Dehydration acid-catalysed

The addition of siliceous material e.g., kieselguhr, Fuller s earth, diatom-aceous earth, or powdered pumice) promotes the acid-catalysed dehydration of the alcohol 2-3 g. of siliceous material for each mole of alcohol is recommended. 

Allylic A" -3-hydroxyls are particularly reactive, although some difficulty arises because this system is prone to acid-catalysed dehydration to the 3,5-diene. A" -3-Methyl ethers are readily prepared by direct, p-toluenesulfonic acid-catalysed reaction with methanol. 

The final acetal is made by the usual acid-catalysed method, using the ortho-formate IIC(0Et)2 as a dehydrating agent. ... 

The concept of extractive reaction, which was conceived over 40 years ago, has connections with acid hydrolysis of pentosans in an aqueous medium to give furfural, which readily polymerizes in the presence of an acid. The use of a water-immiscible solvent, such as tetralin allows the labile furfural to be extracted and thus prevents polymerization, increases the yield, and improves the recovery procedures. In the recent past an interesting and useful method has been suggested by Rivalier et al. (1995) for acid-catalysed dehydration of hexoses to 5-hydroxy methyl furfural. Here, a new solid-liquid-liquid extractor reactor has been suggested with zeolites in protonic form like H-Y-faujasite, H-mordenite, H-beta, and H-ZSM-5, in suspension in the aqueous phase and with simultaneous extraction of the intermediate product with a solvent, like methyl Aobutyl ketone, circulating countercurrently. 

The aldol condensation of acetone to diacetone alcohol is the first step in a three-step process in the traditional method for the production of methyl isobutyl ketone (MIBK). This reaction is catalysed by aqueous NaOH in the liquid phase. (3) The second step involves the acid catalysed dehydration of diacetone alcohol (DAA) to mesityl oxide (MO) by H2S04 at 373 K. Finally the MO is hydrogenated to MIBK using Cu or Ni catalysts at 288 - 473 K and 3- 10 bar (3). 

The reaction mechanism is shown in Figure 4 and is adapted from work by Fiego et al. [9] on the acid catalysed condensation of acetone by basic molecular sieves. The scheme has been modified to include the hydrogenation of mesityl oxide to MIBK. The scheme begins with the self-condensation of acetone to form diacetone alcohol as the primary product. The dehydration of DAA forms mesityl oxide, which undergoes addition of an addition acetone to form phorone that then can cyclise, via a 1,6-Michael addition to produce isophorone. Alternatively, the mesityl oxide can hydrogenate to form MIBK. 

The reaction is reversible, the reverse being the perhaps better known acid-catalysed dehydration of alcohols (p. 247). Protonation can also occur on oxygen in a carbon-oxygen double bond,... 

Acid-catalysed hydration of an alkene is the reversal of the similarly acid-catalysed dehydration (by the El pathway, cf. p. 248) of alcohols to alkenes ... 

Under acid conditions the tertiary alcohol (91) almost always undergoes acid-catalysed dehydration (cf. p. 247) to yield the txf -unsaturated ketone, 2-methylpent-3-ene-2-one (mesityl oxide, 95) ... 

It is well known that heating of honey resrrlts in HMF, which is formed dming acid-catalysed dehydration of hexoses [6]. The presence in honey of simple sugars (glucose and fructose) and mar r acids is a favotrrable condition for the production of this substance. 

Many of the enone substrates used in polyamino acid-catalysed epoxidation reactions can be made via a simple aldol condensation, which leads directly to the desired enone after in situ dehydration. Enones that cannot be synthesised by the above route may often be synthesised using standard Wittig chemistry, (Scheme 6). The above methods of substrate synthesis provide compounds with a variety of groups R and enabling the incorporation of both aliphatic and aromatic moieties into the enone structure. 

The solute benzene radical cation was formed on pulse radiolysis of an acidic aqueous solution of benzene. The transient optical absorption bands (A-max = 310, 350-500 nm) were assigned to the solute benzene radical cation which is formed on acid-catalysed dehydration of the OH adduct. The radical cation is able to undergo an electron-transfer reaction with Br and was found to be a strong electron oxidant. Pulse radiolysis has been used to study the complex reaction that follows electron addition to hydroxybenzophenones (HOBPs). The various radical species involved have been characterized spectrally and their p/fa values evaluated. The differences... 

Hydroxymethylfurfural (HMF) can be readily obtained from acid-catalysed dehydration of carbohydrates. On the other hand, FFA can be further reacted to produce 2,5-furandiacetic acid and 5-carboxyfuran-2-acetic acid which could form polymers, much like tereftahc acid. Therefore the carbonylation of MF can be regarded as the first step of the green manufacture of polymers on the basis of renewable (carbohydrate) raw materials. 

Predict the major product of acid catalysed dehydration of... 

Secondary amines react with aldehydes and ketones via addition reactions, but instead of forming imines, produce compounds known as enamines. Initially, there is the same type of nucleophilic attack of the amine onto the carbonyl system, followed by acid-catalysed dehydration but, since the amine is secondary, the product of dehydration is an iminium... 

Alkenes are obtained hy dehydration of alcohols via elimination reactions (see Section 5.4.3), and esters are prepared conveniently by the acid-catalysed reaction of alcohols and carboxylic acids (see Section 5.5.5). 

Ethers are prepared from alkyl halides by the treatment of metal alkoxide. This is known as Williamson ether synthesis (see Sections 4.3.6 and 5.5.2). Williamson ether synthesis is an important laboratory method for the preparation of both symmetrical and unsymmetrical ethers. Symmetrical ethers are prepared by dehydration of two molecules of primary alcohols and H2SO4 (see Sections 4.3.7 and 5.5.3). Ethers are also obtained from alkenes either by acid-catalysed addition of alcohols or alkoxymercuration-reduction (see Section 5.3.1). 

Addition of water is known as a hydration reaction. The hydration reaction occurs when alkenes are treated with aqueous acids, most commonly H2SO4, to form alcohols. This is called acid-catalysed hydration of alkenes, which is the reverse of the acid-catalysed dehydration of an alcohol. 

Bimolecular dehydration is generally used for the synthesis of symmetrical ethers from unhindered 1° alcohols. Industrially, diethyl ether is obtained by heating ethanol at 140 °C in the presence of H2SO4. In this reaction, ethanol is protonated in the presence of an acid, which is then attacked hy another molecule of ethanol to give diethyl ether. This is an acid-catalysed Sn2 reaction. If the temperature is too high, alkene is formed via elimination. 

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