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Chemistry 125: Lecture 42January 22, 2010
Solvation, Ionophores and
Brønsted Acidity
preliminary This
For copyright notice see final page of this file
Puzzle Answer(s)
H-OCH2
R
i-Pr
i-PrN+H
Cl
B
free-radicalchain
(might fail with 30% H2SO4)
Note: the base that removes H+ could be a very weak one, like ROH or HSO4
-.
CRO
Helimination
B
HOMO-LUMO
i-Pr
i-PrN
H
H-OCH
R
H
Cl
+
i-Pr
i-PrN+H
H
OCH
R
H
Cl elimination
nO *N-Cl
OCH2
R
H
OCH
R
Cl
H
Chapter 6: R-XX = Halogen, OH(R), NH(R)2, SH(R)
Non-Bonded Interactions and Solvation (key for ionic reactions)
Ionic Chemistry of * (pKa and Ch. 7)
(electrostatic - gravity & magnetism are for wimps, and the “strong force” is for physicists)
The theory of organic chemistry became manageable because it is often possible to focus on a simple unit with strong interactions (bonds with well defined geometry
and energy), neglecting the much weaker (and more numerous and complex) intermolecular interactions.
But the weak intermolecular inter-actions give organic materials many of their most valuable properties.
dielectric constant
Non-Bonded “Classical” Energies
R-1+ -R Charge-Charge(Coulomb’s Law)
The ONLY source of true chemical potential energy.
E±Coulomb = -332.2 kcal/mole / dist (Å)
[long-range attraction; contrast radical bonding]
Table 6.7 p. 239
+- +
Non-Bonded “Classical” Energies
- + R-2
+ R-3
- + - + R-3
-+ -+ R-6
R-1+ -R Charge-Charge(Coulomb’s Law)
+ Charge-Dipole(Dipole Moment)
Charge-Induced Dipole(Polarizability)
Dipole-Dipole(Dipole Moments)
Induced-Induced
-+-
+-+ -
+-+
(Cf. Correlation Energy)
What if the dipole orientation is not fixed?
R-4
T
Nonpolar
The latter interactions are weak because dipoles moments and polarizabilities are small - and because of the energies fall off rapidly with increasing distance.
Halide Trends (text sec. 6.2)
Bond Distanceof X-CH3 (Å)
van der WaalsRadius of X (Å)
Dipole Momentof X-CH3
“Charge” of X , CH3 (e)
H F Cl Br Iatom
0
1
2
Debye units = 4.8 charge (electrons) separation (Å)
= Debye / (4.8 dist)
i.e. non-bonded distances are about twice bonded distances.
Non-monotonic
(monotonic)
The dipole moment () is the product of two properties, with opposing trends. Both are monotonic, but one is nonlinear.
conflicting nonlinear trends
Halide Trends (text sec. 6.2)
Bond Distanceof X-CH3 (Å)
van der WaalsRadius of X (Å)
“A-Value” of X Eaxial-Eequatorial
(kcal/mol)another measure
of substituent “size” H F Cl Br Iatom
0
1
2
compare
CH3
larger vdW radiusstands off further
Non-monotonic,like
!
(suggests competition)
Boiling points
from Carey & Sundberg
CH4 isnot polar
and not verypolarizable
polarizability,
(Table 6.2) 0 1.85 1.87 1.81 1.62
not just polarity
- + - +
- +
-+ -+
- +
Boiling points
n-Pentane 36°C
iso-Pentane 28°C
neo-Pentane 10°C
Polarizability does its job well only when the atoms can get
really near one another.
Atoms near surface count!
Intra- vs. Intermolecular“Solvation”
Hf (gas)
-35.1
-36.9
-40.3
n-butane
isobutane
Cf. gas-phase ionic dissociation
R-Cl R+ Cl-
R+ kcal/mole
(CH3)3C+ 176
CH3CH2+
193
CH3+ 229
What does molecular weight have to do with b.p.?
Could be plottedmore informatively
HH-(CH2)n-X
100
-100
0
200
2 4 6 8 10n
Boi
ling
Poi
nt (
°C)
I
BrCl
F
CH3-Cl 1.9 5
CH3-Br 1.8 6
CH3-I 1.6 8
CH3-H
CH3-F
DipoleMoment
(D)
Polarizability(10-24 cm3)
0
1.8 3
3
Like Dissolves Like“Solvophobic” Forces
Hgdoes not “wet” glass
Like Dissolves Like“Solvophobic” Forces
Hg does not “wet” hydrocarbon
Alkanes and water (or Hg and glass) do not repel one
another.
but Hg has good reason to be near Hg, and water near water.
nor does H2OHg attracts H2O
Water Dipoles
Calculated Water Dimer
Lengthened by only ~0.5%
(not much * occupancy)
Klopper, et al., PCCP, 2000, 2, 2227-2234
Water Multipoles
Surface potential -45 to +50Surface potential +35 to +50Surface potential -45 to -35
6-311+G**
Calculated Water Dimer
Klopper, et al., PCCP, 2000, 2, 2227-2234Cf. Goldman, et al., J. Chem. Phys., 116, 10148 (2002)
Dissociation energy = 3.3 kcal/mole
The small size of H allows the unusually close approach that
makes O-H•••O-H worth R R .
calling a “hydrogen bond”.
