Carbonate anions

The structure of barbituric acid was the subject of disagreement for many years, but since 195 2 (52BSB44) the trioxo formulation (57 R = H) has been accepted generally, along with the fact that barbituric acid loses a proton, first from carbon (anion) and subsequently from nitrogen (dianion). Barbital (5,5-diethylbarbituric acid) adopts a similar trioxo form (57 R = Et) (69AX(B)1978). 

One such compound, bropirimine (112), is described as an agent which has both antineo-plastic and antiviral activity. The first step in the preparation involves formation of the dianion 108 from the half ester of malonic acid by treatment with butyllithium. Acylation of the anion with benzoyl chloride proceeds at the more nucleophilic carbon anion to give 109. This tricarbonyl compound decarboxylates on acidification to give the beta ketoester 110. Condensation with guanidine leads to the pyrimidone 111. Bromination with N-bromosuccinimide gives bropirimine (112) [24]. 

As with the reduction of carbonyl compounds discussed in the previous section, we ll defer a detailed treatment of the mechanism of Grignard reactions until Chapter 19. For the moment, it s sufficient to note that Grignard reagents act as nucleophilic carbon anions, or carbanions ( R ), and that the addition of a Grignard reagent to a carbonyl compound is analogous to the addition of hydride ion. The intermediate is an alkoxide ion, which is protonated by addition of F O"1 in a second step. 

We saw in Section 17.5 that treatment of an aldehyde or ketone with a Grignard reagent, RMgX, yields an alcohol by nucleophilic addition of a carbon anion, or carbanion. A carbon-magnesium bond is strongly polarized, so a Grignard reagent reacts for all practical purposes as R - +MgX. 

Carbanion (Section 19.7) A carbon anion, or substance that contains a trivalent, negatively charged carbon atom (R3C -). Carbanions are s/Ahybridized and have eight electrons in the outer shell of the negatively charged carbon. 

Similarly to Mn,Ox and related compounds, the chalcophanite structure can be interpreted as a filled Cdl2-type structure. The space in the octahedral layer is filled by an additional layer of water molecules and some foreign cations. A comparable situation is found in several hy-droxozincates, e.g., Zn5(OH)8Cl2 H20 or Zn5(OH)6(CO)3. In these compounds the layers are formed by edge-sharing zinc hydroxide octahedra, Zn(OH)6, and the space between the layers is filled with chloride and carbonate anions and some Zn2+ cations, which are located above and below vacancies in the Zn - OH layers. 

With growing interest in the chemical behaviour of actinide ions in the environment (1), the complexation of these ions with carbonate anions has been recently attracting particular attention (2-10) due to the ubiquitous presence of carbonate ions in nature (11, 12) and their pronounced tendency to form complexes with heavy metal ions (7, 10-14). In spite of the carbonate complexation of actinides being considered important chemical reactions for understanding the chemistry of actinides in natural fluids, not many experiments have been devoted up to now to the quantitative study of the subject, though numerous qualitative observations are discussed in the literature. Although there are a few papers reporting the formation constants of carbonate complexes... 

It is also attacked by the Lewis base OH in hot, molten sodium hydroxide and by O2- from the carbonate anion of hot molten sodium carbonate ... 

Sodium carbonate Sodium, a Group 1 metal, always forms +1 atomic cations, and carbonate anions are always -2. Thus, the compound formed from sodium and carbonate must contain two Na ions for every C03 , and its formuia is Na2 CO3. 

Ozone and carbon dioxide demonstrate that p orbitals can overlap side by side with more than one neighbor. This feature can lead to a system of 7r bonds that can extend over many atoms. Extended systems can be long chains, or they can be more compact clusters or rings. We begin by describing two examples of four-atom tt systems, butadiene and the carbonate anion. 

In addition to four atoms in a row, a four-atom deiocaiized framework can have a centrai atom surrounded by three other atoms with which it interacts. An exampie is the carbonate anion, whose Lewis structure dispiays resonance ... 

Because 1 is several orders of magnitude larger than 2 or, we identify 1 as dominant. Notice, however, that the water equilibrium generates some hydroxide ions in the solution, so this equilibrium must be used to find the concentration of hydroxide ions. The third reaction involves a minor species, HCO3, as a reactant, so it cannot be the dominant equilibrium. However, just as the water equilibrium generates some hydroxide ions, the hydrogen carbonate equilibrium generates some carbonate anions, whose concentration must be determined. 

Next, we take into account the proton-transfer equilibrium involving hydrogen carbonate anion. To do this, we complete a second concentration table, using as initial concentrations those calculated for the first equilibrium ... 

