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Kinetics isotope effects

It is clear, then, that the measurement of primary kinetic isotope effects will not give a wholly unambiguous clue to mechanism in the absence of other evidence. Nevertheless, the absence of a kinetic isotope effect is most easily understood in terms of the /S 2 mechanism... [Pg.110]

Melander first sought for a kinetic isotope effect in aromatic nitration he nitrated tritiobenzene, and several other compounds, in mixed acid and found the tritium to be replaced at the same rate as protium (table 6.1). Whilst the result shows only that the hydrogen is not appreciably loosened in the transition state of the rate-determining step, it is most easily understood in terms of the S 2 mechanism with... [Pg.110]

One way in which the step of the reaction in which the proton is lost might be slowed down, and perhaps made kinetically important (with i), would be to carry out nitration at high acidities. Nitration of pentadeuteronitrobenzene in 97-4% sulphuric acid failed to reveal such an effect. In fact, nitrations under a variety of conditions fail to show a kinetic isotope effect. [Pg.112]

The cases of pentamethylbenzene and anthracene reacting with nitronium tetrafluoroborate in sulpholan were mentioned above. Each compound forms a stable intermediate very rapidly, and the intermediate then decomposes slowly. It seems that here we have cases where the first stage of the two-step process is very rapid (reaction may even be occurring upon encounter), but the second stages are slow either because of steric factors or because of the feeble basicity of the solvent. The course of the subsequent slow decomposition of the intermediate from pentamethylbenzene is not yet fully understood, but it gives only a poor yield of pentamethylnitrobenzene. The intermediate from anthracene decomposes at a measurable speed to 9-nitroanthracene and the observations are compatible with a two-step mechanism in which k i k E and i[N02" ] > / i. There is a kinetic isotope effect (table 6.1), its value for the reaction in acetonitrile being near to the... [Pg.115]

By protodetritiation of the thiazolium salt (152) and of 2 tritiothiamine (153) Kemp and O Brien (432) measured a kinetic isotope effect, of 2.7 for (152). They evaluated the rate of protonation of the corresponding yiides and found that the enzyme-mediated reaction of thiamine with pyruvate is at least 10 times faster than the maximum rate possible with 152. The scale of this rate ratio establishes the presence within the enzyme of a higher concentration of thiamine ylide than can be realized in water. Thus a major role of the enzyme might be to change the relative thermodynamic stabilities of thiamine and its ylide (432). [Pg.118]

Kinetic isotope effects are an important factor in the biology of deuterium. Isotopic fractionation of hydrogen and deuterium in plants occurs in photosynthesis. The lighter isotope is preferentially incorporated from water into carbohydrates and tipids formed by photosynthesis. Hydrogen isotopic fractionation has thus become a valuable tool in the elucidation of plant biosynthetic pathways (42,43). [Pg.6]

In CLTST there appears a kinetic isotope effect owing to the difference in partition functions in the initial state [see eq.(2.12)], and at 2Pf < o > I5... [Pg.31]

A special type of substituent effect which has proved veiy valuable in the study of reaction mechanisms is the replacement of an atom by one of its isotopes. Isotopic substitution most often involves replacing protium by deuterium (or tritium) but is applicable to nuclei other than hydrogen. The quantitative differences are largest, however, for hydrogen, because its isotopes have the largest relative mass differences. Isotopic substitution usually has no effect on the qualitative chemical reactivity of the substrate, but often has an easily measured effect on the rate at which reaction occurs. Let us consider how this modification of the rate arises. Initially, the discussion will concern primary kinetic isotope effects, those in which a bond to the isotopically substituted atom is broken in the rate-determining step. We will use C—H bonds as the specific topic of discussion, but the same concepts apply for other elements. [Pg.222]

