Electronegative substituents, changes

Why do free-radical reactions involving neutral reactants and intermediates respond to substituent changes that modify electron distribution One explanation has been based on the idea that there would be some polar character in the transition state because of the electronegativity differences of the reacting atoms ... 

Dealkylation is the main direction in the decomposition of mixed dications 124-126. Stability of these dications strongly depends on the nature of the chalcogen and substituents.119 In general, the order of stability is consistent with the difference in electronegativities and changes as follows Te > > Se > S (Scheme 46).95 119... 

The results provide a clear indication of the importance of the change in hybridization between cyclopropane ( sp2 for the external bonds) and propane ( sp3). Electropositive groups prefer to be bonded to the more electron-withdrawing cyclopropane ring, whereas electronegative substituents prefer to be bonded to C2 of propane. The one anomaly in Table 3 is found with the BH2 substituent which leads to the most exothermic group transfer reaction. This is presumably a result of the interaction of the cyclopropane ring with the empty p orbital at boron. This type of interaction will be discussed in the next section. 

The terms of Equation 1 that are expected to be changed most as groups on phosphorus are varied are SP(0) 2 and the s character of the phosphorus donor orbital aP, which contributes directly to (P smsp)2 Both of these terms are expected to increase with increasingly electronegative substituents on phosphorus— SP(0) 2 by contraction of the... 

It is evident that the electron density on the olefin and the resultant changes in bond length, nuclear shielding, and vibrational modes arc related to the ionization potential of the metal atoms. It is not clear whether this effect is caused by inhibition of the a forward donation or enhancement of the 7r back donation as the ionization potential is lowered. When electronegative substituents are placed on the olefin to activate it, there is also the question of the extent to which the backbonded electron density remains localized on the double bond or is delocalized to the substituent groups. Any means of probing the electron density on the metal or olefin would be useful in ascertaining the relative importance of the [Pg.44]

The NMR chemical shifts of some substituted thiophenes are listed in Table 37. Highly electronegative substituents cause the largest downfield shift on the ipso-csahon atom but the adjacent carbons are upfield as compared to thiophene. For substituents at the 2-position, the C-4 carbon resonances are the least affected, while for 3-substituted thiophenes, the C-5 resonances do not change much. 

The most electronegative substituents are always located in the axial position. In this respect there is no difference between compounds of penta-coordinate silicon and isoelectronic phosphorus compounds. Thus, in adducts of the type SiCljX NMCj (X = F, Cl) the substituent X is always in the axial position (C3V symmetry), but if X = H the molecular symmetry changes to since the hydrogen atom is always in the equatorial plane In solution of [(C6H5)3P]2Pt Sip4... 

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