3 Determination of Mechanism

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3 Determination of Mechanism

• Philosophy of mechanistic studies
• No reaction could be determined with 100
  certainty.
• One can only disproof a hypothetical mechanism,
  not proof.
• As the result, an approved, last mechanism is
  said to be reasonable, not correct.
• More than one method would be needed to confirm,
  and their results must all be consistent.
• Gather information from many experiments until
  enough to induce or extrapolate to a general
  conclusion.
• Occams razor In the event that several
  hypotheses are found to fit the facts, the
  simplest one is given preference.

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3. Determination of Mechanism

• 3.1 Identification of products
• 3.2 Determination of the presence of
  intermediates
• 3.2.1 Isolation of intermediates
• 3.2.2 Detection of intermediates
• 3.2.3 Trapping of intermediates
• 3.2.4 Addition of a suspected intermediate
• 3.3 Study of catalysis
• 3.3.1 General acid catalysis
• 3.3.2 Specific acid catalysis
• 3.4 Labeling study
• 3.4.1 Group labeling
• 3.4.2 Isotope labeling
• 3.4.3 Crossover experiments

3.5 Isomeric selectivity study 3.5.1
Regiochemical evidences 3.5.2 Stereochemical
evidences 3.6 Kinetic studies 3.6.1 Measurement
of rate 3.6.2 Mechanistic information obtained
from kinetic studies 3.6.3 Rate law 3.7 Kinetic
isotope effects 3.7.1 Deuterium isotope
effects 3.7.2 Primary isotope effects 3.7.3
Secondary isotope effects 3.7.4 Solvent isotope
effects
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3. Determination of Mechanism

• 3.1 Identification of products
• Mechanism must be compatible with its products
  including the by-product.
• e.g. von Richter Rearrangement

At the first glance, the mechanism was though as
a simple nucleophilic substitution of NO2 by CN-
followed by the hydrolysis of CN- to CO2H
However,
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Early proposed mechanism
But, from its product study, none of the NO2 or
NH3 gas was found, instead, the N2 gas was
detected.
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The mechanism was then fixed as follow
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3.2 Determination of the presence of intermediates

• 3.2.1 Isolation of intermediates
• Isolate the intermediate which can give the same
  products when subjected to the same reaction
  conditions at a rate no slower than the starting
  compound
• e.g. Hofmann rearrangement
• Neber rearrangement

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3.2 Determination of the presence of intermediates

• 3.2.2 Detection of an intermediate
• In many cases, intermediate cannot be isolated
  but can be detected by IR, NMR, UV-Vis or other
  spectra.
• Radical and triplet species can be detected by
  ESR and by Chemically Induced Dynamic Nuclear
  Polarization (CIDNP).
• Radicals can also be detected by cis-trans
  isomerization of stilbene.

Caution Beware of non-intermediate species and
impurities which may give interference signals.
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3.2 Determination of the presence of intermediates

• 3.2.3 Trapping of an intermediate
• In some cases, the suspected intermediate is
  known to be one that reacts in a given way with a
  certain compound.
• Benzynes react with dienes in the Diels-Alder
  reaction

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3.2 Determination of the presence of intermediates

• Trapping an anion to determine if the elimination
  of alkenes is E2 or E1cb.

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3.2 Determination of the presence of intermediates

• Examples of free radical trapping agents are
  DPPH, oxygen (O2), triphenylmethylradical
  (Ph3C?), nitric oxide (NO), imine oxide, iodine,
  hydroquinone and dinitrobenzene.
• A radical reaction may proceed slower in the
  presence of air if the free radical intermediate
  can be trapped by O2.