Fig. 4.9. DifiBoing zero-point energies ofprotium- and deuterium-substituted molecules as the cause of primary kinetic isotope effects. Fig. 4.9. DifiBoing zero-point energies ofprotium- and deuterium-substituted molecules as the cause of primary kinetic isotope effects.
Scheme 4.2. Some Representative Kinetic Isotope Effects... Scheme 4.2. Some Representative Kinetic Isotope Effects...
The details of proton-transfer processes can also be probed by examination of solvent isotope effects, for example, by comparing the rates of a reaction in H2O versus D2O. The solvent isotope effect can be either normal or inverse, depending on the nature of the proton-transfer process in the reaction mechanism. D3O+ is a stronger acid than H3O+. As a result, reactants in D2O solution are somewhat more extensively protonated than in H2O at identical acid concentration. A reaction that involves a rapid equilibrium protonation will proceed faster in D2O than in H2O because of the higher concentration of the protonated reactant. On the other hand, if proton transfer is part of the rate-determining step, the reaction will be faster in H2O than in D2O because of the normal primary kinetic isotope effect of the type considered in Section 4.5. [Pg.232]

A number of studies of the acid-catalyzed mechanism of enolization have been done. The case of cyclohexanone is illustrative. The reaction is catalyzed by various carboxylic acids and substituted ammonium ions. The effectiveness of these proton donors as catalysts correlates with their pK values. When plotted according to the Bronsted catalysis law (Section 4.8), the value of the slope a is 0.74. When deuterium or tritium is introduced in the a position, there is a marked decrease in the rate of acid-catalyzed enolization h/ d 5. This kinetic isotope effect indicates that the C—H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone ... [Pg.426]

The distribution of a-bromoketones formed in the reaction of acetylcyclopentane with bromine was studied as a function of deuterium substitution. On the basis of the data given below, calculate the primaiy kinetic isotope effect for enolization of... [Pg.448]

Consider the kinetic isotope effect that would be observed in the reaction of semicarbazide with benzaldehyde ... [Pg.506]

Table 10.6. Kinetic Isotope Effects in Some Electrophilic Aromatic Substitution Reactions... Table 10.6. Kinetic Isotope Effects in Some Electrophilic Aromatic Substitution Reactions...
A substantial body of data, including reaction kinetics, isotope effects, and structure-reactivity relationships, has permitted a thorough understanding of the steps in aromatic nitration. As anticipated from the general mechanism for electrophilic substitution, there are three distinct steps ... [Pg.571]

Bromination has been shown not to exhibit a primary kinetic isotope effect in the case of benzene, bromobenzene, toluene, or methoxybenzene. There are several examples of substrates which do show significant isotope effects, including substituted anisoles, JV,iV-dimethylanilines, and 1,3,5-trialkylbenzenes. The observation of isotope effects in highly substituted systems seems to be the result of steric factors that can operate in two ways. There may be resistance to the bromine taking up a position coplanar with adjacent substituents in the aromatization step. This would favor return of the ff-complex to reactants. In addition, the steric bulk of several substituents may hinder solvent or other base from assisting in the proton removal. Either factor would allow deprotonation to become rate-controlling. [Pg.578]

Friedel-Crafts acylation sometimes shows a modest kinetic isotope effect. This observation suggests that the proton removal is not much faster than the formation of the (j-complex and that the formation of the n-complex may be reversible under some conditions. [Pg.586]

When one of the ortho hydrogens is replaced by deuterium, the rate drops from 1.53 X 10 " s to 1.38 X lO s. What is the kinetic isotope effect The product from such a reaction contains 60% of the original deuterium. Give a mechanism for this reaction that is consistent with both the kinetic isotope effect and the deuterium retention data. [Pg.599]

Indicate mechanisms that would account for the formation of each product. Show how the isotopic substitution could cause a change in product composition. Does your mechanism predict that the isotopic substitution would give rise to a primary or secondary deuterium kinetic isotope effect Calculate the magnitude of the kinetic isotope effect from the data given. [Pg.602]

Consider a reactant molecule in which one atom is replaced by its isotope, for example, protium (H) by deuterium (D) or tritium (T), C by C, etc. The only change that has been made is in the mass of the nucleus, so that to a very good approximation the electronic structures of the two molecules are the same. This means that reaction will take place on the same potential energy surface for both molecules. Nevertheless, isotopic substitution can result in a rate change as a consequence of quantum effects. A rate change resulting from an isotopic substitution is called a kinetic isotope effect. Such effects can provide valuable insights into reaction mechanism. [Pg.292]

