HX addition

Hydrogen halides add to carbon-carbon double and triple bonds to yield haloge-nated hydrocarbons. The reactivity of the four hydrogen halides in the addition is in the order HI HBr HC1 HF. HI and HBr, the two most reactive compounds, add readily at room temperature to unsaturated hydrocarbons. HC1 requires heating or catalyst, while HF often exhibits irregular behavior. Most studies of hydrohalo-genations have focused on the use of HC1 and HBr. 

Alkenes. The reaction of hydrogen halides with alkenes is generally an electrophilic addition. It occurs by a variety of mechanisms and involves a carbocation 

Concentrated HI in water or acetic acid is usually applied in hydroiodination of alkenes. Markovnikov products are exclusively formed and rearrangements may be observed. 

Detailed wide-ranging studies are available on the addition of HC1 and HBr to alkenes. The most useful procedure is to react dry HC1 gas and the alkene neat or in an inert organic solvent. Water or acetic acid may also be used. Alkenes yielding tertiary or benzylic alkyl chlorides react most readily. Styrene, however, adds HC1 only at — 80°C to give a-chloroethylbenzene without polymerization.101 At more elevated (room) temperature polymerization prevails. HBr adds to alkenes in an exothermic process more rapidly than does HC1. Rearrangements may occur during addition indicating the involvement of a carbocation intermediate 102 

A similar conclusion can be drawn from the higher reactivity of 1,1-dialkylalkenes compared with 1,2-dialkylalkenes. Norbomene and related bicyclic olefins were 

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Neutral HX addition X = P, S, Se, Si Allylic bromination Carbon-halogen addition... 

Water adds to alkenes to yield alcohols, a process called hydration. The reaction takes place on treatment of the alkene with water and a strong acid catalyst (HA) by a mechanism similar to that of HX addition. Thus, protonation of an alkene double bond yields a carbocation intermediate, which reacts with water to yield a protonated alcohol product (ROH2+). Loss of H+ from this protonated alcohol gives the neutral alcohol and regenerates the acid catalyst (Figure 7.2). 

The stereochemistry of HX addition is varied. Examples are known of predominant syn, anti, and nonstereoselective addition. It was found that treatment of 1,2-dimethylcyclohexene (4) with HBr gave predominant anti addition, ° while addition of water to 4 gave equal amounts of the cis and trans alcohols ... 

In order to understand the regiochemistry and stereochemistry of HX addition, we must analyze the proposed mechanism. When adding HX across a double bond, there are two key steps involved in the mechanism ... 

In order to understand the regiochemistry of HX additions to alkenes, we focused our attention on the intermediate carbocation. We argued that the reaction would proceed via the more stable carbocation. This all-important principle will also help explain why some reactions will involve a rearrangement. For example, consider the following reaction ... 

The regiochemical reversal induced by the acidic compounds led them to propose the mechanisms illustrated in Scheme 11. In both catalytic cycles, the alkyne linkage inserts into the H-Pd bond the left cycle (no acidic additive) forms linear alkenylpalladium complex, while the right cycle (with HX additive) generates branched alkenyl species. The provenance of the difference in the insertion regioselectivity has not been clearly addressed. 

For the HX additions above, we noted that, in aqueous solution, water would be the most abundant nucleophile, and the predominant product would thus be an alcohol derivative. A similar situation holds if we use aqueous bromine or chlorine, for example. The product is going to be a halo... 

Dihalocyclobutanes 5. key starting materials in an attempted generation of cy-clobutylidenes, were prepared by HX addition (X = Cl, Br or I) to 1-bromocyciobutene.11... 

These substituents usually are strongly electronegative relative to hydrogen, and this often causes diminished reactivity of the double bond toward electrophiles. Nonetheless, the preferred orientation of HX addition situates the positive charge of the intermediate carbocation next to the substituent ... 

The modern view of HX addition is that H+ is transferred from HX to the alkene to give a carbocation. The major product is the one derived from the more stable carbocation. Compare the energies of 1-propyl and 2-propyl cations (protonated propene), 2-methyl-1-propyl and 2-methyl-2-propyl cations (protonated 2-methylpropene), and 2-methyl-2-butyl and 3-methyl-2-butyl cations (protonated 2-methyl-2-butene). Identify the more stable cation in each pair. Is the product derived from this cation the same product predicted by Markovnikov s rule Is the more stable carbocation also the one for which the positive charge is more delocalized Compare atomic charges and electrostatic potential maps for one or more pairs of carbocations. 

Markovnikov Rule predicts the regiochemistry of HX addition to unsymmetrically substituted alkenes. 

One of the first mechanistic proposals for the hydrocarboxylation of alkenes catalyzed by nickel-carbonyl complexes came from Heck in 1963 and is shown in Scheme 24. An alternate possibility suggested by Heck was that HX could add to the alkene, producing an alkyl halide that would then undergo an oxidative addition to the metal center, analogous to the acetic acid mechanism (Scheme 19). Studies of Rh- and Ir-catalyzed hydrocarboxylation reactions have demonstrated that for these metals, the HX addition mechanism, shown in Scheme 24, dominates with ethylene or other short-chain alkene substrates. Once again, HI is the best promoter for this catalytic reaction as long as there are not any other ligands present that are susceptible to acid attack (e g. phosphines). 

One currently accepted mechanism for the addition of two equivalents of HX to an alkyne involves two steps for each addition of HX addition of H (from HX) to form a carboca-tion, followed by nucleophilic attack of X . Mechanism 11.1 illustrates the addition of HBr to... 

Because of the instability of a vinyl carbocation, other mechanisms for HX addition that avoid formation of a discrete carbocation have been proposed. It is likely that more than one mechanism occurs, depending in part on the identity of the alkyhe substrate. 

When anhyd HCl is bubbled through trans-IrCl(CO)(PPh3)2 in ether, the product HIr(Cl)2(CO)(PPh3)2 is formed rapidly and quantitatively. In nonpolar solvents or in the solid state the cis isomer is obtained. In polar solvents mixtures of cis and trans adducts formT When the phosphine is tri-o-tolylphosphine, HX addition to trans-IrCl(CO)(PR3)2 is slow because the apical sites are blocked. When the sterically bulky but strongly basic tricyclohexylphosphine is incorporated into trans-IrCl(CO)(PR3)2, the HCl adduct is formed easily. Moreover, when the phosphine of the complex is PMCjCo-MeOCjHp, interaction of the methoxy oxygen with the Ir center increases its nucleophilicity and so facilitates oxidative addition. ... 

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