Dehydration of Secondary and Tertiary Alcohols

The dehydration of the title alcohols was made over alumina in the presence of piperidine (84). The experimental results listed in Table VII give the primary products. In all experiments the cis-olefins predominate over the trans-. 

The method of preparation of the alumina has a marked effect on the product distribution as shown in Table VIII (47). Over the pure alumina (P) the olefinic products are nearly equilibrated. The alkali-containing catalysts, however, give kinetically controlled products. The very low activity of these catalysts for olefin isomerization had been ascertained independently. It may, therefore, be concluded that the compo.sition of the olefins produced at 350° is very nearly that of the primary dehydration products. 

Experiments 3-5 show a small trend toward more 1-alkene as the alumina becomes more basic. From a plot of product composition versus contact time and extrapolation to zero time Pines and Haag (49) 

The primary products obtained from 2-butanol are of mechanistic. significance and may be compared with other eliminations in the sec-butyl system 87). The direction of elimination does not follow the Hofmann rule 88) nor is it governed by statistical factors. The latter would predict 60% 1-butene and 40% 2-butene. The greater amount of 2-alkene and especially the unusual predominance of the cis-olefin over the trans isomer rules out a concerted cis elimination, in which steric factors invariably hinder the formation of cis-olefin. For example, the following ratios oicisjtrans 2-butene are obtained on pyrolysis of 2-butyl compounds acetate, 0.53 89, 90) xanthate, 0.45 (S7) and amine oxide, 0.57 86) whereas dehydration of 2-butanol over the alkali-free alumina (P) gave a cisjtrans ratio of 4.3 (Fig. 3). 

The kinetic preference for cis- over imns-olefin elimination from acyclic compounds is rare. Cope and co-workers 91) reported a slight preference for cis- over irans-2-butene and 2-pentene in the thermal decomposition of the quaternary ammonium hydroxides, and Andr u and co-workers 92,93) found a preponderance of cis- over trons-2-butene in the elimination of hydrogen chloride from 2-chlorobutane over solid catalysts. Neureiter and Bordwell 94) found the formation of cis-2-butene rather than ra s-2-butene in the release of chloride ion during the formation of alkene from a-chlorosulfone on treatment with alkali  

---

To circumvent the need for strong acid and allow the dehydration of secondary alcohols, reagents have been developed that are effective under mild, basic conditions. One such reagent, phosphorus oxychloride (POCI3) in the basic amine solvent pyridine, is often able to effect the dehydration of secondary and tertiary alcohols at 0 °C. 

Mechanism of the dehydration of secondary and tertiary alcohols by reaction with POCI3 in pyridine. The reaction is an E2 process. 

The method is suitable for the preparation of ethers having primary alkyl groups only. The alkyl group should be unhindered and the temperature be kept low. Otherwise the reaction favours the formation of alkene. The reaction follows S l pathway when the alcohol is secondary or tertiary about which you will learn in higher classes. However, the dehydration of secondary and tertiary alcohols to give corresponding ethers is unsuccessful as elimination competes over substitution and as a consequence, alkenes are easily formed. 

Ring enlargement of cyclobutanols.1 The reagent effects dehydration of secondary and tertiary alcohols. Dehydration of the tertiary cyclobutanol (1) results in dehydration and ring enlargement to give isolaurene (2) in quantitative yield. 

Dehydration of secondary and tertiary alcohols with phosphorus oxychloride (POCI3) in pyridine leads directly to alkenes without isolating the dichlorophosphate intermediate. ... 

Dehydration of secondary and tertiary alcohols involves the formation of a carbocation intermediate, so be sure to check the stmcture of the carbocation for the possibility of rearrangement. Remember that a carbocahon will rearrange if rearrangement produces a more stable carbocation (Section 4.6). For example, the intitially formed secondary carbocation in the following reaction rearranges to a more stable tertiary carbocation ... 

A Mechanism for Dehydration of Secondary and Tertiary Alcohols An El Reaction... 

In the dehydration of secondary and tertiary alcohols the slowest step is formation of the carbocation as shown in step 2 of the A Mechanism for the Reaction box in this section. The first and third steps involve simple acid—base proton transfers, which occur very rapidly. The second step involves loss of the protonated hydroxyl as a leaving group, a highly endergonic process (Section 6.7), and hence it is the rate-determining step. 

On the basis of the relative ease of dehydration of alcohols (3° > 2° > 1°), chemists propose a three-step mechanism for the acid-catalyzed dehydration of secondary and tertiary alcohols. This mechanism involves the formation of a carbocation intermediate in the rate-determining step and therefore is an El mechanism. 

It is necessary to use different procedures to prepare secondary alkyl bromides from secondary alcohols because such alcohols are easily dehydrated by concentrated sulfuric acid to give alkenes by way of Equations 14.13 and 14.14. In fact, the acid-catalyzed dehydration of secondary and tertiary alcohols is a common method for synthesizing alkenes (Sec. 10.3). This problem may be circumvented by using concentrated hydrobromic acid however, it is better to prepare secondary alkyl bromides by the reaction of secondary alcohols with phosphorus tribromide, PBrg (Eq. 14.16). 

The mechanism for acid-catalyzed dehydration depends on the structme of the alcohol dehydrations of secondary and tertiary alcohols are El reactions. 

Because dehydration of secondary and tertiary alcohols occurs via carbocation intermediates, rearrangement reactions are common. For example, in the dehydration of 3,3-dimethyl-2-butanol, only 3% of the dehydration product maintains the original carbon skeleton. The remaining 97% is a mixture of two isomeric alkenes with a rearranged carbon skeleton. 

---