Hydrogen-bonding substitution

Dearden, Rahman 1988 Hydrogen bonding Substituted anilines... 

Figure 1 Model for the chemical treatment of wood. (A) Cellular level. (l)-(3) Untreated cell wall, (4)-(6) treated cell wall (1) untreated (4) no chemical deposits in lumen (2) and (5) deposits on cell wall surface (3) and (6) filling of lumen. (B) Modification of lignocellulosic material at molecular level, (o) Hydroxyl group available for hydrogen bonding ( ) substitution of hydroxyl group ( ) bulking agent.

Several carbon-hydrogen bond substitutions are involved in the palladium-catalysed reaction of arylsulfonic acids with arenes to yield aromatic sulfones. A plausible mechanism, shown in Scheme 9, involves the initial formation of the palladacycle (99) which, after eoupling with the arene, yields the biphenyl derivative (100). Further coordination and carbon-hydrogen activation gives the seven-membered palladacycle (101) that affords the sulfone, (102), after reductive elimination. ... 

Breslow studied the dimerisation of cyclopentadiene and the reaction between substituted maleimides and 9-(hydroxymethyl)anthracene in alcohol-water mixtures. He successfully correlated the rate constant with the solubility of the starting materials for each Diels-Alder reaction. From these relations he estimated the change in solvent accessible surface between initial state and activated complex " . Again, Breslow completely neglects hydrogen bonding interactions, but since he only studied alcohol-water mixtures, the enforced hydrophobic interactions will dominate the behaviour. Recently, also Diels-Alder reactions in dilute salt solutions in aqueous ethanol have been studied and minor rate increases have been observed Lubineau has demonstrated that addition of sugars can induce an extra acceleration of the aqueous Diels-Alder reaction . Also the effect of surfactants on Diels-Alder reactions has been studied. This topic will be extensively reviewed in Chapter 4. 

The picture of the process of substitution by the nitronium ion emerging from the facts discussed above is that of a two-stage process, the first step in which is rate-determining and which leads to a relatively stable intermediate. In the second step, which is relatively fast, the proton is lost. The transition state leading to the relatively stable intermediate is so constructed that in it the carbon-hydrogen bond which is finally broken is but little changed from its original condition. 

The occurrence of a hydrogen isotope effect in an electrophilic substitution will certainly render nugatory any attempt to relate the reactivity of the electrophile with the effects of substituents. Such a situation occurs in mercuration in which the large isotope effect = 6) has been attributed to the weakness of the carbon-mercury bond relative to the carbon-hydrogen bond. The following scheme has been formulated for the reaction, and the occurrence of the isotope effect indicates that the magnitudes of A j and are comparable ... 

In 2-aminothiazole the two N-bonded hydrogens are substituted by the trimethylsilyl group with the reagent trimethylsilyl chloride-triethylamine (Scheme 105) (348). 

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

Once m the organic phase cyanide ion is only weakly solvated and is far more reactive than It IS m water or ethanol where it is strongly solvated by hydrogen bonding Nude ophilic substitution takes place rapidly... 

Some ortho substituted phenols such as o mtrophenol have significantly lower boiling points than those of the meta and para isomers This is because the intramolec ular hydrogen bond that forms between the hydroxyl group and the substituent partially compensates for the energy required to go from the liquid state to the vapor... 

Halogen exchange with KF is not successful ia acetic acid (10). Hydrogen bonding of the acid hydrogen with the fluoride ion was postulated to cause acetate substitution for the haUde however, the products of dissolved KF ia acetic acid are potassium acetate and potassium bifluoride (11). Thus KF acts as a base rather than as a fluorinating agent ia acetic acid. 

Olefin Complexes. Silver ion forms complexes with olefins and many aromatic compounds. As a general rule, the stabihty of olefin complexes decreases as alkyl groups are substituted for the hydrogen bonded to the ethylene carbon atoms (19). 

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