Alkyl groups stabilizing effect

About this time, it was postulated that the transition state was a trivalent siliconium ion followed by the rapid addition of water. In 1982, this view was supported by Schmidt et al. [15], They studied the hydrolysis of tetra-alkoxy-silanes and various alkoxyalkylsilanes. It was noted that the alkyl-containing silanes reacted significantly faster than the analogous alkoxysilanes. They attributed this result to the stabilizing effect that the electron donating character of the alkyl group (+1 effect) would have on a siliconium ion. 

Substituents with a electron-donating inductive (+1) effect (i.e., alkyl groups) stabilize the C=0 double bond of aldehydes and ketones. They increase the importance of the zwitterionic resonance form by which carbonyl compounds are partly described. The driving force for the formation of addition products from carbonyl compounds therefore decreases in the order H—CH(=Q) > R—CH(=0) > R R2c(=0). 

We know how to estimate the stability of a carbonium ion, on the basis of inductive effects and resonance. Because of the electron-releasing inductive effect of alkyl groups, stability and hence rate of formation of the simple alkyl cations follows the sequence 3 > T > l ... 

A point of contrast with the data for base-catalyzed removal of a proton (see Table 6.8) is the tendency for acid-catalyzed enolization to result in preferential formation of the more-substituted enol. Eor 2-butanone, the ratio of exchange at CH2 to that at CH3 is 4.2 1, after making the statistical correction for the number of hydrogens. The preference for acid-catalyzed enolization to give the more-substituted enol is the result of the stabilizing effect that alkyl groups have on carbon-carbon double bonds. To the extent that the TS resembles product, alkyl groups stabilize the... 

Kinetic protectors we first investigated were alkyl groups since it was our initial hope to stabilize and hopefully isolate triplet carbene consisting of only hydrocarbons. However, the use of alkyl groups as effective kinetic protectors was supposed to be very difficult in the light of the strong affinity of carbenes for electrons since they react even with very weak sources of electrons, e. g., C-H bond a electrons. 

More recently, quaternary ammonium halides such as methyl tri-n-butyl ammonium chloride were used in the polymerization of PPTA in NMP [80]. Effective shielding of the anunonium cation by bulky alkyl groups stabilizes the ionized species in an organic medium so that it can facilitate the polarization of NMP (Equation 13.23). 

Alkyl groups stabilize carbocations by a resonance effect in which a fiUed 0 orbital of the alkyl group overlaps with the... 

If alkyl groups stabilize a free radical by an electron-donating inductive effect, then we should see a difference in the DH° values for the two different C—bonds in propane since one is to a primary carbon and the other to a secondary carbon. And that is exactly what we see. 

We have seen that alkyl groups stabilize double bonds and carbocations by an inductive effect. We have also seen that an sp -hybridized carbon is electron withdrawing with respect to an sp -hybridized carbon. Therefore, it follows that alkyl groups also transfer electron density to a benzene ring by an inductive effect. 

Hydrogen abstraction can occur at three different positions (Cl, C2, or C3), giving a primary, secondary, or tertiary radical, respectively. The tertiary radical is the most stable, because alkyl groups stabilize the unpaired electron via a delocalization effect, called hyperconjugation. The primary radical is the least stable, because it lacks the stabilizing effect provided by multiple alkyl groups. 

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... 

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... 

Consider first the electronic effect of alkyl groups versus hydrogen atoms attached to C=0 Recall from Section 17 2 that alkyl substituents stabilize C=0 making a ketone carbonyl more stable than an aldehyde carbonyl As with all equilibria factors... 

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