alkenes

In the early days of organic chemistry, when it was found that the alkenes, but not the alkanes, readily undergo addition reactions with substances such as halogens, hydrogen halides, sulfuric acid, and oxidizing agents, the chemical affinity of alkanes was said to be " saturated" while that of the alkenes was said to be " unsaturated." Now, even though we recognize that no chemical entity (even the noble gases such as helium and xenon) can surely be classified as saturated, the description of alkanes and alkenes as saturated and unsaturated is still commonly used. However, in place of a nebulous chemical affinity, we ascribe the unsaturation of alkenes to the ease of cleaving half of a carbon-carbon double bond in an addition reaction. Additions occur with alkenes much more easily than with alkanes because (1) the carbon-carbon bonds of a double bond are individually weaker (more strained) than a normal carbon-carbon single bond and (2) the double-bond electrons are generally more accessible than single-bond electrons to an attacking reagent (see Section 2.6).

The great variety and specificity of the addition reactions that compounds with double bonds undergo make these substances extremely important as intermediates in organic syntheses. We have already examined two of these reactions (addition of halogens and hydrogen) in connection with our study of ethene, the simplest alkene, in Chapter 2.

nomenclature

Open-chain alkenes containing one double bond have the general formula C_nH_2n, and are sometimes called olefins. According to the IUPAC system for naming alkenes, the longest continuous chain containing the double b

ond is given the name of the corresponding alkane with the ending -ane changed to -ene. This chain is then numbered so that the position of thefirst carbon of the double bond is indicated by the lowest possible number.

Other, less systematic names are often used for the simpler alkenes. By one method, alkenes are named as substituted ethylenes. This nomenclature,

based on the older name "ethylene," is given here because, even though not in accord with modern practices, it has been widely used in the literature. A little reflection will show that attempts to name alkenes as derivatives of propylene (propene) or butylene (as will be seen, there are four open-chain C₄,H₈, isomers) will require special rules or be hopelessly ambiguous.

The hydrocarbon groups derived from alkenes carry the suffix -enyl, as in alkenyl, and numbering of the group starts with the carbon atom with the free bond :

However, there are a few alkenyl groups for which trivial names are commonly used in place of systematic names. These are vinyl, allyl, and isopropenyl groups :

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

More complex nomenclature systems are required when the double bond is exocyclic to the ring, especially if a ring carbon is one terminus of the double bond. Usually the parent compounds of this type are called methylenecycloalkanes

Many compounds contain two or more double bonds and are known as alkadienes, alkatrienes, alkatetraenes, and so on, the suffix denoting the number of double bonds. The location of each double bond is specified by appropriate numbers.

A further classification is used according to the relationships of the double bonds, one to the other. Thus, 1,2-alkadienes and similar substances are said to have cumulated double bonds. 1,ZAlkadienes and other compounds

with alternating double and single bonds are said to have conjugated double bonds, and this arrangement leads, as we shall see in Chapter 6, to compounds having rather special properties.

Compounds with double bonds that are neither cumulated nor conjugated are classified as having isolated double-bond systems.

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