Dehydration, alcohol, elimination

A large variety of methods is applicable to the formation of isolated double bonds. This permits selection of reagents compatible with other functionality present. Alcohol dehydration, ester elimination and other nonreductive p eliminations are the most common methods. Reductive elimination of halo-hydrins, vic-dihalides, etc., and of a variety of ketone derivatives has also been used. 

Alkenes are generally prepared by an elimination reaction, such as dehydrohalo-genation, the elimination of FIX from an alkyl halide, or dehydration, the elimination of water from an alcohol. 

You will recall that this reaction can also be described as an elimination reaction, i.e. a reaction In which the elements of a simple molecule, such as water, are removed from the organic molecule and not replaced. Concentrated phosphoric acid is preferred to concentrated sulfuric acid when dehydrating alcohols because more side reactions occur with the latter and it tends to lead to charring. 

In the last chapter, you saw alkenes being made qh dehydration El elimination in acid from alcohols by El elimination—dehydration—under acid catalysis. The question we are going to answer in this section is how can you make this elimination run backwards—in other words, how can you hydrate a double bond ... 

The internal salt (Me0-C0 N-S02-NEt3), derived from methyl(carboxy-sulphamoyl)triethylammonium hydroxide, is effective in dehydrating alcohols (36) to give olefins (38), in anhydrous benzene at room temperature. " The cis proton is eliminated, apparently through an ion-pair mechanism (37). Tertiary... 

Alcohols undergo elimination and substitution reactions. Dehydration, the elimination of H and OH, requires acid and forms alkenes ... 

In synthetic practice, it is more common to dehydrate alcohols using thionyl chloride or phosphorus oxychloride in pyridine. Generally, the mechanism involved in such reactions is the formation of chlorosulhnyl or dichlorophosphoryl esters, followed by El or E2 elimination ... 

The fact that primary carbenium ions are unstable suggests that the exchange with solvent is an Sn2 process with primary alcohols. If true, then are primary carbenium ions ever intermediates in dehydration reactions Studies have shown that it depends upon the case. Neopentyl alcohol does form a primary carbenium ion, whereas 1-propanol does not. Acid-catalyzed elimination of 1-propanol to form propene occurs by a concerted E2 reaction (Eq. 10.90). Similarly, whether a secondary alcohol eliminates in acid via an El or E2 pathway depends on the case. 

Likewise, alcohols undergo dehydration, the elimination of H2O, from two adjacent atoms. Unlike fragmentations discussed thus far, dehydration results in the cleavage of two bonds and forms H2O and the radical cation derived from an alkene. 

Methods for the preparation of alkenes generally involve elimination reactions, such as dehydrohalogenation—the elimination of HX from an alkyl halide—and dehydration—the elimination of water from an alcohol. The flip side of that elimination reaction to prepare alkenes is the addition of various substances to the alkene double bond to give saturated products. 

Dehydrogenation of alkanes is not a practical laboratory synthesis for the vast majority of alkenes The principal methods by which alkenes are prepared m the labo ratory are two other (3 eliminations the dehydration of alcohols and the dehydrohalo genation of alkyl halides A discussion of these two methods makes up the remainder of this chapter... 

Except for the biochemical example just cited the stractures of all of the alcohols m Section 5 9 (including those m Problem 5 13) were such that each one could give only a single alkene by p elimination What about ehmmahon m alcohols such as 2 methyl 2 butanol m which dehydration can occur in two different directions to give alkenes that are conshtutional iso mers Here a double bond can be generated between C 1 and C 2 or between C 2 and C 3 Both processes occur but not nearly to the same extent Under the usual reachon con dihons 2 methyl 2 butene is the major product and 2 methyl 1 butene the minor one... 

Dehydration of this alcohol is selective m respect to its direction Elimination occurs m the direction that leads to the double bond between C 2 and C 3 more than between C 2 and C 1 Reactions that can proceed m more than one direction but m which one direction is preferred are said to be regioselective... 

Zaitsev s rule as applied to the acid catalyzed dehydration of alcohols is now more often expressed in a different way elimination reactions of alcohols yield the most highly substituted alkene as the major product Because as was discussed in Section 5 6 the most highly substituted alkene is also normally the most stable one Zaitsev s rule is sometimes expressed as a preference for predominant formation of the most stable alkene that could arise by elimination... 

Like the reaction of tert butyl alcohol with hydrogen chloride step 2 m which tert butyloxonium ion dissociates to (CH3)3C and water is rate determining Because the rate determining step is ummolecular the overall dehydration process is referred to as a ummolecular elimination and given the symbol El... 

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... 

Like alcohol dehydrations El reactions of alkyl halides can be accompanied by carbocation rearrangements Eliminations by the E2 mechanism on the other hand nor mally proceed without rearrangement Consequently if one wishes to prepare an alkene from an alkyl halide conditions favorable to E2 elimination should be chosen In prac tice this simply means carrying out the reaction m the presence of a strong base... 

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... 

The point was made earlier (Section 5 9) that alcohols require acid catalysis in order to undergo dehydration to alkenes Thus it may seem strange that aldol addition products can be dehydrated in base This is another example of the way in which the enhanced acidity of protons at the a carbon atom affects the reactions of carbonyl com pounds Elimination may take place in a concerted E2 fashion or it may be stepwise and proceed through an enolate ion... 

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. 

Chem Systems Inc. proposed a process in which ben2yl alcohol obtained by an undisclosed direct oxidation of toluene is homologated with synthesis gas to yield 2-phen5iethyl alcohol, which is then readily dehydrated to styrene (57). This process eliminates the intermediate formation of methanol from synthesis gas but does require the independent production of ben2yl alcohol. 

Addition and elimination processes are the reverse of one another in a formal sense. There is also a close mechanistic relationship between the two reactions, and in many systems reaction can occur in either direction. For example, hydration of alkenes and dehydration of alcohols are both familiar reactions that are related as an addition-elimination pair. 

The dehydration of alcohols is an important elimination reaction that takes place under acidic rather flian basic conditions. It involves an El mechanism." The function of the acidic reagent is to convert the hydroxyl group to a better leaving group by protonation ... 

Aldehydes and ketones undergo reversible addition reactions with alcohols. The product of addition of one mole of alcohol to an aldehyde or ketone is referred to as a hemiacetal or hemiketal, respectively. Dehydration followed by addition of a second molecule of alcohol gives an acetal or ketal. This second phase of the process can be catalyzed only by acids, since a necessary step is elimination of hydroxide (as water) from the tetrahedral intermediate. There is no low-energy mechanism for base assistance of this... 

Ehminations of HX to give double bonds offer considerable scope for selectivity and choice of reaction conditions. The dehydration of alcohols is the most common example of this class and may be achieved directly or through intermediate derivatives. In most cases, such derivatives are transient species formed in situ, but sometimes e.g. sulfonates, certain other esters and halides) they are isolated and characterized. Eliminations from jS-substituted ketones are very facile. The dehydration of jS-hydroxy ketones has been covered in section V. 

The principal complications in the dehydration of simple alcohols arise through the possibilities of rearrangement and alternative directions for elimination. 

When, in the phosphorous oxychloride-pyridine dehydration of an alcohol these rules conflict, the stereoelectronic factor determines the direction of elimination ... 

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