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

Cryptands

Nonactin

a bacterium-generated antibiotic

Nonactin

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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.

Factors that Influence Acidity

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)

Brønsted AcidityChapter 3

BDE 105 108 119 136

91 103

88

71

Overlap!

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+

Titration of Alanine

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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)

1st of 6 pages from http://evans.harvard.edu/pdf/evans_pKa_table.pdf

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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