Dehydration of pinacols

The pinacol used was the commercially available material, or that obtained by dehydration of pinacol hydrate. This dehydration may be accomplished by adding 2 1. of benzene to 1 kg. of pinacol hydrate (Org. Syn. Coll. Vol. 1, 1st Ed. (1932), p. 448 2nd Ed. (1941), p. 459), and distilling the water-benzene mixture. The lower layer is separated, and the upper benzene layer is returned to the distilling flask. This is repeated until the benzene distillate is clear. The anhydrous pinacol is then distilled, and the fraction boiling from 168° to 173° is collected. Depending upon the quality of the material used, 1 kg. of pinacol hydrate yields about 500 g. of anhydrous pinacol. 

The most convenient method for preparation of 2,3-dimethylbutadiene involves the dehydration of pinacol. Many catalysts have been used, among them hydrobromic,1 hydriodic,1 and sulfuric acids 2,3,4 sulfonic acids of the benzene5 and naphtha-... 

Dehydration of pinacol (sealed tube, 398 K, 1 hour, catalyst/diol = 50 mg/10 mg)... 

To be separately considered are the MT s of substituted phenyl groups, particularly meta- and para-substituted ones for which, supposedly, there is no complicating steric effect. As model compounds for quantitatively establishing the migration ability of substituted phenyl groups, symmetrical pinacols (4) have been studied This model is preferred since the formation of carbonium ions upon dehydration of pinacols of this type seems to arouse no doubts (see Ref. 1 7, iss) however. Ref. Besides, in this case the detachment of a water molecule from any carbon atom results in the same carbonium ion (5). 

Dimethyl-1,3-butadiene is produced by dehydration of pinacol, which in turn is made by reductive coupling of acetone, followed by purification via sulfur dioxide adducts [313]. It can be polymerized radically (emulsion polymerization), anionically, cationically, or by coordinative catalysts [314-319]. Due to the sterical hindrance of two methyl groups. 

The catalytic vapor phase dehydration of pinacol can proceed in two directions ... 

Pinacol (tetramethylethyleneglycol). Pinacol hydrate may be dehydrated in the following manner (compare Section 11,39). Mix 100 g. of pinacol hydrate with 200 ml. of benzene and distil a mixture of water and benzene passes over. Separate the lower layer and return the upper layer... 

Dehydration of diols pinacol rearrangement. Preparation of pinacolone... 

Pinacol rearrangement is a dehydration of a 1,2-diol to form a ketone. 2,3-drmethyl-2,3-butanediol has the common name pinacol (a symmetrical diol). When it is treated with strong acid, e.g. H2SO4, it gives 3,3-dimethyl-2-butanone (methyl r-butyl ketone), also commonly known as pinacolone. The product results from the loss of water and molecular rearrangement. In the rearrangement of pinacol equivalent carbocations are formed no matter which hydroxyl group is protonated and leaves. 

Superacid dehydrative cyclization of pinacols such as (93) gives condensed aromatic compounds as shown, presumably via dicationic species like (94).137... 

The dehydration of ditertiary alcohols in the presence of hydrobromic acid may lead to dienes (e.g. pinacol to 2,3-dimethylbuta- 1,3-diene, cognate preparation in Expt 5.12), although in this case some concomitant rearrangement to t-butyl methyl ketone (pinacolone, Expt 5.98) occurs under the acidic conditions employed. 

Pinacol. Pinacol hydrate may be dehydrated in the following manner (compare Section 2.23, p. 168, Drying by distillation). Mix 100 g of pinacol hydrate with 200 ml of benzene (CAUTION) and distil a mixture of water and benzene passes over. Separate the lower layer and return the upper layer of benzene to the distilling flask. Repeat the process until the benzene distillate is clear. Finally distil the anhydrous pinacol and collect the fraction boiling at 169-173 °C (50 g). The pure pinacol has m.p. 43 °C, but on exposure to moist air the m.p. gradually falls to 29-30 °C and then rises to 45-46 °C when hydration to the hexahydrate is complete. 

In the 1-electron reduction of A1 4-3-ketosteroids (164), various stereoisomers of pinacols are formed according to the pH. The protonized form of the ketosteroid, reduced in acidic solution, gives rise to a pinacol with hydroxyl groups in the a-position. In alkaline media, the unprotonized ketosteroid is reduced with the formation of the isomer with the hydroxyls in the p-position. The structure of the products prepared by controlled potential electrolysis, are supported by the rates of dehydration and periodic acid oxidations. For A4-3-ketosteroids, the difference in the composition of products obtained in acidic and alkaline media is less pronounced. 

The pinacol rearrangement is a dehydration of an alcohol that results in an unexpected product. When hot sulfuric acid is added to an alcohol, the expected product of dehydration is an alkene. However, if the alcohol is a vicinal diol, the product will be a ketone or aldehyde. The reaction follows the mechanism shown, below. The first hydroxyl group is protonated and removed by the acid to form a carboca-tion in an expected dehydration step. Now, a methyl group may move to fonn an even more stable carbocation. This new carbocation exhibits resonance as shown. Resonance Structure 2 is favored because all tire atoms have an octet of electrons. The water deprotonates Resonance Structure 2, forming pinacolone and regenerating the acid catalyst. 

