NAMING OF ALKENES

From the third member (butene ) of the alkene family onward, there is a need to specify the location of the double bond to recognize the correct structure denoted by the name. When naming alkenes, the numbering should begin from the carbon chain end which gives the double bond position the lowest number. 

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The names of alkenes are derived from those of the corresponding alkanes with the same number of carbon atoms per molecule. There are two modifications ... 

Alkenes and alkynes are unsaturated hydrocarbons. Alkenes contain at least one carbon-carbon double bond and alkynes have at least one carbon-carbon triple bond. The names of alkenes and alkynes use the alkane prefixes, but add ene and yne endings, respectively (Table 15.3). 

Hydrocarbons with one or more carbon-to-carbon double bonds are called alkenes. The names of alkenes end with -ene. Ethene, or ethylene, the simplest alkene, has two carbon atoms connected by a double bond. The double bond consists of two pairs of shared electrons, one pair from one carbon atom and one pair from the other carbon atom. Each of the carbons also has two single bonds with hydrogen atoms so that the formula for the compound can be written as... 

The IUPAC Rules are exactly the same for naming alkynes as for naming of alkenes except that die suffix -yne replaces -ene. 

The names of alkenes and alkynes contain the suffixes -ene and -yne, respectively. With some alkenes it is necessary to define the stereochemistry of the double bond ... 

Alkenes contain one or more double bonds. Alkenes are also called olefins. The suffix used in the naming of alkenes is -ene, and the number roots are those used for alkanes of the same length. 

Alkynes are named in a manner nearly identical to the naming of alkenes except that the suffix is -yne. The same rules for numbering apply. Compounds with several triple bonds use the suffixes -diyne, -triyne, and so on. For compounds that contain both a double bond and a triple bond, both suffixes are used, as in -enyne. Some examples follow ... 

The names of alkenes follow the same format as the names of alkanes prefix + root + suffix. The prefixes and the steps for locating and identifying branches are the same, too. The greatest difference involves the double bond. The suffix -ene immediately tells you that a compound has at least one double bond. The rest of the necessary information—the location of the double bond, and the number of carbon atoms in the main chain—is communicated in the root. Follow the steps below to find out how to name the compound in Figure 13.20. 

Common names are seldom used except for three simple alkenes ethylene, propylene, and isobutylene. The various alkenes of a given carbon number are, however, sometimes referred to collectively as the pentylenes amylenes), hexylenes, heptylenes, and so on. (One sometimes encounters the naming of alkenes as derivatives of ethylene as, for example, tetramethylethylene for (CH3)20—C(CH3)2.) Most alkenes are named by the lUPAC system. 

For more complicated alkynes the lUPAC names are used. The rules arc exactly the same as for the naming of alkenes, except that the ending -yne replaces... 

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 names of alkenes are based on the longest continuous chain of carbon atoms that contains the double bond. The chain is named by changing the ending of the name of the corresponding alkane from -ane to -ene. The compound on the left in Figure 24.7, for example, has a double bond as part of a three-carbon chain thus, the parent alkene is propene. 

SECTION 24.3 The names of alkenes and alkynes are based on the longest continuous chain of carbon atoms that contains the multiple bond, and the location of the multiple bond is specified by a numerical prefix. Alkenes exhibit not only structural isomerism but geometric (cis-tmiis) isomerism as well. In geometric isomers the bonds are the same, but the molecules have different geometries. Geometric isomerism is possible in alkenes because rotation about the C=C double bond is restricted. 

Write the lUPAC names of alkenes from their molecular stmctures. (Section 2.1)... 

Write the lUPAC names of alkenes from their molecular structure. 

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 names of alkenes and alkynes are derived from the corresponding alkanes. 

We form lUPAC names of alkenes by changing the -an- infix of the parent alkane to -en-(Section 3.5). Hence, CH2 = CH2 is named ethene, and CH3CH=CH2 is named propene. In higher alkenes, where isomers exist that differ in the location of the double bond, we use a numbering system. We number the longest carbon chain that contains the double bond in the direction that gives the carbon atoms of the double bond the lower set of numbers. We then use the number of the first carbon of the double bond to show its location. We name branched or substituted alkenes in a manner similar to the way we name alkanes (Section 3.3). We number the carbon atoms, locate the double bond, locate and name substituent groups, and name the main (parent) chain. 

As these two examples demonstrate, the names of alkanes end with -ene and nnmbers are nsed to indicate the position of the double bond. The parent chain is nnmbered so that the lowest nnmber possible is given to one of the carbon atoms in the donble bond, regardless of any other substitnents present in the compound (for example, alkyl gronps or halides). 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. Because of restricted rotation abont the carbon-carbon double bond, alkenes can form geometric isomers (Section 4.4). In this case, the name of an alkene must also specify whether the isomer is cis or trans ... 

The naming of alkenes or double-bonded molecules is very much like that of the alkanes. 

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