Gem diol

This behavior is rather typical of gem-diols (gem = geminal, that is, with both hydroxyl groups on the same carbon atom). The few gem-diols of this kind that can be isolated are those that carry strongly electron-attracting substituents such as the following ... 

Water adds to an aldehyde or a ketone to form a hydrate. A hydrate is a molecule with two OH groups on the same carbon. Hydrates are also called ge/n-diols (gem comes from geminus, Latin for twin ). Hydrates of aldehydes or ketones are generally too unstable to be isolated because the tetrahedral carbon is attached to two oxygen atoms. 

In the second step of the reaction, water acts as a nucleophile and adds to the Lewis acid (the protonated carbonyl) to generate a protonated diol (Fig. 16.25). In the final step of this acid-catalyzed reaction, the protonated diol is deprotonated by a water molecule to generate the neutral diol and regenerate the add catalyst, H30. A diol with both OH groups on the same carbon is called a em-diol (gem stands for geminal) or a hydrate. Note how closely related are the acid-catalyzed additions of water to an alkene and to a carbonyl group. Each reaction is a sequence of three steps protonation, addition of water, and deprotonation. 

In an aqueous acid solution, chromic acid converts aldehydes to carboxylic acids. Before rhi.s second occurs, the aldehyde reacts with water to give a 1,1-diol, cai[ed a gem-diol. gem-dio s result from an addition reaction to the carbonyl group, a process that we will discuss in Chapter 18. 

Step 1 The peroxy acid adds to the carbonyl group of the ketone This step is a nucleophilic addition analogous to gem diol and hemiacetal formation... 

Neopentyl glycol, or 2,2-dimethyl-1,3-propanediol [126-30-7] (1) is a white crystalline soHd at room temperature, soluble ia water, alcohols, ethers, ketones, and toluene but relatively iasoluble ia alkanes (1). Two primary hydroxyl groups are provided by the 1,3-diol stmcture, making this glycol highly reactive as a chemical intermediate. The gem-A methy configuration is responsible for the exceptional hydrolytic, thermal, and uv stabiUty of neopentyl glycol derivatives. 

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 position of the equilibrium between a gem diol and an aldehyde or ketone depends on the structure of the carbonyl compound. The equilibrium generally favors the carbonyl compound for steric reasons, but the gem diol is favored for a few simple aldehydes. For example, an aqueous solution of formaldehyde consists of 99.9% gem diol and 0.1% aldehyde, whereas an aqueous solution of acetone consists of only about 0.1% gem diol and 99.9% ketone. 

The alkoxide ion is protonated by water to give the gem diol product and regenerate the hydroxide ion catalyst. 

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). 

Addition of water to the protonated carbonyl compound gives a protonated gem diol intermediate. 

Deprotonaiion of the intermediate by reaction with water yields the neutral gem diol and regenerates the acid catalyst. 

Nucleophilic addition of an alcohol to the carbonyl group initially yields a hydroxy ether called a hemiacetal, analogous to the gem diol formed by addition of water. HcmiacetaJs are formed reversibly, with the equilibrium normally favoring the carbonyl compound. In the presence of acid, however, a further reaction occurs. Protonation of the -OH group, followed by an El-like loss of water, leads to an oxonium ion, R2C=OR+, which undergoes a second nucleophilic addition of alcohol to yield the acetal. The mechanism is shown in Figure 19.12. 

Soil die Carboxy-Gruppe mit Lithiumalanat selektiv hydriert werden, so mu8 die Keto-Gruppe geschiitzt werden. Bei ungeschiitzter Keto-Gruppe werden mit uberschiissi-gem Lithiumalanat normalerweise unter Verbrauch von vier Hydrid-Aquivalenten Diole erhalten ... 

Das gleichzeitig gebildete vie. und gem. Diamino-diol wird bei der alkalischen Aufarbeitung untcr Rclro-al-dol-Addition zum Aldehyd aufgespalten,... 

There is no way in which dehydration of alcohols can be used to prepare triple bonds gem-diols and vinylic alcohols are not normally stable compounds and vic-diols give either conjugated dienes or lose only 1 mol of water to give an aldehyde or ketone. Dienes can be prepared, however, by heating alkynyl alcohols with triphenyl phosphine. ... 

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. 

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... 

This, and the tetraketone resulting from dehydration of the gem-diol groups, both explode under a hammer blow. 

A solvent-free pinacol-pinacolone rearrangement has been reported using micro-wave irradiation. The process involves the irradiation of the gem diols with Al3+-mon-tmorillonite K 10 clay for 15 min to afford the rearrangement product in excellent yields (Scheme 6.25) [24]. The comparative studies performed by conventional heating in an oil bath showed that the reaction times are too long (15 h). 

Most gem diol compounds of carbon (containing two OH groups on the same carbon atom) are unstable. Using bond energies, show that this is expected. 

The clearest example of rapid turnover of a functional micelle is a reaction of a micellized gem-diol. Attack of the anion of the gem-diol gives an acetylated or phosphorylated intermediate which rapidly breaks down regenerating the catalytic gem-diol (Scheme 8) (Menger and Whitesell, 1985). 

Treatment of the optically active gem-borazirconocene alkanes with deuterium oxide followed by alkaline oxidation affords the corresponding optically active 1-deuterio primary alcohols. The enantiomeric excess of the resulting primary alcohols represents the diaster-eoselectivity of the asymmetric hydrozirconation (Scheme 7.13). Based on the cost and availability of optically active ligands, three types were explored monoterpenes, 1,2-diols, and 1,2-amino alcohols. Hydrozirconation of optically pure 1-alkenyl boranes 39 provided optically active 1,1-bimetallics 40. 

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). 

Diol or glycol (two -OH) gem-diol 1,1 diol vic-diol 1,2 diol... 

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