Hydrogen halides, HX

The intense blue colour of starch-iodine was mentioned on p. 790. 

It is common practice to refer to the molecular species HX and also the pure (anhydrous) compounds as hydrogen halides, and to call their aqueous solutions hydrohalic acids. Both the anhydrous compounds and their aqueous solutions will be considered in this section. HCl and hydrochloric acid are major industrial chemicals and there is also a substantial production of HF and hydrofluoric acid. HBr and hydrobromic acid are made on a much smaller scale and there seems to be little industrial demand for HI and hydriodic acid. It will be convenient to discuss first the preparation and industrial uses of the compounds and then to consider their molecular and bulk physical properties. The chemical reactivity of the anhydrous compounds and their acidic aqueous solutions will then be reviewed, and the section concludes with a discussion of the anhydrous compounds as nonaqueous solvents. 

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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... 

Hydrogen Chloride (HCl) EPA Method 26 is the reference method used to measure hydrogen chloride emissions from stationaiy sources. The method is applicable for determining emissions of hydrogen halides (HX) such as hydrogen chloride (HCl), hydrogen... 

It is known that the order of acidity of hydrogen halides (HX, where X = F, Cl, Br, I) in the gas phase can be successfully predicted by quantum chemical considerations, namely, F < Cl < Br < I. However, in aqueous solution, whereas hydrogen chloride, bromide, and iodide completely dissociate in aqueous solutions, hydrogen fluoride shows a small dissociation constant. This phenomenon is explained by studying free energy changes associated with the chemical equilibrium HX + H2O + HjO in the solu-... 

Alcohols react with metal hydrides, MH, and with hydrogen halides, HX, but in very different ways. Proton transfer is involved in both reactions, but different molecules act as the proton donor and acceptor. 

The hydrogen halides, HX, can be prepared by the direct reaction of the elements ... 

We will now explore the details of adding a hydrogen halide (HX) across a double bond. Here are two examples ... 

Table 3 Nudeophilicities Nb of Lewis bases B and electrophilicities hx of hydrogen halides HX ...

When hydrogen halides, HX(g), dissolve in water, the hydronium ion, H301 (aq) is always produced. 

This paper was presented at the Symposium on Polymer Chemistry as Applied to Plastics on the 22nd of September 1950. In it the author recognises the possible types of reaction by which a co-catalyst may initiate polymerisation in a mixture of an alkene and a metal halide. It is stated, probably not for the first time, that the hydrogen halides, HX, do not form protonic acids with metal halides, but nonetheless these ghosts kept on spooking around the literature for decades, and they have still not been exorcised completely. 

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

Problem 6.27 Why are dry gaseous hydrogen halides (HX) acids and not their aqueous solutions used to prepare alkyl halides from alkenes ... 

Alkyl halides are almost always prepared from corresponding alcohols by the use of hydrogen halides (HX) or phosphorus halides (PX3) in ether (see Section 5.5.3). Alkyl chlorides are also obtained by the reaction of alcohols with thionyl chloride (SOCI2) in triethylamine (Et3N) or pyridine (see Section 5.5.3). 

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). 

Alcohols react with hydrogen halides (HX) to give alkyl halides. Primary alcohols undergo Sn2 reactions with HX. Primary alcohols with branching on the P-carbon give rearranged products. The temperature must be kept low to avoid the formation of E2 product. 

Hydrogen fluoride in aqueous solution is a weak acid, characterized by its pKa value of 3.2. By comparison, the other hydrogen halides are extremely strong acids in aqueous solution all three are fully dissociated in dilute solution, and their pA", values may be estimated by thermochemical cycle calculations. The thermochemical cycle shown in Figure 3.1 represents the various processes as the aqueous hydrogen halide, HX, is converted to a solution containing hydrated protons and hydrated halide ions. The enthalpy of acid dissociation of the HX(aq) compound is given by ... 

The -elimination of hydrogen halides HX from organic halogen compounds yields olefins or acetylenes, depending on the structure of the starting substance and the number of HX molecules which have split off, viz. 

The mechanism of this type of reaction has been studied extensively, and much evidence has accumulated in support of a stepwise process, which proceeds first by base-catalyzed elimination of hydrogen halide (HX) from the aryl halide—as illustrated below for the amination of bromobenzene ... 

Unequal sharing of electrons in a covalent bond is by no means restricted to molecules where the dot structure would indicate formal charges. Take, for example, the hydrogen halides, HX, where X represents a halogen atom. 

Reaction with Hydrogen Halogens react with hydrogen gas to yield hydrogen halides, HX ... 

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... 

Spin-Orbit/Fermi Contact Effects. While scalar relativistic effects seem to be sufficient for some systems like the metal carbonyls of Table I (even though it has been speculated (9) that spin-orbit might improve the agreement with experiment even further), there are other cases where this is not the case. We have chosen as an example the proton NMR absolute shielding in hydrogen halides HX, X = F, Cl, Br, I (7,9), Figure 1. This series has also been studied by other authors (34-38), and it may well be the most prominent example for spin-orbit effects on NMR shieldings and chemical shifts. 

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