chemistry 125: lecture 45 january 29, 2010 nucleophilic substitution and mechanistic tools: rate...
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Chemistry 125: Lecture 45January 29, 2010
Nucleophilic Substitutionand Mechanistic Tools:
Rate Law, Rate Constant,Structure This
For copyright notice see final page of this file
Stereochemistry
Rate Law
Rate Constant
StructureX-Ray and Quantum Mechanics
Tools for Testing(i.e. Excluding) Mechanisms:
LeavingGroupSubstrate
Rate Constant Dependance onSolvent
Nu: R-L Nu-R L(+) (-)
Product
80
1,000
10,000
16,000
126,000
Nucleophile
[1]
krel
Br-
F-
H2O
HO-
Cl-
Nu
HS-Sec. 7.4d, Table 7.3
I- 80,000
-8
-9
7
-10
3.2
15.7
-1.7
pKa (NuH+)
For first-row elementsnucleophilicity (attack
C-L )
parallels basicity (attack H+).Both require high HOMO.
But as atoms get bigger, they get better at attacking
C-L (compared to attacking H+)
Solvent
LeavingGroupSubstrate
Rate Constant Dependance onNu: R-L Nu-R L
(+) (-)
Nucleophile
80
1,000
10,000
16,000
126,000
[1]
krel
Br-
F-
H2O
HO-
Cl-
Nu
HS-Sec. 7.4dg
I- 80,000
-8
-9
7
-10
3.2
15.7
-1.7
pKa (NuH+) krel
CH3I in H2O
[1]
14
160
krel
CH3Br in Acetone
11
5
[1]
harderto break H-bonds
to smaller ions
Polar solvents accelerate reactions that generate (or concentrate) charge,
and vice versa.
Sen
sibl
e
Backw
ards
Nucleophile
Solvent
Rate Constant Dependance onNu: R-L Nu-R L
(+) (-)
Sec. 7.4e
Substrate
goodRSO2O- -3
LeavingGroup
bad
good
good
v. bad
bad
good
Br-
F-
H2O
HO-
Cl-
HS-
I- v. good
-8
-9
7
-10
3.2
15.7
-1.7
L pKa (LH+)Weak bases are
good leaving groups(H like R,
as expected)
R-OH v. bad
R-OH2+ good
R-OSO2R’ good(Kenyon/Phillips)
(acid catalysis)
OH Leaving-Group-Trick Lore (sec 7.4f)pKa ~16 pKa -1.7
OH2+
Ether / HBr
Good Leaving Group
Good Nucleophile
OH Leaving-Group-Trick Lore (sec 7.4f)
Cf. Kenyon & Phillips (1923)
OSO2R
pKa -3
OH Leaving-Group-Trick Lore (sec 7.4f)
OSOCl
gases
OH Leaving-Group-Trick Lore (sec 7.4f)
OPXn
OH Leaving-Group-Trick Lore (sec 7.4f)
OP+
1
2
Maximizing Synthetic Speed to support PET scanning (from Loudon, Org. Chem.)
e- e+ 18O= + 7 MeV proton
- neutron 18F- t1/2
110 min
http://en.wikipedia.org/wiki/Positron_emission_tomography
Need to get 18F where cancer isor
11C t1/2 ~
20 min
13N t1/2 ~
10 min
15O t1/2 ~
2 min
positron
Connecting coincident scintillations tells where 18F’s were.
and you have to do it within a couple
hours.
Yale PET
What tosynthesize?
Problems :K+
trifluormethanesulfonate
(Triflate)
Maybe it would suck up 2-Fluoroglucose as well.
Rapid metabolism of tumor sucks up glucose.
AcO = CH3C-O-(acetate protecting group)
O
F tied up by H-bondingand by K+cation
inversion gives wrong configuration
HO a horrid leaving group
wrong C-OH could be attacked
Glucose
start withMannose
2-FluoroglucoseKF18
SN2?
Cl-SO2CF3
well known for sugarsK+ F18-
CH3CN(aprotic solvent)
H2O H+
Mannose to 2-F18Glucose - ASAP
F18
Office Hours 3-4:30
this afternoon181 SCL
Loudon, Organic Chemistry
Stereochemistry
Rate Law
Rate Constant
StructureX-Ray and Quantum Mechanics
Tools for Testing(i.e. Excluding) Mechanisms:
So far we’ve just been beating up on the D/A mechanism
(trivalent C intermediate)
though there are cases (SN1) where it in fact applies.
