Acetal/hemiketal

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. 

Carbohydrates are either polyhydroxy aldehydes and ketones or substances that form these compounds after hydrolysis. The general formula is [C (H20) ]. Normally carbohydrates occur as hemiacetals or acetals (hemiketals or ketals). 

The reduction occurs by interception of the open chain form present in the aldehyde/ketone-hemi-acetal/hemiketal equilibrium. Although only a small amount of the open chain form is present at any given time, that small amount is reduced. Then more is produced by opening of the pyranose form, and that additional amount is reduced, and so on until the entire sample has undergone reaction. 

For our present purposes, we are interested in the ways in which hemi-acetals, acetals, hemiketals, and ketals are formed. Hemiacetals and hemi-ketals can be regarded as products of the addition of alcohols to the carbonyl... 

Note that some textbooks use the term acetal to describe only the product that forms when an alcohol and an acid catalyst are mixed with an aldehyde, and the term ketal to describe the product that forms when an alcohol and an acid catalyst are mixed with a ketone. Based on this terminology, draw an example of each of the following hemiacetal, acetal, hemiketal, ketal. 

Aldehydes and ketones undergo reversible addition reactions with alcohols. The product of addition of one mole of alcohol to an aldehyde or ketone is referred to as a hemiacetal or hemiketal, respectively. Dehydration followed by addition of a second molecule of alcohol gives an acetal or ketal. This second phase of the process can be catalyzed only by acids, since a necessary step is elimination of hydroxide (as water) from the tetrahedral intermediate. There is no low-energy mechanism for base assistance of this... 

The equilibrium constants for addition of alcohols to carbonyl compounds to give hemiacetals or hemiketals show the same response to structural features as the hydration reaction. Equilibrium constants for addition of metiianoHb acetaldehyde in both water and chloroform solution are near 0.8 A/ . The comparable value for addition of water is about 0.02 The overall equilibrium constant for formation of the dimethyl acetal of... 

The second step in acetal and ketal hydrolysis is conversion of the hemiacetal or hemiketal to the carbonyl compound. The mechanism of this step is similar to that of the first step. Usually, the second step is faster than the initial one. Hammett a p plots and solvent isotope effects both indicate that the transition state has less cationic character than... 

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

Glycosides are mixed acetals formally arising by elimination of water between the hemiacetal or hemiketal hydroxy group of a sugar and a hydroxy group of a second compound. The bond between the two components is called a glycosidic bond. 

Reduction of Acetals, Ketals, Hemiacetals, Hemiketals, and Orthoesters... 

Table 20. organosilane reduction of acetals, ketals, and hemiketals... 

TABLE 20. ORGANOSILANE REDUCTION OF ACETALS, KETALS, AND HEMIKETALS (Continued)... 

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