Paired Group and Neighboring Group Effects

When random, irreversible, and intramolecular reactions occur on polymeric backbones with the functional groups adjacent to each other, they can be expected to react. There is, however, an upper limit to conversions. This upper limit is due to statistical probability that some functional groups are bound to become isolated. The limit for conversion was calculated to be 86.5% [11]. Theoretically, quantitative conversions should be possible with reversible reactions of paired functional groups on macromolecular backbones. The ability, however, of isolated reactive groups to find each other and then pair off depends either upon particularly high driving forces, or upon the time required to accomplish complete conversions [12]. For reactions initiated randomly, at different sites, the probability is high that two groups on the terminal units will eventually meet and react. Since the reactions are reversible, at least in theory, very high conversions are possible. Neighboring group participation can usually be deduced from three types of evidence: 1. If the reactions occur more rapidly during the rate-determining step than can be expected from other considerations. 2. If the stereochemistry of the reactions suggests neighboring group involvement. 3. If molecular rearrangements occur and the groups remain bonded to reaction centers, but break away from the atoms to which they were originally attached on the substrates. There are many examples in the literature that describe neighboring group effects in reactions of polymers. One example is hydrolysis of poly (p-nitrophenyl methacrylate-co-acrylic acid). The high reaction rate at a neutral pH is due to attacks by the carboxylic moieties upon the neighboring carbonyl carbons [13–15]. Decomposition rates of t-butyl acrylate-styrene copolymers [16] can serve as an another example. Experimental data show pronounced acceleration for all samples. This is interpreted in terms of both intra and intermolecular interactions of the esters and the carboxylic groups. It follows a suggestion of Cherkezyan and Litmanovich [16, 17] that the instantaneous reactivity of any group depends on its microenvironment. That includes (for reactions of polymer in molten condition) two nearest units on the same chain (internal neighbors) and two units belonging to two different chains (external neighbors).

Another example of the neighboring group effect is the behavior of polyacrylamides in hydrolyses. There are two distinct and successive rates [18]. After conversions of up to 40–50% are reached, the reactions slow down. This is due to accumulations of negative electrostatic charges on the polymeric backbones [18]. In alkaline media, the increasing negative charges along the chains exert electrostatic repulsions toward the hydroxyl ions. This results in rate decreases.

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Halogenation

Reactions That Favor Large Molecules

Effects of Changes in Solubility

Effect of Molecular Size

Reactivity of Macromolecules

Polyalkanoates

Synthetic Methods for the Preparation of Nucleic Acids

DNA and RNA

Nucleic Acids

Chemical Modification of Proteins

Synthetic Methods for the Preparation of Polypeptides

Structures and Chemistry of Proteins