Alkyl groups electron-releasing

Kinetic data are available for the nitration of a series of p-alkylphenyl trimethylammonium ions over a range of acidities in sulphuric acid. - The following table shows how p-methyl and p-tert-h xty augment the reactivity of the position ortho to them. Comparison with table 9.1 shows how very much more powerfully both the methyl and the tert-butyl group assist substitution into these strongly deactivated cations than they do at the o-positions in toluene and ferf-butylbenzene. Analysis of these results, and comparison with those for chlorination and bromination, shows that even in these highly deactivated cations, as in the nitration of alkylbenzenes ( 9.1.1), the alkyl groups still release electrons in the inductive order. In view of the comparisons just... 

Free radicals like carbocations have an unfilled 2p orbital and are stabilized by substituents such as alkyl groups that release electrons Consequently the order of free radical stability parallels that of carbocations... 

The increase in the values of aliphatic amines (thus making them stronger bases as compared to ammonia) is due to the electron-releasing nature of alkyl groups. This release of electrons pumps electron density back to the nitrogen atom, which stabilizes the positive charge. 

If our generalization about dispersal of charge applies in this case, alkyl groups must release electrons here possibly through an inductive effect, possibly through resonance (hyperconjugation. Sec. 8.21). 

Any inductive effect (such as that of —CH3 or another alkyl group), which releases electron density toward the ring, activates the ring toward further substitution. 

The isolated double bonds in the dihydro structure are much less easily reduced than the aromatic ring, so the reduction stops at the dihydro stage. The rate of reduction is affected in a predictable way by substituent groups. Electron-releasing groups retard electron transfer whereas electron-withdrawing groups facilitate reduction. Alkyl- and alkoxyaromatics, phenols, and benzoate anions are the most useful... 

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... 

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... 

Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... 

As shown m Table 6 4 electron releasing alkyl groups on the double bond increase the rate of epoxidation This suggests that the peroxy acid acts as an electrophilic reagent toward the alkene... 

The carbonyl carbon of a ketone bears two electron releasing alkyl groups an aldehyde carbonyl group has only one Just as a disubstituted double bond m an alkene is more stable than a monosubstituted double bond a ketone carbonyl is more stable than an aldehyde carbonyl We 11 see later m this chapter that structural effects on the relative stability of carbonyl groups m aldehydes and ketones are an important factor m then rel ative reactivity... 

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. 

The nonspecialist reading Table 7-7 will probably be impressed by the substantial consistency among cti values evaluated by different methods, but the specialist tends to concentrate on the differences. There is one very interesting difference in Table 7-7, that for cti of alkyl groups based on Eq. (7-33) compared with cti based on the ionization of 3, the latter values showing practically no effect of inductive electron release and certainly no trend with increased branching. (The uncertainties associated with these substituent constants can be found in the original literature.) Swain... 

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