Electron-withdrawing effect of a phenyl group

Another important effect in explaining the lower basicity of aromatic amines is the electron-withdrawing effect of a phenyl group. Because the carbon atoms of a phenyl... 

A more detailed exploration of the reactivity of biphenyl resolves the problem. The ra-phenyl substituent reduces the rate of substitution in the benzene nucleus (Table 7). Qualitatively, this effect is in agreement with the predictions based on the rate of solvolysis of ra-phenylphenyl-dimethylcarbinyl chloride (Brown and Okamoto, 1958) and with the expected electron-withdrawing properties of the phenyl group. The data conform to the Selectivity Relationship with reasonable precision (Fig. 31). In view of the activation of the ortho and para positions, direct evaluation of the partial rate factors for the deactivated meta position is not always possible. Hence, indirect kinetic procedures were employed in several cases, halogenation and acylation, to estimate the values. Graphical analysis of the data shows that mfb is independent of the reagent selectivity. Deviations from the relationship are no greater than for the ordinary side-chain reactions. 

If the alkyl groups in any R SnX4 series are replaced by phenyl, the U9Sn signal moves to lower frequencies (Table XI), which is inconsistent with the greater electron withdrawing capability of a phenyl substituent on tin. Similar effects are found with benzyl, vinyl, and allyl groups (Tables XXX and XXXI) and may be due to the increased polarizability of these unsaturated substituents. (23) This topic is discussed further in Section III.B.3. 

A limitation of the scope of this reaction has been observed when attempting its application to 6-nitro-2-(trimethylsilyl)phenyl triflate. The excessively strong electron-withdrawing effect of the nitro group promotes nucleophilic attack by the fluoride on the sulphur atom of the triflate, cleaving the S-0 bond and expelling the phenoxide. 

Another important effect in explaining the lower basidty of aromatic amines is the electroii-widultawing effect of a phenyl group. Because the carbon atoms of a phenyl group are sp hybridized, they are more electronegative (and therefore more electron withdrawing) than the. -hybridized carbon atoms of alkyl gtoups. We shall discuss this effect further in Section 21.5A. 

---