Alkenes Hydrocarbons with Double Bonds

A hydrocarbon that contains at least one C=C bond is called an alkene (general formula C H2 ). The double-bonded C atoms are sp hybridized. Because their carbon atoms are bonded to fewer than the maximum of four atoms each, alkenes are considered unsaturated hydrocarbons. 

Alkene names differ from those of alkanes in two respects  

The main chain (root) must contain both C atoms of the double bond, even if it is not the longest chain. The chain is numbered from the end closer to the C=C bond, and the position of the bond is indicated by the number of the first C atom in it. 

For example, there are three four-C alkenes (C4H8), two unbranched and one branched (see Sample Problem 15.1b). The branched isomer is 2-methyl-propene the unbranched isomer with the C=C bond between C-1 and C-2 is 

1- butene the unbranched isomer with the C=C bond between C-2 and C-3 is 

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Alkenes Hydrocarbons with Double Bonds 469 Alkynes Hydrocarbons with Triple Bonds 470... 

Hydrocarbons with double bonds are called alkenes. Ethene, CH2=CH2, is the simplest example of an alkene. It used to be (and still widely is) called ethylene it is used in the manufacture of polyethylene. Benzene is a hydrocarbon with double bonds that has such distinct properties that it is regarded as the parent hydrocarbon of a whole new class of compounds called—for historical reasons—aromatic compounds. The benzene ring is exceptionally stable and can be found in many important compounds. 

Alkenes (unsaturated hydrocarbons with double bonds) are the oldest and most studied carbon ligands, with interest in their study dating from around 1827 [3,185-189]. An important aspect is the structure of 71-alkene and similar n-alkyne complexes. Their structural data is summarized in reviews [186-189] and presented in... 

Gasoline contains several hydrocarbons with double bonds. A hydrocarbon in which one or more double bonds link carbon atoms together is called an alkene. The most common alkenes found in gasoline are pictured in Figure 18.5. 

OBJECTIVES To learn to name hydrocarbons with double bonds (alkenes) and triple bonds (alkynes). To understand addition reactions. 

Unsaturated hydrocarbons are formed by double and triple bonds between adjacent C-atoms. Hydrocarbons with double bonds derived from alkanes are called alkenes hydrocarbons with triple bonds derived from alkanes are called alkynes. The same holds for the cyclo-alkanes, where double bonds lead to c-alkenes. Benzene is the basic molecule for the class of aromatic compounds. Examples are... 

Alkadienes, alkatrienes, and alkatetraenes (poly-enes). These are unsaturated aliphatic hydrocarbons containing two, three, or four C = C double bonds, respectively. Alkadienes are also called diolefins or dienes, and alkatrienes are also known as triolefins or tricncs. Alkenes containing multiple double bonds fall under the general class of poly-enes. Double bonds that alternate with single bonds in a straight chain are said to be conjugated. Examples are... 

Our discussion has centered around hydrocarbon molecules that consist of carbon atoms bonded to each other in long chains. These chains are called normal, straight, or unbranched. The chains may have only single bonds (alkanes), double bonds (alkenes), or triple bonds (alkynes). Hydrocarbons with only single bonds are called saturated hydrocarbons with double or triple bonds are called unsaturated. Not all hydrocarbons want to form straight chains. After all, who among us wants to be straight and serious all the time Some of us will always be comedians. In the world of hydrocarbon molecules, the comedians are the branched and cyclic molecules. 

The number of hydrocarbons in the atmosphere is potentially very large, since vapor pressures are favorable and the heavier species admit many isomers. In urban areas several hundred different hydrocarbons have been identified by gas chromatography (Appel et al, 1979 Louw el al, 1977). They include saturated compounds (alkanes) unsaturated species with one carbon-carbon double bond (alkenes) or two double bonds (alkadienes), acetylene type compounds (alkynes), and benzene derivatives or aromatic compounds (arenes). To separate that many different compounds requires... 

