Carbonyl functional groups hemiacetals

Although carbohydrates exist almost entirely as cyclic hemiacetals m aqueous solution they are m rapid equilibrium with their open chain forms and most of the reagents that react with simple aldehydes and ketones react m an analogous way with the carbonyl functional groups of carbohydrates... 

In accordance with the alternating selectivity as a function of the distance of carbonyl groups, the 1,4-diketones give meso products when reduced with lithium borohydride in the absence of titanium tetrachloride - similar to the 1,2-diketones (Scheme 3.22). Selectivities are very high, which is astonishing because of the distance of the two carbonyl functional groups. The reaction is much slower than in the 1,2- and 1,3-diketones furthermore, hemiacetals can easily be formed. The addition proved to be somewhat problematic, as tetrahydrofurans can be formed. No reversal of diastereoselectivity was observed with titanium tetrachloride. 

Compounds that contain both carbonyl and alcohol functional groups are often more stable as cyclic hemiacetals or cyclic acetals than as open chain compounds Examples of several of these are shown Deduce the structure of the open chain form of each... 

Aldoses incorporate two functional groups C=0 and OH which are capable of react mg with each other We saw m Section 17 8 that nucleophilic addition of an alcohol function to a carbonyl group gives a hemiacetal When the hydroxyl and carbonyl groups are part of the same molecule a cyclic hemiacetal results as illustrated m Figure 25 3 Cyclic hemiacetal formation is most common when the ring that results is five or SIX membered Five membered cyclic hemiacetals of carbohydrates are called furanose forms SIX membered ones are called pyranose forms The nng carbon that is derived... 

Although the structures of many carbohydrates appear to be quite complex, the chemistry of these substances usually involves only two functional groups —ketone or aldehyde carbonyls and alcohol hydroxyl groups. The carbonyl groups normally do not occur as such, but are combined with hydroxyl groups to form hemiacetal or acetal linkages of the kind discussed in Section 15-4E. 

Now, if the hydroxyl group on carbon 5 attacks the carbonyl carbon, we get a six-membered ring, as shown in Figure 12.13. The hemiacetal functional group is shown in blue. 

In general, carbonyl compounds do not polymerize by themselves. It is only the exceptional reactivity of formaldehyde as an electrophile that allows repeated nucleophilic addition of hemiacetal intermediates. A more common way to polymerize carbonyl compounds is to use two different functional groups that react together by carbonyl substitution to form a stable functional group such as an amide or an ester. Nylon is just such a polymer. 

Since water adds to (at least some) carbonyl compounds, it should come as no surprise that alcohols do too. The product of the reaction is known as a hemiacetal, because it is halfway to an acetal, a functional group, which you met in Chapter 2 (p. 35) and which will be discussed in detail in Chapter 14. The mechanism follows in the footsteps of hydrate formation just use ROH instead of HOH. 

Sugars, especially those with five or six carbon atoms, normally exist as cyclic molecules rather than as the open-chain forms we have shown so far. The cycUzation takes place as a result of interaction between the functional groups on distant carbons, such as G-1 and G-5, to form a cyclic hemiacetal (in aldohexoses). Another possibility (Figure 16.5) is interaction between G-2 and G-5 to form a cyclic hemiketal (in ketohexoses). In either case, the carbonyl carbon becomes a new chiral center called the anomeric carbon. The cyclic sugar can take either of two different forms, designated a and P, and are called anomers of each other. 

Similarly, carbohydrates often exist in an equilibrium between a cyclic h niacetal form and a carbonyl form with the latter in an extremely low quantity (14). Therefore, for alkylation of carbohydrate derivatives in organic solvent, extensive functional group transformation and derivatization is required to generate the desired carbonyl. However, in the aqueous reaction, the hemiacetal-form and the carbonyl form is in a constant equilibrium. And the carbonyl form will react, which drives the equilibrium until all the starting material has reacted (eq. 7). 

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