Hemiacetals and hemiketals

FIGURE 7.16 Acetals and ketals can be formed from hemiacetals and hemiketals, respectively. 

Figure 4.18 The reaction of alcohols with aldehydes (a) and ketones (b) to form hemiacetals and hemiketals. (c) The reaction between the alcohol on carbon 5 and the aldehyde of glucose forms two hemiacetals, ot-D-glucopyranose and P-D-glucopyranose (pyranose by comparison with pyran the simplest compound containing this six-membered ring).

The addition of 1 mol of an alcohol to an aldehyde gives a bemiacetal, and to a ketone a hemiketal. However, most chemists do not now differentiate between hemiacetals and hemiketals these are both termed hemiacetals. This reaction is usually catalysed... 

The position of equilibrium, i.e. whether the carbonyl compound or the addition product is favoured, depends on the nature of the reagents. The equilibrium constant is often less than 1, so that the product is not favoured, and many simple hemiacetals and hemiketals are not sufficiently stable to be isolated. However, stable cyclic hemiacetals and hemiketals... 

The cyclic hemiacetal and hemiketal forms of monosaccharides are capable of reacting with an alcohol to form acetals and ketals (see Section 7.2). The acetal or ketal product is termed a glycoside, and the non-carbohydrate portion is referred to as an aglycone. In the nomenclature of glycosides we replace the suffix -ose in the sugar with -oside. Simple glycosides may be synthesized by treating an alcoholic solution of the monosaccharide with an acidic catalyst, but the reaction mixture usually then contains a mixture of products. This is an accepted problem with many carbohydrate reactions it is often difficult to carry out selective transformations because of their multifunctional nature. 

Hemiacetals and hemiketals have the important structural feature of -OH and -OR attached to the same carbon as shown above. Through cyclization, sugars attain a pyranose and/or a fumnose form. 

Hemiacetal Acetal FIGURE 7-5 Formation of hemiacetals and hemiketals. An aldehyde or ketone can react with an alcohol in a 1 1 ratio to yield a hemiacetal or hemiketal, respectively,... 

Although the crystalline forms of a- and /3-D-glucose are quite stable, in solution each form slowly changes into an equilibrium mixture of both. The process can be observed as a decrease in the optical rotation of the a anomer (+112°) or an increase for the /3 anomer (+18.7°) to the equilibrium value of 52.5°. The phenomenon is known as mutarotation and commonly is observed for reducing sugars. Both acids and bases catalyze mutarotation the mechanism, Equation 20-1, is virtually the same as described for acid- and base-catalyzed hemiacetal and hemiketal equilibria of aldehydes and ketones (see Section 15-4E) ... 

B is correct. Alcohols add to aldehydes and ketones to form hemiacetals and hemiketals, respectively. You should be able to recognize this reaction for the MCAT. 

H20 or alcohols as nucleophiles give low molecular weight compounds when they add to the C=0 double bond of carbonyl compounds. These addition products are called aldehyde or ketone hydrates (Section 9.1.1) and hemiacetals or hemiketals (Section 9.1.2), respectively, depending on whether they result from the addition to an aldehyde or a ketone. Today, one no longer distinguishes systematically between hemiacetals and hemiketals, but the expression hemiacetal is frequently used to cover both. 

There is yet another level of isomerism of sugars, because aldehydes (and ketones) form freely reversible adducts with alcohols (hemiacetals and hemiketals) in aqueous solutions. In the presence of... 

Five- and six-membered ring hemiacetals and hemiketals are common configurations of many sugars. Ribose, which forms a part of the structure of RNA, closes to form a five-membered ring preferentially. 

Monosaccharides, polyhydroxy aldehydes or ketones, are either aldoses or ketoses. Sugars that contain four or more carbons primarily have cyclic forms. Cyclic aldoses or ketoses are hemiacetals and hemiketals, respectively. 

Hemiacetals and hemiketals react with alcohols to form acetals and ketals, respectively. When the cyclic hemiacetal or hemiketal form of a monosaccharide reacts with an alcohol, the new linkage is called a glycosidic linkage, and the compound is called a glycoside. 

Hemiacetals and hemiketals are readily formed in carbohydrates. Monosaccharides contain several hydroxyl groups and one carbonyl group. The linear form of a monosaccharide quickly undergoes an intramolecular reaction in solution to give a cyclic hemiacetal or hemiketal. 

Alcohols add to the carbonyl groups of aldehydes and ketones to yield hemiacetals and hemiketals, respectively. The reaction requires an acid catalyst in the laboratory or an enzyme in biological systems. 

Some texts do not use the terms hemiketal and ketal. Those texts use hemiacetal for both hemiacetals and hemiketals, and acetal for both acetals and ketals. 

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