We have seen that alkanes are saturated hydrocarbons—each of the carbon atoms is bound to four atoms by single bonds. Hydrocarbons that contain carbon-carbon double bonds are called alkenes. Hydrocarbons with carbon-carbon triple bonds are called alkynes. Alkenes and alkynes are unsaturated hydrocarbons. 

Bond energy Amount of energy required to break one mole of bonds between a specified pair of atoms Alkenes Hydrocarbons with one or more carbon-carbon double bonds... 

Alkenes Hydrocarbons with one or more carbon-carbon double bonds... 

In a catalytic cracking plant ( cat crackers, as they re called in Texas), fractions in the C12 to C20 range are heated in the absence of air with a catalyst. This process causes the long alkanes (compounds of carbon and hydrogen with only carbon-to-carbon single bonds, which are covered in glorious detail in Chapter 14) to break apart into smaller alkanes and alkenes (hydrocarbons with at least one carbon-to-carbon double bond, covered in equally glorious detail in Chapter 14). 

Dummy variable for alkane, alkene, cycloalkane, or cyclu-alkene (hydrocarbons with 0 or 1 double bond)... 

Important reactions of O3 in the polluted atmosphere are those with organic compounds with double bonds such as alkenes, dienes, terpenes, etc. Here, the most fundamental elemental reactions with ethylene (C2H4) are described. Reactions with other alkenes and biogenic hydrocarbons are treated in Sects. (7.2.4) and (7.2.6). 

Unsaturated acyclic hydrocarbons with double and/or triple bonds are generally designated as alkenes, alkynes, and alkenynes multiple unsaturation is indicated by the numerical prefixes di, tri, etc. alkatrienes, alkenediynes etc. 

Open chain hydrocarbons which are undersaturated, i.e. having at least one carbon-carbon double bond are part of the olefin series, and have the ending -ene . Those with one carbon-carbon double bond are called mono-olefins or alkenes, for example ethylene CH2 = CH2. 

Carbon can also form multiple bonds with other carbon atoms. This results in unsaturated hydrocarbons such as olefins (alkenes), containing a carbon-carbon double bond, or acetylenes (alkynes), containing a carbon-carbon triple bond. Dienes and polyenes contain two or more unsaturated bonds. 

Alkenes are hydrocarbons that contain a carbon-carbon double bond A carbon-carbon double bond is both an important structural unit and an important func tional group m organic chemistry The shape of an organic molecule is influenced by the presence of this bond and the double bond is the site of most of the chemical reactions that alkenes undergo Some representative alkenes include isobutylene (an industrial chemical) a pmene (a fragrant liquid obtained from pine trees) md fame sene (a naturally occurring alkene with three double bonds)... 

Hydrocarbons that contain a carbon-carbon triple bond are called alkynes Non cyclic alkynes have the molecular formula C H2 -2 Acetylene (HC=CH) is the simplest alkyne We call compounds that have their triple bond at the end of a carbon chain (RC=CH) monosubstituted or terminal alkynes Disubstituted alkynes (RC=CR ) have internal triple bonds You will see m this chapter that a carbon-carbon triple bond is a functional group reacting with many of the same reagents that react with the double bonds of alkenes... 

What about a substance wrth the molecular formula 71414 Thrs compound can not be an alkane but may be erther a cycloalkane or an alkene because both these classes of hydrocarbons correspond to the general molecular formula C H2 Any time a ring or a double bond is present in an organic molecule its molecular formula has two fewer hydrogen atoms than that of an alkane with the same number of carbons... 

The important hydrocarbon classes are alkanes, alkenes, aromatics, and oxygenates. The first three classes are generally released to the atmosphere, whereas the fourth class, the oxygenates, is generally formed in the atmosphere. Propene will be used to illustrate the types of reactions that take place with alkenes. Propene reactions are initiated by a chemical reaction of OH or O3 with the carbon-carbon double bond. The chemical steps that follow result in the formation of free radicals of several different types which can undergo reaction with O2, NO, SO2, and NO2 to promote the formation of photochemical smog products. 

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