Hydrohalogenation

Examples of addition of HCl to a triple bond have been reported in (93EUP 571326). Aeetie aeid esters of formula 44 may be obtained from alkynylpyrazoles 43 by aleoholysis with HCl in the presenee of methanol (Seheme 85). 

A study applying core-electron spectroscopy on the addition of HC1 to ethylene, propylene, and isobutylene in the gas phase concludes that a significant portion of the difference between Markovnikov and anti-Markovnikov addition is also 

Mixtures of HC1 and 2-butyne were reacted in the gas phase in a Pyrex IR cell between 23 and 63°C to yield only (Z)-2-chloro-2-butene.564 The process is suggested to be initiated and the rate determined by surface-assisted proton-alkyne interaction. Donation of the proton occurs from within a multilayer found on the wall, that is, from an HC1 molecule not directly attached to the wall. 

Hydrogen halides may add to acetylenes in a similar way to afford alkenyl halides.565 The use of silica and alumina, in this case, provides a simple means for facilitating addition of hydrogen halides to alkynes that does not occur readily in solution. Arylalkylacetylenes yield the corresponding syn-addition products (45) which undergo isomerization on extended treatment  

Addition of HBr or HI to alkylacetylenes, such as 4-octyne, in turn, affords primarily the anfi-addition products. The change in mechanism may be due to the difficulty in forming an alkyl cation lacking stabilization of the Ph substituent, thereby requiring simultaneous nucleophilic trapping from the opposite side of the alkyne by a HHlg molecule (AdE3-type addition). 

The MesSiCl and water system is a convenient reagent for the selective hydrochlorination of alkenes. The anti selective manner of addition was shown to occur by reacting 9(10)-octalin to afford 4a-chloro-frans-decahydronaphthaline exclusively.566 

de la Mare and R. Bolton, Electrophilic Additions to Unsaturated Systems, Elsevier, New York (1966), Chpt 5 

Buehler and D. E. Pearson, Survey of Organic Chemistry, Wiley Interscience, New York (1970), Vol 1, p 356 

L1AIH4, Cp2 nCl2/Br2 HS1CI3, cat (it-C3H5PdCl)2, cat chiral ligand/KF/NBS (chiral) 

/ CuX2 UAIH4, TiCL, or ZiCU/X2 HSiCl, cat HjPtCl /KF/CuXj 

various catalysts Me3SiCl, A1203 TiCl4, ROH 

TiCl4 or ZrCl4/X2 HSiCl3, cat H2PtCl6/KF/CuX2 

Br continued) (COBr)2, AI2O3 Me3SiBr, Sip2 Me3SiBr, AI2O3 JACS 112 7433 (1990) JACS 115 3071 (1993) JACS 115 3071 (1993) 

Now let s consider the entropy term — T AS. This term will always be positive for an addition reaction. Why In an addition reaction, two molecules are joining together to produce one molecule of product. As described in Section 6.2, this situation represents a decrease in entropy, and AS will have a negative value. The temperature component, T (measured in Kelvin), is always positive. As a result, — T AS will be positive for addition reactions. 

Now let s combine the enthalpy and entropy terms. The enthalpy term is negative and the entropy term is positive, so the sign of AG for an addition reaction will be determined by the competition between these two terms  

Not all addition reactions are reversible at high temperature, because in many cases high temperature will cause the reactants and/ or products to undergo thermal degradation. 

In order for AG to be n ative, the enthalpy term mnst be larger than the entropy term. This competition between the enthalpy term and the entropy term is temperatnre dependent. At low temperature, the entropy term is small, and the enthalpy term dominates. As a resnlt, AG will be negative, which means that prodncts are favored over reactants (the eqniUbrinm constant K will be greater than 1). In other words, addition reactions are thermodynamically favorable at low temperature. 

However, at high temperature, the entropy term will be large and will dominate the enthalpy term. As a result, AG will be positive, which means that reactants will be favored over products (the equihbrium constant A will be less than 1). In other words, the reverse process (elimination) will be thermodynamically favored at high temperature  

In 1900 Weber passed hydrogen chloride into a chloroform solution of natural rubber to produce what is commonly known as rubber hydrochloride . While natural rubber, the synthetic 1,4- and 3,4-polyisoprenes and poly-1,4-dimethyl butadiene add hydrochloric acid with ease (a, b, and c, respectively), 1,2-polyisoprene and the 1,2- and 1,4-polybutadienes (d, e and f, respectively) are relatively unreactive. 

This is a consequence of the well-established ability of branched olefins to undergo electrophilic attack. 

Such an addition is according to Markovnikov s rule. Evidence for this mode of addition, at least in the case of c/s-1,4-poIyisoprenes was provided by X-ray analysis (Bunn and Garner, 1942) which showed that the chlorine was attached to the t-carbon atom. In addition Salomon et al. (1948) showed that the chemical reactivity of the C—Cl bond was consistent with that of a tertiary carbon atom. 

