Acetals and hemiacetals

The c.d. spectra of these 21 monosaccharides that were studied contain a wealth of information, although proper analysis of the data is not always obvious. However, c.d.-difference spectra between pairs of sugars that differ at only one carbon atom can be used to simplify the analysis. Each of the chromophores in a monosaccharide (hydroxyl, methoxyl, hydroxymethyl, hemiacetal, and acetal) are symmetric and obtain their c.d. by interaction... 

Butyl alcohol is not the principal use of butanal obtained by propene hydro formylation. Rather its major market is 2-ethylhexanol that is prepared via aldol condensation followed by hydrogenation. [4] Thus formation of alcohols when aldehydes are desired is not only a direct efficiency loss, but also the alcohol impurity will form hemiacetals and acetals that complicate refining and lead to increased operating costs. 

Figure 10-19 shows the general structures of hemiacetals and acetals. 

If you look in older texts, you may still see the terms hemiketal and ketal. At one time, four terms were used for the products of alcohols with carbonyl groups hemiacetal, acetal, hemiketal, and ketal. A hemiketal is now a type of hemiacetal and a ketal is now a type of acetal. Originally, acetals and hemiacetals came from aldehydes and ketals and hemiketals came from ketones. The structures of hemiacetals and acetals contained a C-H bond, but ketals and hemiketals did not. 

Formation of hemiacetals and acetals, as well as of hemiketals and ketals, is reversible under acidic conditions, as we already have noted for acid-catalyzed esterification. The reverse reaction is hydrolysis and the equilibrium for this reaction can be made favorable by having an excess of water present ... 

Aldehydes can add H20 to form hydrates or can add alcohols to form hemiacetals and acetals. 

Hemiacetals and acetals play an important role in the chemistry of carbohydrates. Consider what would happen, for example, if the —OH group on the fifth carbon atom in a glucose molecule attacked the aldehyde at other end of this molecule. 

Know the meaning of nucleophilic addition, hemiacetal and acetal, aldehyde hydrate, cyanohydrin. 

Ketones react with alcohols to form hemiketals and then ketals. This pair of reactions is exactly analogous to the formation of hemiacetals and acetals. In the... 

Aldehyde starches are prepared by treatment with periodic acid/periodate ions, which selectively oxidize the adjacent hydroxyl groups on carbon atoms 2 and 3 to aldehyde groups. Dialdehyde starch can react with cellulose by forming covalent hemiacetal and acetal bonds.40 It is primarily used as a wet strength agent in the production of tissue and other sanitary grades. 

We shall discuss the reasons why Intramolecular reactions are more favourable, and why cyclic hemiacetals and acetals are more stable, In Chapter 14,... 

Figure 111-4 (a) Hemiacetal and acetal formation and acetyl hydrolysis. (b) Internal hemiacetal formation with the OH on carbon 5 of hexoses yields two six-membered rings. 

Carbonyl addition reactions include hydration, reduction and oxidation, the al-dol reaction, formation of hemiacetals and acetals (ketals), cyanohydrins, imines (Schiff bases), and enamines [54]. In all these reactions, some activation of the carbonyl bond is required, despite the polar nature of the C=0 bond. A general feature in hydration and acetal formation in solution is that the reactions have a minimum rate for intermediate values of the pH, and that they are subject to general acid and general base catalysis [121-123]. There has been some discussion on how this should be interpreted mechanistically, but quantum chemical calculations have demonstrated the bifunctional catalytic activity of a chain of water molecules (also including other molecules) in formaldehyde hydration [124-128]. In this picture the idealised situation of the gas phase addition of a single water molecule to protonated formaldehyde (first step of Fig. 5) represents the extreme low pH behaviour. 

Cyclic hemiacetals and acetals are readily formed from y-hydroxyaldchydes," ketones, or acetals.Either the carbonyl compound or the alcohol is deblocked before spontaneous cyclization occurs. Aldehydes are regularly synthesized from alkeiies via ozonolysis or periodate cleavage. Acetal 7. a starting material for nucleoside analogs, is prepared directly by transacctalization of alcohol 6 with 1 % ethanol/hydrogen chloride. 

Figure 4 shows the results for sugars and sugar derivatives. The sorbitol, which lacks the hemiacetal and acetal groups of the sugars, shows weak chlorine release only during... 

We have not previously considered stereochemistry. Hemiacetal (and acetal) formation is thermodynamic control as aU the reactions are reversible. The dimer and trimer crystallize frc-liquid so the stereochemistry may be governed by the formation of the most stable p . compound or by the fact that the crystallization of the least soluble diastereoisomer removes the equilibrium and so more is formed. We can see some reasons why the diastereoisomer i might be the most stable. The as ring junction between the two five-membered rings is much m stable than the trans, the two acetyl groups may prefer to be trans to each other, and there ma. > H bond in the crystal. We cannot be sure of these reasons but they are explored more in Chap . ... 

We shall find the chemistry of hemiacetals and acetals to be fundamental to the study of carbohydrates (Chaps. 34 and 35). 

When this reaction is conducted on ketones, instead of aldehydes, the equivalent species to the hemiacetal and acetal are now called hemiketal and ketal respectively. However, when simple ketones are reacted with normal monofunctional alcohols, the reaction rarely goes to completion, unlike the situation with aldehydes where the reaction proceeds smoothly. Yet, when a 1,2-diol is reacted with a ketone, then the reaction goes to completion much more readily. Write the complete reaction sequence for the formation of the cyclic ketal starting from propanone and 1,2-ethandiol, under acidic conditions. Also, suggest why the product between one molecule of each reagent is favoured over the alternative product that would result from the reaction between two molecules of the diol and one of the ketone. 

Fig. 5.24 Hemiacetal and acetal formation from ethanol with aldehydes...

MIBK in the analysis [41]. In test chamber investigations hemiacetals and acetals such as 1-hydroxy-1-ethoxycyclohexane and 1,1-diethoxycyclohexane can sometimes be demonstrated qualitatively in the chamber air. These are typical reaction products of cyclohexanone with ethanol. ... 

Mean, Hemiacetal and Acetal Geometries in a- and D-Pynmoses, Methyl a- and /l-D-Pyianosides, and Oli( osaccharides ... 

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