Reactivity substitution

The main problem encountered during electrophilic substitution reactions of aromatic amines is that of their veiy high reactivity. Substitution tends to occur at ortho- and para-positions. If we have to prepare monosubstituted aniline derivative, how can the activating effect of -NH group be controlled This can be done by protecting the -NH group by acetylation with acetic anhydride, then carrying out the desired substitution followed by hydrolysis of the substituted amide to the substituted amine. 

Attack at the 2-position has been effected with stabilized radicals such as benzyl and triphenylmethyl. Substitution has also been observed with alkylthio, phenylethynyl and thienyl radicals since the latter two are very reactive, substitution occurs at both positions 2 and 3 (73US295). Thiophene can also be homolytically aminated with amino cation-radicals, leading to 2-dialkylamino derivatives (74AHC(16)123>. 

Silicone polymer technology rests in practice upon the preparation of reactive substituted silanes from silicon metal and the subsequent conversion of these reactive substances, usually through stepwise hydrolysis and condensation reactions, into polysiloxanes. Thus the hydrolysis of these reactive intermediates is a fundamental process, the nature and implications of which have demanded increasing attention as organo-silicon chemistry and technology have developed. 

The network structure at any conversion may be obtained by joining the different fragments at random, with a probability given by the concentration of every fragment in the mixture. This is a mean-field approach and is not valid when nonidealities are present. Unequal reactivities, substitution effects, and intramolecular cycles give place to preferred nonrandom combinations. 

Results expressed by Eqs (3.100) and (3.102) are strictly valid for ideal systems. Unequal reactivities, substitution effects and the formation of intramolecular cycles will affect them. The first two nonidealities may be conveniently taken into account using the fragment approach described in the previous sections. 

For the anion-catalyzed heterolysis reaction with the complex (H20)5Cr(CMc20H), the catalytic effect increases with basicity of the anion. The activation volumes are all positive, in support of a dissocia-tively activated heterolysis mechanism. Overall, the decomposition mechanism involves parallel pathways whereby hydrolytic cleavage occurs at both (H20)sCr(CMe20H) + and the more-reactive substituted complex (H20)4XCr(CMe20H)+ (Scheme 2). 

R-M Organo- metallics CHjLi CHaMgl (CH3)2Cu-Li+ The more ionic RM bond is more reactive Substitutions Additions. Deprotonates acidic H s See enolates (allylic sources)... 

MH4- Complex metal hydrides LiAlH4 LiAlH(OR)3 NaBH4 NaBHjCN The more ionic MH bond is more reactive Substitutions Additions. Deprotonates acidic Hs on heteroatoms NaH and KHas MH bases... 

C=C-Z Allylic sources Enolates C=C-0 Enamines C=C-NR2 Enol ethers C=C-OR The better donor on the pi bond is more reactive Substitutions Additions. Carbon vs. heteroatom decision Extended enolates. Allylic alkyne sources... 

C-L Leaving group on sp carbon Sulfonates R-OSO2R Alkyl iodides R-I Protonated alcohols ROH2 Alkyl bromides R-Br Alkyl chlorides R-Cl The better L is more reactive Substitution versus Elimination Decision Section 9.5 2 Ls on C R2CL2 3 Ls on C RCLj... 

Zn alkyls, generated in situ, are very versatile reagents for alkyl-alkyl, alkyl-aryl and aryl-aryl couplings [73]. Arylzinc bromides and benzylzinc iodides may serve as more reactive substitutes for boronates or stannanes in Pd-mediated couplings. 

Raw Materials Base-Catalyzed Reactions Acid-Catalyzed Reactions Classification of Phenolic Resins Unsubstituted and Heat Reactive Unsubstituted and Nonheat Reactive Substituted and Heat Reactive Substituted and Nonheat Reactive Applications... 

Adjacent carbonyl groups also affect reactivity. Substitution by the ionization mechanism proceeds slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 304, such substituents destabilize a carbocation intermediate, but substitution by the direct displacement mechanism proceeds especially readily in these systems. Table 4.10 indicates some representative relative rate accelerations. 

The strained oxirane ring can cleave readily, exhibiting high reactivity substitution can occur on the benzene ring attached to the epoxide through ethereal linkage. 

In the past, impedance measurements using reactively substituted Wheatstone bridges at audio frequencies have been the easiest to accomplish. Consequently, great emphasis has been placed historically on electrochemical processes having characteristic impedance spectra in the audio frequency range 20-20,000 Hz, namely, double-layer capacitive and moderately fast reaction kinetic effects at plane parallel electrodes. 

The high-frequency limitation imposed on the operation of reactively substituted Wheatstone bridges by unavoidable stray capacitances prompted the development of the transformer ratio arm bridge (Calvert [1948]). By substituting a transformer for orthodox ratio arms, a bridge was produced for which the impedance ratio is proportional to the square of the number of turns and which was capable of accepting heavy capacitive loads with virtually no effect on the voltage ratio. 

In order to overcome issues such as inductive and steric effects in polymerizing substituted monomers, less-reactive substituted monomers can be either copolymerized with unsubstituted monomers or homopolymerized under more controlled conditions. In addition, it is important for the substitution to avoid locations that will impede polymer growth. For example, in the case of aniline, the substitution should only be in the meta and/or ortho positions and in the case of aromatic heterocyclics such as thiophene and pyrrole, substitution should only occur in the p position. ... 

Figure 10.4 Diversity of chemistiy structures utilized to create biomimetic adhesive polymers. Catechol side chain (A) modification alters its interfacial binding strength and reactivity. Substitution can be achieved by replacing -H at the para position with chloro-(B), nitro- (C) and hydrojyl (D) groups or a hydro)yl group at the meta position (E). The benzene ring can be substituted with a pyridine group (F). Linking the catechol with a polymer can be achieved via reaction of the amino acid (G), acid (H), or amine (I) groups. Catechol modified with a bromide propionamide Initiator to initiate polymerization (J) or functionalized with polymerizable methacrylamide (K), vinyl (L), and M-carboxyanhydride (NCA, M) groups.

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