Dehydration 4-methyl-2-pentanol

Dehydration of 1-pentanol or 2-pentanol to the corresponding olefins has been accompHshed, in high purity and yields, by vapor-phase heterogeneous catalyzed processes using a variety of catalysts including neutral gamma —Al Og catalyst doped with an alkah metal (23), zinc aluminate (24,25), hthiated clays (26), Ca2(P0 2 montmorillonite clays (28). Dehydration of 2-methyl-1-butanol occurs over zinc aluminate catalyst at... 

C to give the expected 2-methyl-1-butene in high selectivites (24). The AI2O2 catalyzed process can be optimized to give di- -pentyl ether as the exclusive product (23). Dehydration of 1-pentanol over an alkah metal promoted AI2O2 catalyst at 300—350°C provides 1-pentene at selectivities of 92% (29,30). Purification produces polymerization grade (99.9% purity) 1-pentene. A flow chart has been shown for a pilot-plant process (29). 

Solution 2-Methyl-3-pentanol, an open-chain alcohol, has M+ = 102 and might be expected to fragment by o- cleavage and by dehydration. These processes would lead to fragment ions of m/z = 84, 73, and 59. Of the three expected fragments, dehydration is not observed (no m/z = 84 peak), but both a clea iges take place (m/z = 73, 59). 

Write the structure of the three olefins produced by the dehydration of 3-methyl-3-pentanol. 

Few detailed studies of olefinic dehydration products have been reported. Rapoport ei al. (105) found cis- and major products from n-pentanol dehydration over NaX only traces of 2-methyl-1-butene and 2-methyl-2-butene were reported. Thus, it is evident that double bond isomerization has accompanied or followed the dehydration reaction. Several authors (99,101,106) have suggested that diffusion processes may be rate controlling, or at least of some significance, in zeolite-catalyzed dehydration reactions. Molecular-shape selective alcohol dehydration (7) was discussed earlier. 

Pentene. p-e-Amylene sym-methylethylethyl -ene. C,H mol wt 70.13. C 85.63%, H 14.37%. CH3CH2-CH=CHCHj. Prepn of mixture of cis and trims isomers by dehydration of 2-pentanol Norris, Org. Syn. coll, vol. I, (2nd ed., 1941) p 430. Prepn of the geometrical isomers from cis- and matts-a-methyl-d-ethylacrylic acids Lucas, Prater, J. Am. Chem. Soc. S9, 1682 (1937). Prepn of the cis form from 1-ethyl-2-iodobutyric acid with quinoline, of the trans form from I-ethyl-2-iodobutyric acid with sodium carbonate Sherrill, Matlak, ibid. 2134 prepn of both isomers from sec-amyl alcohol with HjSO( and diatomaceous earth at 90-110" for 3 hrs Lucas et al, ibid. 63, 22 (1941). Absorption spectra Carr, Stridden, ibid. 59, 2138 (1937). 

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. 

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. 

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 ... 

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. 

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. 

Typic2il examples of acid-catalysis of heteropoly compounds are as follows Dehydration of methanol, - > ethanol, - - propanol - - - -"- "- > and butanol, conversion of metanol or dimethyl ether to hydrocarbons, etheration to form methyl /-butyl ether, esterifications of acetic acid by ethanol and pentanol, decomposition of carboxylic acid and formic acid, alkylation of benzene by ethylene and isomerization of butene, o-xylene and hexane. ... 

Dehydration of alcohols is catalyzed by boron phosphorous oxide. The reactivity of alcohols in the dehydration decreases as follows /ert-amyl alcohol > 3-pentanol > > 2-propanol > 1-pentanol > ethanol.The catalytic activities of the oxides composed of different amounts of B and P for propanol dehydration correlate with the total amount of acid sites.Butanol undergoes dehydration on boron phosphorous oxide. The maximum activity is obtained at the P/B ratio of 0.6. The activity correlates with the sum of Lewis and Bronsted acid sites. 2-Butanol, 2-methyl — 2-butanol, and 3-methyl — 2-butanol also undergo dehydration. The formation of tnw -2-butene and 2-methyl — 2-butene increases with increasing surface acidity. In these reactions, the carbenium ion mechanism is operating. 

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. ... 

The dehydration of three hexanol isomers, 1-hexanol, 2-hexanol and 2-methyl — 2-pentanol over LazOs and ThOa was studied. Though the product selectivity clearly indicated the ElcB mechanism, the order of reactivity was found to be primary < secondary < tertiary alcohol, as in the 1 and 2 mechanism, indicating that the rupture of C — O bond is the rate-determining step. This result is in conformity with the reversibility between the reactant alcohol and the carbanion, as evidenced by H —D exchange experiments. 

Acid-catalyzed dehydration of 2-methyl-2-pentanol (dilute H2SO4, 50°C) gives one major product and one minor product. Elemental analysis reveals that both contain only carbon and hydrogen in a 1 2 atom ratio, and high-resolution MS gives a mass of 84.0940 for the molecular ions of both compounds. Spectroscopic data for these substances are as follows ... 

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.)... 

Two aUtenes are produced in the dehydration of 2-methyl-2-butanol, but three are produced in the dehydration of 2-pentanol. Write the structures of the products of both reactions. 

---