determination of reaction mechanisms
TRANSCRIPT
INTRODUCTIONReaction mechanism is the step by step
sequence of elementary reactions in a chemical change
It describes about the reactant involved,intermediates and products
4
Methods 0f Determining of Mechanism1 Identification of products
2 Determination of the presence of intermediates
2.1 Isolation of intermediates
2.2 Detection of intermediates
2.3 Trapping of intermediates
2.4 Addition of a suspected intermediate
3 Study of catalysis
3.1 General acid catalysis
3.2 Specific acid catalysis
4 Labeling study
4.1 Group labeling
4.2 Isotope labeling
.4.3 Crossover experiments5
5 Stereochemical study
6 Kinetic studies
7 Kinetic isotope effects7.1 Deuterium isotope effects
DETERMINATION OF MECHANISM
IDENTIFICATION OF PRODUCTS Mechanism must be compatible with its products including the by-product.
6
7
CH4 + Cl Cl CH3 Clh ν
This reaction fail to account for the formation of small amount of ethane
Eg:
CH3
O
NH2 N a O B r
H 2 O
- C O 2
CH3 NH2
1
Eg:2
Determination of the presence of intermediates
Isolation of intermediatesIsolate the intermediate which can give the same products
when subjected to the same reaction conditions
8
Determination of the presence of intermediates
Detection of an intermediateIn 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).
10
11
H H
Cis - stilbene
R S .
C
HRS H
- R S .
H
H
trans -stilbene
Eg:-Free radical can be determined by addition of double bonded compound in the reaction mixture and it’s fate is traced
Determination of the presence of intermediates
Trapping of an intermediateIn 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
12
F
Br
O
O
Lithium
Benzyne
(trap)
Determination of the presence of intermediatesAddition of a suspected intermediate
Perform a reaction by using a suspected intermediate obtained by other means can be used for a negative evidence.
Eg : Von Richter rearrangement
13
CO2H
CO2H
von Ritchercondition
von Ritchercondition
NO2
CN
Study of catalysis
Mechanism must be compatible with its catalysts , initiator and inhibitors.
Utilization of catalytic amount of peroxide and iodine usually suggests a radical mechanism.
15
Kinetic study of acid-base catalyzed reaction can reveal the rate determination step (RDS.) if it is involved with the proton transfer process
A) General acid (or base) catalysis usually indicates that the proton transfer process is the RDS
B) Specific acid (or base) catalysis usually indicates that the proton transfer process is not the RDS
16
A) General acid (or base) catalysis
17
• In general acid catalysis all species capable of donating protons contribute to reaction rate acceleration.
• The strongest acids 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 catalysis.)
18
B) 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 .
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-
Eg: Acid catalysed aldol reaction
19
Labeling study
A) Isotope labeling: Difficult to obtain the starting materials but no group alteration to affect the mechanism.
B) Crossover experiments: The experiments are closely related to either group or isotope labeling.
20
D can be detected by NMR, IR and MS13C can be detected by 13C-NMR and MS14C can be traced by its radio activity15N can be detected by 15N-NMR18O can be detected by MS
e.g.
21
RCOO- + BrCN RCN* *
Isotope labeling
N C Br
R C O-
O
N
C O
C Br
O-
R C
N C O
R R C
N C
OO
Oisolatedintermediate
R
C
N
O
C
O
+
+
Isotope labeling The hydrolysis of ester proceed through “alkyl” or
“acyl” cleavage
22
R O
O
R' H218O R 18OH
O+ R'OH
R 18O
O
R' H2O R OH
O+ R'18OH
In these cases, the products can be easily identified by MS.
Crossover ExperimentsUse for distinguishing between intra- and intermolecular reaction
23
A B AB
A' B' A'B'
+ +
A B AB
A' B' A'B'
+ +
+
+
A'B
AB'
+
crossoverproducts
• Crossover products indicate intermolecular reaction.
• The method requires identification of products in the mixture.
.
No crossover product
Crossover ExperimentsIs benzidine rearrangement an inter- or intramolecular process?
24
HN
HN
H2N NH2
HN
HN
H2N NH2
OR ORRO OR
HN
HN
OR' OR'
H2N NH2
R'O OR'
No crossover product indicates an intramolecular rearrangement
R=OCH3 R`=OC2H5
Stereochemical evidencesSN2 reaction
25
OTs OAc
KOAc
OTs OAc
KOAc
and
• 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.
Ph
H CH3
OTsAcOδ− δ−
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)
26
Kinetic isotopic effectExchange of an atom in the starting product of a
reaction for one of its heavier isotope leads to reduction in reaction rate
When a hydrogen in a reactant molecule is replaced by deuterium, there is a change in the rate
Such changes are known as deuterium isotope effect
It is expressed by KH/kD
27
Types of kinetic isotope effects
Primary kinetic isotope effectsSecondary kinetic isotope effects
i.Normal
ii.Inverse Kinetic solvent isotope effects
28
Primary kinetic isotopic effectsWhen reaction rate is decreased by the
exchange of D for a H from the X-H(X=C, N, O, S, etc..) that is cleaved during rate-determining step
Elevation of activation energy of RDS
KH/kD is up to 7Extent of bond cleavage or formation can be
inferred
29
Secondary kinetic isotope effectsThe chemical bond varied by the isotopic exchange is not
cleaved during the reaction
KH/kD is 0.7 to 1.5
Normal secondary kinetic isotope effect (KH/kD > 1) reaction rate is reduced by exchanging an atom for one its heavier isotopes
Inverse secondary kinetic isotope effect(KH/kD <1)
reaction rate is increases by exchanging an atom for one its heavier isotopes
30
Kinetic solvent isotope effectsReaction rate is influenced by an isotopic
exchange of the solvent, rather than that of a reactant that has been exchanged for one of its isotopes
Eg: H2O is exchanged for D2O
31
RefeRenceJeRRy maRch; “advanced oRganic
chemistRy” 4th edition page no:297-
327
www.chemgapedia.com
www.wikipedia.com