We now carry the argument over to transition state theory. Suppose that in the transition state the bond has been completely broken then the foregoing argument applies. No real transition state will exist with the bond completely broken—this does not occur until the product state—so we are considering a limiting case. With this realization of the very approximate nature of the argument, we make estimates of the maximum kinetic isotope effect. We write the Arrhenius equation for the R-H and R-D reactions... [Pg.294]

A kinetic isotope effect that is a result of the breaking of the bond to the isotopic atom is called a primary kinetic isotope effect. Equation (6-88) is, therefore, a very simple and approximate relationship for the maximum primary kinetic isotope effect in a reaction in which only bond cleavage occurs. Table 6-5 shows the results obtained when typical vibrational frequencies are used in Eq. (6-88). Evidently the maximum isotope effect is predicted to be very substantial. [Pg.295]

Table 6-5. Calculated Hydrogen/Deuterium Primary Kinetic Isotope Effects" ... Table 6-5. Calculated Hydrogen/Deuterium Primary Kinetic Isotope Effects" ...
A more rigorous theory of kinetic isotope effects begins with the transition state equation k = (kTlh)K. Writing this for and ito leads to... [Pg.296]


See other pages where Kinetics isotope effects is mentioned: [Pg.228]    [Pg.6]    [Pg.94]    [Pg.109]    [Pg.110]    [Pg.110]    [Pg.110]    [Pg.114]    [Pg.115]    [Pg.116]    [Pg.116]    [Pg.57]    [Pg.5]    [Pg.6]    [Pg.13]    [Pg.224]    [Pg.224]    [Pg.555]    [Pg.566]    [Pg.625]    [Pg.632]    [Pg.53]    [Pg.272]    [Pg.292]    [Pg.295]   
See also in sourсe #XX -- [ Pg.188 , Pg.190 ]




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1,4-addition kinetic isotope effects

12C/13C kinetic isotope effects

A-Deuterium kinetic isotope effects

Activation energy kinetic isotope effects

Adiabatic Proton Transfer Kinetic Isotope Effects

Alkyl chlorides kinetic isotope effects

Anomalous Kinetic Isotope Effects

Apparent activation energies and kinetic isotope effects using the reaction order approach

Aqueous Solvent Equilibrium and Kinetic Isotope Effects

Arene oxides kinetic deuterium isotope effect

Arylation kinetic isotope effects

Bond breaking kinetic isotope effect

Bond making kinetic isotope effect

Calculations of heavy atom kinetic isotope effect in phosphate monoester hydrolysis

