Hydrates gem -diols

Monoketones are poor ligands. Di-2-pyridylketone ((py)2C O) accommodates this deficiency in an unusual way when it reacts with Co(OAc)2 to form clusters, including [Co4(HQ)4(0 Ac)4] II20, where the monodeprotonated hydrated gem-diol form of the ketone (HQ-) (Equation (5)) binds as an ionic ligand.423 This cluster adopts a tetranuclear cubane shape with four deprotonated O atoms of the diol occupying the alternating vertices to the Co ions. 

Pyridinecarboxaldehyde, 3. Possible hydration of the aldehyde group makes the aqueous solution chemistry of 3 potentially more complex and interesting than the other compounds. Hydration is less extensive with 3 than 4-pyridinecarboxaldehyde but upon protonation, about 80% will exist as the hydrate (gem-diol). The calculated distribution of species as a function of pH is given in Figure 4 based on the equilibrium constants determined by Laviron (9). 

Problem 15.37 In acid, most aldehydes form nonisolable hydrates (gem-diols). Two exceptions are the stable chloral hydrate, Cl3CCH(OH)2, and ninhydrin. 

Aldehydes can become hydrated in water [i.e., establish an equilibrium between the hydrated (gem-diol form) and the unhydrated form]. Therefore, hydration should be considered in evaluating AWPCs. Betterton and Hoffmann [14] have investigated the correlation of AWPCs with Taft s parameter for substituted aldehydes. 

NADP+ containing an acyclic 2, 3 - dialdehyde nicotinamide sugar moiety (3) can be synthesized hy periodate oxidation (43) and exists primarily as the hydrated gem-diol. This analog has been used as an affinity label [it inactivates glucose-6-phosphate dehydrogenase (44)], and is active with some dehydrogenases (45). 

The oxidation of aldehydes to carboxylic acids has been most extensively investigated with horse liver alcohol dehydrogenases (65-67, 69, 73). There are two distinct reactions the direct oxidation of aldehydes as their hydrated gem-diol form [reaction (9)] and the oxidation of hemiacetals to esters [oxidative esterification, reaction (10)]. 

Aldehydes and ketones react with water to yield 1,1-diols, or geminal (gem) diols. The hydration reaction is reversible, and a gem diol can eliminate water to regenerate an aldehyde or ketone. 

The acid-catalvzed hydration reaction begins with protonation of the carbonyl oxygen atom, which places a positive charge on oxygen and makes the carbonyl group more electrophilic. Subsequent nucleophilic addition of water to the protonated aldehyde ot ketone then yields a protonated gem diol, which loses H+ to give the neutral product (Figure 19.5). 

The monoketone bis(2,2, /V,/V -bipyridyl)ketone forms a [CoinL2]+ complex on reaction with [Co(NH3)4(C03)]+ in water.981 As reported for a quite different Co11 complex, the ketone is hydrated to form the gem diol which binds as a monodeprotonated O-donor along with the two pyridine groups in a tridentate chelate, with very little distortion from octahedral observed in the complex. This appears to represent a facile route for this type of inherently poor donor to achieve coordination. Chelated /3-diketonate anions are long-studied examples of O-donor chelates, and continue to be examined. A simple example is the m-[Co(acac)2(NH 3)2]1 (acac = 2,4-pentane-dionate), structurally characterized and utilized to produce molecular mechanics force field parameters for /3-diketones bound to Co111.982... 

Chemical/Physical. Oxidation in air yields acetic acid (Windholz et ah, 1983). In the presence of sulfuric, hydrochloric, or phosphoric acids, polymerizes explosively forming trimeric paraldehyde (Huntress and Mulliken, 1941 Patnaik, 1992). In an aqueous solution at 25 °C, acetaldehyde is partially hydrated, i.e., 0.60 expressed as a mole fraction, forming a gem-diol (Bell and McDougall, 1960). Acetaldehyde decomposes at temperatures greater than 400 °C, forming carbon monoxide and methane (Patnaik, 1992). 

Formaldehyde reacted with hydrogen chloride in moist air to form 5ym-dichloromethyl ether. This compound may also form from an acidic solution containing chloride ions and formaldehyde (Frankel et al, 1974 Travenius, 1982). In an aqueous solution at 25 °C, nearly all the formaldehyde added is hydrated forming a gem-diol (Bell and McDougall, 1960). May polymerize in an aqueous solution to trio methylene (Hartley and Kidd, 1987). 

These observations emphasize the fact that gem-diols are usually unstable and decompose to carbonyl compounds. However, it can be demonstrated that hydrate formation does occur by exchange labelling of simple aldehyde or ketone substrates with 0-labelled water. Thus, after equilibrating acetone with labelled water, isotopic oxygen can be detected in the ketone s carbonyl group. 

Buschmann, H.-J., H.-H. Fuldner, and W. Knoche, The Reversible Hydration of Carbonyl Compounds in Aqueous Solution. Part I. The Keto/Gem-Diol Equilibrium, Ber. Bunsenges. Phys. Chem., 85, 41- 44 (1980). 

Aldehyde and ketone reacts with water in the presence of aqueous acid or base to form hydrate. A hydrate is a molecule with two hydroxyl groups on the same carbon. It is also called gem-diol. Hydration proceeds through the two classic nucleophilic addition mechanisms with water in acid... 

In aqueous solution the (acyclic) aldehydo form is partially hydrated, to give a gem-diol, often referred to as an aldehydrol. The extent of hydration is sensitive to the inductive effect of the substituent on C-2. 

In 0.1 M methanolic sodium hydroxide, the 6-chloropurine ketonu-cleoside 41b was cleaved after 30 min, whereas the theophylline derivative 41a was unaffected.26,27 The stability of the theophylline ketonu-cleoside 41a maybe explained by the establishment of a hydrogen bond between the 4 -gem-diol system (hydrate form of 41a) and the 2-keto group of theophylline. The assigned 4Ct (l) conformation would facilitate this bonding, whereas it could not occur with the 6-chloropurine derivative regardless of its conformation.27... 

When there is a branch at C-3 in an aldose, the aldehydo form is hydrated to a much lesser extent than in an unbranched sugar the branching causes a 1,3-parallel interaction with one of the hydroxyl groups of the gem-diol. Whereas the ratio of aldehydrol to aldehyde is 10 1 for threose and 5 1 for erythrose, it is only 1.7 1 and 1.5 1, respectively, for their 3-C-methyl derivatives, and 0.4 1 for 3,3-dimethyl-DL-g/ym-o-tetrose.13... 

Pyridyl aldehydes are reduced by a mechanism involving a preequilibrium in aqueous electrolytes.62,63 The rate-determining step for reduction was found to be the ionization of the gem-diol, or hydrated aldehyde. As a consequence, buffers were found to have an effect on the reduction of the 2-aldehyde (41 ).64... 

Hydration of formaldehyde by H2170 produces a gem-diol in which the labeled and unlabeled hydroxyl groups are equivalent. When this gem-diol reverts to formaldehyde, loss of either of the hydroxyl groups is equally likely and leads to eventual replacement of the mass-16 isotope of oxygen by 170. 

Ketones and aldehydes have electrophilic carbonyls that significantly contribute to the chemistry of these functional groups. In general, aldehydes tend to be more electrophilic and will often exist in aqueous solutions in hydrated form as a gem-diol (Fig. 20). This is an important consideration when attempting to characterize by NMR an unknown that may contain an... 

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