Cycloalkanes shape

In this chapter we explored the three dimensional shapes of alkanes and cycloalkanes The most important point to be taken from the chapter is that a molecule adopts the shape that minimizes its total strain The sources of strain m alkanes and cycloalkanes are... 

Section 16 2 The oxygen atom m an ether or epoxide affects the shape of the mole cule in much the same way as an sp hybridized carbon of an alkane or cycloalkane... 

The Baeyer strain theory is useful to us in identifying angle strain as a destabilizing effect. Its fundfflnental flaw is its assumption that the rings of cycloalkanes are planar-. With the exception of cyclopropane, cycloalkanes are nonplanar. Sections 3.5-3.13 describe the shapes of cycloalkanes. We ll begin with cyclopropane. 

We JI look in the next chapter at cycloalkanes—saturated cyclic hydrocarbons— and we ll see that the molecules generally adopt puckered, nonplanar conformations. Cyclohexane, for instance, has a puckered shape like a lounge chair rather than a flat shape. Why ... 

An alkane is a hydrocarbon that has only single bonds. Alkanes that do not contain rings have the formula C H2n + 2 An alkane in the shape of a ring is called a cycloalkane. Cycloalkanes have the formula CnH2n- An alkene is a compound that has at least one double bond. Straight-chain alkenes with one double bond have the same formula as cycloalkanes, Cj,H22,. 

In the early studies, the cycloalkane holes were viewed as molecular radical cations that undergo rapid resonant charge transfer. At any given time, the positive charge was assumed to reside on a single solvent molecule and, once in 0.5-2 psec, to hop to a neighboring molecule. The low activation energy was explained by the similarity between the shapes of cycloalkane molecules and their radical cations [60]. 

Alkanes have similar chemical properties, hut their physical properties vary with molecular weight and the shape of the molecule. The low polarity of all the bonds in alkanes means that the only intermolecular forces between molecules of alkanes are the weak dipole-dipole forces (see 2.5.1), which are easily overcome. As a result, compared with other functional groups, alkanes have low melting and boihng points, and low solubility in polar solvents, e.g. water, but high solubility in nonpolar solvents, e.g. hexane and dichloromethane. Most cycloalkanes also have low polarity. 

Alkanes and cycloalkanes IUPAC, shapes, properties, reactions 11.6-11.11... 

Aromatic hydrocarbons are nonpolar, and their physical properties resemble those of alkanes of similar molecular mass. However, as was the case with cycloalkanes, the symmetrical shapes of many aromatic hydrocarbons often result in higher melting points. For example, the melting and boiling points of benzene are nearly identical to those of cyclohexane. (Recall that cyclohexane melts at considerably higher temperatures than does hexane.) As expected, a mixture of benzene and water forms two layers, with benzene as the upper layer. 

Most cycloalkanes resemble the acyclic (noncyclic), open-chain alkanes in their physical properties and in their chemistry. They are nonpolar, relatively inert compounds with boiling points and melting points that depend on their molecular weights. The cycloalkanes are held in a more compact cyclic shape, so their physical properties are similar to those of the compact, branched alkanes. The physical properties of some common cycloalkanes are listed in Table 3-4. 

Density functional theory (DFT) studies on [A]chalcogena[iV]pericyclynes (n = 0-3, 5) demonstrate their relative stability and hence their possible existence as stable species. By minimizing repulsive interactions between the chalcogens lone pairs, the molecules adopt structures that resemble, in shape, cycloalkanes or elemental chalcogens. [3]Chalcogena[3]pericyclynes may be interconverted with their valence tautomers, benzene derivatives with three fused three-membered rings <20040L589>. 

In contrast to the radical cations of strained-ring cycloalkanes, the cyclopentane radical cation, c-CsHio , formed by electron transfer to radiolysis-induced holes in halocarbon matrices, had a simpler spectrum. A triplet with uh = 2.5 mT (2H) was attributed to a localized species with Cj symmetry. The unpaired electron was assigned to a W-shaped cr-orbital, involving C5-C1-C2, and the two equatorial protons at C5 and C2 [80, 88, 89]. At temperatures above 77 K, all ring protons become equivalent, most probably as a result of processes such as ring inversion, or pseudo-rotation around the C5-axis [89]. 

RuFiePc in NaX warrants corroborating evidence for zeolite inclusion, such as shape selective catalysis. Results for the o dation of cycloalkanes presented below support our conclusions. 

It is fairly apparent that encapsulation of the RuFiePc complex in NaX dramatically alters the catalytic activity and selectivity, however, that in itself is not evidence for the intrazeolite location of the complex. Therefore, we examined the oxidation of the much larger cyclododecane using the same reaction conditions as for cyclohexane. We found the homogeneous RuFisPc catalyst had virtually no preference for either cycloalkane, showing approximately the same number of turnovers per day. In contrast, the RuFiePc-NaX catalyst exhibited relatively low activity ( 300 tumovers/day) for the larger cyclododecane. The acti dty of the zeolite encapsulated complex was nearly 10 times greater for the smaller cyclohexane. This shape selectivity is consistent with the active sites located inside the zeolite. 

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