Addition of hydrogen halides HX

The addition of HX occurs so as to give the most substituted alkyl halide, following Markovnikov s rule (see Section 6.2.2.1). One or two equivalents of HX can be used to form vinyl halides or dihaloalkanes, respectively. 

The addition proceeds via the most stable (or substituted) vinylic carbocation 

Vinylic carbocations are generally less stable than alkyl carbocations, as there are fewer +1 alkyl groups to stabilise the positive charge. As a consequence, alkynes (which give vinylic carbocations) generally react more slowly than alkenes (which give alkyl carbocations) in electrophilic addition reactions. 

Alkynes will react with water in the presence of a mercury(II) catalyst. Water adds in a Markovnikov addition to form an enol, which tauto-merises to give a ketone (see Section 8.4.1). 

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Other methods for the preparation of alkyl halides are electrophilic addition of hydrogen halides (HX) to alkenes (see Section 5.3.1) and free radical halogenation of alkanes (see Section 5.2). 

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

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

Hydrogenolysis (Section 28.7) A reaction that cleaves a o bond using H2 in the presence of a metal catalyst, a Hydrogens (Section 23.1) The hydrogen atoms on the carbon bonded to the carbonyl carbon atom (the a carbon). Hydrohalogenation (Section 10.9) An electrophilic addition of hydrogen halide (HX) to an alkene or alkyne. 

As shown in this equation, addition of both the first and second moles of HBr is regiose-lective. Addition of hydrogen halides follows Markovnikov s rule (Section 6.3A) hydrogen adds to the carbon that has the greater number of hydrogens. We can account for this regioselectivily of addition of HX by a two-step mechanism for each addition. 

The addition of hydrogen halides to alkenes gives us a way to make allg l bromides, chlorides, and iodides (fluorides don t work very well) as long as we have the appropriate starting alkene. We will have to be careful when using unsymmetrical alkenes, because there is a choice to be made here. As you saw in Section 3.18, the addition of HX will only lead to the more substituted alkyl halide, as the regiochem-istry of the addition is controlled by the stability of the intermediate carbocation. 

Numerous ionic compounds with halogens are known but a noble gas configuration can also be achieved by the formation of a covalent bond, for example in halogen molecules, X2, and hydrogen halides, HX. When the fluorine atom acquires one additional electron the second quantum level is completed, and further gain of electrons is not energetically possible under normal circumstances, i.e... 

Let s compare the carbocation intermediates for addition of a hydrogen halide (HX) to an unsymmetrical alkene of the type RCH=CH2 (a) according to Markovnikov s rule and (b) opposite to Markovnikov s rule (a) Addition according to Markovnikov s rule... 

In the ternary systems aromatic substance (A)-Lewis acid (MX3)-hydrogen halide (HX) the formation of a proton addition complex can be formulated analogously. 

Alkenes are converted to alkyl halides by the addition of HX (HCl, HBr or HI). Addition of HX to unsymmetrical alkenes follows Markovnikov s rule. The reaction is regioselective, and occurs via the most stable carbocation intermediate. For example, addition of hydrogen bromide (HBr) to propene yields 2-bromopropane as the major product. 

There have been both experimental and theoretical studies to probe the degree of concertedness in gas-phase substitutions as shown in Scheme 1. Is (2) an intermediate with a finite lifetime, or are the addition and elimination steps concerted so that (2) is a transition state Experimental molecular beam studies on the femtosecond time-scale have been reported for the reaction of chloride ions with the iodobenzene cation to yield chlorobenzene and iodine. The results show an 880 fs reaction time for the elimination process, indicating a highly non-concerted process, so that here the er-complex is an intermediate rather than a transition state.12 The reactions of halobenzene cations with ammonia have been interpreted in terms of the formation of an addition complex which may eliminate either halogen, X, or hydrogen halide, HX, depending on the nature of the halogen.13... 

The most important addition reactions are the addition of hydrogen (H2), the halogens (X2), the hydrogen halides (HX) and water (H—OH). 

The hydrocarboxylation reactions discussed above have been proposed to involve direct addition of water to the metal center prior to elimination of the product, analogous to the oxidative addition of hydrogen to a metal center at the end of a hydroformylation catalytic cycle. Another class of hydrocarboxylation reactions is more analogous to the haUde-promoted Monsanto acetic acid process, where one has a reductive elimination of an acyl halide species that is rapidly hydrolyzed with free water to generate the carboxylic acid and HX. 

FIGURE 9.5 (a). Curved arrow notation and (b) transition-state representation for electrophilic addition of a hydrogen halide HX to an alkyne. 

Let us consider how independent /i(i ) 2 effects contribute to the v E) for the hydrogen halides, HX (X = I, Br, and Cl). The curves shown on Fig. 7.6 correspond to relativistic adiabatic potential energy curves (respectively 0 dotted, 0+ dashed, 1 and 2 solid) for HI obtained after diagonalization of the electronic plus spin-orbit Hamiltonians (see Section 3.1.2.2). The strong R-dependence of the electronic transition moment reflects the independence of the relative contributions of the case(a) A-S-Q basis states to each relativistic adiabatic II state. The independent experimental photodissociation cross sections are plotted as solid curves in Fig. 7.7 for HI and HBr. Note that, in addition to the independent variations in the A — S characters of each fl-state caused by All = 0 spin-orbit interactions, all transitions from the X1E+ state to states that dissociate to the X(2P) + H(2S) limit are forbidden in the separated atom limit because they are at best (2Pi/2 <— 2P3/2) parity forbidden electric dipole transitions on the X atom. In the case of the continuum region of an attractive potential, the energy dependence of the dissociation cross section exhibits continuity in the Franck-Condon factor density (see Fig. 7.18 Allison and Dalgarno, 1971 Smith, 1971 Allison and Stwalley, 1973). 

In contrast, the reaction of decamethylsilicocene with protic substrates follows three different pathways (Schemes 2 and 3) [6]. Oxidative addition is observed with the hydrogen halides HX, and the protonated silicocene is formed in the reaction with two equivalents of trifluorosulfonic acid. In the reaction with HBF4, the wanted MesCsSr cation might be present together with the BF4" anion as a highly reactive intermediate which easily eliminates BF3 under formation of the polymer (Me5C5SiF) in a final step. 

Blake and Kubota have convincingly shown that in anhydrous benzene, chloroform and ether, as in the gaseous reaction on the solid complex, hydrogen halides (HX) and /m -[Iry(CO)(PPh3)2] (T = halogen) react to give octahedral cw-addition products (i.e. H and X cis). In contrast in wet solvents, and in such solvents as dimethylformamide, acetonitrile and ethanol, mixtures of cis and trans products are formed. Whether these solvents cause rapid halide-ion exchange before the addition reaction or in the products is yet to be demonstrated. ... 

Energy diagrams comparing addition of a hydrogen halide HX with an alkene H2C=CHR according to Markovnikov s rule (solid red) and opposite to Markovnikov s rule (dashed blue). The energy of activation is less and the reaction is faster for the reaction that proceeds through the more stable secondary carbocation. 

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