Neopentyl alcohol dehydration

Pillai and Pines (84) found that neopentyl alcohol, mixed with 10% by weight of piperidine and passed over alumina prepared from aluminum isopropoxide, yielded 2-methyl-l-butene and 2-methyl-2-butene, in a maximum ratio of 3, and small amounts of 1,1-dimethylcyclo-propane. However, lert-pentyl alcohol yielded these two olefins in a maximum ratio of only 1.4, and none of the cyclopropane was produced (Table VI). Because of these facts a carbonium ion mechanism which is applicable to ferf-pentyl alcohol is not adequate to explain the rearrangement taking place during the dehydration of neopentyl alcohol,... 

Dehydration of Neopentyl Alcohol and terl-Pentyl Alcohols" over Alumina ... 

The dehydration of neopentyl alcohol can best be explained by a concerted mechanism involving the removal of the proton from the y-carbon atom by the basic sites and of the hydroxyl group by the acid sites of the alumina, with migration of the methyl group ... 

Siddan and Narayan also employed 7-AI2O3 and Th02 for the dehydration of a number of model alcohols and observed that if the basicity of the alumina was increased by Na -ion doping, 7-elimination was enhanced using both neopentyl and pinacolyl alcohol. It appeared that as the alumina became less acidic and more basic, there was a shift from El/E2-like behaviour to an ElcB-type mechanism, which manifested itself in a concerted 7-elimination (Scheme 10 for neopentyl alcohol A,B = acid, base sites respectively reproduced by permission from J. Catal, 1979, 59, 405). This tendency was also observed by use of erythrof f/ireo)-3-methylpentan-2-ol. 

In the case of thoria, 7-elimination was almost negligible, despite the latter s more basic nature with respect to 7-AI2O3. Again, geometric consideration led to the conclusion that dehydrations with thoria require the presence of 3-hydrogens and there is no suitable site-pairing (see Scheme 11 reproduced by permission from J. Catal, 1979, 59, 405) to allow 7-elimination with an alcohol such as neopentyl alcohol, which contains no 3-hydrogens and was unreactive on thoria. 

Write down the first formed carbonium ion that is derived from the protonation, and subsequent dehydration, of neopentyl alcohol. 

Decarboxylative dehydration 5-Hydroxy-/J,7-unsaturated cyclohexenecar-boxylic acids (1) when treated with this acetal of DMF at room temperature or somewhat higher are converted into 1,3-cyclohexadienes (2) in high yield coproducts are DMF and neopentyl alcohol. a-Methylnaphthalene (b.p. 80°/0.05... 

Cis and trans isomers of 2-methylcyclohexanol were used by Kuhlmann et al. [103] to probe whether the eliminations occurred via an Ei or E2 mechanism. That is, higher reactivity of the cis isomer could indicate the bimolecular E2 pathway because only in this isomer could the leaving group assume the required antiperiplanar conformation. Equal conversion rates would be expected for Ei reactions. Cis- and /rans-2-methylcyclohexanol were eliminated to 1-methylcyclohexanol in low yield but high selectivity (100%) in pure superheated water at 300 °C [103]. However, the treatment of the trans isomer at 270 °C yielded a mixture of methylcyclohexenes at a conversion of about 70%. Similar results were obtained for c -2-raethylcyclohexanol however, 1-methycyclohexene was more predominant than double-bond migration products. These results are still not sufficient to elucidate the mechanism or the function of water in the dehydration. Dehydration of neopentyl alcohol or pentaerythritol, concomitant with the carbon-bond migration, did not occur within 60 min at 250-300 °C. None of the alcohols examined were dehydrated to ethers. 

The different nature of the intermediate in the dehydration of primary and tertiary alcohols, is substantiated by the contrasting dehydration behaviour of neopentyl and -pentyl alcohols . The carbonium ion mechanism for the tertiary alcohol leads only to 2-methyl-1-butene (57%) and 2-methyl-2-butene (42%), but neopentyl alcohol gives different proportions of these olefins (64-73% 23-32%) along with small amounts of 1,1-dimethylcyclopropane. These latter facts are accommodated better in terms of a concerted mechanism for the dehydration of neopentyl alcohol, viz-... 

Acid-catalyzed dehydration of neopentyl alcohol, (CH3)3CCH20H, yields 2-methyl-2-butene as the major product. Outline a mechanism showing all steps in its formation. PRACTICE PROBLEM 7.14... 

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. 

Since the pyrolysis was carried out under a dynamic vacuum, the organic products formed were removed quickly thus preventing secondary reactions. In control experiments, the product alcohols were passed over the residual Ti02 under pyrolytic conditions. With the exception of neopentyl alcohol, no significant dehydration was observed. Thus, our study differs from that reported by earlier by Bradley [3d]. Since the latter was done under a static vacuum, the alcohol formed underwent dehydration to generate water which, in turn, caused hyrolysis of the M-OR bonds. As a result, an autocatalytic decomposition of the metal alkoxide ensued. 

From the similarity of the activation energy of dehydration of the aliphatic alcohols and of some of the cyclohexanols it can be assumed that the mechanism of the dehydration of the two groups of alcohols is identical. The presence of a neopentyl-type carbon atom as in 2,2-dimethylcyclohexanol diminishes the reactivity only slightly, but among stereoisomeric alkylcyclohexanols the cis isomer reacts much faster than the trans. The distinct dissimilarity in activation energies of the two... 

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