Reactivity I, substitution reactions

Reactivity I Substitution Reactions—The Reaction of Aquapentacyanoferrate(ll) Ion [Fe(CN)5(H20)]3 with Amino Acids1,2... 

The results in table 2.6 show that the rates of reaction of compounds such as phenol and i-napthol are equal to the encounter rate. This observation is noteworthy because it shows that despite their potentially very high reactivity these compounds do not draw into reaction other electrophiles, and the nitronium ion remains solely effective. These particular instances illustrate an important general principle if by increasing the reactivity of the aromatic reactant in a substitution reaction, a plateau in rate constant for the reaction is achieved which can be identified as the rate constant for encounter of the reacting species, and if further structural modifications of the aromatic in the direction of further increasing its potential reactivity ultimately raise the rate constant above this plateau, then the incursion of a new electrophile must be admitted. 

S+3C] Heterocyclisations have been successfully effected starting from 4-amino-l-azadiene derivatives. The cycloaddition of reactive 4-amino-1-aza-1,3-butadienes towards alkenylcarbene complexes goes to completion in THF at a temperature as low as -40 °C to produce substituted 4,5-dihydro-3H-azepines in 52-91% yield [115] (Scheme 66). Monitoring the reaction by NMR allowed various intermediates to be determined and the reaction course outlined in Scheme 66 to be established. This mechanism features the following points in the chemistry of Fischer carbene complexes (i) the reaction is initiated at -78 °C by nucleophilic 1,2-addition and (ii) the key step cyclisation is triggered by a [l,2]-W(CO)5 shift. 

The last topic to be treated is unequal reactivity by substitution effects. As a first example, the effect of an infinitely negative substitution effect in C due to a reaction with an h group (so I CD Kqj = 0) is compared with the case of equal (random) reactivity of the two functional groups in C for formulation F40. This is suggested as an example of polyesterification with an anhydride and a carboxylic acid, respectively. Figure 15 gives the dramatic effect on... 

Molecular structural changes in polyphosphazenes are achieved mainly by macromolecular substitution reactions rather than by variations in monomer types or monomer ratios (1-4). The method makes use of a reactive macromolecular intermediate, poly(dichlorophosphazene) structure (3), that allows the facile replacement of chloro side groups by reactions of this macromolecule with a wide range of chemical reagents. The overall pathway is summarized in Scheme I. 

From the investigation of all these data it is clear that the aromaticity of phosphinine is nearly equal to that of benzene. Their chemical reactivity, however, is different. Most important is the effect of the in-plane phosphorus lone pair, which (i) is able to form a complex and (ii) can be attacked by electrophiles to form A -phosphinines. Thus, electrophilic substitution reaction on the ring carbon is impossible. In Diels—Alder reactions, phosphinines behave as dienes, providing similar products to benzene but under less forcing condition (the reaction with bis(trifluoromethyl) acetylene takes place at 100 °C with 3, while for benzene 200 °C is required). 

The 5-position of the nonprotonated 1,2,4-thiadiazole system was calculated to be the most reactive in nucleophilic substitution reactions using a simple molecular orbital method with LCAO approximation (84CHEC-I(6)463>. 

The 5-position in 1,2,4-thiadiazoles is most reactive in nucleophilic substitution reactions, for example, halogens may be displaced by a variety of nucleophiles (84CHEC-i(6)463>. However, halogens in the 3-position are inert towards most nucleophilic reagents. 

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