Acidic hydrogen atom

It has not been determined which hydroxyl group contains the more acidic hydrogen atom corresponding to p Dyes of this type would behave as indicators and exhibit large color shifts with the pH range normally encountered in textile processing. Hence they are always stabUized by coordination with metal ions. 

For a given a hydrogen atom to be acidic, the C-H bond must be parallel to the p orbitals of the C=0 bond (that is, perpendicular to the plane of the adjacent carbonyl group). Identify the most acidic hydrogen atom in the conformation shown for the following structure. Is it axial or equatorial ... 

As usual, R is a hydrocarbon group or, in the simplest case, a hydrogen atom. The acidic hydrogen atom is the one bonded to oxygen. The IUPAC name of a carboxylic add can be obtained by substituting the suffix -oic acid for the final e in the name of the corresponding alkane. In practice, such names are seldom used. For example, the first two members of the series are commonly referred to as formic add and acetic add. 

By completely deprotonated, we mean that each acid molecule or ion has lost the proton of its acidic hydrogen atom by transferring it as a hydrogen ion to a solvent molecule. Completely protonated means that each base species has acquired a proton. By incompletely deprotonated or incompletely protonated, we mean that only a fraction (usually a very tiny fraction) of the acid molecules or ions have lost acidic hydrogen atoms as protons or only a tiny fraction of the base species have acquired protons. 

The term proton in these definitions refers to the hydrogen ion, H+. An acid is a species containing an acidic hydrogen atom, which is a hydrogen atom that can be transferred as its nucleus, a proton, to another species acting as a base. The same definitions were proposed independently by the English chemist Thomas Lowry, and the theory based on them is called the Bronsted-Lowry theory of acids and bases. 

FIGURE 10.15 In a solution of a weak acid, only sonic of the acidic hydrogen atoms are present as hydronium ions (the red sphere), and the solution contains a high proportion of the original acid molecules (HA, gray spheres. The green sphere represents the conjugate base of the acid and the blue spheres are water molecules. The overlay shows only the solute species. 

The conjugate base of a polyprotic acid is amphiprotic it can act as either an acid or a base because it can either donate its remaining acidic hydrogen atom or accept an acidic hydrogen atom and revert to the original acid. For example, a hydrogen sulfide ion, HS-, in water acts as both an acid and a base ... 

Because many biological systems use polyprotic acids and their anions to control pH, we need to be familiar with pH curves for polyprotic titrations and to be able to calculate the pH during such a titration. The titration of a polyprotic acid proceeds in the same way as that of a monoprotic acid, but there are as many stoichiometric points in the titration as there are acidic hydrogen atoms. We therefore have to keep track of the major species in solution at each stage, as described in Sections 10.16 and 10.17 and summarized in Figs. 10.20 and 10.21. 

The titration of a polyprotic acid has a stoichiometric point corresponding to the removal of each acidic hydrogen atom. The pH of a solution of a polyprotic acid undergoing a titration is estimated by considering the primary species in solution and the proton transfer equilibrium that determines the pH. 

Mond process The purification of nickel by the formation and decomposition of nickel carbonyl, monomer A small molecule from which a polymer is formed. Examples CH2=CH2 for polyethylene NH2(CH2)6NH2 for nylon, monoprotic acid A Bronsted acid with one acidic hydrogen atom. Example CH COOI I. monosaccharide An individual unit from which carbohydrates are considered to be composed. Example C6H(206, glucose, multiple bond A double or triple bond between two atoms. 

This bifunctionnal amino-thiourea organocatalyst led to high selectivity because it was activating both the nitrone and the malonate, in its enol form, due to the acidic hydrogen atoms of the thiourea. Thus, the amino-thiourea catalyst promoted the Michael reaction of malonates to various nitroolefins... 

These transition-metal catalysts contain electronically coupled hydridic and acidic hydrogen atoms that are transferred to a polar unsaturated species under mild conditions. The first such catalyst was Shvo s diruthenium hydride complex reported in the mid 1980s [41 14], Noyori and Ikatiya developed chiral ruthenium catalysts showing excellent enantioselectivity in the hydrogenation of ketones [45,46]. 