Carbonyl carbon kinetic isotope effects

Claisen rearrangement kinetic isotope effect

Cope rearrangements kinetic isotope effects

Dehydration kinetic isotope effect

Deuterium in kinetic isotope effect

Deuterium isotope effects kinetics

Deuterium kinetic isotope effects

Deuterium kinetic isotope effects secondary, and transition state

Deuterium kinetic isotope effects structure

Deuterium kinetic isotope effects theoretical calculations

Deuterium kinetic isotope effects, secondary, and transition state structure

Deuterium solvent kinetic isotope effects

Diels-Alder Reaction Kinetic isotope effect

E2 elimination kinetic isotope effect

Electrophilic aromatic substitution kinetic isotope effects

Elementary reactions kinetic isotope effect

Elimination reactions kinetic isotope effects

Ene Reaction. Kinetic Isotope Effects

Enzyme kinetic isotope effects

Enzyme reactions isotope kinetic effects

Experimental Determination of Kinetic Isotope Effects

Extremely high kinetic isotope effects and tunneling

Further Discussion of Nontunneling Kinetic Isotope Effects

General Predictions on Kinetic Isotope Effects

Glycosyl transfer kinetic isotope effects

Group 14 elements kinetic isotope effects

Hammett equation kinetic isotope effects

Hydrogen kinetic isotope effect studie

Hydrogen kinetic isotope effects

Hydrogen-deuterium kinetic isotope effect

Hydrogen-tritium kinetic isotope effects

Hydrogen/deuterium reaction with kinetic isotope effect

Hydrolysis solvent kinetic isotope effects

INTRAMOLECULAR KINETIC ISOTOPE EFFECT

Induced kinetic isotope effect

Intermolecular kinetic isotope effect

Inverse kinetic isotope effect

Inverse secondary kinetic isotope effect

Isotope effects in kinetics

Isotope effects reaction kinetics

Isotope effects, kinetic limits regarding

Isotope effects, kinetic mass regarding

Isotope effects, kinetic phenomenon

Isotope effects, kinetic, Group 14 hydrides

Isotope kinetic

Isotopes Kinetic isotope effect

Isotopes kinetic effects, 276 applications

Isotopes, 69-70, Table kinetic isotope effect

Isotopic Labeling and Kinetic Isotope Effects

Isotopic kinetic

Isotopic labeling kinetic isotope effects

Kinetic H/D isotope effect

Kinetic Isotope Effect (KIE)

Kinetic Isotope Effect Magnitude and Variation with Reaction Asymmetry

Kinetic Isotope Effect for Metals with High Hydrogen Overpotentials

Kinetic Isotope Effects (KIEs)

Kinetic Isotope Effects Continued Variational Transition State Theory and Tunneling

