Primary alcohols dehydration

Fig. 1. Correlation of the rates of primary alcohol dehydration (40) (series 1 from Table II) in the coordinates of the Taft equation (8) for different separations of the reactants molecules into the parts substituent-link-reaction center.

Tertiary alcohols are more readily dehydrated than secondary alcohols, whilst primary alcohols are dehydrated with comparative difficulty. Thus the reaction proceeds easily with 33 per cent, sulphuric acid (1 acid 2 water, by volume) for amyl alcohol, but 50 per cent, (by volume) is required for aec.-amyl alcohol. Higher concentrations of acid tend to lead to increasing polymerisation of the olefine and are therefore usually avoided. 

As noted earlier (Section 4 10) primary carbocations are too high m energy to be intermediates m most chemical reactions If primary alcohols don t form primary car bocations then how do they undergo elimination s A modification of our general mech amsm for alcohol dehydration offers a reasonable explanation For primary alcohols it is... 

Hydnde shifts often occur during the dehydration of primary alcohols Thus although 1 butene would be expected to be the only alkene formed on dehydration of 1 butanol It IS m fact only a minor product The major product is a mixture of cis and trans 2 butene... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

Primary alcohols do not dehydrate as readily as secondary or tertiary alcohols and their dehydration does not involve a primary carbocation A proton is lost from the (3 carbon m the same step m which carbon-oxygen bond cleavage occurs The mechanism is E2... 

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. 

The butyl alcohols undergo esterification with organic acids in the usual manner in the presence of trace amounts of mineral acid catalysts. Esterification is fastest with /-butyl alcohol and slowest with the primary alcohols although /-butyl alcohol undergoes substantial dehydration in the presence of the typical acid esterification catalysts. 

The resulting 4-methylhexanone-2 oxime separates and is dried by any suitable means, such as with a dehydrating agent, for example, sodium sulfate or magnesium sulfate. After drying, 4-methylhexanone-2 oxime is reduced with hydrogen by means of a catalyst, such as Raney nickel, or by reaction of sodium and a primary alcohol, such as ethanol. The resulting 2-amino-4-methylhexane may be purified by distillation, as described in U.S. Patent 2,350,318. 

The reaction is an F.1 process and occurs through the three-step mechanism shown in Figure 17.6). As usual for El reactions (Section 11.10), only tertiary alcohols are readily dehydrated with acid. Secondary alcohols can be made to react, but the conditions are severe (75% H2S04,100 °C) and sensitive molecules don t survive. Primary alcohols are even less reactive than secondary ones, and very harsh conditions are necessary to cause dehydration (95% H2S04,150 °C). Thus, the reactivity order for acid-catalyzed dehydrations is... 

Diethyl ether and other simple symmetrical ethers are prepared industrially by the sulfuric acid-catalyzed dehydration of alcohols. The reaction occurs by SN2 displacement of water from a protonated ethanol molecule by the oxygen atom of a second ethanol. Unfortunately, the method is limited to use with primary alcohols because secondary and tertiary alcohols dehydrate by an El mechanism to yield alkenes (Section 17.6). 

The above procedure describes the only known preparation of the inner salt of methyl (carboxysulfamoyl)triethylammonium hydroxide and illustrates the use of this reagent to convert a primary alcohol to the corresponding urethane.2 Hydrolysis of the urethane would then provide the primary amine. The method is limited to primary alcohols secondary and tertiary alcohols are dehydrated to olefins under these conditions, often in synthetically useful yields.2... 

The addition of Grignard reagents to aldehydes, ketones, and esters is the basis for the synthesis of a wide variety of alcohols, and several examples are given in Scheme 7.3. Primary alcohols can be made from formaldehyde (Entry 1) or, with addition of two carbons, from ethylene oxide (Entry 2). Secondary alcohols are obtained from aldehydes (Entries 3 to 6) or formate esters (Entry 7). Tertiary alcohols can be made from esters (Entries 8 and 9) or ketones (Entry 10). Lactones give diols (Entry 11). Aldehydes can be prepared from trialkyl orthoformate esters (Entries 12 and 13). Ketones can be made from nitriles (Entries 14 and 15), pyridine-2-thiol esters (Entry 16), N-methoxy-A-methyl carboxamides (Entries 17 and 18), or anhydrides (Entry 19). Carboxylic acids are available by reaction with C02 (Entries 20 to 22). Amines can be prepared from imines (Entry 23). Two-step procedures that involve formation and dehydration of alcohols provide routes to certain alkenes (Entries 24 and 25). 

Formation of a Rearranged Alkene During Dehydration of a Primary Alcohol... 

The primary alcohol initially undergoes acid-catalyzed dehydration by an E2 mechanism... 

Mild benzhydrylation (diphenylmethylation) of primary and secondary saccharide alcohols, and tritylation of the primary alcohols, were promoted by 4A AW-300 MS at room temperature through a dehydration mechanism in the absence of any strong protic or Lewis acid. While protection of a single primary or secondary hydroxyl... 

Ions of the later transition metals such as Pt+ may not form [MO]+ ions with water and alcohols as shown in Table I for the reaction of Pt+ with methanol, where the formation of Pt+-CO or Pt+-H2 ions are preferred (102). As previously mentioned, Cr+ and Mn+ appear to be much less reactive than any of the other transition metals. The Cr+ ion was reported to be unreactive to primary alcohols but initiated dehydration of branched-chain alcohols it was also described as being unreactive toward propanal and acetone (9). The Mn+ ion has received scant attention due to its reduced activity. The reactions of Fe+, Co+, and Ni+ with alcohols, ethers, aldehydes, and ketones have been extensively covered (9). These ions are more reactive than Cr+ and Mn+ and generally react with alcohols causing dehydration. 

In spite of the fact that alumina is an excellent and widely used catalyst for the dehydration of alcohols, there is no agreement in the literature with regard to the mechanism of this reaction or the nature of the olefinic products. For example, 1-alkenes have been obtained from primary alcohols such as 1-butanol (19-22), 1-pentanol (23), 1-hexanol (24-26), 1-heptano (27), and 1-octanol (25) but mixtures of olefins differing in the position of the double bond (13, 26, 28) or even in the carbon skeleton (29) have been reported from other primary alcohols. 

Before discussing the mechanism of dehydration of primary alcohols, it might be worthwhile to consider some of the published results on the dehydration of ethyl alcohol. Chiefly, two products result ethyl ether and ethylene. Most of the discussions over the years have centered around the problem whether ether is formed simultaneously, in-... 

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