The tougher problem is to distinguish between concerted and A/D with a very weakly stabilized intermediate.
(see supplementary reading on Course website)
Might there be Pentavalent A/D Intermediate instead of a Concerted SN2 Transition State?
PentavalentIntermediate
Nu LCNu LC
TransitionState
Quantum Mechanics says Transition State
for OH- attackingless crowded CH3OH.
1.88 Å 1.88 Å
Quantum Mechanics says Transition State
for H2O attacking protonated t-BuOH.
2.64 Å 2.64 Å
But neither reaction is practical in the laboratory!What does experiment say? X-ray?
Might there be Pentavalent A/D Intermediate instead of a Concerted SN2 Transition State?
Problem: Neither Transition State nor Intermediate would hold together long enough to study.
PentavalentIntermediate
Nu LCNu LC
TransitionState
HOYWAT
JACS 4354-4371 (2005)
Nu LC Held in place by molecular framework
+
HOYWAT+
JACS 4354-4371 (2005)
CH3
O OC
+CH3-O(CH3)2 BF4
-
: :ARE THERE BONDS HERE?
OCH3+NOT elongated to reflect superposed
average of two “bell-clapper” structures.
F3BF- BF3
2.64 Å 2.64 Å
Powerful alkylating agent like
“Meerwein’s . Reagent”
Et3O+ BF4-
5.02 Å
4.86 Å shortened by 0.16 Å
BUT without central C+ etc. shortened by 0.21 Å!
4.96 Å
4.75 Å
125° 114°125° 113°Pentavalent C
attraction?
Eclipsed repulsion
bent inbent in
etc.
Pentavalence seemed to be a safe inference
5.02 Å
4.86 Å
5.08 Å
5.00 ÅO
SiO2
CF3
124° 116°125° 113°
Central O only slightly repulsive compared to C+.
Eclipsed repulsion
126° 112°4.92 Å
4.56 ÅBF2
BF2 does seem to suck in CH3O groups.
Eclipsed repulsion
125° 113°5.02 Å
4.86 Å
Double minimum with stronger nucleophile O-
(higher HOMO& lower LUMO)
127° 109°
4.84 Å
1.47 Å 2.99 Å
CF3CF3
-O
K+and . nearby
1.88 Å 1.88 Å
Range
Bonded O (or S)
seems to “use up” the vacant AO.
CCH3O OCH3+
CCH3O OCH3
+
Higher neighbor HOMOs favor tetravalence.
For F withdrawal dominates donation.
Compared to what?
AX
A-X
dis
tanc
es (
Å)
Compound
Short & Long X A X Distances
Hnonbondreference
B“tight”Bonded?
Btetracoordinate
C+
C+
~ equalsymmetrical
very differentunsymmetrical
B“loose”
like H
No sign of stabilityfor pentavalent
SN2 “intermediate”
transistion state transistion state (as calculated by q. mech.)
Pressed in
by HCH3 repulsion
F CH2
CH2 H :OH
"E2 Elimination"
ABNABN
AON
F
H OH
CH2
CH2
E2 -Elimination Text sec. 7.9
Rate influenced by [base], nature of leaving group,
H isotope (kinetic isotope effect)
C HH OC H O
DD D
kH > kD but only if bond is weakened in rate-determining transition state
Heavier atom, lower ZPE
see Lecture 8: frames 21-22
ZPE(kinetic)
E2 -Elimination Text sec. 9.5
Rate influenced by [base], nature of leaving group,
H isotope (kinetic isotope effect)
Stereochemistry (Anti) sec 9.5G
Regiochemistry (Zaitsev/Hammond) sec 9.5F
E2 vs. SN2 (Sterics & Base Strength) sec 9.5G
N C and RC C as Nucleophiles
Cyanide Table 9.6 p. 396
Acetylide Sec. 14.7B pp. 665-666
Ethylene Oxide as C2 Source
Synthesis GamesStudy Problem 14.2 p. 666-667
Problems 14.18-14.23 p. 666-668
0
-1
1
-2
-3
-4
log
(fra
ctio
n of
R-B
r co
nver
ted
to H
OR
/min
)
(CH3)3C(CH3)2CH
SN1
Hughes Ingold
(1933-1940)
CH3 CH3CH2
EtOH/H2O (4:1)55°C
NaOH + R-Br HO-R + NaBr
k2 (M-1min-1)
concerted displacement
slowed by crowding
k1 (min-1)
D/Aaccelerated
by crowding,
(CH3)3C+cation stabilization,
polar solvent
(0.01 M)
plus ~19%
E2
Rate extrapolated from lower temperature.
