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

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