Additions Hydrohalogenation

Electrophilic (and Markovnikov) Additions Hydrohalogenation, Halogenation, and Hydration (Section 13-7)... 

Substituted N-vinylpyrroles with excess hydrogen halides HX (X=C1, Br) form (0°C, CD2CI2) 3-halodihydropyrrolium ions 5 (Scheme 2.5). This reaction represents the first example of additive hydrohalogenation of the pyrrole ring [424]. 

Halogenation and Hydrohalogenation. Halogens add to the triple bond of acetylene. FeCl catalyzes the addition of CI2 to acetylene to form 1,1,2,2-tetrachloroethane which is an intermediate in the production of the industrial solvents 1,2-dichloroethylene, trichloroethylene, and perchloroethylene (see Chlorocarbons and chlorohydrocarbons). Acetylene can be chlorinated to 1,2-dichloroethylene directiy using FeCl as a catalyst... 

Addition. Addition reactions of ethylene have considerable importance and lead to the production of ethylene dichloride, ethylene dibromide, and ethyl chloride by halogenation—hydrohalogenation ethylbenzene, ethyltoluene, and aluminum alkyls by alkylation a-olefms by oligomerization ethanol by hydration and propionaldehyde by hydroformylation. 

In the early days of alkene chemistry, some researchers found that the hydrohalogenation of alkenes followed Markovnikov s rule, while others found that the same reaction did not. For example, when freshly distilled but-l-ene was exposed to hydrogen bromide, the major product was 2-bromopropane, as expected by Markovnikov s rule. However, when the same reaction was carried out with a sample of but-l-ene that had been exposed to air, the major product was 1-bromopropane formed by antl-Markovnikov addition. This caused considerable confusion, but the mystery was solved by the American chemist, Morris Kharasch, in the 1930s. He realised that the samples of alkenes that had been stored in the presence of air had formed peroxide radicals. The hydrohalogenation thus proceeded by a radical chain reaction mechanism and not via the mechanism involving carbocation intermediates as when pure alkenes were used. 

The addition of alcohols to ketene acetals allows the synthesis of mixed ortho esters [96, 120a-c, 121a, b, 124, 125a, b]. a-Haloaldehydes may be converted to ortho esters by the following process (a) acetal formation, (b) de-hydrohalogenation, and (c) reaction with alcohols via addition reaction (33). In general, the method above, using ketene acetals, is not practical since ketene acetals are not readily available and are difficult to prepare. However, the method is useful because it allows the synthesis of mixed ortho esters and other ortho esters more difficult to synthesize [122-127]. Recently a simple one-step synthesis of ketene acetals and ortho esters has been reported (see p. 56). 

Special reagents introduced more recently allow significant improvements in hydrohalogenation of alkenes. HC1, HBr, and HI in aqueous-organic two-phase systems under phase-transfer conditions readily add to alkenes affording halides in nearly quantitative yields.123 Appropriately prepared silica and alumina have been found to mediate the addition of HC1, HBr, and HI to alkenes.124 The method is very convenient since it uses hydrogen halides prepared in situ 124 125... 

The addition of hydrogen halides to unsaturated organic compounds is called here hydrohalogenation in order to stress that it is the reverse reaction to dehydrohalogenation. Two types of the hydrohalogenation reaction have to be considered, the addition to a carbon carbon triple bond... 

Hydrohalogenation (see Addition reactions to carbon-carbon multiple bonds)... 

All of die halides except tile hexafluoride and the triiodide may be prepared by the hydrohalogenation of rhe dioxide or of the oxalate of plutonium(lll) at a temperature of about 700°C. With hydrogen fluoride the reaction product is PUF4, unless hydrogen is added to the gas stream, in which case the trifluoride is produced. With hydrogen iodide the reaction product is PuOi, and the other oxyhalides may be formed by the addition of appropriate quantities of water vapor to the hydrogen halide gas. Plutonium triiodide is produced by the reaction of the metal with hydrogen iodide at about 400°C. The hexafluoride is produced by direct combination of the... 

Alkenes also undergo a variety of other addition reactions in which a reagent is added across the double bond. Hydration and hydrohalogenation are classic examples. 

Problem 6.23 Arrange the following alkenes in order of increasing reactivity on addition of hydrohalogen acids a) H2C=CH2, (b) (CH3)2C=CH2, (r) CH3CH=CHCH,. ... 

The addition products 125 cyclized upon treatment with base to form the corresponding cyclopropane carboxylates which underwent spontaneous de-hydrohalogenation at room temperature to form dihaloalkenes 126 as a mixture of cis/trans diastereoisomers. Hydrolytic cleavage from the support then gave the product acids 127 [ 18]. 

Chiral Ligand for other Stereoselective Reactions. The effect of the addition of dihydroquinidine-derived alkaloids on the product enantioselectivity has also been investigated in the addition reaction of Diethylzinc to aldehydes, in the addition of aromatic thiols to conjugated cycloalkenones, and in the heterogeneous hydrohalogenation of a,a-dichlorobenzazepinone-2. In these cases, the dihydroquinidine derivatives were not the optimal ligands. 

Hydrohalogenation is the addition of hydrogen halides HX (X = Cl, Br, and 1) to aikenes to form alkyl halides. 

Many hydrohalogenation reactions begin with an achiral reactant and form an achiral product. For example, the addition of HBr to cyclohexene, an achiral alkene. forms bromocyclohexane. an achiral alkyl halide. 

Because addition converts sp hybridized carbons to s[ hybridized carbons, however, sometimes new stereogenic centers are formed from hydrohalogenation. For example, Markovnikov addition of HCl to 2-ethyl-1-pentene, an achiral alkene, forms one constitutional isomer, 3-chloro-3-methylhexane. Because this product now has a stereogenic center at one of the newly formed sp hybridized carbons, an equal amount of two enantiomers—a racemic mixture—must form. 

The mechanism of hydrohalogenation illustrates why two enantiomers are formed. Initial addition of the electrophile (from HCl) occurs from either side of the planar double bond to form a carboca-... 

Because hydrohalogenation begins with a planar double bond and forms a planar carbocation, addition of H and Cl occurs in two different ways. The elements of H and Cl can both be added from the same side of the double bond— that is, syn addition—or they can be added from opposite sides—that is, anti addition. Both modes of addition occur in this two-step reaction mechanism. 

Hydrohalogenation (Section 10.9) An electrophilic addition of hydrogen halide (HX) to an alkene or alkyne. 

---