Dehydration of 4-methyl-2-pentanol

The acidotalyzed dehydration of 4-methyl-2-pentanol (22), which is one of the experiments in this section, is shown in Equation 10.12 and nicely illustrates the preceding principles. Dehydration of 22 produces a complex mixture of isomeric alkenes, including 4-methyl-l-pentene (23), frans-4-methyl-2-pentene (24), cis-4-methyl-2-pentene (25), 2-methyl-2-pentene (26), and 2-methyl-l-pentene (27). 

GLC trace of the product mixture from the dehydration of 4-methyl-2-pentanol. Assignments and peak areas (in parentheses) peak 1 diethyl ether (solvent for analysis) peak 2 4-methyl-l-pentene (3030) peak 3 cis- and trans-4-methyl-2-pentene (51693) peak 4 2-meihyl-l-pentene (2282) peak 5 2-meihyl-2-pentene (9733). Column and conditions 0.25-mm X 30-m APT-Hep-Tex 37 C, 35 mL/min. 

In principle, the equilibrium in the dehydration of an alcohol could be shifted to the right by removal of water. Why is this tactic not a good option for the dehydration of 4-methyl-2-pentanol and cyclohexanol ... 

Give a detailed mechanism explaining the formation of 23-25 from the dehydration of 4-methyl-2-pentanol (22). Use curved arrows to symbolize the flow of electrons. 

Near the end of the dehydration of 4-methyl-2-pentanol, a white solid may precipitate from the reaction mixture. What is the solid likely to be ... 

Alcohols undergo dehydration to form 1-alkenes. The formation of thermodynamically unstable 1-olefins contrasts with the formation of stable 2-olefins observed in the dehydration over acidic catalysts. The results of dehydration of 4-methyl-2-pentanol... 

Table 3.4 Dehydration of 4-methyl-2-pentanol and other oxides...

As described above, silica gel is very weakly acidic. The basicity is also small. Thus, the activity of silica gel for dehydration of 4-methyl —2-pentanol is about 20 times lower than alumina. Though both the acidic and basic strength of silica gel is weak, this is sometimes profitable as cattilysts for many organic rjeactions. The reactants can be activated (or polarized), and extensive side reactions avoided. The use... 

The variations of the activity for decomposition of diacetone alcohol, dehydration of 4-methyl-2-pentanol, and alkylation of phenol are shown as a function of the composition of the binary oxide in Fig. 3.52. The catalytic activity for decomposition of diacetone alcohol correlates with the number of base sites, while that for the dehydration of 4-methyl-2-pentanol correlates with the number of acid sites. The alkylation of phenol with methanol is most effectively catalyzed by a binary oxide possessing both acid and base sites, the oxide containing Ti02 and MgO in a 1 1 ratio being the most active. 

Discuss the differences observed in the IR and NMR spectra of 4-methyl-2-pentanol and the various methylpentenes that are consistent with dehydration occurring in this experiment. 

Compare and contrast the major products of dehydrohalogena-tion of 2-chloro-4-methylpentane with (a) sodium ethoxide in ethanol and (b) potassium ferf-butoxide in 2-methyl-2-propanol (fert-butyl alcohol). Write the mechanism of each process. Next consider the reaction of 4-methyl-2-pentanol with concentrated H2SO4 at 130°C, and compare its product(s) and the mechanism of its (their) formation with those from the dehydrohalogenations in (a) and (b). (Hint The dehydration gives as its major product a molecule that is not observed in the dehydrohalogenations.)... 

Rare Earth Oxides. Rare earth oxides such as La203 act as solid bases (2,9,15). The catalytic activities of rare earth oxides depend on their pretreatment temperature and the maximum activities of La203 for 1-butene isomerization, H-D exchange between CH4 and D2, and hydrogenation of 1,3-butadiene appear at a pretreatment temperature of923 K. Rare earth oxides show unique selectivity in dehydration of alcohols. 1-Alkenes are selectively produced from 2-alkanols such as 4-methyl-2-pentanol. 

A number of unexplained factors warrant mention. Orientation of elimination differs for secondary and tertiary structures. The peculiar predominance of cis- rather than /ra/ii-olefin may arise from the relative stabilities of the proton-olefin complexes. but a more certain conclusion would be possible if the stereochemistry of the dehydration in the acyclic series had been determined. Assumption of the anti stereospecificity known to be favoured by the cyclohexyl systems may be unsound especially in the light of the recent stereochemical findings in base-catalysed elimination reactions (Section 2..1.1(e)). The solution of the problem of the cis/trans ratios may lie in the duality of mechanism, namely the syn-clinallanti complexity. Certainly recent results on the dehydration of threo- and eo t/iro-2-methyl-4-deutero-3-pentanols on thoria show syn-clinal rather than anti stereospecificity as indicated by deuterium analysis of the cis- and /rn/iJ-4-methyl-2-pentenes, but in these cases the trans isomer was formed in a three-fold excess over the m-olefin . Of course, the dehydration reactions on the less acidic thoria may not be good models for alumina but a knowledge of stereochemistry in the acyclic series might prove an invaluable aid in the elucidation of the mechanism. There is obviously plenty of scope for future kinetic investigations which at the moment sadly lag behind preparative studies. 

Molybdenum oxide is also an El-type oxide. The dehydration of 2,2-dimethyl —3-pentanol over M0O3 at 453 K gives primarily products requiring methyl migration, 2,3-dimethylpent-1-ene, 2,3-dimethylpent-2-ene and 3,4-dimethylpent-2-ene. On the other hand, only jS-elimination products, cis- and /raw-4,4-dimethylpent-1-ene are obtained by the dehydration over alumina at the same temperature. ... 

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