Imine Oxide
DPPH
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3.2 Determination of the presence of intermediates

• Kinetic requirement of intermediate trapping

- The intermediate B can be efficiently trapped
by X? when k2? ? k2. - The detection of D does
not always guarantee the formation of B
intermediate as A may directly react with X? to
form D.
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3.2 Determination of the presence of intermediates

• 3.2.4 Addition of a suspected intermediate
• Perform a reaction by using a suspected
  intermediate obtained by other means can be used
  for a negative evidence.
• e.g. von Ritcher reaction

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3. Determination of Mechanism

• 3.3 Study of catalysis
• Mechanism must be compatible with its catalysts ,
  initiator and inhibitors.
• Utilization of catalytic amount of peroxide, AIBN
  and iodine usually suggests a radical mechanism.
• Kinetic study of acid-base catalyzed reaction can
  reveal the rate determination step (rds.) if it
  is involved with the proton transfer process
• 3.3.1 General acid (or base) catalysis usually
  indicates that the proton transfer process is the
  rds.
• 3.3.2 Specific acid (or base) catalysis usually
  indicates that the proton transfer process is not
  the rds.

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3.3.1 General acid (or base) catalysis

• In general acid catalysis all species capable of
  donating protons contribute to reaction rate
  acceleration.
• The strongest acids (SH) are most effective (k1
  is the highest).
• Reactions in which proton transfer is
  rate-determining exhibit general acid catalysis,
  for example diazonium coupling reactions.
• When keeping the pH at a constant level but
  changing the buffer concentration a change in
  rate signals a general acid catalysis. (A
  constant rate is evidence for a specific acid
  catalyst.)

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3.3.2 Specific acid (or base) catalysis

• In specific acid catalysis taking place in
  solvent S , the reaction rate is proportional to
  the concentration of the protonated solvent
  molecules SH.
• The acid catalyst itself (AH) only contributes to
  the rate acceleration by shifting the chemical
  equilibrium between solvent S and AH in favor of
  the SH species. S AH ? SH A-
• For example, in an aqueous buffer solution the
  reaction rate for reactants R depends on the pH
  of the system but not on the concentrations of
  different acids.
• This type of chemical kinetics is observed when
  reactant R1 in a fast equilibrium with its
  conjugate acid R1H which proceeds to react
  slowly with R2 to the reaction product for
  example in the acid catalyzed aldol reaction.

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3.3 Study of catalysis

• Diazonium coupling shows general base catalysis.
  Which step is the rds.?
• Aldol reaction shows specific acid catalysis.
  Which step is the rds.?

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3. Determination of Mechanism

• 3.4 Labeling study
• 3.4.1 Group labeling Easy to obtain starting
  materials but the group change may alter the
  mechanism.
• 3.4.2 Isotope labeling Difficult to obtain the
  starting materials but no group alteration to
  affect the mechanism. (Isotopic scrambling can
  complicate the interpretation of the results.)
• 3.4.3 Crossover experiments The experiments are
  closely related to either group or isotope
  labeling.

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3.4.1 Group labeling

• Is Claisen rearrangement a 1,3 or 3,3
  sigmatropic process?

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3.4.2 Isotope labeling

• D can be detected by NMR, IR and MS
• 13C can be detected by 13C-NMR and MS
• 14C can be traced by its radio activity
• 15N can be detected by 15N-NMR
• 18O can be detected by MS
• e.g.

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3.4.2 Isotope labeling

• Does the hydrolysis of ester proceed through
  alkyl or acyl cleavage?

Labeled water is easier to find than the labeled
ester.
In these cases, the products can be easily
identified by MS.
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Exercises

• Do the following ethanolyses of ?-lactone involve
  alkyl or acyl cleavage?
• Do the following hydrolyses of ?-lactone involve
  alkyl or acyl cleavage?

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3.4.3 Crossover Experiments

• Use for distinguishing between intra- and
  intermolecular reaction

• Crossover products indicate intermolecular
  reaction.
• The method requires identification of products in
  the mixture.
• The method cannot distinguish between an
  intramolecular and solvent cage reactions.

No crossover product
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3.4.3 Crossover Experiments

• Is benzidine rearrangement an inter- or
  intramolecular process?

No crossover product indicates an intramolecular
rearrangement
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3.4.3 Crossover Experiments

• Is 1,2 rearrangement of alkyl lithium an inter-
  or intramolecular process?

