Hemiacetals from reaction of alcohols with aldehydes and ketones

The chart shows the extent of hydration (in water) of a small selection of carbonyl compounds hexafluoroacetone is probably the most hydrated carbonyl compound possible The larger the equilibrium constant, the more the equilibrium is to the right. 

Hemiacetals from reaction of alcohols with aldehydes and ketones 

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 

Intermolecular reactions occur between two molecules. Intramolecular reactions occur within the same molecule. We shall discuss the reasons why Intramolecular reactions are more favourable and why cyclic hemi-acetals and acetals are more stable In Chapters 11 and 12. 

What is certain is that proton transfers between oxygen atoms are very last and are reversible, and for that reason we don t need to be concerned with the details—the proton can always get to where it needs to be for the next step of the mechanism. As with all these carbonyl group reactions, what is really important is the addition step, not what happens to the 

Since water adds to (at least some) catbonyl compounds, it should come as no surprise that alcohols do too. The product of the reaction is known as a hcmiacetal, 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. 

Hemiacetal formation is reversible, and hemiacetals are stabilized by the same special structural features as those of hydrates. However, hemiacetals can also gain stability by being cyclic—when the carbonyl group and the attacking hydroxyl group are part of the same molecule. The reaction is now an intramolecular (within the same molecule) addition, as opposed to the intermolecular (between two molecules) ones we have considered so far. 

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HEMIACETALS FROM REACTION OF ALCOHOLS WITH ALDEHYDES AND KETONES... 

Alcohols also undergo reversible addition reactions with aldehydes and ketones. The product of addition of one mole of alcohol to an aldehyde or ketone is referred to as a hemiacetal or hemiketal, respectively. Dehydration and addition of a second mole 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 ion from a tetrahedral intermediate. There is no low-energy mechanism for base assistance of this elimination step. For this reason, acetals and ketals are stable toward hydrolysis in alkaline aqueous solution. 

Aldehydes and ketones react with alcohols to form hemiacetols and hemiketah, respectively. In this reaction the alcohols react in typical fashion as the nucleophile. When aldehydes and ketones are attacked by a nucleophile, they undergo addition. Aldehydes and hemiacetals, and ketones and hemiketals, exist in equilibrium when an aldehyde or ketone is dissolved in an alcohol however, usually the hemiacetal or hemiketal is too unstable to isolate unless if exists as a ring structure. If a second molar equivalent of alcohol is added, an oceiuf is formed from a hemiacetal, or a ketal is formed from a hemiketal. 

When this reaction is conducted on ketones, instead of aldehydes, the equivalent species to the hemiacetal and acetal are now called hemiketal and ketal respectively. However, when simple ketones are reacted with normal monofunctional alcohols, the reaction rarely goes to completion, unlike the situation with aldehydes where the reaction proceeds smoothly. Yet, when a 1,2-diol is reacted with a ketone, then the reaction goes to completion much more readily. Write the complete reaction sequence for the formation of the cyclic ketal starting from propanone and 1,2-ethandiol, under acidic conditions. Also, suggest why the product between one molecule of each reagent is favoured over the alternative product that would result from the reaction between two molecules of the diol and one of the ketone. 

Benzaldehyde is reduced about 400 times faster than acetophenone in isopropanoT (Chapter 6) While amines react with acetic anhydride quite rapidly at room temperature (reaction complete in a few hours), alcohols react extremely slowly in the absence of a base (Chapter 10) Secondary and tertiary amides are difficult to hydrolyse but a similar mechanism is successful with only a little water and plenty of a strong base (Chapter 10) Acyclic hemiacetals form relatively slowly from an aldehyde or ketone plus an alcohol, but their rate of formation is greatly increased either by acid or by base (Chapter 11). 

Acetalization and ketalization, like esterification, are also important candidate reactions for RD. It is a reversible reaction between aldehyde/ketone and alcohol that generates one molecule of water with one molecule of acetal/ketal. Various acetals, such as methylal and dioxalane, are useful solvents in the chemical industry. Ma-samuto and Matsuzaki (1994) first prepared methylal from formaldehyde and methanol in the presence of cation-exchange resins using a laboratory scale RD column conveniently packed in the form of tea-bag structures [31]. Kolah et al. studied this reaction in both batch and continuous RD column, as shown in Fig. 1.6, with a theoretical analysis of multiple reactions in the RD column [32]. Along with the acetal, formation of dimers and hemiacetals also takes place with substantial con-... 

The hemiacetal has a central carbon atom (from the aldehyde) attached to a hydrogen, a hydroxyl group, a hydrocarbon group (CH3), and an alkoxy group (C2H5O). If a ketone is used rather than an aldehyde, the resulting hemiacetal contains two hydrocarbon groups. For example, reaction of the ketone R COR with the alcohol R OH is ... 

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