
C
C
H
one structure, with all of the charge located on the S. The compound
on
the C
right isC the strongest
structure
that
O acid,
C withCan alternate resonance
C
C
C
N
H
C
C
H
CmuchHof the negative
H
spreads
charge
onto
the electronegative
O:
O
H
(f)
C
C
H
H
H
(e)
H
S
HCCH, H2CCH2, H3CCH3
N
H
H
O
O
Tropolone. Tropolone (pKa = 5) is muchC more acidic thanCphenol (pKa = 10).
H 3C
H 3C
S
S
H
The compound onOthe left has more Oresonance
but they are not
O structures,
H
as good, asHthey spread
less-electronegative Cs:
C onto
C
C
Hnegative charge
C
H
C
S
C
C
C
H
Phenol
(a)
(b)
(c)
(d)
H
H
C
C
HS
C
C
H
C
H
S
S
H
Tropolone
Rationalize the difference in acidity between these acids (acidic proton circled).
Why are the other protons in the two molecules much less acidic than the circled
protons?
1
How many signals would be expected in the 1H NMR spectra of the conjugate
acid forms of phenol and tropolone (pictured), and in what integral ratios?
How many signals would be expected in the 1H NMR spectra of the conjugate
base forms of phenol and tropolone, and in what integral ratios?
(a)
(b)
HI, HCl, HF.
H4SiO4, H3AsO4, H2SeO4.
(c)
NH4+, NH3, NH2–.
H
H
C
(d)
H
C
H
C
C
S
C
H
(b)
O
C
H
C
C
H
S
H
H
H
C
C
H
H
H
H
H
C
(e)
H
O
C
O
C
C
O
C
C
O
H
H
C
C
C
C
N
H
H
C
C
H
H
(2)
H
S
H
H
(f)
C
C
H
C
N
H
O
C
H
C
C
H
H
HCCH, H2CCH2, H3CCH3
The compound
on the
left is the least acidic, as it only has the three minor
Tropolone.
Tropolone (pK
a = 5) is much more acidic than phenol (pKa = 10).
resonance structures that put negative charge onto the carbon.
O
H
H
(a)
(b)
(c)
(d)
O
C
C
C
C
H
O
C
C
H
C
O
H
O
C
H
O
C
C
C
H
H
C
O
H
C
H
The other two compounds have these resonances
structures as well (and I
H
H
will not redraw them),
but
also
have
alternate
major
structures that make
Phenol
Tropolone
the conjugate bases more stable and thus the acids stronger:
Rationalize the difference in acidity between these acids (acidic proton circled).
O
Why are the other protons in the two molecules much
less acidic than the circled
O
N
O
and
protons?N C
O
1
How many signals would be expected in the H NMR spectra of the conjugate
acid forms of phenol and tropolone (pictured), and in what integral ratios?
I would reason that the NO2 compound
spreads negative charge onto
1
How
many
signals
would
be
expected
in
the
H
NMR
of thethe
conjugate
another oxygen, and thus resulted in a stronger spectra
acid than
CN combase
forms
of
phenol
and
tropolone,
and
in
what
integral
ratios?
pound (which spreads it onto the nitrogen.) They are actually quite close
(pKa =7.23 for the NO2 compound, and 7.95 for the CN compound.)
2
H
H
H
H
H
H
H
C
C
(e)
C
C
C
C
H
O
C
C
H
H
O
O
H
O
C
C
N
H
H
H
H
C
C
C
C
C
H
C
N
H
O
H
C
C
C
H
H
(f)
HCCH, H2CCH2, H3CCH3
2. Tropolone (pKa =5) is much more acidic than phenol (pKa =10).
(2)
Tropolone. Tropolone (pKa = 5) is much more acidic than phenol (pKa = 10).
H
O
H
H
C
C
C
C
O
C
C
O
C
H
H
H
H
Phenol
C
C
C
C
H
H
C
H
C
H
H
Tropolone
(a)
Rationalize the difference in acidity between these acids (acidic proton circled).
(a)
Explain
the the
difference
in acidity
these two
(b)
Why are
other protons
in thebetween
two molecules
muchcompounds.
less acidic than the circled
protons?
We’ve seen the resonance stabilization for phenol’s
conjugate base. Tropolone
(c)
How many signals would be expected in the 1H NMR spectra of the conjugate
hasacid
a resonance
spreads
the negative
charge
forms of structure
phenol andthat
tropolone
(pictured),
and in
what equally
integral between
ratios?
the two nitrogens:
1
(d)
How many signals would be expected in the H NMR spectra of the conjugate
base forms of phenol and tropolone, and in what integral ratios?
O
O
O
O
(b) Sketch an expected NMR spectrum for the conjugate base form of tropolone.
Because the resonance structures make the molecule completely symmetric, there will only be three sets of peaks. The two hydrogens closest to
the oxygens have one neighbor (each) and will give a doublet at the left
(unshielded) part of the spectrum. The bottom hydrogen has two neighbors and will give a 1:2:1 triplet shifted right (shielded). Between them
will be the other hydrogens, which together give a triplet, close to 1:2:1.
3
3. The amino acids tyrosine and histidine are shown below:
tyrosine
histidine
For each molecule, rank the protons shown by acidity, and explain your reasoning.
Resonance effects are important, but remember that in most cases, OH hydrogens are more acidic than NH hydrogens, and CH hydrogens rarely have any
significant acidity at all.
O
most acidic (carboxylic acid)
resonance structure spreads charge
between both O atoms.
OH
NH 2
HO
second most. minor resonance
spreads negative charge to
carbons on ring.
NH protons are very weakly acidic;
nothing else has measurable acidity.
O
most acidic
(COOH
ftw).
OH
N
NH 2
HN
resonance spreads
negative charge between
two N atoms; still only weakly acidic.
4
very weak.
Graded problem:
This should be written up and handed in before class begins on Wednesday the 18th.
This problem may also be discussed in class.
1. The structure of vitamin C (ascorbic acid) is:
Only one of the protons in this molecule is noticeably acidic in water. Which
one, and why?
Again, the CH hydrogens will not be acidic, so only the OH groups need to
be considered. Resonance is then the most important effect, with only one OH
having an alternate major resonance structure:
OH
OH
H
H
O
HO
O
O
HO
O
OH
O
O
OH
This spreads the negative charge (mostly) equally between two oxygens, increasing the stability of the conjugate base and making this the only acidic proton.
5