Aldehydes can react with alcohols to form hemiacetals

You have, in fact, already met some less important reactions in which the carbonyl oxygen atom can be lost, but you probably didn t notice at the time. The equilibrium between an aldehyde or ketone and its hydrate (p. 134) is one such reaction. 

By stirring the normal compound with a large excess of isotopically labelled water for a few hours in the presence of a drop of acid they were able to make the required labelled compound. Without the acid catalyst, the exchange is very slow. Acid catalysis speeds the reaction up by making the carbonyl group more electrophilic so that equilibrium is reached more quickly. 

Aldehydes can react with alcohols to form hemiacetals 

When acetaldehyde is dissolved in methanol, a reaction takes place we know this because the IR spectrum of the mixture shows that a new compound has been formed. Most dramatically, the carbonyl frequency is no longer there. However, isolating the product is impossible it decomposes back to acetaldehyde and methanol. 

The product is in fact a hemiacetal. Like hydrates, most hemiacetals are unstable with respect to their parent aldehydes and alcohols, for example the equilibrium constant for reaction of acetaldehyde with simple alcohols is about 0.5. 

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ALDEHYDES CAN REACT WITH ALCOHOLS TO FORM HEMIACETALS... 

In Section 12.6, we saw that aldehydes and ketones react with alcohols to form hemiacetals. We also saw that cyclic hemiacetals form very readily when hydroxyl and carbonyl groups are parts of the same molecule and their interaction can form a five- or six-membered ring. For example, 4-hydroxypentanal forms a five-membered cyclic hemiacetal. Note that 4-hydroxypentanal contains one stereocenter and that a second stereocenter is generated at carbon 1 as a result of hemiacetal formation ... 

Because they contain alcohol functional groups and aldehyde or ketone functional groups, the reactions of monosaccharides are an extension of what you have already learned about the reactions of alcohols, aldehydes, and ketones. For example, an aldehyde group in a monosaccharide can be oxidized or reduced and can react with nucleophiles to form imines, hemiacetals, and acetals. As you read this section and those that follow, you will find cross-references to earlier discussions of simpler organic compounds that undergo the same reactions. Go back and look at these earlier discussions when you see a cross-reference it will make learning about carbohydrates a lot easier. 

The predominant forms of ribose, glucose, fructose, and many other sugars in solution are not open chains. Rather, the open-chain forms of these sugars cyclize into rings because the ring forms are energetically more stable. The basis for ring formation is the fact that an aldehyde can react with an alcohol to form a hemiacetal. 

Aldehydes can react with one molecule of an alcohol to form a hemiacetal (see Chapter 11). Because the catalytic site of elastase contains an active serine hydroxyl group, it is reasonable that an aldehyde derivative of a peptide substrate of elastase would react with the serine -OH group to form a hemiacetal, which is a tetrahedral analogue of the transition state of the peptide hydrolysis reaction. (See also Robert C. Thompson and Carl A. Bauer. [1979]. Biochemistry 18, 1552-1558.)... 

The hydroxyl groups of monosaccharides can react with its own aldehyde of ketone functional group to form Hemiacetals or Hemiketals respectively. Therefore, when the aldehyde group of glucose at Ci reacts with the alcohol group at C5,... 

Ring structures The aldehyde or ketone group can react with a hydroxyl group to form a covalent bond. Formally, the reaction between an aldehyde and the hydroxyl group of a sugar (an alcohol) creates a hemiacetal (Equation 1) whereas a ketone reacts... 

Just as protonated carbonyl groups are much more electrophilic than unprotonated ones, these oxonium ions are powerful electrophiles. They can react rapidly with a second molecule of alcohol to form new, stable compounds known as acetals. An oxonium ion was also an intermediate in the formation of hemiacetals in acid solution. Before reading any further, it would be worthwhile to write out the whole mechanism of acetal formation from aldehyde or ketone plus alcohol through the hemiacetal to the acetal, preferably without looking at the fragments of mechanism above, or the answer below. 

For an aldohexose such as glucose, the C-1 aldehyde in the open-chain form of glucose reacts with the C-5 hydroxyl group to form an intramolecular hemiacetal. The resulting cyclic hemiacetal, a six-membered ring, is called pyranose because of its similarity to pyran (Figure 11.4). Similarly, a ketone can react with an alcohol to form a hemiketal. 

The cyclic hemiacetal (or hemiketal) can react with an alcohol to form an acetal (or ketal), called a glycoside. If the name pyranose or furanose is used, the acetal is called a pyranoside or a furanoside. The bond between the anomeric carbon and the alkoxy oxygen is called a glycosidic bond. The preference for the axial position by certain substituents bonded to the anomeric carbon is called the anomeric effect. If a sugar has an aldehyde, ketone, hemiacetal, or hemiketal group, it is a reducing sugar. 

Aldehydes can undergo reactions with alcohols under acidic conditions. The product of this nucleophilic addition reaction is called hemiacetal. Hemiacetal is formed by the nucleophilic addition of alcohol to the carbonyl carbon. Hemiacetal can react with one more mole of alcohol to form an acetal. 

How can o-glucose exist in a cyclic form In Section 17.12, we saw that an aldehyde reacts with an alcohol to form a hemiacetal. The reaction of the alcohol group bonded to C-5 of D-glucose with the aldehyde group forms two cyclic (six-membered ring) hemiacetals. 

When allenyl aldehydes are allowed to react with DMDO, the aldehyde moiety is not oxidized to the acid except for monosubstituted allenes [21]. In all other cases, the carbonyl oxygen participates as a nucleophile in the opening of the intermediate epoxide. From 2,2,5-trimethy]-3,4-hexadienal 67, for example, five different products can be synthesized selectively under different reaction conditions (Scheme 17.22). When p-toluenesulfonic acid (TsOH) is present or DMDO is formed in situ, then the initially formed allene (mono)oxide reacts with the aldehyde moiety to give 68 or 69. In the presence of excess DMDO and the absence of acid, three other products (70-72) can be formed via the spirodioxide intermediate. These reactions, however, seem to be less general compared with similar reactions of allenyl acids and allenyl alcohols. y-Allenylaldehydes 73 can be cyclized to five-membered hemiacetals 74 via the spirodioxide intermediate. 

An explanation for this behaviour can be deduced fi om Scheme 1. Ethanol decomposes at the surface of the catalyst, forming ethoxy then acyl species. This reaction is an equilibrium, the concentration of ethoxy and acyl species dependent on temperature and operating pressure. For ethyl ethanoate formation, an adsorbed acyl species reacts with an adsorbed ethoxy species to form an intermediate hemiacetal subsequent dehydrogenation forms ethyl ethanoate. By-products, such as ketones, aldehydes and alcohols are formed by... 

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