The Nomenclature of Alkenes

We have seen that the lUPAC system uses a suffix to denote certain functional groups (Sections 3.6 and 3.7). The double bond is the functional group of an alkene its presence is denoted by the suffix ene. Therefore, the systematic (lUPAC) name of an alkene is obtained by replacing the ane ending of the corresponding alkane with ene. For example, a two-carbon alkene is called ethene, and a three-carbon alkene is called propene. Ethene also is frequently called by its conunon name ethylene. 

Most alkene names need a number to indicate the position of the double bond. (The four names above do not because there is no ambiguity.) The lUPAC mles you learned in Chapter 3 apply to alkenes as well  

Notice that 1-butene does not have a common name. You might be tempted to call it butylene, which is analogous to propylene for propene. Butylene, however, is not an appropriate name because it could signify either 1-butene or 2-butene, and a name must be unambiguous. 

The stereoisomers of an alkene are named using a cis or trans prefix (in italics). 

Number the longest continuous chain containing the functional group in the direction that gives the functional group suffix the lowest possible number. 

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The nomenclature of alkenes and alkynes is very similar to that of the alkanes with two major differences. (1) The longest carbon chain must contain the double or triple bond and number the chain from the end closest to the double/triple bond. A number indicates the position of the double/triple bond. 

Exercise 5-1 Draw structures showing the configuration of the following compounds. You may wish to review the nomenclature of alkenes, given in Section 3-3. 

The nomenclature of alkenes and alkynes follows the same rules as alkanes, except the double or triple bond must be numbered. The multiple bond is numbered on the first number to which it is assigned. Also, because double bonds have a rigid configuration, they can exhibit a cis or trans isomerism. A cis structure is one with substituents on the same side of the double bond, and the trans is one with the substituents are on opposite sides of the double bond. 

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

Finally, if you have not studied the material already, you may wish to return to the last part of Chapter 3 and become acquainted with the nomenclature of cycloalkanes, alkenes, cycloalkenes, and alkynes (Sections 3-2 to 3-4). 

The Orbital Description of the Alkene Double Bond 286 7-3 Elements of Unsaturation 287 7-4 Nomenclature of Alkenes 289 7-5 Nomenclature of Cis-Trans Isomers 291 Summary Rules for Naming Alkenes 293 7-6 Commercial Importance of Alkenes 294 7-7 Stability of Alkenes 296 7-8 Physical Properties of Alkenes 302... 

In the nomenclature of alkanes, we started to number the carbon atoms starting from the end nearest to the substituent. Note that when we are numbering the carbon atoms in alkenes, the double bond is more important. 

Epoxides are also named as alkene oxides, since they are often prepared by adding an O atom to an alkene (Chapter 12). To name an epoxide this way, mentally replace the epoxide oxygen by a double bond, name the alkene (Section 10.3), and then add the word oxide. For example, the common name for oxirane is ethylene oxide, since it is an epoxide derived from the alkene ethylene. We will use this common method of naming epoxides after the details of alkene nomenclature are presented in Chapter 10. 

Alkenes, arenes and other unsaturated molecules attach to central atoms, using some or all of their multiply bonded atoms, to give organometallic complexes. While there are many similarities between the nomenclature of coordination and organometallic compounds, the latter differ from the former in clearly definable ways. Organometallic complexes are therefore treated separately in Chapter IR-10. 

The con figuration, relative stability and nomenclature of alkenes, both acyclic and cyclic... 

The numbers in the names of alkenes refer to the lowest numbered carbon atom in the chain that is part of the C=C bond of the alkene. The name butene means that there are four carbon atoms in the longest chain. Alkene nomenclature must specify whether a given molecule is cis or trans if it is a geometric isomer, such as... 

The nomenclature of dienes is analogous to that of alkenes, with certain modifications to indicate the extra double bond. The numbering of carbons is the same as that of alkenes. The numbers of the olefinic carbons are placed at the beginning of the name, which is suffixed by -diene instead of -ene. For example, a compound of the structure... 

The nomenclature of FA reflects the long history of their smdy and description. Most FA were originally described under trivial names prior to the adoption of the international molecular nomenclature rules in 1892 (Table 3.1). Even after adopting the International Union of Pure and Applied Chemistry (lUPAC) system for nomenclature (lUPAC-IUB, 1977), the habit of assigning trivial names to FA continues. The basis of the systematic nomenclature system is an extension of that accepted for hydrocarbon (alkane/alkene) naming. Hence, the descriptive name is based on the number of carbon atoms contained in the molecule, with the suffix -e replaced with -oic acid. However, in most instances, FA are referred to by their formula notations. As is the case with systematic and trivial names, there exists an lUPAC accepted formula notation nomenclature and several earher versions (Table 3.2). Each system... 

Some compounds contain more than one double bond per molecnle. Molecnles of this type are important components of natural and synthetic rubber and other useful materials. The nomenclature of these compounds is the same as for the alkenes with one donble bond, except that the endings -diene, -triene, and the like are used to denote the number of double bonds. Also, the locations of all the mnltiple bonds must be indicated in all molecules, including those with rings ... 

In contrast to the relatively unreactive alkanes, alkenes and alkynes are chemically reactive because they have double and triple carbon-carbon bonds as functional groups. In the lUPAC nomenclature the names of alkenes end with the suffix -ene and the position of the double bond is labeled by number of the C-atom on which this double bond begins. In analogy with alkanes with branched chains, the root of the word is the name of the longest chain of C-atoms and the atom numbering is arranged... 

The bromination of alkenes is an example of an electrophilic addition reaction. (See Experiments [A2b] and [D2] for detailed discussions of the mechanism involved in this reaction. In particular, refer to Experiment [D2], which very closely resembles this reaction, for a discussion of the erythro and threo nomenclature used in this experiment.) In the present reaction, bromination of 4-nitrochalcone yields eiyfl2ro-2,3-dibromo-3-(4-nitrophenyl)propiophenone. 

Cyclitols are polyhydroxycycloalkanes and -alkenes. The most abundant of these carbocyclic compounds are the hexahydroxycyclohexanes, commonly called inositols, and their methyl ethers. The nomenclature of cyclitols is problematic several systems have been proposed and used. The rules used here are those of lUPAC and lUB (97). 

As in the nomenclature of alkanes, many groups derived from alkenes have common names. Three of the most often encountered are the vinyl, allyl, and isopropenyl groups. 

In lUPAC nomenclature, the suffix for the class of alkenes is -ene. Some of the lower members of the class are shown in Figure 3.6. In working out the formal name of the alkene, we now have two new factors to consider—where is the double bond and what is the geometry about the double bond Remember this geometry is fixed, as there is no rotation about the double bond. While the cis- and tra s-designations will serve for simple alkenes such as 2-butene, we will need a more sophisticated set of rules for larger, more complex molecules. For example, how would compound 3.7 be named ... 

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