Hydrochlorination of natural rubber usually yields a polymer with slightly less than the theoretical chlorine content for complete reaction (e.g. 33 5 cf. 33-9%). NMR studies (Golub and Heller, 1964) have established that some cyclization occurs during hydrohalogenation and this can account for the discrepancy. 

SBR hydrochlorides have been prepared by reaction under pressure and the products are claimed to have better stability than those of the cis-polyisoprenes (Salomon and Koningsberger, 1958). 

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When the preceding rules lead to inconvenient names, then (1) the unaltered name of the base may be used followed by the name of the anion or (2) for salts of hydrohalogen acids only the unaltered name of the base is used followed by the name of the hydrohalide. An example of the latter would be 2-ethyl-p-phenylenediamine monohydrochloride. 

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... 

Halosdanes are very reactive toward protic chemicals. They generally react violendy with water, forming siUcon dioxide and the respective hydrohalogens. Other examples include reaction with alcohols and amines as follows ... 

Health and Safety. Halosilane vapors react with moist air to produce the respective hydrohalogen acid mist. Federal standards have not set exposure to halosilanes, but it is generally beheved that there is no serious risk if vapor concentrations are maintained below a level that produces an irritating concentration of acid mist. The exposure threshold limit value (TLV) for HCl is 5 ppm, expressed as a ceiling limit. Because most people experience odor and irritation at or below 5 ppm, HCl is considered to have good warning properties. 

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. 

Halogenation—Hydrohalogenation. The most important iatermediate is ethylene dichloride [107-06-2] (EDC) which is produced from ethylene either by direct chlorination or by oxychloriaation. Direct chlorination is carried out ia the Hquid or vapor phase over catalysts of iron, alumiaum, copper, or antimony chlorides, and at conditions of 60°C. Oxychloriaation is carried out ia a fixed or fluidized bed at 220°C with a suitable soHd chloride catalyst. 

Like NR, SBR is an unsaturated hydrocarbon polymer. Hence unvulcanised compounds will dissolve in most hydrocarbon solvents and other liquids of similar solubility parameter, whilst vulcanised stocks will swell extensively. Both materials will also undergo many olefinic-type reactions such as oxidation, ozone attack, halogenation, hydrohalogenation and so on, although the activity and detailed reactions differ because of the presence of the adjacent methyl group to the double bond in the natural rubber molecule. Both rubbers may be reinforced by carbon black and neither can be classed as heat-resisting rubbers. 

ChLorofluorocarbene generated by the thermal decomposition of dichlorofluoro-methylphenylmercury reacts with 2,3-dimethylindole to give 3-fluoro-2,4-di-methylquinoline, 3-chloro-2,4-dimethylqumoline and 3-(chlorofluoromethyl)-2,3-dimethylindole [f] (equation 1). Similar results are obtained when the chloro-fluorocarbene is generated from dichlorofluoromethane by base-catalyzed de-hydrohalogenation using a phase-transfer catalyst in an aqueous-organic solvent system [21 (equation 1). 

Thienylacetylenes have been prepared in good yield through de-hydrohalogenation of 1,2-dichloroethylthiophenCs or 1-chlorovinyl thiophenes, which are obtained from acetylthiophenes and PCI5, with sodium amide and in liquid ammonia. " The 3-isomers show... 

Although the chlorosultones can be isolated by the same procedure, the isolation is tedious at concentrations below 5 ppm. The chlorosultones are best determined by performing two analyses, one on the intact surfactant and one in which the sultone-containing concentrates are treated with collidine to de-hydrohalogenate the chlorosultones before measurement of the total quantity of unsaturated sultones. 

Compare the reaction enthalpies for the hydrohalogenation of ethene by HX, where X = Cl, Br, 1. What trend, if any, exists in these numbers Use bond enthalpies to estimate rhe enthalpies of reaction. 

Butene reacts with hydrogen chloride in a hydrohalogenation reaction. Estimate the standard reaction enthalpy from bond enthalpies. 

In each mechanism above, the first step involves protonation of the alkene to form a carbocation. Then, in both cases, a nucleophile (either X or H2O) attacks the car-bocation to give a product. The difference between these two reactions is in the nature of the product. The first reaction above (hydrohalogenation) gives a product that is neutral (no charge). However, the second reaction above (hydration) produced a charged species. Therefore, one more step is necessary at the end of the hydration reaction— we must get rid of the positive charge. To do this, we simply deprotonate ... 

In complexing acid media (e.g. hydrohalogenic acids HX) the reduction of pertechnetate ions proreeds in a different way. In the works [11,13,15] it was shown that such a reduction occurred according to the scheme (1). 

A number of studies [42,43,52,114] have shown that [TcX6]2 " entered into reduction reactions more readily in hydrohalogenic acids at a concentration of 4-7 M than in concentrated hydrohalogenic acids. It was assumed [9,11,52] that the reduction of [TcC16]2 was promoted by their acidic hydrolysis (anation reaction) (2). 