Upon an addition of14C-labeled phenyl lithium
(Ph-Li), no 14C-labeled product was detected,
indicating no intermolecular process involved.
Upon an addition of14C-labeled benzyl lithium
(PhCH2-Li), the 14C-labeled product was
detected, indicating an intermolecular process.
This is called labeled fragment addition technique
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3. Determination of Mechanism

• 3.5 Isomeric selectivity study
• Selectivity Non-statistical distribution of
  products
• Specificity Correspondence between isomeric
  ratios of starting materials and products
• Level of isomeric selectivity chemoselectivity ?
  regioselectivity ? diastereoselectivity ?
  enantioselectivity
• 3.5.1 Regiochemical study
• 3.5.2 Stereochemical study

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3.5.1 Regiochemical evidences

• HX addition on alkenes

• Regioselectivity suggests radical mechanism.
• Solvent polarity has no effect on the reaction
  rate supporting the radical mechanism.

• Regioselectivity suggests cationic mechanism.
• Polar solvents increase the reaction rate
  supporting the polar mechanism.

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3.5.1 Regiochemical evidences

• Aromatic substitution by strong basic nucleophiles

Possible mechanisms SNAr or benzyne
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3.5.1 Regiochemical evidences

• The benzyne mechanism was supported by
  regiochemical evidences obtained from group and
  isotope lebeling

11 ratio
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3.5.1 Stereochemical evidences

• SN2 reaction

• The reaction is stereospecific with 100
  inversion indicating that the reaction is
  concerted and the nucleophile attacks from the
  back side of the leaving group.
• The proposed transition state is a trigonal
  bipyramid.

and
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3.5.1 Stereochemical evidences

• Neighboring group participation (NGP)

The reaction is not stereospecific but
diastereoselective. Both diastereomers give the
same major product. The results suggest a common
intermediate for all diastereomers.
The stereochemistry is controlled by the
intermediate not by the starting material.
Which one is the major product?
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3.5.1 Stereochemical evidences

• Neighboring group participation (NGP)

Each reaction involves NGP in which an
intermediate with 2 reactive sites is formed.
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3.5.1 Stereochemical evidences

• Addition

Anti addition in which a bromonium ion was
proposed as an intermediate.
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3.5.1 Stereochemical evidences

• Photorearrangement of spirofuran

Possible mechanisms pericyclic or biradical
Stereospecific product
Racemic product
?
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3. Determination of Mechanism

• 3.6 Kinetic studies
• 3.6.1 Measurements of rate
• 3.6.2 Mechanistic information obtained from
  kinetic studies
• 3.6.3 Rate law

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3.6.2 Measurement of rate

• Real Time Analysis by Periodic or Continuous
  Spectral Readings
• Quenching and Analyzing
• Removal of Aliquots at Intervals

(Rate Expression)
k rate constant kobs rate constant
directly obtained experimentally molecularity
number of molecules come together in a single step
N stoichiometric number nA order of
reaction for reactant A ?ni order of overall
reaction
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3.6.2 Measurement of rate
Zeroth order
First order
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3.6.2 Measurement of rate

• Second order

Use pseudo first order B0gtgtA0 B constant
B0 Treat like first order
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3.6.3 Mechanistic information obtained from
kinetic studies

• Order of reaction can give information about
  which molecules take part in rate determining
  step and the previous steps.
• Changes in rate constants upon structural and
  condition changes can give much information about
  mechanisms. (Linear free energy relationships)
• From transition state theory, rate constants
  measured at various temperature can lead to
  important energetic parameters.

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3.6.3 Rate law

• First order Rate kA (rds. is unimolecular
  process)
• Second order Rate kA2 or Rate kAB
• Order is for the whole reaction while
  molecularity is the order for each step.
• Rate law depends on the rate-determining step.
• The first step is the rate-determining step
• Rate kAB

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3.6.1 Rate Law

• The first step is a rapid equilibrium
• Rate -dA/dt k1AB - k-1I
• dI/dt k1AB - k-1I - k2IB
  k1AB - (k-1 k2B)I
• Steady state assumption dI/dt 0 
• I k1AB/(k-1 k2B)
• Therefore
• Rate k1AB - k1k-1AB/(k-1 k2B)
• Rate k1k2AB2/(k-1 k2B)
• For rapid equilibrium in the first step k-1I
  k2IB or k-1 k2B
• Thus Rate K1k2AB2

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Exercise

• Using the steady state assumption, derive a rate
  expression for the following reaction if (a) the
  first step is a rate determining step, (b) the
  first step is a fast equilibrium.

