Involving Two Functional Groups

Non-redox reactions where water is formed as a product are reactions of dehydration. Such reactions can occur between two substrate molecules, or they can involve two functional groups in a single substrate, either creating a new bond (e.g., lactone formation), or transforming a single into a double bond. In xenobiotic metabolism, dehydration is usually in dynamic equilibrium with hydrolysis or hydration and is of relatively modest significance (Chapt. 11). 

The ratio of a first-order rate constant for an intramolecular reaction (involving two functional groups or moieties within the same molecular entity) to the second-... 

Cyclization reactions can involve two functional groups of dihydroazines. For example, treatment of 2-methyl-3,5-biscarbethoxy derivatives of 1,4-dihy-dropyridine 404 and 405 sequentially with sodium hydride and 1,3,5-triazine yields either l,4,5,6-tetrahydro-l,6-naphthyridine-3-carboxylate 406 [402, 403, 404, 405] or pyrido[2,3-J pyrimidines 407 [406] (Scheme 3.129). 

However, this is considerably more complicated than for thermochemistry, because most chemical reactions involve two functional groups, not just one, and some types of reactions have a rather complex dependence on the extended molecular structure. When representing functional groups for chemical reactions, one usually needs to specify several special atoms rather than just one... 

The step polymers that are treated in this text are cellulose, starch, vegetable gums, nylon, epoxy resins, phenol- methanal network polymers, and most polymeric liquid crystals. The reactions which form these polymers are summarized in Table 3. For the synthetic polymers of Table 3, there are often several reactions which produce the polymer. Only one representative reaction is shown in the Table. Note that all of these reactions involve two functional groups on different molecules reacting to form another link and ultimately thereby form a polymer. 

Koshland (1962) has calculated, however, that such a propinquity effect will not explain the large rate enhancements observed with enzymes unless there are more than two functional groups involved with utilization of five functional groups (2 substrates and 3 catalytic groups) a rate increase of 10 would be possible. Such multifunctional catalysis would, of course, be impossible to demonstrate... 

Kinetic analysis of a step polymerization becomes complicated when all functional groups in a reactant do not have the same reactivity. Consider the polymerization of A—A with B—B where the reactivities of the two functional groups in the B—B reactant are initially of different reactivities and, further, the reactivities of B and B each change on reaction of the other group. Even if the reactivities of the two functional groups in the A—A reactant are the same and independent of whether either group has reacted, the polymerization still involves four different rate constants. Any specific-sized polymer species larger than dimer is formed by two simultaneous routes. For example, the trimer A—AB—B A—A is formed by... 

The discussion to this point has involved copolymers in which both repeating units have the same functional group. A second catagory of copolymer involves different functional groups in the two repeat units, for example, an amide-ester copolymer such as... 

Although the structures of many carbohydrates appear to be quite complex, the chemistry of these substances usually involves only two functional groups —ketone or aldehyde carbonyls and alcohol hydroxyl groups. The carbonyl groups normally do not occur as such, but are combined with hydroxyl groups to form hemiacetal or acetal linkages of the kind discussed in Section 15-4E. 

Two routes can be explored with a view to modify the surface properties of cellulosics. The first one involves chemical reactions involving the functional groups of cellulose. The second route can be achieved through irradiation or thermal grafting of monomers onto cellulose. 

Changes in ring conformation vastly alter the relative position of groups within the same molecule. In cases involving interaction between or reaction with two functional groups, ring shapes may determine the course of reactions. The formation of certain hexosans, (1,5)/3(1,6), has been explained upon the basis of the conformational behavior of the aldohexose.10... 

The distances between the neighbouring atoms of the two functional groups (AH. .. B) involved in the hydrogen bond are substantially smaller than the sum of their van der Waals radii RHB < K + < ... 

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