1-methylcyclohexanol, dehydration

Methylcyclohexanol, dehydration of, 183 1-Methylcyclopentene addition of hydrogen chloride, 215 hydroboration-oxidation, 230-233 Methylenecyclohexane, 677 Methylene group, 57 prefix, 170... 

Assume you are carrying out the dehydration of 1-methylcyclohexanol to yield L-methylcyclohexene. How could you use infrared spectroscopy to determine when the reaction is complete ... 

Tire acid-catalyzed dehydration of 1-methylcyclohexanol yields a mixture o( two alkenes. How could you use 1H NMR to help you decide which was which ... 

Because of the usefulness of the reaction, a number of ways have been devised for carrying out dehydrations. One method that works particularly well for tertiary alcohols is the acid-catalyzed reaction discussed in Section 7.1. For example, treatment of 1-methylcyclohexanol with warm aqueous sulfuric acid in a solvent such as tetrahydrofuran results in loss of water and formation of 1-methylcydohexene. 

The geometry of the cis-alkylcyclohexanol is favorable for trans elimination since the hydroxyl and the neighboring trans hydrogen are coplanar, but this is not true for the l,i-trans isomer hence the molecular conformation has to flip over, to set the hydroxyl group in the axial position for the trans elimination to occur. This would require a few kilocalories of energy and for frans-lert-butylcyclohexanol it would be more difficult to achieve than for IroMs-methylcyclohexanol. It is, therefore, possible that the trans elimination from a boat conformation, or possibly even an epimerization from the trans to the cis isomer which then undergoes a trans elimination reaction. Such an epimerization was found to occur under conditions of dehydration of certain alcohols over alumina, as will be seen under 1,4-cyclohexanediol. The more facile elimination of the cis-i-tert-butylcyclohexanol system as compared with the trans system in solution was also reported in the literature 63). 

Methylcyclohexene is easily synthesized by the dehydration of 1-methylcyclohexanol. The most substituted alkene is the desired product. 

Dehydration is often carried out by treatment of an alcohol with a strong acid. For example, loss of water occurs and 1-methylcyclohexene is formed when l-methylcyclohexanol is wanned with aqueous sulfuric acid in tetra-hydrofuran (THF) solvent ... 

From an industrial point of view, gas-phase reactions are often preferred due to their ease of operations. The reduction of 4-methylcyclohexanone and the oxidation of cis- and trans-4-methylcyclohexanol over Ti-beta and Al-beta in the gas phase were studied at 100°C. As can be seen from Fig. 3, both Ti-beta and Al-beta are active, but Ti-beta has a considerably lower rate of deactivation. The deactivation of Al-beta is probably caused by the higher acidic strength of the protonic aluminium site compared to the non-protic titanium site. Another difference between the two catalysts is the pronounced dehydration of the alcohols formed over Al-beta. Two important differences between the gas-phase and liquid-phase reaction were observed. The most striking is the selectivity towards the cis-alcohol. Under liquid phase conditions (Table 1.) a selectivity of 99 % towards the cis-alcohol was observed, while in the gas-phase over Ti-beta only 53 - 62 % cis-alcohol was obtained. 

Zeolites are also excellent catalysts in the selective dehydration of tertiary alcohols. Shape selectivity, i. e. selective dehydration of 1-butanol to 1-butene over CaA zeolite in the presence of 2-butanol has also been reported [36]. The bulky 2-methylcyclohexanol, in turn, undergoes dehydration over zeolite Y, although resulting in a mixture of alkenes [37]. 

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 most common way to carry out the reaction is by heating the alcohol in the presence of a catalytic amount of a strong mineral acid such as sulfuric acid or phosphoric acid. The dehydration of 2-methylcyclohexanol is illustrated in... 

When r< m-2-methylcyclohexanol (see the following reaction) is subjected to acid-catalyzed dehydration, the major product is 1 -methylcyclohexene ... 

Outline a mechanism for the dehydration of 4-methylcyclohexanol catalyzed by phosphoric acid. 

Obtain a sample of another alcohol such as 2-methylcyclohexanol, which is typically available as a mixture of cis- and frans-isomers, from your instructor. Perform the acid-catalyzed dehydration of 2-methylcyclohexanol according to the procedure in Part B. Calculate your yield and analyze the products by capillary GLC to identify and to determine the ratio of the three possible alkenes. Consult with your instructor before undertaking any experiments. 

Problem 9.30. As pointed out in Scheme 9.74, methylenecyclohexane is the minor product of the dehydration of 1-methylcyclohexanol while 1-methylcyclohexene is the major product. Propose a synthesis of the former in which the latter might not be found at aU. 

Dehydration of alcohols proceeds over aluminum phosphorous oxides. Correlation between 1-butanol dehydration activity and amount of acid sites stronger than Ho - 1.5 has been reported. The dehydration of cis- and phosphorous oxides of different Al/P ratios. While the cis isomer is converted more extensively than the irans with all compositions, the relative amount of the two olefinic products (l-/3-methylcyclohexene) increases markedly on increase in the amount of P. The formation of 3-methylcyclohexene from either alcohol takes place on strong acid sites by the 1 process in which carbenium intermediates are involved. On the other hand, the formation of 1-mediylcydohexene takes place on pairs of acid and base sites by the 2 process. The aluminum atoms and aluminum atoms with hydroxy] groups attached function as Lewis add sites and Bransted acid sites, respectively. 

Draw the structures of the two dehydration products of 1-methylcyclohexanol and describe how infrared spectroscopy can be used to establish their structures. 

Problsm 9.22 Predict the major product formed in the dehydration of each of the following alcohols, (a) 1-methylcyclohexanol (b) 3-ethyl-2-pentanol (c) 1-isopropylcyclohexanol... 

Dehydration of ch-2-methylcyclohexanol yields two products in a 5 1 ratio. What are the structures of the two products ... 

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