Alkanes electrophilic addition reactions

The Lead-Off Reaction Addition of HBr to Alkenes Students usually attach great-importance to a text s lead-off reaction because it is the first reaction they see and is discussed in such detail. 1 use the addition of HBr to an alkene as the lead-off to illustrate general principles of organic chemistry for several reasons the reaction is relatively straightforward it involves a common but important functional group no prior knowledge of stereochemistry or kinetics in needed to understand it and, most important, it is a polar reaction. As such, 1 believe that electrophilic addition reactions represent a much more useful and realistic introduction to functional-group chemistry than a lead-off such as radical alkane chlorination. 

Alkane functionalization by electrophilic addition reactions is also possible for example, the particularly stable tertiary adamaniyl cation must be involved in equation (34), a reaction which gives an excellent 75% yield of adduct In a similar way, a variety of alkenes and arenes can be alkylated by alkanes, or alkanes acylated by RCOCl/AlBra. ... 

Key point. Alkenes and alkynes are unsaturated hydrocarbons, which possess a C=C double bond and a C=C triple bond, respectively. As (weak) re-bonds are more reactive than (strong) o-bonds, alkenes and alkynes are more reactive than alkanes. The electron-rich double or triple bond can act as a nucleophile, and most reactions of alkenes/ alkynes involve electrophilic addition reactions. In these reactions, the re-bond attacks an electrophile to generate a carbocation, which then reacts with a nucleophile. Overall, these reactions lead to the addition of two new substituents at the expense of the re-bond. 

This reactivity makes alkenes an important class of organic compounds because they can be used to synthesize a wide variety of other compounds. For example, alkyl halides, alcohols, ethers, and alkanes all can be synthesized from alkenes by electrophilic addition reactions. The particular product obtained depends only on the electrophile and the nucleophile used in the addition reaction. 

Electrophilic addition reactions of alkenes lead to the synthesis of alkyl halides, vicinal dihalides, halohydrins, alcohols, ethers, and alkanes. 

Cyclopropane is one notable exception to the generalization that cyclic and acyclic compounds undergo the same reactions. Although it is an alkane, cyclopropane undergoes electrophilic addition reactions as if it were an alkene. 

Besides the addition of halides and hydrogen-halide adds to alkenes or alkynes, other industrially relevant electrophilic addition reactions involve hydratization reactions (addition of water to alkenes and alkynes, forming alcohols), cationic polymerization (addition of carbocation to an alkene), hydrogenation (addition of hydrogen to alkenes to form alkanes), and Diels-Alder reactions (addition of an alkene to a conjugated diene to form complex, unsaturated hydrocarbon structures). 

Various side-reactions may complicate the course of the Nef reaction. Because of the delocalized negative charge, the nitronate anion 2 can react at various positions with an electrophile addition of a proton at the a-carbon reconstitutes the starting nitro alkane. 1. The nitrite anion can act as leaving group, thus leading to elimination products. 

The selective oxidation of C—H bonds in alkanes under mild conditions continues to attract interest from researchers. A new procedure based upon mild generation of perfluoroalkyl radicals from their corresponding anhydrides with either H2O2, m-CPBA, AIBN, or PbEt4 has been described. Oxidation of ethane under the reported conditions furnishes propionic acid and other fluorinated products.79 While some previously reported methods have involved metal-mediated functionalization of alkanes using trifluoroacetic acid/anhydride as solvent, these latter results indicate that the solvent itself without metal catalysis can react as an oxidant. As a consequence, results of these metal-mediated reactions should be treated with caution. The absolute rate constants for H-abstraction from BU3 SnH by perfluorinated w-alkyl radicals have been measured and the trends were found to be qualitatively similar to that of their addition reactions to alkenes.80 a,a-Difluorinated radicals were found to have enhanced reactivities and this was explained as being due to their pyramidal nature while multifluorinated radicals were more reactive still, owing to their electrophilic nature.80... 

One of the simplest classes of nucleophiles that attacks borane is that of alkenes. The result, described as hydroboration, is an overall addition of borane across the double bond. Unlike most electrophilic additions to alkenes that occur in a stepwise manner via charged intermediates (Chapter 20), this addition is concerted so that both new bonds are formed more or less at the same time. The result is a new borane in which one of the hydrogen atoms has been replaced by an alkane. This monoalkyl borane (RBH2) is now able to undergo addition with another molecule of the alkene to produce a dialkyl borane (R2BH) which in turn undergoes further reaction to produce a trialkyl borane (R3B). All these boranes have a vacant p orbital and are flat so that repeated attack to produce the trialkyl borane is easy and normal if an excess of alkene is present. 

