Alkyl groups electronegativity

Fig. 35. (a) Correlation between the photolysis rate of halomethyl-cobalamins and the electronegativity of the alkyl ligand. (The electronegativity values are shown as the sums of the Pauling electronegatives of the atoms attached to the methyl carbon.) (b) Correlation between pKa for displacement of benzimidazole and alkyl group electronegativity for a series of halomethyl-cobalamins... 

Steric and inductive effects determine the rate of formation of the pentacovalent siUcon reaction complex. In alkaline hydrolysis, replacement of a hydrogen by alkyl groups, which have lower electronegativity and greater steric requirements, leads to slower hydrolysis rates. Replacement of alkyl groups with bulkier alkyl substituents has the same effect. Reaction rates decrease according to ... 

The stabihty of these products varies (38). When R is methyl, the compound decomposes below room temperature, but when R is phenyl it is stable up to 40—45°C. Increasing the molecular weight of the alkyl group and introducing an electronegative group on the phenyl increases the stabihty. The aUphatic mixed anhydrosulfide can be decomposed by two different routes (53) ... 

The equilibrium situation for simple substituted 2-ulkenyl alkali metal derivatives can be estimated by a rule of thumb electron-accepting and electropositive substituents ( ) prefer the exo position, but electron-donating and electronegative substituents ( ), including alkyl groups, tend to occupy the endo position. With increasing steric demand of the substituent, the exoisomer becomes more favored. 

In this case, the diminution in Lewis acidity caused by the introduction of the two, less-electronegative, sulfur ligands prevents any reaction with the dithiol enzyme, regardless of the nature of the alkyl group. 

Now we know the effect that electronegative atoms (N, O, Cl, Br, etc.) can have if they are near a negative charge. It is a stabilizing effect. But what effect do carbon atoms have (alkyl groups) For example, is there a difference in acidity between the following two compounds ... 

Are there any inductive effects (electronegative atoms or alkyl groups) that stabilize or destabilize any of the conjugate bases ... 

The relative insensitivity of Sp to changes in electronegativity of substituents which is observed for P fluoro-compounds is also evident for penta-arylphosphoranes. Thus the placement of />ara-substituents on the phenyl rings of pentaphenylphosphorane has very little effect and Sp is -f 88 1 p.p.m. for (16 Y = H, Me, or Cl). The effect on Sp of the introduction of an amino-group in the bis-biphenylenephosphorane system (17) is similar to that of an alkyl group (see Table 3). ... 

Polymerization in electrostatic systems like the ones mentioned above is stericaUy inhibited by alkyl substitution at the a-carbon which must assume a coordination number greater than 4. Coates and Glockhng have treated this inhibition of polymerization in terms of decreased electronegative character of the branched alkyl groups. Therefore, stimulated by the idea that f-afkylhthium compounds may exist as low polymers or even as monomeric molecules, Weiner and coworkers and Kottke and Stalke have isolated f-butyllithium as a pure substance for the first time and characterised it by spectroscopic methods and X-ray diffraction. The colourless crystalline solid was found to be tetrameric over a range of concentrations in both benzene and hexane ... 

---