Electron releasing substituent

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)... 

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 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. 

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). 

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... 

The pATr+ values allow for a comparison of the stability of carbocations. The carbocations that can be studied in this way are all relatively stable carbocations. The data in Table 5.1 reveal that electron-releasing substituents on the aryl rings stabilize the carbocation (more positive pA r+) whereas electron-withdrawing groups such as nitro are destabilizing. This is what would be expected from the electron-deficient nature of the carbocation. 

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-... 

In the bromination of styrene, a po-+ plot is noticeably curved. If the extremes of the curves are taken to represent straight lines, the curve can be resolved into two Hammett relationships with p = —2.8 for electron-attracting substituents and p = —4.4 for electron-releasing substituents. When the corresponding -methylstyrenes are examined, a similarly curved ap plot is obtained. Furthermore, the stereospecificity of the reaction in the case of the -methylstyrenes varies with the aryl substituents. The reaction is a stereoespecific anti addition for strongly electron-attracting substituents but becomes only weakly stereoselective for electron-releasing substituents, e.g., 63% anti, 37% syn, forp-methoxy. Discuss the possible mechanistic basis for the Hammett plot curvature and its relationship to the stereochemical results. 

There are alternatives to the addition-elimination mechanism for nucleophilic substitution of acyl chlorides. Certain acyl chlorides are known to react with alcohols by a dissociative mechanism in which acylium ions are intermediates. This mechanism is observed with aroyl halides having electron-releasing substituents. Other acyl halides show reactivity indicative of mixed or borderline mechanisms. The existence of the SnI-like dissociative mechanism reflects the relative stability of acylium ions. 

It has long been known that the Diels-Alder reaction is particularly efficient and rapid when the dienophile contains one or more electron-attracting groups. These substituent effects are illustrated by the data in Table 11.3. In the case of the diene, reactivity is increased by electron-releasing substituents. Some illustrative data are given in Table 11.4. 

Simple alkyl radicals such as methyl are considered to be nonnucleophilic. Methyl radicals are somewhat more reactive toward alkenes bearing electron-withdrawing substituents than towards those with electron-releasing substituents. However, much of this effect can be attributed to the stabilizing effect that these substiments have on the product radical. There is a strong correlation of reaction rate with the overall exothermicity of the reaction. Hydroxymethyl and 2-hydroxy-2-propyl radicals show nucleophilic character. The hydroxymethyl radical shows a slightly enhanced reactivity toward acrylonitrile and acrolein, but a sharply decreased reactivity toward ethyl vinyl ether. Table 12.9 gives some of the reactivity data. 

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

Like the 5/) -hybridized carbons of carbocations and free radicals, the sp -hybridized carbons of double bonds are electron attracting, and alkenes are stabilized by substituents that release electrons to these carbons. As we saw in the preceding section, alkyl groups are better electron-releasing substituents than hydrogen and aie, therefore, better able to stabilize an alkene. 

T -Values were used for electron-withdrawing substituents, (t+-values for electron-releasing substituents. 

The nature and position of substituents on the arylidene ring markedly affect the ratio of addition products to cleavage products. As expected, electron-releasing substituents enhance the cleavage reaction. 

Electron-releasing substituents stabilize the isoindolenine tautomer 7, whereas eleetron-withdrawing groups have the opposite effect. In Table IV some data are given. 

In general, electron-releasing substituents R (NH2, SCH3) shift the equilibrium 27a 27b toward the IH tautomers 27a. With respect to the sub-... 

Previous extended Hiickel calculations on the bis(trifluoromethyl) derivative 6 show the favored isomer to be 6b, while the CNDO/2 method favors 6a [75JCS(P2)559]. More recent ab initio calculations for different R substituents show that electron-releasing substituents favor the ring-opened dithione (6b), whereas electron-withdrawing substituents favor the cyclic structure (6a) (80JA6687). These conclusions are supported by electron diffraction (ED) (76JA899) and photoelectron (PE) spectral data... 

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