Rate, and substitution

Calculating the average reaction rate and substituting into the mass balance, after integration ... 

Replacing the rate and substituting these valnes for the same conversion, we obtain t = 4.71 min... 

This is the most common and stable state of chromium in aqueous solution. The Cr ion, with 2d electrons, forms mainly octahedral complexes [CrX ], which are usually coloured, and are kweticallv inert, i.e. the rate of substitution of X by another hgand is very slow consequently a large number of such complexes have been isolated (see below, under chromium(III) chloride). 

A point in case is provided by the bromination of various monosubstituted benzene derivatives it was realized that substituents with atoms carrying free electron pairs bonded directly to the benzene ring (OH, NH2, etc) gave 0- and p-substituted benzene derivatives. Furthermore, in all cases except of the halogen atoms the reaction rates were higher than with unsubstituted benzene. On the other hand, substituents with double bonds in conjugation with the benzene ring (NO2, CHO, etc.) decreased reaction rates and provided m-substituted benzene derivatives. 

TABLE 6.2 Stabilities of complexes of alkylbenzenes and rates of substitutions ... 

We have seen ( 6.2.3) hat there is a close relationship between the rates of electrophilic substitutions and the stabilities of tr-complexes, and facts already quoted above suggest that no such relationship exists between those rates and the stabilities of the 7r-complexes of the kind discussed here. These two contrasting situations are further illustrated by the data given in table 6.2. As noted earlier, the parallelism of rate data for substitutions with stability data for o"-complexes is not limited to chlorination ( 6.2.4). Clearly, rr-complexes have no general mechanistic or kinetic significance in electrophilic substitutions. 

If, on the other hand, the encounter pair were an oriented structure, positional selectivity could be retained for a different reason and in a different quantitative sense. Thus, a monosubstituted benzene derivative in which the substituent was sufficiently powerfully activating would react with the electrophile to give three different encounter pairs two of these would more readily proceed to the substitution products than to the starting materials, whilst the third might more readily break up than go to products. In the limit the first two would be giving substitution at the encounter rate and, in the absence of steric effects, products in the statistical ratio whilst the third would not. If we consider particular cases, there is nothing in the rather inadequate data available to discourage the view that, for example, in the cases of toluene or phenol, which in sulphuric acid are nitrated at or near the encounter rate, the... 

The best-known equation of the type mentioned is, of course, Hammett s equation. It correlates, with considerable precision, rate and equilibrium constants for a large number of reactions occurring in the side chains of m- and p-substituted aromatic compounds, but fails badly for electrophilic substitution into the aromatic ring (except at wi-positions) and for certain reactions in side chains in which there is considerable mesomeric interaction between the side chain and the ring during the course of reaction. This failure arises because Hammett s original model reaction (the ionization of substituted benzoic acids) does not take account of the direct resonance interactions between a substituent and the site of reaction. This sort of interaction in the electrophilic substitutions of anisole is depicted in the following resonance structures, which show the transition state to be stabilized by direct resonance with the substituent ... 

Relative rates and isomer ratios for substitution in benzene and toluene ... 

Solvent Effects on the Rate of Substitution by the S l Mechanism Table 8 6 lists the relative rate of solvolysis of tert butyl chloride m several media m order of increasing dielectric constant (e) Dielectric constant is a measure of the ability of a material m this case the solvent to moderate the force of attraction between oppositely charged par tides compared with that of a standard The standard dielectric is a vacuum which is assigned a value e of exactly 1 The higher the dielectric constant e the better the medium is able to support separated positively and negatively charged species 8olvents... 

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

Regardless of the alkyl halide raising the temperature increases both the rate of substitution and the rate of elimination The rate of elimination however usually increases faster than substitution so that at higher temperatures the proportion of ehm mation products increases at the expense of substitution products... 

Rate of substitution is independent of both concentration and nature of nucleophile Nucleophile does not participate until after rate determining step (Section 8 8) Rate depends on both nature of nucleophile and its concentration (Sections 8 3 and 8 7)... 

Rate increases with increasing po larity of solvent as measured by its dielectric constant e (Section 8 12) Polar aprotic solvents give fastest rates of substitution solvation of Nu IS minimal and nucleophilicity IS greatest (Section 8 12)... 

The regioselectivity of substitution like the rate is strongly affected by the sub stituent In the following several sections we will examine the relationship between the structure of the substituent and its effect on rate and regioselectivity of electrophilic aro matic substitution... 

