Dimethyl ether, model structure

C09-0078. Write the Lewis structure of dimethyl ether, (CH3)2 O. Draw a ball-and-stick model of this molecule, showing it as a water molecule with each hydrogen atom replaced by a CH3 group. 

Figure 1.1 Ball-and-stick models and structural formulas for ethyl alcohol and dimethyl ether...

The infancy of these first-principles methods as applied to periodic zeolite lattices means that further detailed work is necessary, particularly in the area of verification of the ability of the pseudopotential to reproduce dynamic as well as static structural properties. However, the results found with these methods demonstrate that the debate concerning the modeling of the activation of methanol within a zeolite is far from concluded. The proton transfer to methanol as a reaction in its own right is, however, of relatively little interest. It does not govern the pathway or energetics of reactions such as dehydration to give dimethyl ether (DME). These are governed instead by the individual transition states that lead to the products, as we discuss in the next section. 

The rate constants for the reactions between OH and a range of ethers and hydroxy ethers have been reported at 298 K233 as well as those for reactions between dimethyl ether and methyl f-butyl ether over the range 295-750 K.234 Data from the former study show deviations from simple structure-activity relationships which were postulated to arise due to H-bonding in the reaction transition states.233 The atmospheric lifetime of methyl ethyl ether has been determined to be approximately 2 days.235 Theoretical studies on the H-abstraction from propan-2-ol (a model for deoxyribose) by OH have been reported using ab initio methods (MP2/6-31G ).236 The temperature dependence (233-272 K) of the rate coefficients for the reaction of OH with methyl, ethyl, n-propyl, n-butyl, and f-butyl formate has been measured and structure-activity... 

FIGURE 7.24 Structure of dimethyl ether, (a) Lewis diagram, (b) Ball-and-stick model. 

Also used as a spectral comparison model for the elucidation of the B and G aflato-xinsss this compound was prepared in a multi-step synthesis. The von Pechmann condensation of phloroglucinol dimethyl ether (59)133 with diethyl cyclopentane-4,5-dione-l, 3-dicarboxylate (41) in acidic solution afforded the /J-ketoester (42), which readily underwent decarboalkoxylation, in a seperate step, to give the keto-coumarin (5). The beauty of this methodology is illustrated by the use of the symmetrical diketoester (41), which of course, only allows for the formation of a single coumarin (von Pechmann) product (42). The regiochemistry of the final product, however, was demonstrated to be the incorrect isomer insofar as the aflatoxin structures were concerned. 

This difference in molecular structure gives rise to a difference in properties it is the difference iti properties which tells us that we. are dealing with different compounds. In some casesj the difference in structure—and hence the difference in properties—is so marked that the isomers are assigned to different chemical families, as, for example, ethyl alcohol and dimethyl ether. In other cases the difference in structure is so subtle that it can be described only in terms of three-dimensional models. Other kinds of isomerism fall between these two extremes. 

The fourth area in which we have introduced students to computational chemistry at the first year level is in the field of molecular model building and molecular mechanics. We provide exercises in which the student is required to build and optimize the molecular structure of water and of dimethyl ether using the MM2 force field. Students compare the bond angles around the O atom in the two cases and examine wire frame, ball and stick, and CPK models of the molecules. They also explore the stereoisomers of carvone. One of these stereoisomers has dill scent and the other spearmint. Samples of the two isomers are available in the laboratory where the students do the molecular modeling. 

The two isomers of Example 1.1 are quite different. Ethyl alcohol (grain alcohol) is a liquid at room temperature, whereas dimethyl ether is a gas. As we ve seen before, the structural differences exert a significant influence on properties. From this example, we can see that molecular formulas such as C2H5O provide much less information about a compound than do structural formulas. > Figure 1.6 shows ball-and-stick models of these two molecules. 

In the model system for (/ ,5)-2 in a polar solvent with a coordinating dimethyl ether molecule at the lithium center (Epimerization 2 in Scheme 1), the energy difference between MIN-7 and TS-4 is smaller by 25 kJ/mol. Furthermore, in contrast to the nonpolar case described, there is an energy difference of 9 kJ/mol between the two diastereomeric minimum structures MIN-7 and MIN-8. The conclusion from these studies is that from ether solution, a highly diastereomerically enriched (,R,S)-2 should be obtained by epimerization at room temperature, while a diastereotopos-differentiating deprotonation without noticeable epimerization should be possible in nonpolar solution (under the reaction conditions described). 

The most generally useful data for structure elucidation have been provided by very comprehensive studies of the proton magnetic resonance spectra of the natural arylphenalenones and the related metabolites, together with a large number of synthetic model compounds. As shown for structures (54) and (55) the H-NMR spectra (6 values in CDCI3) easily distinguish between the dimethyl ethers obtained from 2,6-di-hydroxy-5-methoxy-9-phenyl-1 H-phenalen-1 -one (haemocorin aglycone). 

Computational methods can also be used to describe enolate structure. Most of the structural features of enolates are correctly modeled by B3LYP computations with dimethyl ether as the solvent molecule. Although semiemprrical PM3 calculations give adequate representations of the geometries of the aggregates, the energy values are not accurate. 

DMF, see Dimethylformamide DM SO, see Dimethyl sulfoxide DMT (dimethoxytrilyl ether), DNA synthesis and, 1114 DNA, see Deoxyribonucleic acid DNA fingerprinting, 1118-1119 reliability of, 1119 STR loci and, 1118 Dopamine, molecular model of. 930 Double bond, electronic structure of, 16... 

We report on the reaction of 2,2-dimethyl-1,3-propanediol catalyzed by various solid acids in the gas phase at temperatures > 250 °C. Originally, we tried to synthesize four membered cyclic ethers since several oxetanes are of synthetic interest [7,8], E g. 3-hydroxy-oxetane can undergo ring opening polymerization, leading to a water soluble polymer Since 3-hydroxy-oxetane is not very stable, we choose 2,2-dimethyl-1,3-propanediol as model substrate. In this communication, we describe the effect of catalyst structure (various zeolites. 

It was the purpose of this investigation to synthesize suitable monomers which could conceivably undergo cyclopolymerization or cyclocopolymerization to lead to large ring-containing polymers. A more specific purpose was to synthesize polymers which would contain ring structures in the polymer backbone which may simulate the properties of the crown ethers (2). Thus a variety of monomers, including l,2-bis(ethenyloxy)benzene, l,2-bis(eth-enyloxy)-4-methylbenzene, 1,2-bis(2-ethenyloxyethoxy)benzene and l,t-butyl-3,4-bis(ethenyloxyethoxy)benzene, as well as model compounds 2,3-dimethyl-1,4-benzodioxane and cis- and trans-2,4-dimethyl -1,5-benzodioxepane, were synthesized and studied. This paper deals with cyclopolymerization studies of the four monomers. 

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