chemistry 125: lecture 42 january 22, 2010 solvation, and ionophores this for copyright notice see...
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Chemistry 125: Lecture 42January 22, 2010
Solvation, and Ionophores
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
78
49
33
25
21
5
4
2
H2O
(CH3)2S=O
CH3OH
CH3CH2OH
(CH3)2C=O
CHCl3
(CH3CH2)2O
n-hexane
+- +
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
End of Lecture 42Jan. 22, 2010
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