chemistry 125: lecture 43 january 25, 2010 solvation, ionophores and brønsted acidity this for...
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Chemistry 125: Lecture 43January 25, 2010
Solvation, Ionophores and
Brønsted Acidity
This
For copyright notice see final page of this file
Text Section 6.10
Crown Ethers andTailored Ionophores
Nobel Prizein Chemistry
1987
“ion carriers”
18-c-6
Relative binding constants for 18-crown-6 with various
alkali metal ions
K = [M+•18-c-6]
[M+] [18-c-6] (mol-1)
23,000
1,150,000
in MeOH at 25°C29106 stronger
than MeOH !0.79 g/ml mol.wt. 32
25 molarH -TS
-13.4 5.2 kcal/mole
-8.4 2.5
>1/2 complexed at 1 M [18-c-6]
By making cation large 18-c-6 “destabilizes” solid or aqueous KMnO4 allowing the salt to dissolve in hydrocarbons. (“purple benzene”)
Phase-Transfer Catalysis
H2O
organicsolvent
KMnO4
organic substanceto oxidize
Similar effect from adding other salts with large organic cations,
e.g.R4N+ Cl-
R4P+ Cl-
Avoids need for expensive, dangerous solvents
like (CH3)2SOthat dissolve both reagents
Nonactin
QuickTime™ and aH.264 decompressor
are needed to see this picture.
Keq (MeOH)
Na+ 512 K+ 31,000
moves K+ selectivelythrough a membrane
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(G) 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
Single indicators work best over ~2.5 pH units (95:5 - 5:95).
Bootstrap with overlapping indicators for wide coverage.
Learning frompKa Values
HOH 15.716
12
8
4
0
pKa
*
-4
H2OH -1.7+
HSH 7.0
FH 3.2
H3NH 9.2+
(BDE 119)
(BDE 91)
(BDE 136)
Learning frompKa Values
16
12
8
4
0
pKa
*
-4
4.8 CH3-COH
O
2.9 ClCH2-COH
O
HOH 15.7
H2OH -1.7+
HSH 7.0
FH 3.2
H3NH 9.2+
9 CH3-C-CH-C-CH3
O
H
O
H3NCH-COH
OCH3+
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)
1) List factors that help determine pKa for an acid.
2) Choose a set of several acids from the Ripin-Evans Tables or from the text (inside back cover) and explain what they teach about the relative importance of these factors.
3) Explain your conclusions to at least one other class member and decide together how unambiguous your lesson is.
Problems for Wednesday:
Feel free to consult a text book and its problems or the references at the end of the Tables.
Hint: this could provide a good question.
End of Lecture 43Jan. 25, 2010
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