• Rate -dA/dt k1A - k-1B
• dB/dt k1A - k-1B - k2B
• Steady state assumption dB/dt 0
• B k1A/(k-1 k2)
• Rate k1A - k-1k1A/(k-1 k2)
• Rate k1k2A /(k-1 k2)
• k-1 ltlt k2 Rate k1A
• k2 ltlt k-1 Rate Kk2A

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Exercise

• Condensations

a)
Rate kCH2(COOEt)2 CH2O OH-
The reaction between the enolate and formaldehyde
is the rds.
b)
Rate kCH3CHO OH-
The formation of the enolate is the rds.
Write a reasonable mechanism and specify the rds.
of each reaction.
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3.7 Kinetic Isotope effects

• 3.7.1 Deuterium isotope effects (kH/kD) is the
  ratio between the rate of reaction of the
  protonic substrate and that of the corresponding
  deutero substrate.
• A normal isotope effect has kH/kD gt 1 indicating
  that the reaction of the protonic substrate is
  faster than the reaction of the corresponding
  deutero substrate.
• An inverse isotope effect has kH/kD lt 1
  indicating that the reaction of the protonic
  substrate is slower than the reaction of the
  corresponding deutero substrate.
• 3.7.2 Primary isotope effect is observed in the
  reaction that its rate determining step involves
  the breaking of the bond connecting to the
  isotopic H.
• The primary isotope effects usually have 2
  kH/kD 7.

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3.7.2 Primary isotope effects

• Origin of the primary isotope effects

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3.7.2 Primary isotope effects

• Alcohol oxidation

Gives kH/kD 6.9 The transition state proposed
for the rds. is as follow
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Exercise

• Write a reasonable mechanism and specify the rate
  determining step for the following reaction which
  shows kH/kD ? 7

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3.7 Kinetic Isotope effects

• 3.7.3 Secondary isotope effect is observed in
  the reaction that its rate determining step does
  not involve the breaking of any bond connecting
  to the isotopic H.
• ?-secondary isotope effect usually has kH/kD in
  the range 0.7-1.5. It is the result of the
  greater vibration amplitude of C-H bond comparing
  to C-D bond.
• A normal ?-secondary isotope effect (kH/kD gt 1)
  generally suggests a rehybridization of the
  carbon connecting to the isotopic H from sp3 to
  sp2 in the rate determining step.
• An inverse ?-secondary isotope effect (kH/kD lt 1)
  generally suggests a rehybridization of the
  carbon connecting to the isotopic H from sp2 to
  sp3 in the rate determining step.
• ?-secondary isotope effect has kH/kD gt 1. It is
  mainly attributed to hyperconjugation.

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3.7.3 Secondary isotope effect

• Solvolysis of cyclopentyl tosylate
• normal
• Addition on aldehyde

sp3C-H sp2C-H (kH/kD 1.17)
sp2C-H sp3C-H (kH/kD 0.833)
inverse
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Summary of primary and secondary kinetic isotope
effects
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3.7 Kinetic Isotope effects

• 3.7.4 Solvent isotope effects
• Generally observed when a protic solvent e.g. D2O
  or ROD is used.
• kH/kD lt 1 when the reaction involves a rapid
  equilibrium protonation because the acidity of
  D3O is greater than H3O (specific acid
  catalysis can be used for confirmation)
• kH/kD gt 1 when proton transfer is the rate
  determining step (general acid catalysis can be
  used for confirmation)
• Secondary solvent isotope effect can interfere
  the interpretation. Solvent isotope effect is
  thus used only as a supporting evidence.

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Exercise

• Write a reasonable mechanism for hydration of
  styrene and predict which step is the rate
  determining step. Suggest 3 experiments and the
  expected results that can support the proposed
  mechanism and rate determining step.