Kinetic Isotope Effects in Electron-Transfer Reactions

Kinetic Isotope Effects in Mixed Solvents

Kinetic Isotope Effects on Chemical Reactions

Kinetic deuterium isotope effects primary

Kinetic isotope effect , glycosylation

Kinetic isotope effect bromination

Kinetic isotope effect determination

Kinetic isotope effect examples

Kinetic isotope effect general theory

Kinetic isotope effect hydrogen isotopes

Kinetic isotope effect lithium enolates

Kinetic isotope effect profile

Kinetic isotope effect quantum mechanical tunneling

Kinetic isotope effect second-order

Kinetic isotope effect table

Kinetic isotope effect types

Kinetic isotope effect usual range

Kinetic isotope effect, decarbonylation

Kinetic isotope effect, equilibrium

Kinetic isotope effect, mechanism

Kinetic isotope effect. See

Kinetic isotope effects

Kinetic isotope effects

Kinetic isotope effects 1,4-dihydropyridine

Kinetic isotope effects 1,6-cuprate additions

Kinetic isotope effects Aldol-Tishchenko reaction

Kinetic isotope effects aerobic oxidation

Kinetic isotope effects alkane activation

Kinetic isotope effects alkenylation

Kinetic isotope effects alkyne reactions

Kinetic isotope effects alpha carbon

Kinetic isotope effects apparent

Kinetic isotope effects approach

Kinetic isotope effects aromatic substitution

Kinetic isotope effects benzylamine reactions

Kinetic isotope effects carbon

Kinetic isotope effects carbon-hydrogen insertions

Kinetic isotope effects chloride

Kinetic isotope effects chloride/nucleophile

Kinetic isotope effects chlorine

Kinetic isotope effects cyclization

Kinetic isotope effects dependence

Kinetic isotope effects deuterium effect

Kinetic isotope effects diminished

Kinetic isotope effects electronic structure calculations

Kinetic isotope effects enyne-allene cyclization

Kinetic isotope effects enzymes effect determination using

Kinetic isotope effects enzymic bond cleavage

Kinetic isotope effects fluorination

Kinetic isotope effects halide reactions

Kinetic isotope effects heavy-atom

Kinetic isotope effects hydride transfer

Kinetic isotope effects hydrogen shifts

Kinetic isotope effects hydroxylation

Kinetic isotope effects in phosphoryl transfer reactions

Kinetic isotope effects in proton-transfer reactions

Kinetic isotope effects internal

Kinetic isotope effects intramolecular reactions

Kinetic isotope effects intrinsic

Kinetic isotope effects laser flash photolysis

Kinetic isotope effects limiting

Kinetic isotope effects measurement

Kinetic isotope effects methyl transfers

Kinetic isotope effects methyl-transfer reactions

Kinetic isotope effects model calculation

Kinetic isotope effects nitration

Kinetic isotope effects nitrogen

Kinetic isotope effects nomenclature

Kinetic isotope effects normal temperature dependence

Kinetic isotope effects nucleophile carbon

Kinetic isotope effects nucleophilic substitution

Kinetic isotope effects observed

Kinetic isotope effects of deuterium

Kinetic isotope effects origin

Kinetic isotope effects ortholithiation

Kinetic isotope effects oxidation

Kinetic isotope effects oxygen

Kinetic isotope effects primary

Kinetic isotope effects primary hydrogen-deuterium

Kinetic isotope effects product dependence

Kinetic isotope effects radical

Kinetic isotope effects remote

Kinetic isotope effects secondary

Kinetic isotope effects secondary alpha hydrogen-deuterium

Kinetic isotope effects secondary hydrogen-deuterium

Kinetic isotope effects singlet carbenes

Kinetic isotope effects substitutions

Kinetic isotope effects substrate dependence

Kinetic isotope effects temperature dependence

Kinetic isotope effects theoretical calculations

Kinetic isotope effects theory

Kinetic isotope effects transition state structures

Kinetic isotope effects transition-state variation

Kinetic isotope effects tunneling

Kinetic isotope effects variation with

Kinetic isotope effects, benzophenoneA/iV-dimethylaniline proton-transfer

Kinetic isotope effects, benzophenoneA/iV-dimethylaniline proton-transfer classical model

Kinetic isotope effects, benzophenoneA/iV-dimethylaniline proton-transfer reactions

Kinetic isotope effects, benzophenoneA/iV-dimethylaniline proton-transfer semiclassical/quantum model comparisons

Kinetic isotope effects, catalysis rates

Kinetic isotope effects, elucidating reaction

Kinetic isotope effects, elucidating reaction mechanisms with

Kinetic isotope effects, use

Kinetic isotopic effect

Kinetic studies isotope effects

Kinetic, diameter isotope effects

Kinetics and Isotope Effects

Labeling and Kinetic Isotope Effects

Lysozyme kinetic isotope effect

Mechanistic kinetic isotope effect

Mutarotation kinetic isotope effects

Nitrogen-15 kinetic isotope effects, values

Non-unit kinetic isotope effects

Nonadiabatic Proton Transfer Kinetic Isotope Effects

Normal kinetic isotope effect

Normal secondary kinetic isotope effect

Nucleophilic displacement reactions kinetic isotope effects

Origin of kinetic isotope effects

Possible contribution of tunneling to the high secondary a-deuterium kinetic isotope effect

Potential energy surfaces kinetic isotope effect studies

Primary and secondary kinetic isotope effects

Primary kinetic hydrogen isotope effects

Primary kinetic isotope effect for glucose-6-phosphate isomeras

Primary kinetic isotope effect of sulfur

Primary solvent kinetic isotope effect

Protium/deuterium/tritium kinetic isotope effects

Reaction branching and extreme kinetic isotope effects

Reaction kinetic isotope effects

Reductive elimination kinetic isotope effects

Regioselectivity kinetic isotope effect

Relaxation methods kinetic isotope effect

Secondary Kinetic Isotope Effects in Substitution Mechanisms

Secondary a-deuterium kinetic isotope effect

Secondary a-deuterium kinetic isotope effect and the structure of ferrocenylmethyl carbocation type transition state

Secondary deuterium kinetic isotope effects

Secondary deuterium kinetic isotope effects SDKIE)

Secondary kinetic isotope effect on fumarate hydratase

Secondary kinetic isotope effect participation

Secondary kinetic isotope effect reactions

Secondary solvent kinetic isotope effect

Site selectivity kinetic isotope effect

Solvent Kinetic Isotope Effects in Enzyme Reactions (See Also Section

Solvent kinetic isotope effect

Solvolysis secondary kinetic isotope effects

Some representative kinetic isotope effects

Stable isotopes kinetic isotope effects

Structure kinetic isotope effects

Studies on Phenols The Kinetic Isotope Effect

Substantial kinetic isotope effect

Substrate kinetic isotope effects

Symmetry-induced kinetic isotope effects

The Kinetic Isotope Effect

The Origin of Primary Kinetic Isotope Effects

Theoretical studies kinetic isotope effects

Transition state analysis using multiple kinetic isotope effects

Transition state kinetic isotope effects

Transmethylation kinetic isotope effects

Tritium kinetic isotope effects

Tritium-deuterium kinetic isotope effects, relative

Using kinetic isotope effects to determine

Wheland intermediates kinetic isotope effect

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