Sigma-substitution

Two modified sigma constants have been formulated for situations in which the substituent enters into resonance with the reaction center in an electron-demanding transition state (cr+) or for an electron-rich transition state (cr ). cr constants give better correlations in reactions involving phenols, anilines, and pyridines and in nucleophilic substitutions. Values of some modified sigma constants are given in Table 9.4. 

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms ia the alpha positions therefore He ia a plane. The pi bond which ties over the plane of the atoms acts as a source of electrons ia addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the aHyUc resonance stmcture. This stmcture can stabilize both positive and... 

A good correlation with ordinary Hammett a values was based on 16 well-behaved substituents, and p-SOMe conformed well to this. Various other substituents showed deviations which were attributed to enhanced + R effects. These included p-SPh and this was explained in terms of 7t(pd) bonding, which was thus taken to play no part in the effect of p-SOMe on the methyl hyperfine splitting. More recently several 4-substituted benzyl radicals of the type RSO C6H4CH2 (n — 0,1 or 2 R = Me, Ph, Tol, COMe or OMe) have been examined by ESR spectroscopy249. The ability to delocalize spin density onto the substituent decreases in general as n increases and the effect of R depends on the oxidation state of sulfur. These authors have devised a new scale of substituent effects (sigma dot... 

Copper-catalyzed cyclopropanation of benzene and its derivatives by a diazoacetic ester yields a norcaradiene 230 which undergoes spontaneous ring opening to cyclo-heptariene 231. At the temperatures needed for successful cyclopropanation, sigma-tropic H-shifts leading to conjugated isomers of cycloheptatriene carboxylates cannot be avoided. The situation is complicated by the formation of regioisomers upon cyclopropanation of substituted benzenes, and separation of the cycloheptatriene isomers may became tedious if not impossible. 

The inhibited unimolecular decomposition of symmetrically di-substituted benzoyl peroxides into radicals also obeys the Hammett rho-sigma relationship. Unfortunately, no extensive activation parameter data are available. The effect of the substituent changes on the rates at the single temperature has been explained in terms of dipole-dipole repulsion in the peroxide.122... 

The effect of a substituent on the aromatic substitution reaction is similar to its effect on electrophilic side chain reactions, but not precisely parallel. Thus the Hammett relationship using the usual sigma or substituent constants gives considerable scatter when applied to aromatic substitution. The scatter is probably due to an increased importance of resonance effects in the nuclear substitution reaction as compared with the side chain reactions. 

We shall next consider the effect of substitution on sigma and pi orbital energies. 

The above rules are derived on the basis of our discussion of the effect of hetero-atomig substitution (Section 1.1). Further evidence of their validity will be provided in Part IV in connection with our discussion of the intrinsic donor-acceptor properties of lone pairs and pi or sigma bonds. 

The introduction of NO2 into the aniline molecule would be expected to reduce the nucleophilicity of NH2 and for / -N( >2 this should be more or less according to 0. Examples of this are not easy to find, but there are the necessary data for the reactions of picryl chloride with substituted anilines in methanol. Log k for the reaction of aniline itself is about —0.2 at 25 °C (extrapolated from measurements at much lower temperatures). For the reaction of p-nitroaniline log A is —4.06 p is about —3.4. These values give an apparent sigma value of 1.13 for p-NC>2 in this system, a little below the value for [Pg.512]

In Taylor s tabulation of sigma values228, a+ and a for p-NCP are given identical values of 0.79228. No distinction between these two types of substituent constant is to be expected for p-N02 in connection with electrophilic aromatic substitution. Some slight distinction between them is drawn for m-N02 a = 0.72, a+ = 0.73. 

---