Then we shall examine the stereochemistry of several reactions we have already studied—free-radical halogenation of alkanes, and electrophilic addition of halogens to alkenes- and see how stereochemistry can be used to get information about reaction mechanisms. In doing this, we shall take up ... 

The addition reactions take place at a carbon-carbon multiple bond, or carbon-hetero atom multiple bond. Because of this peculiarity, the addition reactions are not common as the first step in pyrolysis. The generation of double bonds during pyrolysis can, however, continue with addition reactions. The additions can be electrophilic, nucleophilic, involving free radicals, with a cyclic mechanism, or additions to conjugated systems such as Diels-Alder reaction. This type of reaction may explain, for example, the formation of benzene (or other aromatic hydrocarbons) following the radicalic elimination during the pyrolysis of alkanes. In these reactions, after the first step with the formation of unsaturated hydrocarbons, a Diels-Alder reaction may occur, followed by further hydrogen elimination ... 

Bromine is an electrophile which will undergo an addition reaction with alkenes in the dark. The lack of a reaction in the dark with Br2 indicates that the molecule is not an alkene. In the presence of light, bromine will undergo a substitution reaction with alkanes. Therefore, the molecule is most likely an alkane and the only alkane with the molecular formula C3H6 is cyclopropane. 

CH proton 2) the ability of the adduct to undergo oxidative addition and 3) the preference, probably of steric origin, for binding at the least-hindered CH bond. The first allows facile deprotonation or H/D exchange in an alkane complex. The second leads to alkylmetal hydrides that can evolve to give alkene or a functionalized product. The third may be a factor in the selectivity (primary > secondary > tertiary) shown by many of the systems described in this article, a selectivity that contrasts with that seen for conventional radical and electrophilic CH reactions (tertiary > secondary > primary). 

It is therefore not surprising that the reactivities of arenes and alkanes in electrophilic substitution reactions are very different, with the former being much more active. At the same time, the mechanism of the interaction (oxidative addition) of both saturated and aromatic hydrocarbons with complexes of metals in a low oxidation state is in principle the same. The reactivities of arenes and alkanes in oxidative addition reactions with respect to low-valent metal complexes therefore usually differ insignificantly. Furthermore, a metal complex via the oxidative addition mechanism can easily cleave the C-H bond in olefin or acetylene. 

Formed in superacid media species exhibiting electrophilic properties are able to attack alkanes primarily via electrophilic addition to C-H bond followed by other reactions. In particular, Olah et al. observed O atom insertion in hydrogen peroxide reaction with methane in Magic Acid above 0 °C to produce methanol with very high (>95%) selectivity [54a]. The particle (OH)"", which may be considered to be a protonated oxygen atom in the singlet state is apparently the active species in the reaction. Methyl alcohol formed is immediately protonated to methyloxonium ion, and this prevents further... 

The simplest reaction of alkenes is the addition of hydrogen to form alkanes. Hydrogenation is a nonpolar process, unlike many of the addition reactions, to be described later, that involve electrophiles attaching to the nucleophilic n electrons of the alkene. Hydrogenation reactions require catalysts, such as platinum oxide (Adams s catalyst), palladium on carbon (Pd-C), or Raney nickel (Ra-Ni). —, ... 

Typical chlorinations of alkanes or alkenes with (dichloroiodo)benzene proceed via a radical mechanism and generally require photochemical conditions or the presence of radical initiators in solvents of low polarity, such as chloroform or carbon tetrachloride. However, the alternative ionic pathways are also possible due to the electrophilic properties of the iodine atom in PhICH or electrophilic addition of CI2 generated by the dissociation of the reagent. An alternative synchronous molecular addition mechanism in the reactions of PhICl2 with alkenes has also been discussed and was found to be theoretically feasible [44]. The general reactivity patterns of ArICh were discussed in detail in several earlier reviews [8, 45, 46]. 

Table 9.3 summarizes reactions that reduce alkynes to alkenes and alkanes. Table 9.4 summarizes electrophilic addition to alkynes. 

---