Depends on [OH-]
SN1 and E1 sec. 9.6
Product Determined After Rate 9.6B
By Competition for Short-Lived Cation
Rearrangement of Short-Lived Cation 9.6C
Net Inversion from Short-Lived Ion Pair 9.6D
EtOH/H2O (4:1)55°C
NaOH + tBu-Br HO-t-Bu + NaBr(0.01 M)
+ CH2=C(CH3)2
E2 or E1? How do you tell?
Overall rate (and % alkene) depends on [OH-]
Kinetic Isotope Effect shows whether H is being transferred in rate-determining step.
CH3-Br + OH-
5. (5 min) Give a real example of the influence of a change of reactant structure on the ratio of SN2 to E2 products. Be as specific and quantitative as you can. (You will need to show the ratios for two different reactants.)
(CH3)3C-Br + OH-
Perspectives on Drastic Product Ratios
Synthetic Organic Chemist : Reliable High-Yield Tool
Physical-Organic Chemist : Definitive Ea DifferenceUnambiguous interpretation of cause
e.g. syn- vs. anti-hydrogenation of acetylene
e.g. Steric retardation of SN2 / 105 acceleration for t-Butyl via SN1
Perspectives on 50:50 Product Ratios
Physical-Organic Chemist : Valuable “Borderline” Reference
Synthetic Organic Chemist : Deadly Influence on 12-Step Synthesis
(1/2)12 = 0.02% Yield
(Might provide optimizable lead)
Allows Sensitive Tests of Subtle Influences.e.g. isotope effect by competition
A lesson from E2 Elimination
If Step 1 (motion) is rate-limiting, H- and D-transfer products should
form in equal amounts. (because their motions should be equally fast)
If Step 2 (atom shift) is rate-limiting, more H-transfer product should form.
kH/kD > 1 (kinetic “isotope effect”)
In a Very Viscous Solvent Can Short-Range Motion Constitute a Rate- (and Product-) Determining Step?
Generates steric hindrance & requires moving radicals past N2
N
N
CH3
CH3
H3C CD3
CD3
CD3
•
•
UV LightCH3
CH3
H3C
CD3
CD3
CD3
Radical-PairCombination
CH3
CH3
H3C CD2 CD3
CD3
•
•
DD
(1) Rotate N2 + C4D9
(2) Shift D atomexothermic/easy/fast
N
N
Radical-Pair“Disproportionation”
(1) Rotate N2 + C4H9
(2) Shift H atomexothermic/easy/faster
CD3
CD3
CD3
CH3
H3CCH3 HCH2
•
•
Jo David’s Question:
NN
NN
t-Butylhydrazine
CH3
CH3
H3C NH
NH2
CH3
CH3
H3C Cl NH2
NH2
(prepare from)
CH3
CH3
H2C
?To do his project, Jo David needed to prepare this compound.
E2 >> SN2
CH3
CH3
H3C N
N
CD3
CH3
CD3
CD3
Smith-Lakritz
It is very common to change a C=X double bond into
C=O and H2X (we’ll discuss this soon)
C=N-R C=O + H2N-R
NN- +
t-Butylhydrazine
CH3
CH3
H3C NH
NH2
CH3
CH3
H3C MgCl
??? Jo David FineApril-October 1971
O
CH3
CH3
H3C N
N
CD3
CH3
CD3
CD3
Jo David Fine
Jo David Fine Notebook p. 91 (October 1971)
Jo David is now a respectedprofessor of dermatology at Vanderbilt University, whose son has graduated from Yale.
Happy Ending:
Crucial Lesson (from S. Nelsen, U. Wisc.)
CH3
CH3
H3C NH
NH2
CH3
CH3
H3C Cl NH2
NH2
CH3
CH3
H2C95%
5% SN1When you need a compound, % yield isn’t everything!
HCl salt easilypurified by
crystallization
E1 / E2
Major product a gas,just “goes away”
CH3
CH3
H3C N
N
CD3
CH3
CD3
CD3
Happy Ending:
Jo David Fine’s successor found that in fluid solvents, there was more H- than D-transfer (atom transfer is rate-limiting), but that in very viscous solvents at low temperature this “kinetic isotope effect” disappeared (there were equal amounts of H- and D-transfer), because motion
had indeed become rate-limiting.
End of Lecture 45Jan. 29, 2010
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