Strong acids hydrogen halides

The reaction is acid catalyzed. Alcohols react with the strongly acidic hydrogen halides HCI, HBr, and HI, but they do not react with nonacidic NaCI, NaBr, or Nal. Primary and secondary alcohols can be converted to alkyl chlorides and bromides by allowing them to react with a mixture of a sodium halide and sulfuric acid ... 

We can gam a general understanding of the mechanism of hydrogen halide addi tion to alkenes by extending some of the principles of reaction mechanisms introduced earlier In Section 5 12 we pointed out that carbocations are the conjugate acids of alkenes Therefore strong acids such as HCI HBr and HI can protonate the double bond of an alkene to form a carbocation... 

The most common hydrogen halides are HF (U.S. production = 3 X 108 kg/yr) and HC1 (3 X 109 kg/yr). They are most familiar as water solutions, referred to as hydrofluoric acid and hydrochloric acid, respectively. Recall (Chapter 13) that hydrofluoric acid is weak, incompletely dissociated in water, whereas HCl is a strong acid. 

Similar mechanisms of course can be written for the reaction with the other hydrogen halide acids, and indeed for the relatively slow decomposition of N-chloroacetanilide in solutions of other strong acids15, viz. 

A number of small molecules that are acids do not fall into the categories mentioned above. These acids have no clear patterns in their structures, so it is best simply to learn their names and stmctures. Three hydrogen halides, HCl, HBr, and HI, are strong acids, but the fourth, HF, is a weak acid. Hydrogen sulfide, H2S, and hydrocyanic... 

All of the hydrogen halides are very soluble in water, and acidic solutions result. Although HF is a weak acid, the others are strong and are almost completely dissociated in dilute solutions. HC1, HBr, and HI form constant-boiling mixtures with water that contain 20.2%, 47.6%, and 53% of the acid, respectively. 

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 ... 

Even though the hydrogen halides HC1, HBr, and HI are strong acids, HF is not a strong acid, as explained in Box 6-3. For most purposes, the hydroxides of the alkaline earth metals (Mg2+, Ca2+, Sr2+, and Ba2+) can be considered to be strong bases, although they are far less soluble than alkali metal hydroxides and have some tendency to form MOH1 complexes (Table 6-3). 

The hydrogen halides HCI, HBr, and HI are all strong acids, which means that the reactions... 

Halide formation proceeds at a useful rate only in the presence of strong acid, which can be furnished by excess hydrogen bromide or, usually and more economically, by sulfuric acid. The alcohol accepts a proton from the acid to give an alkyloxonium ion, which is more reactive in subsequent displacement with bromide ion than the alcohol (by either SN2 or SN1 mechanisms) because H20 is a better leaving group than eOH ... 

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