Destabilising group

The relative stereochemistry of hyperaspine (1) was determined by 2D NMR and MS methods. It has a m-fused bicyclic conformation 82 (01TL4621). The tram-fused one is disfavored by an axial pentyl group at C(8) and by a destabilising dipole-dipole interaction between the N and O atoms, which does not exist in the alternative cis conformation. 

Sulphuric acid at 93% was added to p-nitrotoluene. The temperature reached 160°C due to a failure of the thermal control system. The sulphonic acid formed decomposed violently at this temperature. The post-accident investigation showed that the decomposition started between 160 and 190 C. In fourteen minutes the temperature rose to 190-224°C and in one minute and thirty seconds to 224-270°C. A large volume of gas was then released during the eruption. The phenomena caused by the decomposition of nitrated derivatives in the presence of sulphuric acid will be addressed several times. What these incidents have in common is the formation of large carbonised volumes. This phenomenon is common with sulphonic acids. The nitro group role is to destabilise intermediate compounds and final compounds and to generate... 

Protonation, if forced upon pyrrole, is found to take place not on nitrogen but on the a-carbon atom (19). This occurs because incorporation of the nitrogen atom s lone pair of electrons into the aromatic 6jre system leaves the N atom positively polarised protons tend to be repelled by it, and are thus taken up by the adjacent a-carbon atom. The basicity situation rather resembles that already encountered with aniline (p. 70) in that the cation (19) is destabilised with respect to the neutral molecule (18a). The effect is much more pronounced with pyrrole, however, for to function as a base it has to lose all aromatic character, and consequent stabilisation this is reflected in its related pKa (-0-27) compared with aniline s of 4-62, i.e. pyrrole is a very weak base indeed. It can in fact function as an acid, albeit a very weak one, in that the H atom of the NH group may be removed by strong bases, e.g. eNH2 the resultant anion (20) then retains the aromatic character of pyrrole, unlike the cation (19) ... 

The destabilising influence of the electron-donating inductive effect of alkyl groups is seen in the observed carbanion stability sequence ... 

These features should be useful in attempts to estimate the stability or reactivity of a compound or reaction system which does not appear in this Handbook. The effects on stability or reactivity of changes in the molecular structure to which the destabilising or reactive group(s) is attached are in some cases discussed in the group entry. Otherwise such information may be gained from comparison of the information available from the individual compound entries listed collectively (now in alphabetical order, with serial number) in the group entry. 

Steric effects on both the amide and the acyloxyl side chain are similar. Tert-butyl and adamantyl groups on the amide side chain in 29v, 29x, 29c, and 29e (Table 2 entries 53 and 54, 63 and 65) result in lower stretch frequencies that, on average, are only 40 cm-1 higher than their precurser hydroxamic esters. Streck and coworkers have suggested that such changes in dialkyl ketones can be ascribed to destabilisation of resonance form II through steric hindrance to solvation which, in the case of tert-butyl counteracts the inductive stabilisation.127... 

The need for electrophilicity at the amide nitrogen was demonstrated by comparison of the mutagenic activity of 28a with that of two analogues with similar structure, A-benzoyloxy-A-benzylbenzamide 117, which possesses a leaving group but without anomeric destabilisation and A-benzyl-A-benzyloxybenzamide 118,... 

Electron-donating substituents in conjugation with the carbonyl group have the effect of stabilising the (7r,7r ) triplet state but destabilise the (n,7t ) triplet state. On the other hand, the (n,7r ) triplet state is stabilised relative to the (n,n ) triplet state by the presence of electron-accepting substituents. 

Electron donating group (EDG) destabilises the carbojq late anton and weakens the add... 

Trialkylsilyl substituents have proved very useful for directing the course of cy-clizations involving vinyl ethers. The trialkylsilyl group stabilises a radical centre but destabilises a carbonium ion next to the silicon atom. An example of the di-... 

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