The synthetic oestrogen dienoestrol 43 might be made10 by dehydration of the symmetrical diol 44 and hence by pinacol dimerisation of the ketone 45. Successful pinacol with magnesium metal gave 44 that could be dehydrated with AcCl in AC2O. 

Though restricted by the need for symmetry, this is a useful approach to t-alkyl ketones which are otherwise difficult to make.7 The crowded alkenes 40 can be made by dehydration of alcohols 41 and hence from the ketone 42 and RLi or RMgX. As 42 has a t -alkyl substituent it is a candidate for the pinacol approach. 

This reagent is both a Lewis acid and a dehydrating agent. Thus it effects Pummerer rearrangement of sulfoxides and rearrangement of pinacol to pinacolone via an epoxide.2... 

Hydride Reduction of a Carbonyl Group 454 Reaction of a Tertiary Alcohol with HBr(S[ 1) 480 Reaction of a Primary Alcohol with HBr (SN2) 480 Reaction of Alcohols with PBr3 485 (Review) Acid-Catalyzed Dehydration of an Alcohol 487 The Pinacol Rearrangement 495 Cleavage of an Ether by HBr or HI 639 Acid-Catalyzed Opening of Epoxides in Water 649 Acid-Catalyzed Opening of an Epoxide in an Alcohol Solution 650... 

Acid Form - Pseudoliquid Phase Behavior. Owing to a high affinity for polar molecules, large quantities of molecules such as alcohols and ether are absorbed within the bulk phase of crystalline heteropolyacids. The amounts of pyridine, methanol, and 2-propanol absorbed correspond to 50-100 times that which can be adsorbed on the surface, while nonpolar molecules like ethylene and benzene are adsorbed at the surface only. Catalytic reactions of polar molecules occiu both on the surface and in the bulk, so that the solid heteropolyacid behaves as a highly concentrated solution, called a pseudoliquid phase . The dehydration of alcohols, various conversions of methanol and dimethyl ether to hydrocarbons in gas-solid systems, and the alkylation of phenol and pinacol rearrangements can all occur in the pseudoliquid. The transient response using isotopically labeled 2-propanol provides evidence for the pseudoliquid phase behavior of H3PW12O40. This behavior influences the selectivity, for example, the aUcene/aUcane ratio, in the conversion of dimethyl ether. 

Preparation of dienes is accomplished by dehydration of diols or ole-finic alcohols. Pinacol, (CHjljCOHCOHfCHjlj, is converted to 2,3-di-methyl-1,3-butadiene by heating with 48% hydrobromic acid or by passing the vapors over activated alumina at 420-470°, Yields of the diene are 60% and 86%, respectively. Aniline hydrobroniide is used as a catalyst in the dehydration of 3-methyl-2,4-pentanediol to 3 methyl-l,3-penta-diene (42%). An excellent laboratory preparation of isoprene from acetone in 65% over-all yield has been described. The last step involves catalytic dehydration of dimethylvinylcatbinol over aluminum oxide at 300° to give isoprene in 88% yield. ... 

Heating 2,3-dimethyl-2,3-diol, pinacol (24), at 80°C in the presence of Nafion-H gave a 92% yield of pinacolone (methyl-tert-butyl ketone) (25) (Eqn. 22.18). Similar yields were obtained with other ditertiary vicinal diols. Reaction of pinacol with either La-HY or H-ZSM-5 or aluminum exchanged montmorillonite 2 gave both pinacolone (25) and 2,3-dimethylbutadiene (26), resulting from the dehydration of the diol, in nearly equal quantities. Only a small amount of dehydration was observed when the rearrangement was run over... 

As catalytic tests four reactions, isomerization of 1-butene and methyloxirane, dehydration of 2-propanol and the pinacol rearrangement of 2,3-dimethyl-2,3-butanediol were used. 

As with polystyrene sulfonic resins, Nafion-based acid catalysts are highly efficient for hydration and dehydration processes and, in general, for condensation reactions that occur with the formation of water or similar secondary products. Formation of ethers has been studied for various alcohols [109-111]. Dehydration of 1,4- and 1,5-diols at 135 °C affords the corresponding cyclic ethers such as 20 in excellent yields (Scheme 10.7), while 1,3-diols experience different transformations depending on their structure [112]. The dehydration of 1,2-diols mainly proceeds via the pinacol rearrangement. Further condensation of the initially formed carbonyl compound and unreacted diol affords 1,3-dioxolanes [113]. The catalyst could be efficiently reused following a reactivation protocol. Formation of aryl ethers is also possible, and the synthesis of dibenzofurans 21 (X = O) from 2,2 -dihydroxybiphenyls has been reported (Scheme 10.7) [114]. The related reaction... 

Dehydration is the first step of pinacol rearrangement of vzc-diol. Tertiary alcohols can dehydrate intramolecularly with an acid as a catalyst to form olefins, which provides another mechanism of a reverse polarity change from a polar to nonpolar state [353]. 

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