* Typically ~ 5% as strong as a covalent bond
*
Text Section 6.10
Crown Ethers andTailored Ionophores
Nobel Prizein Chemistry
1987
“ion carriers”
18-c-6
18-Crown-6 • K+Cl-
2.82
2.78
2.83 Å
Radii (Å)
K+ 1.33
O 1.4
18-Crown-6 • Cs+N=C=S-
3.10
3.04
Å
3.04
3.163.27
3.27
Radii (Å)
Cs+ 1.67
O 1.4
18-Crown-6 • Na+N=C=S-
Radii (Å)
Na+ 0.98
O 1.4
2.62
2.55
Å
2.58
2.472.62
2.32
2.45
18-Crown-6 • Li+ClO4-
3.523.11
2.71
3.79
2.07
Å2.12
Radii (Å)
Li+ 0.68
O 1.4
1.911.92
• 2 H2O
Relative binding constants for 18-crown-6 with various
alkali metal ions
K = [M+•Ligand]
[M+] [Ligand](mol-1)
23,000
1,150,000
in MeOH at 25°C
29106 strongerthan MeOH !
0.79 g/ml mol.wt. 32
25 molar
H -TS
-13.4 5.2kcal/mole
-8.4 2.5
By making cation large18-c-6 allows KMnO4 to dissolve in hydrocarbons
Cryptands
Nonactin
a bacterial antibiotic
Nonactin
QuickTime™ and aH.264 decompressor
are needed to see this picture.
Keq (MeOH)
Na+ 512 K+ 31,000
H2O (aq)
kcal
/mol
400
300
200
100
0H2O (g) 6.3
H3O+ (aq)
OH- (aq)
H+ + OH- (g)392
H3O+ (g)
164 !
106
100
Sum = 370
H+(aq) + OH-(aq)
pKa = 15.8
The Importance of Solvent for Ionic Reactions
21.5
E±Coulomb = -332.2 / dist (Å) [long-range attraction; contrast radical bonding]
H+ :OH2 bondingplus close proximity
of + to eight electrons (polarizability shifts e-cloud)
+-+-
+-
28
18
etc,etc,etc
From small difference of
large numbers!K 10-(3/4 386) 10-290BDE HO-H 120
e transfersimilar
Fortunately solvation energies of analogous compounds are similar enough that we can often make reasonably accurate predictions (or confident rationalizations)
of relative acidities in terms of molecular structure.
When pKa = pH
Why should organic chemists bother about pH and pKa, which seem like topics for general chemistry?a) Because whether a molecule is ionized or not is important for predicting reactivity (HOMO/LUMO availability), conformation, color, proximity to other species, mobility (particularly in an electric field), etc.
b) Because the ease with which a species reacts with a proton might predict how readily it reacts with other LUMOs (e.g. *C-X or *C=O).
Ka =[H+] [B-]
[HB]
[B-][HB]
pKa = pH - log = pH, when HB is half ionized
Approximate “pKa” Values
CH3-CH2CH2CH2H ~ 52
CH3-CH2CH=CHH ~ 44
CH3-CH2C CH ~ 25
~ 34 H2NH
= 16 HOH
CH3-CH=C=CHH
CH3-C C-CH2H ~ 38
sp3 C_
sp2 C_ (no overlap)
sp C_ (no overlap)
C_ HOMO - overlap(better E-match N-H)
(bad E-match O-H)
(best E-match C-H)
* Values are approximate because HA1 + A2- = A1
- + HA2 equilibria for bases stronger that HO- cannot be measured in water. One must
“bootstrap” by comparing acid-base pairs in other solvents.
50
40
30
20
10
pKa
*
:
:
(allylic)
(Acidity of 1-Alkynes Sec. 14.7)
(Problems 14.15-17, 14.18acd, 14.21-22)
Brønsted AcidityChapter 3
BDE 105 108 119 136
91 103
88
71
Overlap!
Factors that Influence Acidity
End of Lecture 42Jan. 22, 2010
Copyright © J. M. McBride 2010. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).
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