Alkene atom addition

The region of high electron density between the doubly bonded carbon atoms gives alkenes an additional reactivity and in addition to burning and reacting with halogens, alkenes will add on other molecules for example ... 

The first three members of the olefin series are ethylene, propylene, and butylene (or butene). Structural isomers exist when n > 4, as a consequence of the positioning of the double bond in normal alkenes as a result of branching in branched alkenes. In addition, geometric isomers may be possible owing to restricted rotation of atoms about the C=C bond. For instance, C H (butene) has four possible isomers instead of the expected three ... 

This forms individual hydrogen atoms adsorbed to the surface of the metal. These hydrogen atoms are now available for addition across the alkene. The addition reaction begins when the alkene coordinates with the metal surface ... 

The diborane is generated (in situ, or separately, from NaBH4 and Et2Offi—BF3e), and probably complexes, as the monomeric BH3, with the ethereal solvent used for the reaction. BH3 is a Lewis acid and adds to the least substituted carbon atom of the alkene (Markownikov addition), overall addition is completed by hydride transfer to the adjacent, positively polarised carbon atom ... 

Figure 7.9 The mechanism for the hydrogenation of an alkene as catalyzed by finely divided platinum metal (a) hydrogen adsorption (b) adsorption of the alkene (c) and (d), stepwise transfer of both hydrogen atoms to the same face of the alkene (syn addition).

Acetylene is the most widely used alkyne. Alkynes undergo the same reactions as alkenes. Because a triple bond coimects the two carbon atoms, addition of an atom initially forms a double bond. The addition of a second atom converts the double bond into a single bond. For example, hydrogenation of propyne to propane is represented as ... 

Reaction 8 may, therefore, be the major chain-propagating reaction of H02 between 250° and 400°C. The radicals produced will, of course, undergo the same fates as those produced in Reaction 4, regenerating (eventually) alkyl radicals. The main difference between the alkene-H02 addition route and the alkylperoxy radical isomerization route is that in the former case the hydroperoxyalkyl radicals formed are necessarily a-radicals—i.e., radicals in which the unpaired electron is borne by a carbon atom adjacent to that bearing the hydroperoxy group, such as... 

The upswing in the calculated /4-factors for tetrafluoroethylene is due to the greater symmetry of the alkene which leads to a relatively low entropy. Knox and co-workers also calculated a higher rate constant for chlorine atom addition to tetrachloroethylene, but the experimental result was not in agreement with this finding. 

The multiple alkylation of carbanions with electron-deficient alkenes (Michael addition) only yields the expected products if the carbanion is less basic than the initial product of Michael addition. If the attacking carbanion and the carbanion resulting from Michael addition have similar basicity, oligomerization of the Michael acceptor can occur instead of multiple alkylations of the same carbon atom (Scheme 10.21). 

Hydrogenation of an alkene is an example of an addition, one of the three major reaction types we have studied addition, elimination, and substitution. In an addition, two molecules combine to form one product molecule. When an alkene undergoes addition, two groups add to the carbon atoms of the double bond and the carbons become saturated. In many ways, addition is the reverse of elimination, in which one molecule... 

For alkenes, in addition to structural isomerism, geometrical isomerism can also occur. Geometrical isomerism differs only in the arrangement of the atoms in space. 

The rate of hydrogen atom addition to alkenes grows in the order ethylene < propene 1-butene < 2-methylpropene. 

However, H atom addition to alkenes is ca. IVOkJmoP exothermic, so that C—C homolysis reactions are enhanced through the chemically activated alkyl radicals formed. Relative yields of products are thus very dependent on pressure. 

The addition of dihydrogen to Vaska s compound (Scheme 7.3) is an excellent example of concerted, 3-center, syn addition in which the transition state is a three-membered ring consisting of the metal and the two hydrogen atoms. Addition of H2 to Wilkinson s catalyst, an early step in the homogeneous catalytic hydrogenation of alkenes (to be discussed in Chapter 9), serves as another well-known example (equation 7.29). 

Addition reactions of the 4-tm-butylcyclohexyl radical have been studied with a variety of alkenes and also in atom-abstraction reactions (see Section D.2.2.)3 58. While hydrogen or halide abstraction reactions yield preferentially the axial product, unselective addition occurs with terminally unsubstituted alkenes. In addition reactions of alkenes substituted by alkyl groups at the attacked olefinic center, the preference for axial attack decreases further and the equatorial addition product is formed. This influence of the size of the reaction partner on the selectivity is rationalized on the basis of the simultaneous presence of steric effects (hindering axial attack) and torsional effects (hindering equatorial attack), very similar to those discussed for nucleophilic addition reactions to cyclic ketones59. 

Carbohydrate-derived enones have been employed as chiral, electrophilic alkenes in addition reactions to alcohols, ethers, or aldehydes90. Through benzophenone-sensilized photochemical initiation, hydrogen abstraction occurs from the a-carbon atoms in alcohols and ethers or from the carbonyl atom in aldehydes. Addition then occurs exclusively on the /(-face of the enone,... 

Hydrogen atom addition to various higher alkenes... 

As with a normal alkene, a cyano substituent on the alkene does not alter the ability of the system to undergo , Z-isomerization. Thus a study of the photochemical isomerization of the cyano alkenes (169) has shown that heavy atom additives and quenchers have no effect on the process and that the isomerization arises from a singlet state Other systems such as the vinylthiophenes (170) also undergo isomerization 03... 

---