Sulfuric acid, H2 SO4, is both a strong acid and a weak acid. The compound has two acidic hydrogen atoms, hi aqueous solution, one hydrogen atom undergoes quantitative proton transfer to water ... 

An aqueous solution of potassium hydroxide contains K , OH, and H2 O. Phosphoric acid is a weak acid, so most of its molecules remain as H3 PO4 in aqueous solution. The species present at the beginning of the reaction are K and OH ions and molecules of H3 PO4 and H2 O. The hydroxide ion is a powerful base that removes all of the acidic hydrogen atoms from both strong and weak acids. 

A titration requires a solution whose concentration is known. In Example the NaOH solution used as the titrant was known to be 0.1250 M. A titrant of known concentration is known as a standard solution, and the concentration of such a solution is determined by a standardization titration. In a standardization titration, the solution being titrated contains a known amount of acid or base. An excellent acid for standardization is potassium hydrogen phthalate, KHCg H4 O4. This substance, a carboxylic acid that contains one weakly acidic hydrogen atom per molecule, is easily obtained as a highly pure solid. A known number of moles can be weighed on an... 

Hydrogen phthalate has one acidic hydrogen atom. As the net reaction shows, there is a 1 1 molar ratio between hydrogen phthalate and hydroxide HCg Hq O4 + OH Cg Hq O4 + H2 O The number of moles of hydrogen phthalate anions is the same as the number of moles of KHCg Hq Oq, which is calculated from its mass and molar mass ... 

C04-0032. Carbonic acid, H2 CO3 (molecular model shown below), is a weak oxoacid that forms when carbon dioxide dissolves in water. Carbonic acid contains two acidic hydrogen atoms. Write the net ionic reaction that occurs when carbonic acid reacts with an excess of hydroxide ions. Draw a molecular picture of the process. 

H3 PO4, for example, there are three O—H bonds but no H—P bond. This exception to Guideline d arises because of the convention of listing acidic hydrogen atoms first in the formula of an oxoacid, as we describe in Chapter 17. 

An acid that contains more than one acidic hydrogen atom is caiied a polyprotic acid. Figure 17-8 shows the structures of two poiyprotic oxoacids, suifuric acid (H2 SO4, diprotic) and phosphoric acid (H3 PO4, triprotic). Many carboxyiic acids are poiyprotic, too. Tabie 17-2 lists several examples of polyprotic acids. 

The Lewis structures of these molecules confirm the assignments. The acidic hydrogen atoms are highlighted. 

This proton transfer reaction involves the second acidic hydrogen atom of carbonic acid, so the appropriate equilibrium constant is. a 2 > whose p is found in Appendix E p. a 2 — 10.33. Because this is a buffer solution, we apply the buffer equation ... 

In the titration of a polyprotic acid, the added base reacts first with the more acidic hydrogen atoms of the neutral acid. For example, the titration of maleic acid takes place in two steps. For removal of one acidic hydrogen atom of maleic acid, p = 1.82 (. al = 1-5 X 10 ) ... 

The second acidic hydrogen atom, with a p jQ of 6.59 2 = 2.6 X 10 ), is dramatically less acidic than... 

Sulfurous acid, H2 SO3, has two acidic hydrogen atoms, with p values of 1.85... 

Methylsull anyT I //-tetrazole was found to crystallize in a monoclinic form, and could be sublimed into an orthorhombic form, with the structures differing in the relative polarity of the molecular layers in the two forms [47]. /) -1 o d o a ce t o p h e n one was found to crystallize in two polymorphs that both contained C—H-re points of contact, but where the contacts were shorter in one form than in the other [48]. A second monoclinic modification of the mixed salt benzimidazolium 3-carboxyphe-noxyacetate 3-carboxyphenoxyacetic acid was reported, where the acid hydrogen atom and the two monoanions comprised a carboxylate monoanion/neutral molecule in which the acid proton was disordered between the two anionic units [49]. 

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