Alcohols tertiary elimination

In contrast to the primary alcohols, tertiary alcohols eliminate water smoothly at 0°C in the presence of a 2-10 molar excess of hydrogen fluoride followed by polymerization.259 Reduced polymerization and satisfying yields of tertiary alkyl fluorides are achieved only at low temperatures (— 50 C). The reactivity of secondary aliphatic alcohols is thus interpreted to be between the other two types. High yields of alkyl fluorides are difficult to obtain because of the competing reactions and the effect of temperature on the equilibrium. 

Ferles et al. have studied the crossed hydrocoupling of acetone and cyclopentanone with pyridinium salts.167 The products were similar to those found for hydrocoupling of ketones with the pyridine free bases except for the Af-alkyl group. In the case of cyclopentanone, the tertiary alcohol product eliminated water in some cases to form an olefin. 

Tertiary alcohols undergo elimination the more readily, and most certainly by an El process. 

Tertiary alcohols, tertiary ethers, or carboxylic acid esters of tertiary alcohols can undergo El eliminations, but only in the presence of Bronsted or Lewis acids. Anyone who has prepared a tertiary alkoxide by a Grignard reaction and treated the crude reaction mixture with HC1 and obtained the alkene knows that tertiary alcohols can be converted into alkenes even with dilute hydrochloric acid. 

The reaction of a carbocation with a neutral nucleophile such as water gives a protonated alcohol. Tertiary butyl carbocation, for example, reacts with water (neutral nucleophile) to give protonated tert-butyl alcohol, which eliminates a proton to give tert-butyl alcohol (Scheme 2.5). 

The dehydration of alcohols to alkenes may also occur under add-catalysed conditions. Tertiary alcohols undergo elimination more readily than secondary alcohols, which are in turn dehydrated more easily than primary alcohols. Furthermore, where several elimination products are possible, the hydrogen atom that is lost tends to come from the carbon atom bearing the smaller number of hydrogen atoms. However, in more complex molecules this type of reaction may be accompanied by rearrangements. 

In contrast to the tertiary alcohols, primary (e.g., ethyl alcohol) and secondary alcohols e.g., isopropyl alcohol) decompose to products at temperatures above 800 °K via complex free radical chain processes . This mechanistic inversion is not surprising. Based on the magnitude of substituent effects in four-center elimination reactions, particularly the variations found in the series r-BuCI, i-PrCl, EtCl - , one would estimate that the isopropyl alcohol unimolecular elimination of water should have an activation energy about 6 kcal.mole higher than that for r-butyl alcohol. The. 4-factor can be estimated by transition state methods, and one obtains for the unimolecular decomposition... 

Most alcohols undergo elimination of water in ionizing solution in the presence of either Lewis (e.g., Scheme 8.27) and/or Brpnsted acid catalysts. For alkyl alcohols, the rate of dehydration generally follows carbocation stability, that is, as in the El reactions of alkyl hahdes (Chapter 7) tertiary > secondary > primary. 

Other alcohols behave similarly. Secondary alcohols undergo elimination at lower temperatures than primary alcohols, and tertiary alcohols at lower temperatures than secondary. 

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... 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

The reaction is of the 8 2 type and works best with primary and secondary alkyl halides Elimination is the only reaction observed with tertiary alkyl halides Aryl and vinyl halides do not react Dimethyl sulfoxide is the preferred solvent for this reaction but alcohols and water-alcohol mixtures have also been used... 

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. 

This elimination reaction is the reverse of acid-catalyzed hydration, which was discussed in Section 6.2. Because a carbocation or closely related species is the intermediate, the elimination step would be expected to favor the more substituted alkene as discussed on p. 384. The El mechanism also explains the general trends in relative reactivity. Tertiary alcohols are the most reactive, and reactivity decreases going to secondary and primary alcohols. Also in accord with the El mechanism is the fact that rearranged products are found in cases where a carbocation intermediate would be expected to rearrange ... 

When applied to the synthesis of ethers, the reaction is effective only with primary alcohols. Elimination to form alkenes predominates with secondary and tertiary alcohols. 

---