Chemistry 125: Lecture 41January 19, 2011

Fractional-Order KineticsElectronegativity & Bond Strength

SolvationIonophores

This

For copyright notice see final page of this file

[(CH3Li)4] [CH3Li]4 when Assoc = Dissoc

when Term = Init [Rad•]2 [RO-OR]

Generalization of Fractional-Order Kinetics(suggestion by Ayesha)

Reaction of MeLi (pre-equilibrium)

Number of “Machines” [RO-OR]1/2

1/4 [CH3Li] [(CH3Li)4]

(steady state)

dominantreactive (CH3Li)44 CH3Li

Radical-Chain Initiator

RO-OR 2 RO• ROH + •R’H-R’

prop •XH-R’

H-X

term

X-XX-R’R’-R’

Union of Ideasi.e. Progress

3. Consider the chlorination reaction :

i-Pr2NCl + RH i-Pr2NH + RCl

and these approximate bond dissociation energies (kcal/mole): N-Cl (46), R-Cl (85), N-H (92), R-H (100).

G. (3 min) Why should the BDEs of N-Cl and R-Cl be so different, when those for N-H and R-H

are so similar?

From 2009 Exam

Could e-Pair Repulsion Explain BDEs?

1.79 Å

1.73 Å1.40 Å

1.53 ÅH3C CH3

HO OH H2N Cl

H3C Cl

84BDE 90 kcal/mole

51 46

Total e-Density Contour (a0-3)

0.002

0.01

0.050.25

Drawing proton(s) away from nucleus removes OMO-OMO e-density from overlap region.

isoe

lect

roni

c

isoe

lect

roni

c

from Wikipedia

Lone pair repulsion seems a plausible explanation for weakening O-O vs. C-C or N-Cl vs. C-Cl.

But might electronegativity help explain stronger C-Cl than N-Cl ?

C+ Cl-

Which Bond is Stronger N-Cl or C-Cl?

Cl N

Ele

ctro

n E

nerg

y

separate separate

C

Compared to What?

••••

N-Cl stronger if forming Ions (N+ Cl-)

••

C-Cl stronger if forming Radicals (Cl C)• •

together

mismatch aids Heterolysis

mismatch hinders Homolysis

BDE

•

•

•

First use in English (O.E.D.)

1837 J. D. Dana Syst. Mineral. 82 When chemistry has so far advanced, that the relative electro-negativity, (if I may so call it,) or electro-positivity, of the several elements, is fully known,..we shall probably be able to construct a natural arrangement of minerals on chemical principles.

Dana House24 Hillhouse Avenue

1849

“Electronegativity” and Bond Strength

James Dwight Dana1813-1895

1823-1907

HenriettaBenjamin Silliman1779-1864

Silliman

by k

ind

perm

issi

on o

f th

e ow

ners

by k

ind

perm

issi

on o

f th

e ow

ners

Dana

“Electronegativity” and Bond Strength

http

://no

belp

rize.

org/

Linus Pauling1901-1994

J. Am. Chem. Soc.

H-X“normal”(average of

H-H and X-X)

H+ X-

1932Pauling was

pushing resonance.

Why not use to measure resonance

stabilization?

in Pauling’s theory

(ele

ctro

n vo

lt =

23.

06 k

cal/m

ole

)

Pauling thought F-F was 65 kcal/mole. Actually it is 38.

Observed

or A:B = (A:A * B:B)1/2arithmetic meangeometric mean

1932Pauling was

pushing resonance.

~ additive!O-F polar “resonance stabilization”

BDE (obs)

(units of electron volts)

O-O O-F

F-F(actually 1.65)

OF ≠ (OO+FF)/2

“Normal”[OF = (OO+FF)/2]

C-F – C-O ≠ O-F

≠ polarity difference

C-F½ – C-O

½) ≈ O-F½

C-X½

C-F C-O

Relative to H & F

0.48Relative to O

Relative to C

Relative to H

1.00

1.48

1932Pauling was

pushing resonance.

0.34

0.58

Mapped toa Distorted

Periodic Table

(screened) nuclear charge

HO

AO

nod

e #

1932Pauling was

pushing resonance.

Is it surprising that bond strength correlates with differences in Pauling’s electronegativities, P?

No, his Pscale was defined by differences in bond strength.

“By the 1960s, for all Pauling’s salesmanship, MO theory was generally agreed to be more con-venient [than his resonance theory] for most purposes.” .

pp. 26-28 Beyond the Bond “More than ever before, new techniques show the bond to be a convenient fiction, albeit one that holds the field of chemistry together, finds Philip Ball.”

Jan. 6, 2011 (International Year of Chemistry)

from Wikipedia

Mulliken Electronegativity (1934) average of Atomic Ionization Potential and Electron Affinity

Pau

ling

Ele

ctro

nega

tivity

Still we expect energy-mismatch

to strengthen bonds, so crude correlation of P with IP and EA

is hardly surprising.

A A+ + e A A + e

Chemical & Physical Properties of Alkyl Halides

Non-Bonded Interactions and Solvation (key for ionic reactions)

Ionic Chemistry of * (SN & pKa)

Free-radical halogenation introduces a functional group (*) into alkanes.

X R :NuHSN1SN2pKa

PV = nRTHenri Victor

Regnault(1810-1878)

For chemical affinity to act freely, substances must be dispersed, and since dispersion by mechanical pulverisation is incomplete, they must be taken into the liquid or gaseous state. Previous chemists expressed this fact by saying:

Corpora non agunt,nisi soluta

Premiers Éléments de Chimie ~1850

[

]

The theory of organic chemistry became manageable because it is often possible to focus on a simple unit with strong interactions (molecules & 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.

(gravity & magnetism are for wimps; the “strong force” is for physicists)

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]

78

49

33

25

21

5

4

2

H2O

(CH3)2S=O

CH3OH

CH3CH2OH

(CH3)2C=O

CHCl3

(CH3CH2)2O

n-hexane

Solubility of NaCl vs. Me4NI?

(e.g

. J&

F T

able

6.7

p. 2

39)

NaCl (mp 801°C)

vs. CsI (mp 621°C)

+- +

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 the energies fall off rapidly with increasing distance.

Halide Trends (e.g. J&F 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

- + - +

- +

-+ -+

- +

from J&F

End of Lecture 41Jan. 19, 2011

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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