Electron releasing substituents

Reactions of substituted cumyl benzoates in 50 50 trifluoroethanol-water show no effect of [NaN3] on the rate between 0 and 0.5 M. The product ratio, however, is highly dependent on the cumyl substituent. Electron-releasing substituents favor azide formation whereas electron-withdrawing substituents result in solvent capture. Formu-... 

The conductance of the OPE nitro-16 (X = N02) was monitored by the STM-BJ method, as the nitro group was reduced to NO and NH2, and then protonated to NH3. The resulting data gave an inverse linear Hammett plot with the meta-cr substituent parameter, indicating that substituent electron release into the aromatic core increases conductance [63]. 

Reduction of a, 3-epoxyketones is usually accompanied by hydrogenolysis at the a-position, and the product is a p-hydroxy ketone, 14 as in the conversion of 414 to 415.415 if other substituents (electron releasing or withdrawing) are attached to the p-position, the direction of reduction is less predictable. Reduction of aziridines by hydrogenolysis is similar to that of epoxides and usually occurs at the less hindered C—N bond.416-417... 

Quantitative aspects of l,3-p-<7 overlap across cyclobutane rings in anion radicals have been examined by e.s.r. e.g. the radical anion generated from the cyclobutane (277). The equilibria between the anion radicals and dianions of [ H5]phenyl-, 1,2,5,7-tetramethyl-, t-butoxy-, and ethyl-cyclo-octatetraene in HMPA, have been studied, and the enthalpy changes found to be linearly related to the a-values of the substituents. Electron-releasing substituents remove the degeneracy of the two nonbonding orbitals of the anion radicals. ... 

Carbocations are stabilized by alkyl substituents attached directly to the positively charged carbon Alkyl groups are electron releasing sub stituents Stability increases in the order... 

Section 5 6 Electron release from alkyl substituents stabilizes a double bond In gen eral the order of alkene stability is... 

In general alkyl substituents increase the reactivity of a double bond toward elec trophilic addition Alkyl groups are electron releasing and the more electron rich a dou ble bond the better it can share its tt electrons with an electrophile Along with the observed regioselectivity of addition this supports the idea that carbocation formation rather than carbocation capture is rate determining... 

A vinyl group is an extremely effective electron releasing substituent Resonance of the type shown delocalizes the rr electrons of the double bond and disperses the pos itive charge... 

A methyl group is an electron releasing substituent and activates all of the ring carbons of toluene toward electrophilic attack The ortho and para positions are activated more than the meta positions The relative rates of attack at the various positions m toluene compared with a single position m benzene are as follows (for nitration at 25°C)... 

Unlike a methyl group which is slightly electron releasing a trifluoromethyl group is a powerful electron withdrawing substituent Consequently a CF3 group destabilizes a car bocation site to which it is attached... 

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... 

Electron release from the substituent X not only stabilizes the carbonyl group it decreases the positive character of the carbonyl carbon and makes the carbonyl group less electrophilic... 

Electron releasing substituents attached to the ring have a negligible effect on the acidity of phenols Strongly electron withdrawing groups increase the acidity The compound 4 nitro 3 (tnfluoromethyl)phenol for example is 10 000 times more acidic than phenol... 

The following rules pertain to electrophilic substitution in pyrroles (35) (/) an electron-withdrawing substituent in the a-position directs substitution to the P and a -positions, (2) an electron-releasing substituent in the a-position directs substitution to the neighboring -position or to the a -position, (J) an electron-withdrawing substituent in the -position leads to substitution in the a -position, and (4) an electron-releasing substituent in the P-position tends to direct substitution into the neighboring a-position. 

In general, electron-releasing substituents cause a bathochromic shift of the n band... 

Substitution of the pyrazine ring by electron releasing substituents reduces the reactivity of halopyrazines and more forcing conditions must invariably be employed to bring about displacement of the halogen. 

In many cases, substituents linked to a pyrrole, furan or thiophene ring show similar reactivity to those linked to a benzenoid nucleus. This generalization is not true for amino or hydroxyl groups. Hydroxy compounds exist largely, or entirely, in an alternative nonaromatic tautomeric form. Derivatives of this type show little resemblance in their reactions to anilines or phenols. Thienyl- and especially pyrryl- and furyl-methyl halides show enhanced reactivity compared with benzyl halides because the halogen is made more labile by electron release of the type shown below. Hydroxymethyl and aminomethyl groups on heteroaromatic nuclei are activated to nucleophilic attack by a similar effect. 

Isothiazoles with electron-releasing substituents such as amino, hydroxy, or alkoxy in the 3- or 5-position are brominated in high yield in the 4-position. Alkylisothiazoles give lower yields, but 3-methylisothiazole-5-carboxylic acid has been brominated in 76% yield (72AHC(14)1). Again, thiazoles with an electron-releasing substituent in the 2- or 4-position are brominated at the 5-position (79HC(34-1)5). 

There are also substituents that can act as electron-releasing groups through resonance. Among familiar examples are alkoxy and amino groups in vinyl ethers and enamines, respectively. 

Bicyclo[3.3.1]nonan-9-one is another ketone that exhibits interesting stereoselectivity. Reduction by hydride donors is preferentially syn to electron-attracting substituents at C-5 (X = EWG in the structure shown below) and anti to electron-releasing substituents (X = ERG below). These effects are observed even for differentially substituted phenyl... 

In this equation, the substituent parameters and reflect the incremental resonance interaction with electron-demanding and electron-releasing reaction centers, respectively. The variables and r are established for a reaction series by regression analysis and are measures of the extent of the extra resonance contribution. The larger the value of r, the greater is the extra resonance contribution. Because both donor and acceptor capacity will not contribute in a single reaction process, either or r would be expected to be zero. 

Not all reactions can be fitted by the Hammett equations or the multiparameter variants. There can be several reasons for this. The most common is that the mechanism of the reaction depends on the nature of the substituent. In a multistep reaction, for example, one step may be rate-determining in the case of electron-withdrawing substituents, but a different step may become rate-limiting when the substituent is electron-releasing. The rate of semicarbazone formation of benzaldehydes, for example, shows a nonlinear Hammett... 

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