Structure of cycloalkanes

Skeletal structures of cycloalkanes imply that there is empty space in the middle of the rings. Use SpartanView to look at. space filling models of cyclohexane, cyclodecane, and Qrclooctadecane. Which, if any, rings have empty space ... 

Cis-trans isomerism (Often called geometric isomerism although this term refers to all stereoisomers) is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. Both alkenes and cycloalkanes have restricted rotation around certain bonds. In alkenes, the double bond restricts movement and rotation, as does the looped structure of cycloalkanes. 

Because of the rigid structure of cycloalkanes, stronger dispersion forces can develop between molecules. This increase of intermolecular attraction causes an increase in boiling point. 

Among the spectroscopic methods, MS is perhaps the most useful for the identification of alkanes and cycloalkanes. A simple computer search of reference spectra of thousands of compounds greatly enhances the utility of the method. The NMR method for single compounds is very helpful in structure determination for small molecules. However, the larger the saturated hydrocarbon, the less absolute information can be obtained by this spectroscopical method. Both two-dimensional NMR and NMR can furnish decisive information as to the structure of cycloalkanes. 

Petrov AA, Kasatochldn AN, Emelina EE, Nelyubina YV, Antipin MY (2009) a-Amino azoles in the synthesis of heterocycles. VI. Synthesis and structure of cycloalkane-annulated pyrazolo[l,5-a]pyrimidmes. Russ J Oig Chem 45 1390-1401... 

Next, let us consider the structures of cycloalkanes and ask if there are any differences in their stabilities. To do this, we can compare the standard enthalpy changes that occur when the compounds are formed from their elements, a quantity called the standard heat of formation, AH° Table 4.5 lists the heats of formation of cycloalkanes. 

The replacement of carbon by other elements produces changes in several structural parameters and consequently affects the conformational characteristics of the molecule. In this section, we will first describe some stereochemical features of heterocyclic analogs of cycloalkanes. For the purpose of elaborating conformational principles, the discussion will focus on six-membered rings, so that the properties may be considered in the context of a ring system possessing a limited number of low-energy conformations. 

Petroleum contains hydrocarbons other than the open-chain alkanes considered to this point. These include cycloalkanes in which 3 to 30 CH2 groups are bonded into closed rings. The structures of the two most common hydrocarbons of this type are shown in Figure 22.5 (p. 585). Cyclopentane and cyclohexane, where the bond angles are close to the ideal tetrahedral angle of 109.5°, are stable liquids with boiling points of 49°C and 81°C, respectively. 

Ring strain the instability of cycloalkanes due to their cyclic structures. => angle strain and torsional strain. 

Most interestingly, the U-carhon bond distances to the axial cycloalkane d(U-ClS) in 4 "-Uc and 4 "-Ud were determined to be 3.864 and 3.798 A, with the shortest U-carbon bond distance of 3.731 A found in the solid-state structure of complex 4 -Ue. Considering that the sum of the van der Waals radii for a U-CH2 or... 

The single most important factor that determines whether a cyclic monomer can be converted to linear polymer is the thermodynamic factor, that is, the relative stabilities of the cyclic monomer and linear polymer structure [Allcock, 1970 Sawada, 1976]. Table 7-1 shows the semiempirical enthalpy, entropy, and free-energy changes for the conversion of cycloalkanes to the corresponding linear polymer (polymethylene in all cases) [Dainton and Ivin, 1958 Finke et al. 1956]. The Ic (denoting liquid-crystalline) subscripts of AH, AS, and AG indicate that the values are those for the polymerization of liquid monomer to crystalline polymer. 

Introduction of the allene structure into cycloalkanes such as in 1,2-cyclononadiene (727) provides another approach to chiral cycloalkenes of sufficient enantiomeric stability. Although 127 has to be classified as an axial chiral compound like other C2-allenes it is included in this survey because of its obvious relation to ( )-cyclooctene as also can be seen from chemical correlations vide infra). Racemic 127 was resolved either through diastereomeric platinum complexes 143) or by ring enlargement via the dibromocarbene adduct 128 of optically active (J3)-cyclooctene (see 4.2) with methyllithium 143) — a method already used for the preparation of racemic 127. The first method afforded a product of 44 % enantiomeric purity whereas 127 obtained from ( )-cyclooctene had a rotation [a]D of 170-175°. The chirality of 127 was established by correlation with (+)(S)-( )-cyclooctene which in a stereoselective reaction with dibromocarbene afforded (—)-dibromo-trans-bicyclo[6.1 0]nonane 128) 144). Its absolute stereochemistry was determined by the Thyvoet-method as (1R, 87 ) and served as a key intermediate for the correlation with 727 ring expansion induced... 

In this section we shall consider the results recorded in the literature that pertain to the structures of the adsorbed species. Kinetic or catalytic aspects, as could be relevant to hydrogenation, hydrogenolysis, or metathesis processes, will be treated in Part 11. Spectra of the much-investigated alkenes are discussed in detail in Part I. The spectra of the other principal types of hydrocarbon adsorbates, viz. alkynes, alkanes, cycloalkanes, and aromatics, will be analyzed in Part II. Most results are available for the type-molecules ethene, ethyne, ethane, and benzene as well as for the metals, Pt, Pd, Ni, Rh, and Ru. 

Sulfur is usually the only heteroatom to be found in the naphtha fraction, and then only at trace levels in the form of mercaptans (thiols, R-SH) thiophenols (C6H5SH), sulfides (R-S-R1, alkyl sulfides, and five- or six-ring cyclic thiacyclane structures), and to a lesser extent, disulfides (R-S.S-R1)- In general, the sulfur structure distribution mimics the hydrocarbons i.e., naphthenic oils with high amounts of cycloalkanes have high thiacyclane content. 

Cycloalkanes A chain of carbon atoms can form ring structures. To name a cycloalkane, we simply add the prefix cyclo- to the parent name. Some examples of cycloalkanes are given in Figure 11.7. Cyclopropane has been used as an anesthetic gas. 

Tanaka296 found the relative rates of oxidation of cycloalkanes by Co(III) acetate in acetic acid at 90°C to decrease in the order Cs >C6 > C7-Ci2. He concluded that the rate-controlling step did not involve C—H bond rupture but, instead, formation of a complex between the alkane and Co(III). The relative reactivities were attributed to steric hindrance in the formation of the complex, the structural features of which were not elaborated further. 

Naming and Drawing Structures of Alkanes, Alkenes, and Cycloalkanes (page 161) Coached Tutorial Problem... 

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