Tert-butyl methyl ether, protonated

Studies on the use of proton-exchanged zeolites have also been conducted.12-14 Since tert-butyl alcohol and tert-butyl methyl ether (MTBE) are important gasoline octane enhancers, their synthesis by the addition of water and methanol to isobutylene attracted special attention. Among other processes, clay-catalyzed additions have been studied.15... 

The protonated sulfides are less susceptible to cleavage than the corresponding protonated ethers and also more stable than the protonated thiols.149 Protonated tert-butyl methyl ether is completely cleaved to tot-butyl cation and protonated methanol even at 70°C. On the other hand, protonated tert-butyl methyl sulfide 60 is stable even at —60°C. When the temperature is increased to — 15°C, protonated fert-butyl methyl sulfide very slowly cleaves to tert- butyl cation and protonated methyl thiol149 [Eq. (4.39)]. 

An NMR spectrum plots the intensity of a peak against its chemical shift measured in parts per million (ppm). The common scale of chemical shifts is called the 5 (delta) scale. The proton NMR spectrum of tert-butyl methyl ether [CH30C(CH3)3] illustrates several important features ... 

Figure 13.2. Positive ion APCI mass spectrum of the red carotenoid lycopene in a solution of methanol and tert-butyl methyl ether (1 1 v/v). In this analysis, lycopene formed a protonated molecule instead of a molecular ion.

The properties of solvents has been studied extensively by Snyder (5), who created a dassification of the solvent properties of common solvents. It has been found (7) that (excluding proton donors such as alcohols) the maximum difference in mobile-phase selectivity is obtained if the polar solvents have a large difference in basidty. Thus, for maximum selectivity differences, one solvent should have a low basidty. Solvents of this type are acetonitrile, ethyl acetate or other esters, and acetone or other ketones. The other solvent should have a high basidty examples are ethers such as tert-butyl methyl ether, diethyl ether or tetrahydrofuran, or amines such as triethylamine. Between these groups and alcohols, large differences in chromatographic selectivity can be obtained in normal-phase chromatography (10). 

Here, the reaction between methyl iodide and the anion of 2-methyl-2-propanol (terr-butoxide) to form tm-butyl methyl ether is used to demonstrate the process in which a methyl group has been substituted for the proton originally on the oxygen of 2-methyl-2-propanol (tert-butanol) (Equation 8.21). The example of Equation 8.21 is also used to emphasize that the ether (rm-butyl methyl ether) cannot be prepared from the reaction between methoxide anion and 2-iodo-2-methylpropane (or indeed any 2-halo-2-methylpropane) because elinnnation would intrude (Equation 8.22). 

The photodecomposition of isopropyl alcohol on silica gel produces a seven-line spectrum having a hyperfine separation of 20.7 G and an amplitude ratio of 1 6.7 20.2 31 21.1 7.4 1.5 (68). This spectrum was attributed to SiOCMe2 formed from the ether surface groups. In addition to this spectrum the spectrum of the methyl radical was also observed. Irradiation of adsorbed tert-butyl alcohol produced a three-line spectrum which was attributed to SiOMe2OCH2 (68). Apparently the splitting from the methyl protons was too small to be observed. 

Solvents selected were similar to the solvents that Glajch et al. [35] used for Normal Phase Liquid Chromatography. Methyl tert- butyl ether (a proton acceptor) was selected instead of ethyl ether, since the former one is less volatile. The other two selected solvents were methylene chloride (dipole interactions) and chloroform (proton donor). These three solvents meet all practical requirements. The polarity P [21] of the solvents is 2.5, 3.1 and 4.1, respectively. The solvents were used in pure form no supporting solvent was used. 

Methyl tert-butyl ether has two types of protons, giving two NMR signals. 

Protons in identical chemical environments with the same shielding have the same chemical shift. Such protons are said to be chemically equivalent. This is what is meant whenever we use the term equivalent in discussing NMR spectroscopy. In methyl tert-butyl ether, the three methoxy protons are chemically equivalent, and the nine tert-butyl protons are chemically equivalent. 

Reaction of estrone methyl ether with 2,2-dimethylpropane-l, 3-diol in the presence of a catalytic amount of acid leads to derivative 26-1, in which the ketone at 17 is protected as an acetal (Scheme 3.26). Treatment of this intermediate with pyridinium chlorochromate leads to oxidation of the Cg benzylic carbon atom to a carbonyl group (26-2). Potassium tert-butoxide abstracts a proton from the adjacent methylene at C7 alkylation of the resulting anion with 4-(A, A -dimethyl)butyl iodide gives 26-3 as a mixture of diastereomers. The carbonyl group is next reduced to an alcohol by means of sodium borohydride (26-4). Dehydration of the newly introduced hydroxyl group is arguably facilitated by the adjacent aromatic ring (26-5). Aqueous acid removes the 17-acetal to afford 26-6, which is in essence an equilinin derivative. 

Finally, a word of caution when using [BF4] and [PF ]" ionic liquids - they are not stable and give off HF, particularly when heated in the presence of a proton source or a metal salt [21]. There are many examples of this in this chapter. An example of a HF-catalyzed reaction is ether formation from alcohols is a classic acid-catalyzed reaction. An ether formation reaction was found to occur in a range of [BF4] ionic liquids, with an example being the addition of methanol to ten-butanol to form methyl-tert-butyl ether (MTBE) [306]. The author is of the opinion that [Bp4] ionic liquids (even hydrophobic ones) can dehydrate alcohols to ether and refers to these ionic liquids as dehydrators. All that is happening here is a simple HF-catalyzed reaction. With many authors not aware of this phenomenon, they resort to all kinds of inappropriate explanations for what is occurring. 

In all the above processes, the corresponding symmetrical ethers are coproduced by the reaction of product alcohol with the carbonium ion. Ion-exchange resins protonate iso-butene in methanol (at below 100° C) to produce methyl tert-butyl ether (MTBE). 

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