Monocyclic hydrocarbons, structure

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]. 

The structure of the sex pheromone for the Fucus species, fucoserratene (11), was elucidated in 1973.16 The positions and geometries of alkenes were revealed by comparison of the gas chromatographic behavior with those of the isomeric conjugated 1,3,5- and 2,4,6-octatrienes. To date, a series of hydrocarbons and epoxides 1-11 and their stereoisomers have been identified within the pheromone bouquets of more than 100 different species of brown algae.17-23 Identification of these compounds was based on a combination of gas chromatography-mass spectrometry (GC-MS) analysis and by comparison with authentic synthetic compounds. These sex pheromones were all lipophilic, volatile compounds that consisted of C8 or Cn linear or monocyclic hydrocarbons or their epoxides. The monocyclic compounds have a cyclopropane, cyclopentene, or cyclo-heptadiene structure. Interestingly, the relationships between the chemical structures of pheromones and the taxonomical classifications of algae are unclear (Table 1). 

Fig. 7.2 Classification and structure of carotenoids (a) lycopene - acyclic hydrocarbon (b) 7-carotene - monocyclic hydrocarbon (c) /3-carotene - bicyclic hydrocarbon (d) lutein - bicyclic xanthophyll.

Annulene (Section 14.7B) Monocyclic hydrocarbon that can be represented by a structure having alternating single and double bonds. The ring size of an annulene is represented by a number in brackets, e.g., benzene is [6]annulene and cyclooctatetraene is [8]annulene. 

Figure 7. Hiickel delocalization energies per electron in units of p for the conjugated monocyclic hydrocarbons using the Huckel reference structure and the Hiickel computational method.

Figure 10. Resonance energies per Jt electron in units of electronvolts for the conjugated monocyclic hydrocarbons using the Dewar—de Llano reference structure with two bond types and the Pariser—Parr—Pople computational method.

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.)... 

Compared with monocyclic aromatic hydrocarbons and the five-membered azaarenes, the pathways used for the degradation of pyridines are less uniform, and this is consistent with the differences in electronic structure and thereby their chemical reactivity. For pyridines, both hydroxylation and dioxygenation that is typical of aromatic compounds have been observed, although these are often accompanied by reduction of one or more of the double bonds in the pyridine ring. Examples are used to illustrate the metabolic possibilities. 

Another isomerization reaction of arene oxides is equilibrium with oxe-pins [5], Here, the fused six-membered carbocycle and three-membered oxirane merge to form a seven-membered heterocycle, as shown in Fig. 10.2. An extensive computational and experimental study involving 75 epoxides of monocyclic, bicyclic, and polycyclic aromatic hydrocarbons has revealed much information on the structural factors that influence the reaction rate and position of equilibrium [11], Thus, some compounds were stable as oxepins (e.g., naphthalene 2,3-oxide), while others exhibited a balanced equilibrium... 

Of the various types of monocyclic terpene hydrocarbons, those with the p-menthadiene structure are the most important. Examples are as follows ... 

For avoiding chance correlations, three series of hydrocarbons with wide structural variations were considered 37 alkanes, 36 polyalkylcyclohexanes and 48 monocyclic structures (the full results are reported elsewhere 93 a)). 

Hemispherands (11) are by nature only partially preorganized for complexation, depending on their structural characteristics.30 129 132 Addition of hydrocarbon-based bridges, which essentially converts the monocycles to bicyclic systems, serves to provide a more rigid structural framework as in the cryptands.132... 

The acyclic aliphatic hydrocarbons have the general formula CnH2n + 2 cyclic saturated hydrocarbons (alicyclic hydrocarbons) have the general formula CnH2n (if monocyclic), CnH2n 2 (if bicyclic), etc. The structures of these hydrocarbons may be represented in the chemical literature in various ways some common conventions which are used freely in texts are illustrated here. 

It has generally been concluded that the carboxylic acids in petroleum with less than eight carbon atoms per molecule are almost entirely aliphatic in nature monocyclic acids begin at C6 and predominate above CM. This indicates that the structures of the carboxylic acids correspond with those of the hydrocarbons with which they are associated in the crude oil. Thus, in the range where paraffins are the prevailing type of hydrocarbon, the aliphatic acids may be expected to predominate similarly, in the ranges where the monocycloparaffins and dicycloparaffins prevail, one may expect to find principally monocyclic and dicyclic acids, respectively. 

Stereoisomers among the tetpenes are abundant and exceedingly important to the chemistry of the field. Stereochemical nomenclature therefore cannot be ignored in any complete scheme for systematizing terpene nomenclature for the present, however, the recommendations in this report provide only structural names for the simple acyclic, monocyclic, and bicyclic terpene hydrocarbons. Studies in various fields — e.g., steroids — on preferred methods of designating isomeric configurations are being made by other committees (1, 21, 25). 

In this report the simple acyclic, monocyclic, and bicyclic terpene hydrocarbons will be dealt with on the basis of these structural relationships and not on their natural or synthetic origin. 

Reducing the chemical structures of the remaining simple monocyclic and bicyclic hydrocarbons to five fundamental types. 

Complete fundamental cyclic formulas showing all carbons and hydrocarbons are given only in Charts 1, 2, 4, 8, and 9 in the other charts, the conventional ter-pene skeletal formulas have been used. For those not familiar with these common terpene formulas of the monocyclic and bicyclic structures, Chart 2 is included to show the different forms of identical structures which appear in current chemical literature. Only saturated structures are given. The three-dimensional type of skeletal formulas shown for the bicyclic structures is being used currently more and more because they often permit much more to be expressed concerning the properties of a compound and its relations to a fundamental type than do the classical structural formulas. 

Chart 2 Various Forms of Structural Formulas of Fundamental Types of Monocyclic and Bicyclic Terpene Hydrocarbons... 

The monocyclic terpene hydrocarbons, many of which are formed readily from the acyclic terpenes by ring closure or from the bicyclic terpenes by ring fission, contain a six-carbon ring and may all be considered as derivatives of either cyclohexane or benzene. They may also be classified, on the basis of common larger fundamental structures, into two distinct types of substituted six-carbon ring structures ... 

Type B. Those which contain no iso three-carbon group. Most of the hydrocarbons of this type, for which structures have been proved, contain two one-carbon groups in gem configuration and one or two additional isolated one-carbon or two-carbon groups, one of which is usually in meta position. These structures could be further divided into those that contain the gem-dimethyl configuration and those that do not, but their similar chemical reactivity warrants classification as a single group of compounds. Thus, Type B monocyclics may be considered as derivatives of dimethylcyclohexane, for which no trivial name has been well established. 

Chart 4. Structural Relation of Monocyclic Terpene Hydrocarbons to Aromatic and Saturated Cyclic Hydrocarbons... 

---