The stability of the [Tc2X8]3 ions forming in hydrohalogenic acids is determined by the sum of the competing processes (5-7) [80,87]. 

The cluster formation reactions considered in this review occur in technetium-concentrated solutions of hydrohalogenic acids. From comparison of the data reported in [9,11,44,49,50,53,68,72,73,75,76,118] it follows that the ions [Tc2X6]2b (X = Cl,Br) are immediate precursors to polynuclear technetium... 

The hydrohalogenation of phenyl(3-methyl-l,2-butadienyl) phosphinic esters involved protophilic attack of the reagent, followed by heterocyclization of the allenephosphonate system (Scheme 40) [103, 104],... 

Nearly all methods for preparation of stearolic acid involve de-hydrohalogenation of a 9,10-dihalostearic acid, or its esters, with alcoholic potassium hydroxide the most recent method is that of Adkins and Burks.8 These methods employ drastic conditions, which result in poorer yields than those obtainable on dehydrohalogenation with sodamide.6 Methyl 9,10-dibromostearate, on dehydrobromi-nation with sodamide, yields stearolamide 6 (m.p. 82-83°) which may be hydrolyzed to stearolic acid. For preparative purposes, however, this method offers-no special advantage over that described here. 

Hydrohalogenation, 10 597 Hydroiodic acid (HI), 14 360, 374. See also Hydrogen iodide Hydroisoquinolines, 21 205, 206 Hydrolases, 3 675-676 Hydrologic cycle, 26 2-3. See also Hydrogeochemical cycle(s) carbon circulation in, 26 27—30 chlorine circulation in, 26 31 nitrogen circulation in, 26 32 sulfur circulation in, 26 30-31 Hydrology, in radioactive waste disposal, 25 856, 857... 

Although the synthesis is completed in very few steps, oxidation of 1,4-xylene to the corresponding terephthalic acid does not afford a uniform product. Partial dihalogenation gives rise to side products. The condensation reaction requires two equivalents of arylamine per halogen atom. One equivalent is needed to neutralize the generated hydrohalogen acid, which is subsequently separated as aryl-amine-hydrohalide and recycled as hydrohalide and arylamine. 

Direct hydrohalogenation of 1,2-allenic sulfoxides with MX in HOAc failed. A1X3 was found to be a good X- donor to afford 2-haloallylic sulfoxides in 56-89% yields. In this reaction, an Al3+ species activates the reactivity of the allene moiety by its interaction with the sulfmyl group [87, 98]. 

Allenyl sulfones can also readily undergo hydrohalogenation reactions with MX (M = Na, Li X=C1, Br, I) to afford 2-haloallylic sulfones 236 [87, 126]. The reactivity depends greatly on the substitution pattern of the allene moiety. 

Ma et al. reported the hydrohalogenation of 1,2-allenyl ketones leading to /1-halo-/8,y-unsaturated enones [87, 207]. The reaction of 3-substituted-l,2-allenyl ketone with MX in HOAc affords ( )-2-halo-2-alkenyl ketones as the major product. In HOAc, the reaction of l-substituted-l,2-allenyl ketones affords a mixture of/3-halo-a,/3- and -/3,y-unsaturated enones whereas in CF3C02H-H0Ac (1 1) the same reaction affords /3,y-unsaturated enones. 

The hydrohalogenation reaction can also be performed with MX in HOAc [207a], The reaction of 4-substituted-2,3-allenoates affords a mixture of ( )- and (Z)-/3-halo-/8,y-unsaturated enoates [87]. With 2-substituted-2,3-allenoates, the reaction should be carried out in H0Ac-CF3C02H (1 1) or CF3C02H to form /8-halo-/3,y-unsaturated enoates with high regioselectivity [255],... 

The hydrohalogenation reaction of 2,3-allenoamide with MX also delivers /3-halo-/3,y-unsaturated enamides. The Z- and E-isomers can be easily separated [87]. 

The hydrohalogenation of l-cyano-l,2-allenes with MX affords /i,y-unsaturated enenitriles [87, 277]. For the 4-alkyl-substituted 2,3-allenenitriles, the stereoselectivity is low however, the Z- and E-isomers can be easily separated to provide easy access to the pure (Z)- and (fc)-/)-halo-/),y-unsaturated enenitriles. 

The mechanism for the acid-catalysed hydration reaction is very similar to that for the hydrohalogenation of alkenes and also proceeds via a carbocatlon intermediate. It is outlined below using water and propene. 

Like the hydrohalogenation reaction, the hydration of unsymmetrlcal alkenes, such as propene, leads to two products. As well as propan-2-ol, propan-l-ol is also formed. 

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. 

Figure 3. A 220-MHz NMR spectra of the hydrohalogenated 1,4-polyisoprene samples. Chlorobenzene solutions at 100°C with TMS as reference.

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