Alcohols secondary/tertiary dehydration

Like tertiary alcohols secondary alcohols normally undergo dehydration by way of carbocation intermediates... 

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... 

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... 

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 secondary and tertiary benylic alcohols and tertiary aliphatic alcohols undergo dehydration when refluxed with DMSO at 160-185° for about 9-16 hours. 1-methyl cyclopentanol 1 is dehydrated to 1 methyl cyclopentene. 

Dehydrates secondary and tertiary alcohols to give olefins, but forms ethers with primary alcohols. Cf. Burgess dehydrating reagent. 

Dehydration of alcohols.1 This iron salt, as well as CuS04 and NaHS04, when supported on silica gel effects dehydration of alcohols in various solvents at 100-125°. The order of reactivity of alcohols is tertiary > secondary > primary. Silica gel is essential for dehydration other solid supports are not effective, and the rate of dehydration is increased by increasing amounts of Si02 and then becomes constant. 

In general, the acid catalyzed esterification of organic acids can be accomplished easily with primary or secondary alkyl or aryl alcohols, but tertiary alcohols usually give carbonium ions which lead to dehydration. The structure of the acid is also of importance. As a rule, the more hindered the acid is alpha to the carbonyl carbon the more difficult esterification becomes (20A). 

Symmetrical secondary or tertiary alicyclic alcohols are readily dehydrated to only one olefin in each case. Examples include cyclopentene from cyclopentanol and phosphoric acid, cyclohexene from cyclohex-anol over alumina," cycloheptene from cycloheptanol and /6-naphthalene-... 

Tertiary alcohols are dehydrated instantaneously by sulfurane (1) at room temperature. Most secondary alcohols are also dehydrated rapidly at room temperature. There is evidence for a preferred (rnni-diaxial disposition of leaving groups. Thus cis-4-i-butylcyclohexanol is converted into 4-/-butylcyclohexene at least 150 times more rapidly than the /rons-isomer. Primary alcohols (ROH) are generally not dehydrated but react rapidly and quantitatively with (I) at - 50 to give ethers, R OR dehydration occurs only if the /S-proton is sufficiently acidic. 

Alcohols can be prepared by the hydration of alkenes or reduction of aldehydes and ketones. Alcohols can undergo dehydration to )deld alkenes. Primary and secondary alcohols undergo oxidation reactions to yield aldehydes and ketones, respectively. Tertiary alcohols do not undergo oxidation. 

Alcohols can undergo dehydration to produce ethers in an intermolecular reaction (Scheme 4) [1,3]. Characteristically only symmetrical ethers can be prepared in this way, usually via a bimolecular reaction (8 2 mechanism). When a mixture of two alcohols is reacted a mixture of three ethers is produced. Mixed ethers, however may be synthesized by reacting a tertiary alcohol with a primary or secondary alcohol. In this reaction an S l mechanism is operative, with involvement of the tertiary carbocation formed from the tertiary alcohol. 

The ease of dehydration depends largely on the reaction conditions and still more on the structure of the alcohol in question. Increasing substitution by alkyl groups increases the rate of olefin formation. Also, the ease of dehydration increases in the order primary > secondary > tertiary alcohols for the preparation of olefins it often suffices to warm a tertiary alcohol with a trace of iodine or in no more than 15% sulfuric acid.9 For example, 2-methyl-2-hexene is formed merely by distilling 2-methyl-2-hexanol containing a trace of iodine,10 and refluxing 2-methyl-2-butanol with 15% sulfuric acid affords 2-methyl-2-butene with a little 2-methyl-l-butene 11... 

It is mainly on the industrial scale that catalytic dehydration of alcohols becomes important, but it has been used in the laboratory particularly for preparation of the lower alkenes such as ethylene, propene, and butene. The most suitable catalysts are y-alumina, thorium dioxide, and blue tungsten oxide. Primary and secondary alcohols are usually dehydrated in the gas phase at 250-450° high-boiling tertiary alcohols are dehydrated without need for a catalyst when they are heated at 150-200° (see page 814). 

Because the rate-determining step in the dehydration of a secondary or a tertiary alcohol is formation of a carbocation intermediate, the rate of dehydration parallels the ease with which the carbocation is formed. Tertiary alcohols are the easiest to dehydrate because tertiary carbocations are more stable and therefore are easier to form than secondary and primary carbocations (Section 4.2). In order to undergo dehydration, tertiary alcohols must be heated to about 50 °C in 5% H2SO4, secondary alcohols must be heated to about 100 °C in 75% H2SO4, and primary alcohols can be dehydrated only under extreme conditions (170 °C in 95% H2SO4) and by a different mechanism because primary carbocations are too unstable to be formed (Section 10.5). 

An alcohol can be dehydrated if heated with an acid catalyst dehydration is an E2 reaction in the case of primary alcohols and an El reaction in the case of secondary and tertiary alcohols. Tertiary alcohols are the easiest to dehydrate and primary alcohols are the hardest. El reactions... 

A number of different types of kinetic evidence have been used to show that the dehydration reactions proceed via carbonium-ion intermediates in an El process. Simple rate studies show that the order of reactivity decreases along the series tertiary alcohol > secondary > primary, the order of decreasing carbonium-ion stability. Skeletal rearrangements typical of carbonium ions have also been observed " On a number of occasions, the rate of olefin formation has been shown to be a slower process than the exchange of the hydroxyl function with the reaction medium " -. This can conveniently be demonstrated by studying the reaction in water enriched with " OH2 or, in the case of an optically active alcohol, by comparing the rates of racemisation and dehydration - . These observations indicate that exchange occurs before (184), and not simultaneously with, elimination. 

---