Functional derivatives of carbonic acid

Much of the chemistry of the functional derivatives of carbonic acid is already quite familiar to us through our study of carboxylic acids. The first step in dealing with one of these compounds is to recognize just how it is related to the parent acid. Since carbonic acid is bifunctional, each of its derivatives, too, contains two functional groups these groups can be the same or different. For example ... 

A common reaction theme of functional derivatives of carboxylic acids is nucleophilic acyl addition to the carbonyl carbon to form a tetrahedral carbonyl addition... 

The coenzyme forms of folic acid are derivatives of tetrahydrofolic acid, FH4. See Fig. 7. Folic acid functions as a coenzyme in enzyme reactions which involve the transfer of one-carbon fragments at various levels of... 

More recently, Porta and co-workers [6] applied similar considerations of the polar effects to a new one-pot multicomponent process for the addition of nucleophilic radicals to aldimines, generated in situ in the presence of Ti(IV). In analogy with the Minisci reaction, Ti(IV), which acts as a Lewis acid, coordinates the nitrogen of the imine, strongly increasing the electron-deficient character of the carbon in the a-posilion and thus the reactivity of the imine toward nucleophilic radicals. This reaction, as well as the Minisci one, represents a useful route for the synthesis of a variety of poly-functionalized derivatives of chemical and biochemical relevance. 

The treatment of alkenes with iodine(III) reagents usually results in functionalization of the carbon-carbon double bond. However, 1,1-diphenylethylene affords a low yield of (l,l-diphenylethenyl)phenyliodonium tosylate with HTIB (equation 181)11,138. The cyclic dithiolylidene derivative of malonic acid, shown in equation 182, undergoes decarboxylation with [bis(trifiuoroacetoxy)iodo]benzene in methanol and gives an unusual vinyliodo-nium trifluoroacetate139. Finally, when the allenylphosphonate shown in equation 183 is added to a mixture of (difluoroiodo)benzene and BF3-etherate in dichloromethane, a... 

The manufacture of the large variety of polyamides (commonly referred to as nylons) occurs through polycondensation of amino carboxylic acids (or functional derivatives of them, e.g. lactams) and from diamines and dicarboxylic acids. Labeling the amino groups with A and the carboxyl groups with B allows differentiation of the different chemical structures between the two types AB (from amino carboxylic acids) and AA-BB (from diamines and dicarboxylic acids). The number of C atoms in the monomers acts as a code number for the identification of the polyamides. The polycaprolactam manufactured from caprolactam (type AB) is then called polyamide 6 (PA 6). The number of carbon atoms in the diamine is given first for type AA-BB followed by the number of atoms in the dicarboxylic acid, e.g. PA 66 for polyhexamethylenedia-dipic amide from hexamethylenediamine and adipic acid. For copolymers the components are separated by a slash, e.g. PA 66/6 (90 10) is a copolymer composed of 90 parts PA 66 and 10 parts PA 6. 

Polycarbonates and Polyurethanes The chemistry of carbonic acid derivatives is particularly important because two large classes of polymers are bonded by linkages containing these functional groups the polycarbonates and the polyurethanes. Polycarbonates are polymers bonded by the carbonate ester linkage, and polyurethanes are polymers bonded by the carbamate ester linkage. Lexan polycarbonate is a strong, clear polymer used in bulletproof windows and crash helmets. The diol used to make Lexan is a phenol called bisphenol A, a common intermediate in polyester and polyurethane synthesis. 

We use these functional relationships to carbonic acid simply for convenience. Many of these compounds could just as well be considered as derivatives of other acids, and, indeed, are often so named. For example ... 

Lactic acid (2-hydroxypropanoic acid, a-hydroxypropi-onic acid) is the next shortest AHA after glycolic acid. A methyl group (CH3) replaces the terminal hydrogen atom on the alpha carbon (Figure 6.3). It has a of 3.86, which is close to that of glycolic acid. Lactic acid occurs naturally in sour milk. After penetrating the skin, lactic acid is converted automatically and reversibly into pyruvic acid, the alpha-keto acid derivative of lactic acid a keto (=0) function replaces the hydroxy (-OH) function on the alpha carbon. At identical concentrations, lactic acid destroys the epidermis more slowly than glycolic acid. 

The Chemical Synthesis of Peptides Carboxylic acids and acyl derivatives of the carboxyl functional group are very important in biochemistry. For example, the carboxylic acid functional group is present in the femily of lipids called fatty acids. Lipids called glycerides contain the ester functional group, a derivative of carboxylic acids. Furthermore, the entire class of biopolymers called proteins contain repeating amide functional group linkages. Amides are also derivatives of carboxylic acids. Both laboratory and biochemical syntheses of proteins require reactions that involve substitution at activated acyl carbons. 

The traditional classification of organic compounds, according to the nature of functional groups and their preparations [1], emphasizes phosgene (the dichloride of carbonic acid) as a carbonic acid derivative. Indeed, the reactivity of phosgene in nucleophilic reactions is best understood by conadering the electronic structure of carbonic acid and the electronic and steric effects of the substituents in its derivatives. 

Much of the chemistry of that group of compounds referred to as carbohydrates (compounds of carbon corresponding to, approximately, C(H20) ) is a function of the numerous hydroxyl groups that adorn their structures. However, despite the view that they be considered as polyhydroxy derivatives of carbon (polyols), their rich chemistry must include reference to reactions of the carbonyl group (aldehydes, ketones, and carboxylic acids). Therefore, elaboration of the chemistry of carbohydrates is postponed to Chapter 11. 

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