Any anion of a weak acid, including the anions of polyprotic acids, is a weak base. The acid-base properties of monoanions of polyprotic acids are complicated, however, because the monoanion is simultaneously the conjugate base of the parent acid and an acid in its own right. For example, hydrogen carbonate anions undergo two proton-transfer reactions with water ... 

To determine if the overaii caicuiation is reasonable, notice that the hydronium concentration in acid rain is five orders of magnitude higher than in pure water. This increased hydronium ion concentration can convert a substantiai amount of carbonate anions into hydrogen carbonate anions, promoting the soiubiiity of caicium carbonate. The resuiting equiiibrium concentration of caicium cations is about five orders of magnitude iarger than the concentration in a saturated soiution in pure water. 

To reduce soil acidity and keep cropland productive, farmers amend the soil by liming, which is the application of limestone. Limestone is calcium carbonate, and the carbonate anion is the conjugate base of hydrogen carbonate. Liming therefore increases the concentration of hydroxide in the soil, thereby increasing the pH C03 ((3 q) + H2 0(/) HC03 (<3 g) + OH (<3 q)... 

FeC03 This ionic compound contains the carbonate polyatomic anion, C O3. The -2 charge on carbonate requires that iron have +2 oxidation number (Guideline 2). In the carbonate anion, each oxygen atom is -2, for a total of-6. The oxidation numbers must add up to the net charge on the anion (-2), so the carbon atom must have an oxidation number of +4. 

The presentation finally descends on the very last of the listed chemical precipitation processes, and this pertains to precipitation implemented by carbonate anions (CO-). The dissolution of carbon dioxide takes place with the formation of the weak diabasic acid, H2C03. The overall dissociation of this acid (dissociation constant, KD) is represented by ... 

If S moles of CaCC>3 dissolve in a liter of water, then S moles each of calcium ion and carbonate ion form. With these ion concentrations equal to S, the solubility of CaCC>3 is calculated as 9.3 x 10 5 M. The higher solubility of magnesium carbonate in water, 6.3 x 10 3 M, results from the larger solubility product constant. Nevertheless, both of these carbonate salts are rather insoluble, and the excess carbonate anions provided by the sodium carbonate effectively precipitate the calcium and magnesium ions from solution. 

The determination of the structure of the iron transporter, ferric-binding, protein (hFBP)t from Haemophilus influenzae (Bruns et ah, 1997) at 0.16 nm resolution shows that it is a member of the transferrin superfamily, which includes both the transferrins and a number of periplasmic binding proteins (PBP). The PBPs transport a wide variety of nutrients, including sugars, amino acids and ions, across the periplasm from the outer to the inner (plasma) membrane in bacteria (see Chapter 3). Iron binding by transferrins (see below) requires concomitant binding of a carbonate anion, which is located at the N-terminus of a helix. This corresponds to the site at which the anions are specifically bound in the bacterial periplasmic sulfate- and... 

The iron-binding sites have been characterized by crystallographic studies on several transferrins, and in Figure 5.7 (Plate 7) that of the N-lobe of human lactoferrin is presented. The 3+ charge on the ferric ion is matched by the three anionic ligands Asp-63, Tyr-95 and Tyr-188 (the fourth, His-249, is neutral), while the charge on the carbonate anion is almost matched by the positive charge on Arg-124 and the... 

Due to blockage of the main zeolite channels by anions present in the structure, few applications have been reported for these tectosilicates. However, the presence of carbonate anions could make them suitable as antacids. Our group has worked with a series of natural minerals as antacids, such as clays [4], modified carbons [5] and zeolites[6]. In this way, a carbonated cancrinite and its intermediate phase were tested as an antacid with a synthetic gastric juice. 

The aldol reactions introduced thus far have been performed under basic conditions where enolate species are involved as the reactive intermediate. In contrast to the commonly accepted carbon-anion chemistry, Mukaiyama developed another practical method in which enol species can be used as the key intermediates. He is the first chemist to successfully demonstrate that acid-catalyzed aldol reactions using Lewis acid (such as TiCU) and silyl enol ether as a stable enol equivalent can work as well.17 Furthermore, he developed the boron tri-fluoromethane sulfonate (triflate)-mediated aldol reactions via the formation of formyl enol ethers. 

We must treat with caution one further aspect of the Brpnsted theory multiple proton-donation reactions. Consider the example of the bicarbonate ion HC03 in water. When titrating bicarbonate with a base such as hydroxide, the ion behaves as an acid to form the carbonate anion and water ... 

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