Alcohols reactions with acid catalysts

Alcohols undergo dehydration in supercritical and hot water (41). Tertiary alcohols require no catalyst, but secondary and primary alcohols require an acid catalyst. With 0.01 MH2SO4 as a catalyst, ethanol eliminates water at 385°C and 34.5 MPa to form ethene. Reaction occurs in tens of seconds. Only a small amount of diethyl ether forms as a side reaction. 

Normal Fischer esterification of tertiary alcohols is unsatisfactory because the acid catalyst required causes dehydration or rearrangement of the tertiary substrate. Moreover, reactions with acid chlorides or anhydrides are also of limited value for similar reasons. However, treatment of acetic anhydride with calcium carbide (or calcium hydride) followed by addition of the dry tertiary alcohol gives the desired acetate in good yield. 

Scheme 3b). It is instructive at this point to reiterate that the furan nucleus can be used in synthesis as a progenitor for a 1,4-dicarbonyl. Whereas the action of aqueous acid on a furan is known to provide direct access to a 1,4-dicarbonyl compound, exposure of a furan to an alcohol and an acid catalyst should result in the formation of a 1,4-diketal. Indeed, when a solution of intermediate 15 in benzene is treated with excess ethylene glycol, a catalytic amount of / ara-toluenesulfonic acid, and a trace of hydroquinone at reflux, bisethylene ketal 14 is formed in a yield of 71 %. The azeotropic removal of water provides a driving force for the ketalization reaction, and the presence of a trace of hydroquinone suppresses the formation of polymeric material. Through a Finkelstein reaction,14 the action of sodium iodide on primary bromide 14 results in the formation of primary iodide 23, a substance which is then treated, in crude form, with triphenylphosphine to give crystalline phosphonium iodide 24 in a yield of 93 % from 14.

An interesting extension of the acid-catalysed equilibrium reaction is the process termed transesterifl-cation. If an ester is treated with an excess of an alcohol and an acid catalyst, then the ester OR group becomes replaced by the alcohol OR group. This reaction proceeds through a tetrahedral intermediate... 

Common alcohol oxidation methods employ stoichiometric amounts of toxic and reactive oxidants like Cr03, hypervalent iodine reagents (Dess-Martin) and peracids that pose severe safety and environmental hazards in large-scale industrial reactions. Therefore, a variety of catalytic methods for the oxidation of alcohols to aldehydes, ketones or carboxylic acids have been developed employing hydrogen peroxide or alkyl hydroperoxides as stoichiometric oxygen sources in the presence of catalytic amounts of a metal catalyst. The commonly used catalysts for alcohol oxidation are different MoAV(VI), Mn(II), Cr(VI), Re(Vn), Fe(II) and Ru complexes . A selection of published known alcohol oxidations with different catalysts will be presented here. 

Fatty acids are converted to esters by reaction with an excess of alcohol using an acid catalyst or a lipase. For the preparation of methyl esters for GC analysis, boron trifluoride, sulfuric acid, or anhydrous hydrogen chloride in methanol are commonly used (19). Reaction is complete in 30 minutes at reflux. Propyl and butyl... 

In more recent works, the use of dicyclohexylcarbodiimide has been abandoned because the reaction works satisfactorily with acid catalysts alone, followed by bases such as triethylamine or diisopropylethylamine, which gives, sometimes, even better yields than triethylamine [1012,1023. Several activators are being used, and the best seems to be oxalyl chloride (theSwern oxidation) [1023,1149]. Other activators are mentioned in the section Oxidation of Primary Alcohols to Aldehydes. The advantages of oxidations with dimethyl sulfoxide lie in the mildness of the reagent and in the low temperatures, sometimes -45 °C [1020] or -60 °C [1023], at which the reactions are run. 

Although the final equilibrium in the Fischer reaction favors the pyrano-sides, the furanosides appear to be formed first, and they can sometimes be isolated by performing the reaction under mild conditions and arresting it at an early stage. In the same way, advantage can sometimes be taken, at the expense of the yield, of the fact that the a and /3 anomers (furanose or pyranose) may be formed at different rates. If an alcoholic glycoside is refluxed with the alcohol and an acid catalyst, equihbrium between the different forms is re-established. 

In the same way, treatment of a monosaccharide hemiacetal with an alcohol and an acid catalyst yields an acetal in which the anomeric -OH has been replaced by an -OR group. For example, reaction of -D-glucopyranose with methanol gives a mixture of cv and / methyl u-glucopyranosides ... 

Phenols undergo Friedel-Crafts alkylations with allylic chlorides or allylic alcohols over solid acid catalysts such as acidic KIO clay. For example, 2-buten-l-ol gives 3-aryl-1-butene and 1-ary 1-2-butene, albeit in low yields (12%) (equation 11). Allyl carbocations are involved as the reaction intermediates in these reactions. ... 

Alcohols react with alkenes in the same way that water does. Like the addition of water, the addition of an alcohol requires an acid catalyst. The product of the reaction... 

If a mixture of amyl nitrite and ethyl alcohol containing an acid catalyst is wanned, ethyl nitrite distills out. Friedel and Crafts heated ethyl acetate with amyl alcohol and amyl acetate with ethyl alcohol aijd observed alcoholysis in both cases. They also heated ethyl benzoate and amyl acetate together and obtained ethyl acetate and amyl benzoate, although this reaction was slow below 300°C. 

Fischer s classical 0-glycosylation with alcohols and an acid catalyst was applied by Limousin et al. [6], who described the reaction of peracetylated o-glucopyranose 4 with 1-decanol in the presence of zinc chloride to afford the corresponding a- (44a) and yS-glucoside (44h) in 74% yield (44a 44b = 7 1), with a small amount of the 2-de-O-acetyl a-derivatives 4Sa in 3 min (Scheme 12.21). 

In the first step an amine such as meth)4imidazole is alkylated, e.g. with diethyl sulfate. This is followed by a transesterification step where a long chain alcohol and an acidic catalyst such as methane sulfonic acid are added to the intermediate obtained after quatemization. In order to shift the equilibrium of this reaction to the products, ethanol liberated in the transformation must be efficiently removed by applying mild vacuum. Scheme 2.2-2 shows some of the alcohols converted in this manner. 

As mentioned for reactions with ethanol and heptanol, the cleavage of the oxi-rane group by alcohols requires an acid catalyst. The best results were obtained with PTSA (0.5% w/w) under the following conditions 100°C for 17 h, with a molar es-ters/alcohol ratio of 1 3 for a conversion rate close to 100%. The charaeteristies of the reaction products are reported in Table 11. 

Transesterification of triglycerides can be facilitated by both Br0nsted acid and base catalysts [204, 205]. Essential steps over each catalyst types are shown in Scheme 6.30 and Scheme 6.31, respectively. Reaction over acid catalysts first requires triglyceride adsorption, whereas the reaction over base catalysts is initiated by alcohol adsorption. Thereafter, the carbonyl moiety in the glyceride molecule is attacked to produce the alkyl esters. These steps occur consecutively to form the diglyceride and monoglyceride esters, with three molecules of fatty acid alkyl esters produced for every molecule of glycerol. 

---