Decreased mesomeric effect

M Group viz.,p-N02 (an electron withdrawing function)—its presence as shown in (d) above will further increase frequencies due to decreased mesomeric effect. 

Inductive activation (Scheme II) gives only about one-twentieth as great a rate increase as resonance activation from the adjoining ring. The pertinence of the jjom,pom-quinoid structures (342 and 343) or their para,ortho-isomers (not shown) to the observed inductive activation depends on the relative significance of the field and mesomeric factors in these inductive effects. The large decrease in effectiveness of intemuclear resonance activation suggests that the intemuclear mesomeric factor or resonance parameter 179,345 compounds... 

Groups that reduce the electron density at the carbon atom to which the carboxylic acid group is attached assist proton release and increase the acidity. In cases where the substituent can exert both -I and -M effects, the 4-isomer is a stronger acid than the 3-substituted compound, for which only inductive withdrawal can operate. Thus, 4-nitrobenzoic acid is a stronger acid than 3-nitrobenzoic acid, but both are stronger than benzoic acid. The converse applies for a -I/+M substituents such as methoxy, since the mesomeric effect decreases the acidity of 4-methoxybenzoic acid (pAT 4.47), whereas the electron-withdrawing... 

The photolysis of aniline at high temperatures results in N-H and C-N clea-vage, but at room temperature the major reaction is fluorescence. Below 2800 A some photodecomposition takes place the lifetime and dissociation probability of excited aniline molecules have been calculated Flash photolysis of aniline followed by kinetic mass spectroscopy failed to reveal the presence of dissociation products. Irradiation of halogen derivatives of aniline between 2480 and 2562 A produces halide ions in measurable yields . Positive mesomeric effects increase, and negative mesomeric effects decrease... 

In both cases, i.e., equilibrium complexes [p-X-PhO H F] and phenols, electron-accepting substituents (with cTp > 0) increase the contribution of the quinoid-like structure, which in turn leads to a decrease of charge at the oxygen atom. The greater the value of o-p is, the stronger the mesomeric effect appears. It is worth mentioning that in all equilibrium complexes the proton is transferred to fluoride. In the case of phenols the sensitivity of O-H bond lengths on o-p is 100 times lower that for the O- H interatomic distance in equilibrium complexes. 

Electronic and steric factors exert their impacts on this rearrangement. Thus, the rate of isomerization increased with electron depletion and decreased with electron enrichment of the pyrimidine ring. It was less facile when the triazole ring was substituted (89JHC687). Methyl groups with a positive mesomeric effect at C5 and/or C8 (16, R1 and or R3=Me) diminished the rate of acid-induced isomerization (pH 4) by retarding the approach of the nucleophile to C5 by electronic and (for C5) steric hinderance effects. Methyl groups at C3 and/or C7 (16, R and or R2=Me) exerted little electronic and no steric effects on C5, yet promoted the approach of the... 

Substituents which have an unshared pair of electrons can increase the electron density in the ring by the mesomeric effect, +M, which is indicated by curved arrows. Other substituents can decrease the electron density in the ring by the — M effect. These effects are illustrated in the accompanying <... 

C, indicate that the mesomeric effect e.g. MejC- -F Me2C=F ) of the halogens involved decreases in the order F > Cl > Br. All this and related work by Olah s group on fluorinated carbocations has been reviewed, and the results of MO studies of fluorinated methyl, allyl, or trans-2 vinylcyclopropylmethyP cations and radicals have been presented in detail. The electronic structure of difluoromethane and the partial double-bond character of C— F bonds in CHjF, CHjFj, and CHF, have received attention at the hands of MO pundits. 

However, 1-phenylpropene undergoes hydroboration at the C-1 position. This is not what one would expect based on the steric effect of a phenyl group. This is because of the combined effects of phenyl conjugation (-K) and methyl hyperconjugation (-tK), which act to decrease the amount of electron density at the C-2 position and to increase it at the C-1 position (Fig. 4.1) [6]. Evidently, these mesomeric effects are strong enough to override the steric effects of the phenyl group. 

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