Cycloalkane stability

Considering the role of torsional interactions in determining cycloalkane stability, predict the order of the equihbrium constants for... 

Higher cycloalkanes have angles at carbon that are close to tetrahedral and are sufficiently flexible to adopt conformations that reduce their tor sional strain They tend to be populated by several different conforma tions of similar stability... 

Baeyer strain theory (Section 3 4) Incorrect nineteenth century theory that considered the nngs of cycloalkanes to be planar and assessed their stabilities according to how much the angles of a corresponding regular polygon devi ated from the tetrahedral value of 109 5°... 

Saturated hydrocarbons are stable. Only cycloalkanes with a tight ring are unstable. Alkenes and alkynes have a strong endothermic character, especially the first homologues and polyunsaturated conjugated hydrocarbons. This is also true for aromatic compounds, but this thermodynamic approach does not show up their real stability very well. Apart from a few special cases, the decomposition of unsaturated hydrocarbons requires extreme conditions, which are only encountered in the chemical industry. 

Paraffin waxes are also considered of mineral origin and are obtained from petroleum. The petroleum is distilled and the white colour of the wax is obtained by acid washing and purification. It has a typical melting point between about 47 °C and 64 °C. Its uses include candle making, casting and as a solidifier/stabilizer. The wax is composed of C20 C36 n-alkanes (40 90%), isoalkanes and cycloalkanes. 

Does the 10 kJmol 1 stabilization for adjacent exomethylene groups in cyclobutane arise from conjugative interactions Is the 5 kJmol-1 destabilization for nonadjacent exomethylenes in cyclobutane general for other cycloalkane derivatives ... 

While the conformation of cycloalkanes has been discussed in detail later, it will be worth while to see how the ring strain in such compounds is calculated as this will give us a broad picture about their relative stability. 

We know that all cycloalkanes do not have the same relative stability. Cyclohexane is most stable while cyclopropane and cyclobutane are much less stable, because they have a ring strain in their molecules. 

A necessary precondition of cyclization is that the reaction must be thermodynamically feasible. Cg-alkane isomers have approximately the same stability, and they are much more stable near room temperature than are cycloalkanes or aromatics (28). 

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... 

The relative stabilities of cycloalkanes can be determined by comparing their AH s of combustion (Problem 4.39) on a per-CH2-unit basis. Rings have different AH s of combustion per CH, unit because they have different amounts of ring strain. 

Problem 9.5 (a) Calculate AH of combustion per CHj unit for the first four cycloalkanes, given the following AH s of combustion, in kJ/mol cyclopropane, -2091 cyclobutane, -2744 cyclopentane, -3320 cyclohexane, -3952. (b) Write (i) the thermochemical equation for the combustion of cyclopropane and (ii) the theoretical equation for the combustion of a CH unit of any given ring, (c) How do ring stability and ring size correlate for the first four cycloalkanes ... 

Benzene, naphthalene, toluene, and the xylenes are naturally occurring compounds obtained from coal tar. Industrial synthetic methods, called catalytic reforming, utilize alkanes and cycloalkanes isolated from petroleum. Thus, cyclohexane is dehydrogenated (aromatization), and n-hexane(cycli> zation) and methylcyclopentane(isomerization) are converted to benzene. Aromatization is the reverse of catalytic hydrogenation and, in the laboratory, the same catalysts—Pt, Pd, and Ni—can be used. The stability of the aromatic ring favors dehydrogenation. 

In particular, commercial periluoroalkanes and -cycloalkanes (e.g., Fluorinert-, Flutec-, Fom-blin-, Galden-, Hostinert-type products) have great chemical, biochemical and thermal stability. They are rated, biochemically, as comparably harmless or practically nontoxic, which arc the lowest toxicity ratings.15-25-173 Additional fluoroalkanes are listed in the literature, see, for example, refs 6 and 191. 

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