Sections How substituents control rate and regioselectivity m electrophilic aro 12 10-12 14 matic substitution results from their effect on carbocation stability An electron releasing substituent stabilizes the cyclohexadienyl cation inter mediates corresponding to ortho and para attack more than meta... 

These reversible reactions are cataly2ed by bases or acids, such as 2iac chloride and aluminum isopropoxide, or by anion-exchange resias. Ultrasonic vibrations improve the reaction rate and yield. Reaction of aromatic aldehydes or ketones with nitroparaffins yields either the nitro alcohol or the nitro olefin, depending on the catalyst. Conjugated unsaturated aldehydes or ketones and nitroparaffins (Michael addition) yield nitro-substituted carbonyl compounds rather than nitro alcohols. Condensation with keto esters gives the substituted nitro alcohols (37) keto aldehydes react preferentially at the aldehyde function. 

Monomer Reactivity. The poly(amic acid) groups are formed by nucleophilic substitution by an amino group at a carbonyl carbon of an anhydride group. Therefore, the electrophilicity of the dianhydride is expected to be one of the most important parameters used to determine the reaction rate. There is a close relationship between the reaction rates and the electron affinities, of dianhydrides (12). These were independendy deterrnined by polarography. Stmctures and electron affinities of various dianhydrides are shown in Table 1. 

Delignification Chemistty. The chemical mechanism of sulfite delignification is not fully understood. However, the chemistry of model compounds has been studied extensively, and attempts have been made to correlate the results with observations on the rates and conditions of delignification (61). The initial reaction is sulfonation of the aUphatic side chain, which occurs almost exclusively at the a-carbon by a nucleophilic substitution. The substitution displaces either a hydroxy or alkoxy group ... 

OC-Methylstyrene. This compound is not a styrenic monomer in the strict sense. The methyl substitution on the side chain, rather than the aromatic ring, moderates its reactivity in polymerization. It is used as a specialty monomer in ABS resins, coatings, polyester resins, and hot-melt adhesives. As a copolymer in ABS and polystyrene, it increases the heat-distortion resistance of the product. In coatings and resins, it moderates reaction rates and improves clarity. Physical properties of a-methylstyrene [98-83-9] are shown in Table 12. 

Studies of reaction mechanisms ia O-enriched water show the foUowiag cleavage of dialkyl sulfates is primarily at the C—O bond under alkaline and acid conditions, and monoalkyl sulfates cleave at the C—O bond under alkaline conditions and at the S—O bond under acid conditions (45,54). An optically active half ester (j -butyl sulfate [3004-76-0]) hydroly2es at 100°C with iaversion under alkaline conditions and with retention plus some racemization under acid conditions (55). Effects of solvent and substituted stmcture have been studied, with moist dioxane giving marked rate enhancement (44,56,57). Hydrolysis of monophenyl sulfate [4074-56-0] has been similarly examined (58). 

In contrast to the hydrolysis of prochiral esters performed in aqueous solutions, the enzymatic acylation of prochiral diols is usually carried out in an inert organic solvent such as hexane, ether, toluene, or ethyl acetate. In order to increase the reaction rate and the degree of conversion, activated esters such as vinyl carboxylates are often used as acylating agents. The vinyl alcohol formed as a result of transesterification tautomerizes to acetaldehyde, making the reaction practically irreversible. The presence of a bulky substituent in the 2-position helps the enzyme to discriminate between enantiotopic faces as a result the enzymatic acylation of prochiral 2-benzoxy-l,3-propanediol (34) proceeds with excellent selectivity (ee > 96%) (49). In the case of the 2-methyl substituted diol (33) the selectivity is only moderate (50). 

The reactivity of five-membered rings with one heteroatom to electrophilic reagents has been quantitatively compared in a variety of substitution reactions. Table 2 shows the rates of substitution compared to thiophene for formylation by phosgene and iV,AT-dimethylfor-mamide, acetylation by acetic anhydride and tin(IV) chloride, and trifluoroacetylation with trifluoroacetic anhydride (71AHC(13)235). 

Phenyl radicals attack azoles unselectively to form a mixture of phenylated products. Relative rates and partial rate factors are given in Table 7. The phenyl radicals may be prepared from the usual precursors PhN(NO)COMe, Pb(OCOPh)4, (PhC02)2 or PhI(OCOPh)2. Substituted phenyl radicals react similarly. 

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