Carbonic acids, hydrogen bonding

In liquid nitric acid, hydrogen bonding gives a loose structure similar to that of hydrogencarbonate ions. However, although pure nitric acid does not attack metals readily and does not evolve carbon dioxide from a carbonate, it is a conducting liquid, and undergoes auto-ionisation thus ... 

Carbon dioxide is a symmetrical, linear triatomic molecule (0 = C=0) with a zero dipole moment. The carbon-to-hydrogen bond distances are about 1.16A, which is about 0.06A shorter than typical carbonyl double bonds. This shorter bond length was interpreted by Pauling to indicate that greater resonance stabilization occurs with CO2 than with aldehydes, ketones, or amides. When combined with water, carbonic acid (H2CO3) forms, and depending on the pH of the solution, carbonic acid loses one or two protons to form bicarbonate and carbonate, respectively. The various thermodynamic parameters of these reactions are shown in Table I. 

A quantitative understanding of how enzymes catalyze rapid proton abstraction from weakly acidic carbon acids is necessarily achieved by dissecting the effect of active site structure on the values of AG°, the thermodynamic barrier, and AG int, the intrinsic kinetic barrier for formation of the enolate anion intermediate. The structural strategies by which AG° for formation of the enolate anion is reduced sufficiently such that these can be kinetically competent are now understood. In divalent metal ion-independent reactions, e.g., TIM, KSI, and ECH, the intermediate is stabilized by enhanced hydrogen bonding interactions with weakly acidic hydrogen bond donors in divalent metal-dependent reactions, e.g., MR and enolase, the intermediate is stabilized primarily by enhanced electrostatic interactions with... 

Aldehydes, ketones, esters, and Ai,Ai-disubstituted amides have a second site of reactivity. A hydrogen bonded to a carbon adjacent to a carbonyl carbon is sufficiently acidic to be removed by a strong base. The carbon adjacent to a carbonyl carbon is called an a-carbon. A hydrogen bonded to an a-carbon is called an a-hydrogen. 

A hydrogen bonded to an a-carbon of an aldehyde, ketone, ester, or A A -disubshtuted amide is sufficiently acidic to be removed by a strong base because the base that is formed when the proton is removed is stabilized by delocalization of its neg-ahve change onto an oxygen. A carbon acid is a compound with a relatively acidic hydrogen bonded to an hybridized carbon. Aldehydes and ketones (pi a 16-20) are more acidic than esters 25). j8-Diketones 9) and j8-keto esters (p a 11) are even more acidic. 

Metalation. The exchange occurring between R—M and organic compounds containing an acidic hydrogen bonded to carbon is termed metalation. 

Identify all carbon atoms bonded to a carbonyl group.These are the a-carbons. Any hydrogens bonded to these a-carbon atoms are a-hydrogens and will be more acidic than typical alkane or alkene hydrogens. 

A hydrogen bonded to an sp carbon that is adjacent to a carbonyl carbon, however, is much more acidic than hydrogens bonded to other sp carbons. For example, the pK value for dissociation of a proton from the a-carbon of an aldehyde or a ketone ranges from 16 to 20, and the pK value for dissociation of a proton attached to the a-carbon of an ester is about 25 (Table 18.1). A compound that contains a relatively acidic hydrogen bonded to an sp carbon is called a carbon acid. 

A carbon acid is a compound with a relatively acidic hydrogen bonded to an sp carbon. 

Thiazole disulfides are reported to yield quantitatively A-4-thiazoline-2-thiones under treatment with zinc powder in acetic acid (326). The disulfide bond can be broken on heating at 100 to 260°C and (or) by alkali. This property has been used for photographic emulsions (327). The disulfide (136) (R = 4-(D-arabmo-tetrahydroxybutyD can be cleaved readily by aqueous sodium hydroxyde. carbonate, or hydrogen carbonate (149) to give 135 a by-product, 4-(D-arabino-ietrahydroxybutyl) thiazole... 

Examine the electro r static potential map of butanoic acid on t Learning By Modeling and notice how much more in tense the blue color (positive charge) is on the OH hydro gen than on the hydrogens bonded to carbon... 

In aqueous solution intermolecular association between carboxylic acid molecules IS replaced by hydrogen bonding to water The solubility properties of carboxylic acids are similar to those of alcohols Carboxylic acids of four carbon atoms or fewer are mis cible with water m all proportions... 

Esters can participate m hydrogen bonds with substances that contain hydroxyl groups (water alcohols carboxylic acids) This confers some measure of water solubil ity on low molecular weight esters methyl acetate for example dissolves m water to the extent of 33 g/100 mL Water solubility decreases as the carbon content of the ester increases Fats and oils the glycerol esters of long chain carboxylic acids are practically insoluble m water... 

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