Pathways to the Reaction Centre

The resonance pathway (R) requires K-bonding for transmission of the polar effect and is comprised of at least three distinct processes  

The substituent can also interact with the reaction centre by virtue of its bulk (S) as discussed in Chapter 2. In addition to effects caused by substituents the reaction centre is also subject to interaction with solvent.  

---

The equation predicts the substituent effect if it is transmitted to the reaction centre by two pathways (A and B), for example by the substituent having an effect on both the nucleophile and the electrophile in an intramolecular reaction. 

The p value for base-catalysed hydrolysis (+2-51) is +ve and quite large, reflecting the development of not inconsiderable -ve charge at the reaction centre in the rate-limiting step—attack on this centre by OH (step CD in the BAC2 pathway). By contrast, the p value for acid-catalysed hydrolysis (+0-03) is very nearly zero which means, of course, that the rate of this hydrolysis does not vary significantly from one ester to another, no matter what the m- or... 

Possible pathways of the degradation reaction may be visualized for a linear hydrocarbon chain in which the reaction centre ( ) is formed by the effect of initiation (heat, light, oxygen, shear stress, etc.), see Scheme la. A complementary reaction site is denoted as (-). For example, when ( ) is a free radical site, (-) is also a free radical site, if ( ) is a cation, then (-) is an anion, etc. The three stages of the reaction depicted in Scheme la, are initiation, propagation and termination, respectively. The dissociation energies of bonds situated in a /(-position to the reaction site ( ) are considerably lower than those... 

Treatment of hydrosilanes with potassium hydride is a known route to silyl anions and has been used to generate silacyclopentadienide anions. Reinvestigation of the reaction of KH with C4Ph4SiMeH has shown that the dominant pathway is simple addition of the hydride anion to the silicon centre to give the pentavalent silicate [Ph4C4SiMeH2] K+ (129)189 (equation 75). The Si—H coupling constants in 129 (Jsi-H = 192.5 and... 

---