Relative rates of reaction

The behaviour of benzene is the datum from which any discussion of aromatic compoimds must start the reactivity of an aromatic compound is its rate of reaction relative to that of benzene when both are taking part in reactions occurring under the same conditions and proceeding by the same mechanism. 

A silicon atom might be expected to release electrons inductively, but because of empty 7-orbitals shows the overall character ( + 7 —717). Nitration of trimethylsilylbenzene with nitric acid in acetic anhydride at —10 to o °C gives 25-5,39-8,30-2 and 6-8 %, respectively, of 0-, m-, and /)-nitro-trimethylsilylbenzene and nitrobenzene, with a rate of reaction relative to that of benzene of about 1-5. The figures give no indication of an important conjugative effect. 

We should distinguish between the phrases nucleophilic attack and nucleophilic catalysis. Nucleophilic attack means the bond-forming approach by an electron pair of the nucleophile to an electron-deficient site on the substrate. In nucleophilic catalysis this results in an increase in the rate of reaction relative to the rate in the absence of the catalyst. However, nucleophilic attack may not result in catalysis. Thus, if methylamine is reacted with a phenyl acetate, the reaction observed is amide formation, not hydrolysis, because the product of the nucleophilic attack is more stable than is the ester to hydrolysis. 

Thus the efficiency of the reaction depends only upon the rate of reaction relative to the rates of all processes leading to deactivation of the excited state responsible for the reaction. The same is true for all other processes. Therefore an expression for the quantum yield for any particular process can be written down without going through the above kinetics ... 

These three classes of catalysis are distinctly different from each other in the ideal cases. But the extent of the contribution of the inner bulk of the catalyst depends on the rate of the catalytic reaction relative to the rate of diffusion of reactant and product molecules in bulk type I catalysis and on the rate of reaction relative to the rate of diffusion of redox carriers for the bulk type II catalysis. 

For the above series reactions, the most critical parameters related to the yield of the intermediate product are the Damkdhlcr-Peclct number product (rate of reaction relative to rate of membrane permeation) which should be 0.1 to 10 and the ratio of the permeability of the intermediate product to the permeability of the reactant which should... 

Turning to bicyclic systems, and a study of reaction with ethoxide, a small increase in the rate of reaction relative to pyridines is found for chloroquinolines at comparable positions. In the bicyclic compounds, quatemisation again greatly increases the rate of nncleophilic substitution, having a larger effect (-10 ) at C-2 than at C-4 (-10 ). °... 

The Damkohler number expresses the rate of reaction relative to the advection or fluid flow rate. A large Da value means that reaction is fast relative to transport and that... 

All of these effects are observed when comparing the rates of various electrophilic aromatic substitution reactions. Activating substituents increase the rate of reaction relative to benzene. The rate of reaction for the nitration of anisole, for example, was 9.7 x 10 times faster than nitration of benzene. The reaction of anisole with nitric and sulfuric acids, gave 44% of o-nitroanisole, 56% of p-nitroanisole and < 1% of m-nitro-anisole.2 9 contrasts with reactions involving deactivating substituents, where selectivity for the meta -product is usually very good. Nitration of nitrobenzene, for example, gave 1,3-dinitrobenzene in 94% yield, with only 6% of the ortho product and < 1% of the para product. ... 

Palladium(O) exhibits a degree of nucleophilic character, thus electron-with-drawing substituents increase the reactivity of aryl halides in oxidative additions. This is exemplified in the heterocyclic context the inductive effect of C=N units allows 2-chloropyrimidine (it is slightly less reactive than bromobenzene), and even 3-chloropyridine to react (even the moderate inductive effect at the P-position gives rise to a significantly higher rate of reaction relative to chlorobenzene) although a more reactive catalyst is required for the latter case (cf. section 2.7.2.2). 

Nucleophilicity is a kinetic property measured by relative rates of reaction. Relative nucleophilicities for a series of nucleophiles are established by measuring the rate at which each displaces a leaving group from a haloalkane (e.g., the rate at which each nucleophile displaces bromide ion from bromoethane in ethanol at 25°C). Here is a reaction using ammonia as the nucleophile. 

Ag+-catalysed isomerization of 4-substituted homocubanes (5) to norsnoutanes (6) also proceeds via pre-equilibrium complex formation and follows second-order kinetics. In the case R= Me adherence to Michaelis-Menten kinetics could also be demonstrated. C-4 Substituents capable of resonance interaction in the cationic transition state promote deviations in the rate of reaction relative to substituents which exhibit inductive effects only. With R=Bu bond-switching is reduced in rate, presumably because of steric inhibition of Ag+ attack on the homocubane to give an intermediate analogous to (4). Placement of deuterium or CDg at C-4 produces only a minor inverse kinetic deuterium isotope effect (kH/kD=0.97) which implies that a completely free carbonium ion intermediate is not involved and so argues in favour of a delocalized species analogous to (4). 

The orientational effect is often expressed in terms of yields of isomers, and the rate data as overall rate of reaction relative to that of benzene. Another convenient quantitative expression of rates is the partial rate factor, /, which measures the reactivity of the position concerned, relative to one of the six equivalent positions in benzene. The relative reactivity of the meta z.nd para positions of monosubstituted... 

A feature of condensation polymerisation in dispersion is the fast rate of reaction relative to bulk condensation processeso The heterogeneous polymerisation can often be carried out 50-70 C below the normal bulk polymerisation temperatureo... 

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