Chemical reactions of alkenes

The functional group of an alkene, for example, is its carbon—carbon double bond. When we study the reactions of alkenes in greater detail in Chapter 8, we shall find that most of the chemical reactions of alkenes are the chemical reactions of the carbon-carbon double bond. 

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)... 

The regioselectivity and syn stereochemistry of hydroboration-oxidation coupled with a knowledge of the chemical properties of alkenes and boranes contribute to our under standing of the reaction mechanism... 

We have already discussed one important chemical property of alkynes the acidity of acetylene and terminal alkynes In the remaining sections of this chapter several other reactions of alkynes will be explored Most of them will be similar to reactions of alkenes Like alkenes alkynes undergo addition reactions We 11 begin with a reaction familiar to us from our study of alkenes namely catalytic hydrogenation... 

Our discussion of chemical reactions of alkadienes will be limited to those of conju gated dienes The reactions of isolated dienes are essentially the same as those of individual alkenes The reactions of cumulated dienes are—like their preparation— so specialized that their treatment is better suited to an advanced course m organic chemistry... 

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. 

The most characteristic chemical reaction of an alkene is an addition reaction, in which atoms supplied by the reactant form o-bonds to the two atoms originally joined by the double bond (Fig. 18.9). In the process, the 7r-bond is lost. An example is halogenation, the addition of two halogen atoms at a double bond, as in the formation of 1,2-dichloroethane ... 

Reactions of alkenes with H-Si(l 0 0)-2 x 1 surfaces have been shown to yield films with one-dimensional (ID) molecular lines through Si-C linkages, contrary to formation of the islands observed on H-Si(l 11). The reaction can be initiated from isolated surface silyl radicals created using the tip of the STM. The STM images showed molecular lines running along and across the dimer rows depending on the chemical constituent of R in the CH2 = CH-R molecules. 

In a number of classes of systems, the catalytic and other chemical effects of metal ions on reactions of organic and inorganic molecules are generally recognized the catalysis of nucleophilic reactions such as ester hydrolysis the reactions of alkenes and alkynes in the presence of metal carbonyls (8, 9, 69) stereospecific polymerization in the presence of Ziegler catalysts (20, 55, 56) the activation of such small molecules as H2 (37), 02 (13), H202 (13), and possibly N2 (58) and aromatic substitution reactions of metal-cyclopentadienyl compounds (59, 63). 

As with the alkenes, most chemical reactions of alkynes involve the elimination of the triple bond in favor of a double and a single bond. Normally the reactions continue so that the double bond is eliminated in favor of two more single bonds. 

Now that we re familiar with the structure and preparation of alkenes, let s look at their chemical reactions. The characteristic reaction of alkenes is addition to the double bond according to the general equation ... 

The presence of solids such as clays, zeolites, silica or ion-exchange resins may allow catalysis or control of organic reactions. Often, yields are higher and work-up procedures simpler than for the corresponding homogeneous reactions, and product distributions may also be improved. Examples of selective substitution reactions in aromatic and heteroaromatic systems and of selective reactions of alkenes are discussed, and the wider potential for synthesis of fine chemicals is discussed. 

Asymmetric ene reactions.1 The reaction of alkenes with prochiral, electron-deficient aldehydes can furnish optically active homoallylic alcohols when catalyzed by ( + )- or (-)-l and in the presence of activated 4-A molecular sieves. In the absence of the sieves, a stoichiometric amount of the organoaluminum complex is essential for high chemical and optical yields. 

All alkenes have a common feature a carbon-carbon double bond. The reactions of alkenes arise from the reactivity of the carbon-carbon double bond. Once again, the concept of the functional group helps to organize and simplify the study of chemical reactions. By studying the characteristic reactions of the double bond, we can predict the reactions of alkenes we have never seen before. 

The chemical properties of alkenes are very different from those of alkanes because of the double bond (— C = C —) in the structure. Double bond contains a sigma bond and a pi bond. Since electrons in pi bonds are bonded less strongly than in sigma bonds. This makes alkenes chemically reactive combustion, substitution, oxidation and polymerization reactions are all undergone by alkenes. 

Ethylene shows all the chemical properties of alkenes. It undergoes combustion, addition reactions and polymerization reactions. It burns with a bright yellow flame. 

There is a vast and rapidly increasing literature on transition metal hydrides, their chemical reactions and their role in hydrogen transfers, as in catalytic reactions of alkenes and other substrates. We can only outline the present knowledge which includes also what are called agostic H atoms and new types of H-bonding as well as compounds with M—H and M(H2) bonds. 

Ionic liquids can be used as replacements for many volatile conventional solvents in chemical processes see Table A-14 in the Appendix. Because of their extraordinary properties, room temperature ionic liquids have already found application as solvents for many synthetic and catalytic reactions, for example nucleophilic substitution reactions [899], Diels-Alder cycloaddition reactions [900, 901], Friedel-Crafts alkylation and acylation reactions [902, 903], as well as palladium-catalyzed Heck vinylations of haloarenes [904]. They are also solvents of choice for homogeneous transition metal complex catalyzed hydrogenation, isomerization, and hydroformylation [905], as well as dimerization and oligomerization reactions of alkenes [906, 907]. The ions of liquid salts are often poorly coordinating, which prevents deactivation of the catalysts. 

As seen from this review, CO2 diemistry is likely to have its second rebirth when it is extended to systems with transition metal participation. Even the first results disclose numerous possibilities for using CO2 in different chemical processes in the future. The industrial chemist is especially interested in producing usable compounds which are formed from the cheap chemical CO2. Further research should be done on the hydrocarboxylatton reactions of alkenes whereby saturated or unsalurated carboxylic acids are obtained. Aromatics or alkanes with activated hydrogen should also be possible reaction partners of carb(Hi dioxide. Future work will extend our knowledge of the scope of transition metal-C02 chemistry and of the potential uiUity of carbon dioxide as a feedstock for the production of organic chemicals. 

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