Mesomeric Substituents

Mesomeric substituents possess orbitals of Jt symmetry and can therefore extend the conjugated system. Benzene substituted by a donor with a doubly occupied p AO or substituted by an acceptor with an empty p AO is isoelec- 

The substituent effect on the hydrocarbon MOs may again be estimated by perturbation theory. However, it has to be remembered that first-order contributions vanish according to 

If a substituent possesses only one p orbital which in most cases is doubly occupied as in —NHj or —OH, Equation (2.24) is simplified and becomes 

The discussion given earlier for weak mesomeric effects offers an explanation for the empirical increment rules proposed by Woodward (1942) for estimating the absorption maxima of dienes and trienes. The distinct values for the homo-and heteroannular arrangement of the double bonds illustrate the configurational influence on the polyene spectra that has been discussed in Example 2.2. From the data collected in Table 2.5 the absorption of methylenecyclo-hexene (27) with an exocyclic double bond and two alkyl substituents is calcu- 

In general, substituents are neither purely inductive nor purely mesomeric rather they possess some inductive as well as some mesomeric character. From Equations (2.19) and (2.24) it is seen that inductive as well as mesomeric effects on the HOMO and LUMO are proportional to the square of the LCAO coefficient at the substituted center g. It is therefore difficult to differentiate between these two effects from the absorption spectra. This is possible, however, by means of the MCD spectra that depend essentially on the difference AHOMO - ALUMO. (Cf. Section 3.3.) In the case of uncharged (4N-l-2)-eIectron perimeters and especially benzene, perturbations due to purely inductive substituents always yield AHOMO = ALUMO to first order in perturbation theory. Due to the energy difference in the denominator of Equation (2.24), however, AHOMO and ALUMO can be quite different in the case of mesomeric substituents, depending on the energy 

6ef in Equation (2.24) depends not only on but also on the quantity kot)8p as well as on (ef - ) It is therefore not possible to characterize the mesomeric effect by a single substituent strength. In fact, depending on ef the same substituent can act as a donor with respect to one hydrocarbon and yet as an acceptor with respect to another one if it possesses occupied as well as unoccupied orbitals. An example is provided by the vinyl group with its n and n orbitals, which acts as an acceptor when attached to the C Hj anion and as a donor when attached to the C7H7 cation. 

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The low acidity of methylene groups, which are activated by only one electron-withdrawing mesomeric substituent, generally results in a lower reactivity under phase-transfer catalytic conditions. Monoalkylation normally occurs, sometimes to the complete exclusion of dialkylation, and further alkylation is generally only... 

The cations of quinazolines that have a strong —I substituent (e.g. Cl, NO2) are almost completely hydrated, and those with a weaker —I, or with +1, substituent (e.g. OMe, OH, NHg, Me) may contain a small proportion (about 10-25%) of the anhydrous cation. Two important exceptions are now discussed. Substituents in the 4-position, whether —lov+I, have a strong dehydrating effect (cf. Section II, C). Again, when a strongly tautomeric (-t-T ) substituent (sometimes called a mesomeric substituent) is placedporo (i.e. in position 7) to the carbon atom C-4, the tendency to hydrate is strongly reduced. This example of action at a distance was demonstrated in 7-methoxy-,... 

Most organic compounds that show absorption in the visible or in the near-UV region have a linear or cyclic n system as the chromophoric system. Therefore, the results of the previous sections may be used and extended to discuss light absorption of all those compounds that can be derived from linear and cyclic hydrocarbons by including the influence of substituents in an appropriate way. (Cf. Michl, 1984.) A complete theory of substituent effects comprises all areas of organic chemistry. Here, only the fundamental concepts of the influence of inductive and mesomeric substituents will be considered. In order to simplify the discussion, substituent effects will be called inductive if in the HMO model they can be represented by a variation of the Coulomb integral of the substituted n center p. If they are due to an extension of the n system they will be called mesomeric. 

The fact that the benzyl anion and benzyl cation should have opposite MCD signs according to the mirror-image theorem makes it easy to understand why benzene derivatives with mesomeric donor substituents, which are isoelectronic with the benzyl anion, and benzene derivatives with mesomeric acceptor substituents, which are isoelectronic with the benzyl cation, show opposite MCD signs. This fact can be used for a qualitative and even a quantitative characterization of mesomeric substituent effects. (See also Section 2.4 and Example 3.10.)... 

The effect of introduction of an additional heteroatom (88ZOR1799), inductive substituents (90ZOR1793), mesomeric substituents (88ZOR1806), and effect of ben-zannulation (90ZOR1387) on the position of the tautomeric equilibrium of azines with various functional groups were investigated by PMO semiempirical calculations. It was shown that the transition from pyridine to analogous 2-pyrimidine or... 

Then, for a given mesomeric substituent M t (M,R) correlates with Taft s steric constants j(R) (8). In particular, for phenylallenes r (Ph,R) may be related to the angle of twist 0(Ph,R) of the phenyl group which results from the steric bulk of the neighboring ligand (lb). 

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