Dimethylpropane, model

FIGURE 2 13 Tube (top) and space filling bottom) models of (a) pentane (b) 2 methylbutane and (c) 2 2 dimethylpropane The most branched isomer 2 2 dimethylpropane has the most compact most spherical three dimensional shape... 

A completely different dipolar cycloaddition model has been proposed39 in order to rationalize the stereochemical outcome of the addition of doubly deprotonated carboxylic acids to aldehydes, which is known as the Ivanov reaction. In the irreversible reaction of phenylacetic acid with 2,2-dimethylpropanal, metal chelation is completely unfavorable. Thus simple diastereoselectivity in favor of u f/-adducts is extremely low when chelating cations, e.g., Zn2 + or Mg- +, are used. Amazingly, the most naked dianions provide the highest anti/syn ratios as indicated by the results obtained with the potassium salt in the presence of a crown ether. 

All three have five carbon atoms and 12 hydrogen atoms, so they have the molecular formula C5H12. However, as you can see, these models represent three different arrangements of atoms, pentane, 2-methylbutane, and 2,2-dimethylpropane. These three compounds are isomers. Isomers are two or more compounds that have the same molecular formula but different molecular structures. Note that cyclopentane and pentane are not isomers because cyclopentane s molecular formula is C5H10. 

Different model reactions were used in order to study the interaction between the modifier and the parent metal. It was observed that an inert additive introduced by a redox reaction generally poisons, more or less, the activity of the parent metal or strongly modifies the selectivity of the reaction, which indicates a deposition of the additive on the parent metal. For example, a decrease in activity for structure insensitive reactions, such as toluene hydrogenation [41] or cyclohexane dehydrogenation [43, 78] proves the existence of bimetallic nanoparticles. Likewise, in the case of the 2,2-dimethylpropane reaction, the modification of both the selectivity and the apparent activation energy, demonstrates an interaction between Pd and Au introduced by direct redox reaction. Conversely, no modification was observed on the catalysts prepared by incipient wetness co-impregnation [75]. 

While with linear and singly-branched alkanes there is clear but not extensive evidence that on platinum catalysts the intermediates for isomerisation and for hydrogenolysis differ in their extents of dehydrogenation, with doubly-branched alkanes as exemplified by ncopentane (2,2-dimethylpropane) the situation appears not the same. In an extensive review of Arrhenius parameters for its reactions, activation energies for the two reactions were found to be of the same order, - as were orders of reaction (for Pt/KL and Pt/KY zeolites - ). On EUROPT-1 and on oriented model platinum catalysts, activation energies for total reaction increased markedly with hydrogen pressure, as indeed they should. The two reaction paths thus seem to go via the same intermediate, which might be the ap-diadsorbed species. 

Draw the tetra ethyl analog of 2,2-dimethylpropane as discussed in A Word About... Isomers—Possible and Impossible on page 55. What is the correct lUPAC name for this compound Make models of this compound and 2,2-dimethylpropane and compare how sterically congested they are. 

Ewing, M. E. B., Goodwin, A. R. H., McGlashan, M. L. Trusler, J. P. M. (1987). Thermophysical properties of alkanes from speeds of sound determined using a sphericd resonator I. Apparatus, acoustic model, and results for dimethylpropane. 

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