Cycloalkene, naming

Cycloalkenes and their derivatives are named by adapting cycloalkane termmol ogy to the principles of alkene nomenclature... 

When applying this principle to replacement names generated from fusion nomenclature, it is essential to keep in mind that fusion names for hydrocarbons ending in -cycloalkene are for fully unsaturated skeletons the -ene ending implies whatever number of double bonds may be necessary, without a multiplier. Thus (117) has six double bonds in the twelve-membered ring, and one must add ten hydrogens to saturate it to the stage of a simple benzene derivative, compound (118). 

The reversal of ring-closing metathesis, namely ring-opening metathesis, is also a synthetically useful reaction. With strained (small-ring) cycloalkenes, e.g. 12, the equilibrium of the reaction lies on the side of the open-chain product 13 ... 

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]. 

Cycloalkenes are named similarly to open-chain alkenes but, because there is no chain end to begin from, we number the cycloalkene so that the double bond is between Cl and C2 and the first substituent has as low a number as possible. Note that it s not necessary to indicate the position of the double bond in the name because it is always between Cl and C2. As with open-chain alkenes, newer but not yet widely accepted naming rules place the locant immediately before the suffix in a diene. 

Instead ef the name metathesis, the term disproportionation is frequently applied to the reaction, and sometimes the term dismutation. For historical reasons the name disproportionation is most commonly used for the heterogeneously catalyzed reaction, while the homogeneously catalyzed reaction is usually designated as metathesis. The name disproportionation is correct in the case of the conversion of acyclic alkenes according to Eq. (1) however, this name is inadequate in most other situations, such as the reaction between two different alkenes, and reactions involving cycloalkenes. Similar objections apply to the name dismutation. The name metathesis is not subject to these limitations and, therefore, is preferred. 

H risson and Chauvin (88) examined the metathesis between acyclic alkenes and cycloalkenes (telomerization) in the presence of two other tungsten-based catalysts, namely WOCl4-Sn(n-C4He)4 and WOCI4-... 

For monocyclic components other than benzene the nomenclature is based on that of the corresponding cycloalkanes. Prefixes of the type cyclopenta-, cyclohepta-, etc., are used, and for names of monocyclic base components the termination -ene is employed, this termination indicating maximum unsaturation in the whole system (not just a single double bond in the cycloalkene unit). Thus 15 and 16 are named as shown. 

The convenience of always deriving a name from a fully unsaturated parent is the main reason for abandoning the Stelzner nomenclature systems for fused skeletons. However, it should be emphasized that, as a consequence oflhe rejection of Stelzner nomenclature, all fusion names terminating in -cycloalkene imply full unsaturation (i.e., the ene here does not indicate only one double bond), e.g., 76 is 5-azabenzocyclo-octene, and 77 is the 5,6,7,8,9,10-hexahydro derivative. 

Cycloalkenes are named by the system used for the open-chain alkenes, except that the numbering always is started at one of the carbons of the double bond and continued around the ring through the double bond so as to keep the index numbers as small as possible ... 

The IUPAC system for naming cycloalkanes and cycloalkenes was presented in some detail in Sections 3-2 and 3-3, and you may wish to review that material before proceeding further. Additional procedures are required for naming... 

Although Hantzsch-Widman system works satisfactorily (if you can remember the rules) for monocyclic compounds, it is cumbersome for polycyclic compounds. In the case of oxiranes it is simplest for conversational purposes to name them as oxides of the cycloalkenes or epoxy derivatives of the corresponding cycloalkanes. The oxabicycloalkane names seem preferable for indexing purposes, particularly because the word oxide is used in many other connections. 

Exercise 15-43 Draw structures for the following compounds and name each as an oxa-, aza-,drthiacycloalkane (cycloalkene, cycloalkadiene, and so on, as appropriate). 

Cycloalkenes are also common. Unless the rings are very large, cycloalkenes are always the cis isomers, and the term cis is omitted from the names. In a large ring, a trans double bond may occur, giving a tran.v-cycloalkcnc. 

Trans cycloalkenes are unstable unless the ring is large enough (at least eight carbon atoms) to accommodate the trans double bond (Section 7-7D). Therefore, all cycloalkenes are assumed to be cis unless they are specifically named trans. The cis name is rarely used with cycloalkenes, except to distinguish a large cycloalkene from its trans isomer. 

For compounds that show geometric isomerism, add the appropriate prefix cis- or trans-, or E- or Z-. Cycloalkenes are assumed to be cis unless named otherwise. 

Because of the ring structure, the naming rules for cyclic hydrocarbons, including cycloalkanes and cycloalkenes, are slightly different from those for alkanes and alkenes. Below are four examples to illustrate the naming rules. 

In naming cycloalkenes, the double bond is located between Cl and C2, and the 1 is usually omitted in the name. The ring is numbered clockwise or counterclockwise to give the first substituent the lower number. Representative examples are given in Figure 10.2. 

We thus have the families cycloalkanes, cycloalkenes, and cycloalkynes, as well as the multi-double and triple-bond variants such as cycloalkadienes and -atrienes, and cycloalkadiynes, -atriynes, etc. Naming the cyclo compounds corresponds to the naming of the straight-chain forms except that carbon atoms are numbered such that substituents are on the lowest numbered carbon atoms. This... 

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