Aldehydes can react with alcohols to

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 lias been formed. 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 as we saw in Chapter 13. 

So by making [MeOH] very large (using it as the solvent, for example) we can turn most of the  

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ALDEHYDES CAN REACT WITH ALCOHOLS TO FORM HEMIACETALS... 

In a similar manner, ketones can react with alcohols to form hemiketals. The analogous intramolecular reaction of a ketose sugar such as fructose yields a cyclic hemiketal (Figure 7.6). The five-membered ring thus formed is reminiscent of furan and is referred to as a furanose. The cyclic pyranose and fura-nose forms are the preferred structures for monosaccharides in aqueous solution. At equilibrium, the linear aldehyde or ketone structure is only a minor component of the mixture (generally much less than 1%). 

Some interfering compounds react to produce water, which causes the water results to be too high. Carboxylic acids can react with alcohols to produce an ester and water. To minimize this problem, the alcohol can be eliminated in the reagent, or an alcohol that reacts at a slower rate than methanol can be used. The pH of the reagent can be increased because the formation of esters is usually acid catalyzed. Ketones and aldehydes can react with alcoholic solvents to form ketals and acetals with the production of water according to ... 

The alcohol (1) is transformed into a chromic acid ester (2), which evolves to an aldehyde or a ketone (3). When an aldehyde is generated, it can react with water to form the hydrate (4) that can evolve as in Equation below,5 resulting in the formation of an acid (5). 

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. In general, an aldehyde can react with an alcohol to form a... 

Nonchelation-controlled addition reactions can be very useful in organic synthesis. - An example drawn from carbohydrate chemistry is particularly significant in diis regari (Scheme 8)." Due to chelation, the aldehyde (89) reacts with MeMgBr to give a mixture of the diastereomeric alcohols (91) and (92) in 88 12 molar ratio, whereas, in the reaction with MeTi(OPH)3 under nonchelation control, only the Felkin-Anh product (92) is observed. 2,3-O-Isopropylidene-o-glyceraldehyde (93 equation 33) is also reported to react under noncheladon-controlled conditions with several RTi(OPr )3 reagents. Oddly, PhTi(OPr )3 favors the formation of the syn product, a unique result that is difficult to rationalize. 

Reaction (4.7) leads to a proliferation of hydroxy radicals, which non-selectively abstract hydrogen atoms, see Reactions (4.8) and (4.9). Acids are formed by the following two reactions, which start from a hydroperoxy-peroxy radical, see Reaction (4.4), and an aldehyde [9, 10]. Carboxylic acids (RCOOH), formed according to Reaction sequences (4.18) and (4.19), represent one of the principal products under these oxidation conditions. In a subsequent step they can react with alcohols R OH, produced by Reactions (4.10) and (4.14), to form esters, RCOOR. In addition, when the rate of oxidation becomes limited by diffusion, ethers are formed. Reaction sequence (4.20) ... 

Because monosaccharides 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 formimines, hemiacetals, and acetals. When you read the sections that deal with the reactions of monosaccharides, you will find cross-references to the sections in which the same reactivity for simple organic compounds is discussed. As you study, refer back to these sections they will make learning about carbohydrates a lot easier and will give you a good review of some chemistry that you have already learned about. 

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 alcohol has been a spectator in these events so far but the chlorosulfonium ion now formed can react with it to give a new sulfonium salt. This is the sole purpose of all the reactions up to this point. This sulfonium salt is deprotonated by the base (Et3N) to form an ylid. The final step completes the redox reaction the transfer of a proton to the anionic carbon gives an aldehyde, with overall reduction of dimethylsulfoxide (DMSO) to dimethyl-sulfide (DMS). 

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 ... 

Recall from Section 20.5 (Mechanism 20.5) that an aldehyde can react with an alcohol in the presence of an acid catalyst to produce a hemiacetal. 

Reactions in carbohydrates can occur at the carbonyl group or at one of the several alcohols. Aldehydes can be oxidized or reduced. They also can react with amines to form imines. The alcohols can be alkylated or acylated. Selective protection of the glycoside alcohols is possible with careful manipulation of protecting groups. 

The zwitterion (6) can react with protic solvents to produce a variety of products. Reaction with water yields a transient hydroperoxy alcohol (10) that can dehydrate to a carboxyUc acid or spHt out H2O2 to form a carbonyl compound (aldehyde or ketone, R2CO). In alcohoHc media, the product is an isolable hydroperoxy ether (11) that can be hydrolyzed or reduced (with (CH O) or (CH2)2S) to a carbonyl compound. Reductive amination of (11) over Raney nickel produces amides and amines (64). Reaction of the zwitterion with a carboxyUc acid to form a hydroperoxy ester (12) is commercially important because it can be oxidized to other acids, RCOOH and R COOH. Reaction of zwitterion with HCN produces a-hydroxy nitriles that can be hydrolyzed to a-hydroxy carboxyUc acids. Carboxylates are obtained with H2O2/OH (65). The zwitterion can be reduced during the course of the reaction by tetracyanoethylene to produce its epoxide (66). 

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