Chemistry 125: Lecture 41 January 19, 2011 Fractional-Order Kinetics Electronegativity & Bond...
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Transcript of Chemistry 125: Lecture 41 January 19, 2011 Fractional-Order Kinetics Electronegativity & Bond...
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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