Hydrocarbon cycloalkanes

Method Sensitivity and Accuracy. When we use the described procedure and introduce a 5-mL gas sample into the chromatograph, the method is capable of detecting alkane, alkene, and cycloalkane hydrocarbons down to 1-3 parts in 1012 parts of water by weight (ppt). Aromatic hydrocarbons, because of their lower partitioning into the gas phase, can be detected at 4-12 ppt. Reasonable accuracy requires concentrations... 

Seawater equilibrated with unweathered La Rosa crude oil (from the spill tank) contained hydrocarbons dissolved in proportion to their individual solubilities and amounts in the oil. The C2-C4 alkane concentrations are high, but benzene and toluene concentrations stand out compared with C6 and C7 alkane and cycloalkane hydrocarbons. This hydrocarbon signature is typical for water equilibrated with crude oils reported previously in Table II. 

Today no one professionally involved in chemical science or engineering has any doubts about the importance of thermodynamics and thermochemistry. Now, however, the danger is that the field is being taken for granted it is assumed that all the problems have been solved, all the important data measured, and what cannot be measured has been adequately calculated. In this chapter we review the thermochemistry of the alkane and cycloalkane hydrocarbons, with some attention to the current status of our knowledge in this important field and to the problems that remain to be solved. 

Cycloalkanes—Hydrocarbons that form ring structures are termed alicyclic. Ring strain exists in cycloalkanes containing 3,4, and 5 carbon atoms. A study of heats of combustion shows that cyclohexane is strain free. EHsubstituted cycloalkanes exhibit a form of stereoisomerism known as cis-trans isomerism. Cyclohexanes undergo rapid ring flips (Fig. 26-13). In substituted cyclohexanes, substituents compete for the equatorial positions. 

There are a total of eighteen different hydrocarbon series, of which the most common constituents of crude oil have been presented - the alkanes, cycloalkanes, and the arenes. The more recent classifications of hydrocarbons are based on a division of the hydrocarbons in three main groups alkanes, naphthanes and aromatics, along with the organic compounds containing the non-hydrocarbon atoms of sulphur, nitrogen and oxygen. 

What about a substance wrth the molecular formula 71414 Thrs compound can not be an alkane but may be erther a cycloalkane or an alkene because both these classes of hydrocarbons correspond to the general molecular formula C H2 Any time a ring or a double bond is present in an organic molecule its molecular formula has two fewer hydrogen atoms than that of an alkane with the same number of carbons... 

Para (Section 11 7) Term describing a 1 4 relationship be tween substituents on a benzene nng Paraffin hydrocarbons (Section 2 18) An old name for al kanes and cycloalkanes... 

Most organic compounds, including aromatic hydrocarbons, alcohols, esters, ketones, ethers, and carboxyUc acids are miscible with nitroparaffins, whereas alkanes and cycloalkanes have limited solubiUty. The lower nitroparaffins are excellent solvents for coating materials, waxes, resins, gums, and dyes. 

The alcohol fraction is likewise a complex mixture of both aUphatic and cycHc compounds (Table 10). The principal components are cholesterol (34%), and lanosterol and dihydrolanosterol (38%). The aUphatic alcohols account for about 22% of the unsaponifiable products. Sixty-nine components of ahphatic alcohols had been reported up to 1974 (latest reported work as of ca 1997). The hydrocarbons (ca 0.5%) show stmctural similarity to the wool—wax acids or ahphatic alcohols and contain highly branched alkanes as well as cycloalkanes. 

Values for hydrocarbons other than alkynes and alkadienes can be predicted by the method of Suzuki et al. The best model includes the descriptors T, P, the parachor, the molecular surface area (which can be approximated by the van der Waals area), and the zero-order connectivity index. Excluding alkynes and alkadienes, a studv for 58 alkanes, aromatics, and cycloalkanes showed an average deviation from experimental values of about 30 K. 

Cycloalkanes are one class of alicydic (a// phatic cydic) hydrocarbons. 

Paraffin hydrocarbons (Section 2.18) An old name for alkanes and cycloalkanes. 

Alicyclic Hydrocarbons. These refer to cyclic analogues of aliphatic hydrocarbons and are named accordingly, using the piefix cyclo-." Their properties are similar to their open-chain aliphatic counterparts. Alicyclic hydrocarbons are subdivided into monocyclic (cycloalkanes, cycloalkenes, cycloalkynes, cycloalkadienes, etc.) and polycyclic aliphatic compounds. Monocyclic aliphatic structures having more than 30 carbon atoms in the ring are known, but those containing 5 or 6 carbon atoms are more commonly found in nature [47, p. 28]. 

Cycloalkanes (ornaphthenes). These are also known as cycloparaffins or saturated alicyclic hydrocarbons." They are quite stable compounds with the general formula with n > 3 for rings without substituent groups. The first two members are... 

Physical properties of cycloalkanes [49, p. 284 50, p. 31] show reasonably gradual changes, but unlike most homologous series, different members exhibit different degrees of chemical reactivity. For example, cyclohexane is the least reactive member in this family, whereas both cyclopropane and cyclobutane are more reactive than cyclopentane. Thus, hydrocarbons containing cyclopentane and cyclohexane rings are quite abundant in nature. 

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 ... 

Saturated cyclic hydrocarbons are called cycloalkanes, or alicyclic compounds (aliphatic cyclic). Because cycloalkanes consist of rings of -CH2- units, they have the general formula (CH2) , or and can be represented by polygons... 

A cycloalkane is a saturated cyclic hydrocarbon with the general formula C H2 . In contrast to open-chain alkanes, where nearly free rotation occurs around C, -C bonds, rotation is greatly reduced in cycloalkanes. Disubstituted cycloalkanes can therefore exist as cis-trans isomers. The cis isomer has both substituents on the same face of the ring the trans isomer has substituents on opposite faces. Cis-trans isomers are just one kind of stereoisomers—isomers... 

Alicyclic (Section 4.1) An aliphatic cyclic hydrocarbon such as a cycloalkane or cycloalkene. 

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. 

Cycloalkanes, like alkanes, are saturated hydrocarbons containing only single bonds. Notice, though, that as a result of ring formation, each cycloalkane molecule contains two fewer hydrogen atoms than the corresponding alkane. 

Cycloalkane A saturated hydrocarbon containing a closed ring. General formula = C H2 , 584-585 Cyclopentane, 584 Cysteine, 622t... 

Reviews on the microbial metabolism of hydrocarbons with biochemical aspects are available, and inclnde those of Britton (1984) on alkanes, and of Morgan and Watkinson (1994) that also includes cycloalkanes and some aromatic compounds. Virtually all the issues that are discussed in these recur in the examples that are used as illustration. Some broad generalizations are summarized ... 

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. 

Hydrogenolysis reactions of hydrocarbons on metal catalysts have been investigated in some detail. Extensive studies have been conducted on both alkanes and cycloalkanes. While a number of questions still remain with regard to mechanistic and kinetic details of the reactions, the general features seem reasonably clear. 

The amount of energy required to carry out this process depends on the nature of the hydrocarbon it is the highest for saturated hydrocarbons (alkanes, cycloalkanes) and low for unsaturated and aromatic hydrocarbons (in fact, decomposition of acetylene and benzene are exothermic reactions). Methane is one of the most thermally stable organic molecules. 

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