Removal of hydrogen halide

Two methods of dehydrohalogenation come up for discussion, namely, thermal-catalytic cleavage and cleavage by inorganic or organic bases. 

Fluorinated olefins can be prepared in good yield by dehydrohalogenation of 1,1,1-trihaloethanes on metal oxides or active charcoal, e.g. 52 

Preparation of 2-methyl-2-butene in this way has, however, no practical importance since this compound is obtained more easily from pentyl alcohols. 

Dehydrohalogenating agents range from anhydrous alkali alkoxides and solid alkali hydroxides through solutions of alkali hydroxides in water or organic solvents, then through alkali salts of weak acids, to organic bases such as pyridine, aniline, dimethylaniline, and quinoline. 

Removal of hydrogen halide is often accompanied by side reactions in which the halide is hydrolysed by the inorganic base  

---

The dehydrohalogenation of a yQ-chloroalkylbenzene is readily accomplished by refluxing with excess aqueous methanolic potassium hydroxide. Substituted a-alkylstyrenes which are difficult to obtain by other methods are prepared in this way by a two-step process involving catalytic condensation of aromatic compounds with aliphatic chlorohydrins followed by removal of hydrogen halide from the resulting haloalkylated derivatives. ... 

Unsaturated carbohydrates may be prepared by the removal of hydrogen halide from certain halogenated carbohydrates which, in turn, are available from trityl ethers. Thus far this reaction has been successfully carried out only with compounds containing a primary halogen atom. 

Hydrolysis of esters and ethers, hydrolytic cleavage of C—N single bonds, hydrolytic cleavage of nonaromatic heterocycles, hydration and dehydration at m ultiple bonds, new atomic linkages resulting from dehydration reactions, hydrolytic dehalogenation removal of hydrogen halide molecules, various reactions. 

Removal of hydrogen halide (HX) in the preparation of haloolefins is governed by Saytzeff s rule that the proton is removed preferentially from the carbon carrying the smallest number of hydrogen atoms.344 Removal of HX thermally or by aqueous-alcoholic alkali is important for laboratory work but particularly so on an industrial scale.10 Solid NaOH or KOH is used in the preparation of 2,3-dibromo-l-propene from 1,2,3-tribromopropane as described... 

Further, formation of quaternary salts can interfere in the ready removal of hydrogen halide by tertiary amines such as dimethylaniline, pyridine, or quinoline.58 To overcome this, Hiinig and Kiessel59 successfully applied sterically hindered tertiary amines, e.g., ethyldiisopropylamine and 7V-ethyldi-(cyclohexyl)amine for instance, removing hydrogen bromide from 1,2-di-bromo-2-methylpropane by potassium hydroxide in ethylene glycol60 gave unsatisfactory yields of l-bromo-2-methyl-l-propene ... 

Removal of hydrogen halide from an unsaturated halide finds application, for instance, in the preparation of 2-phenyl-l,3-butadiene 108... 

Diphenyl-1,3-butadiene is easily prepared by removal of hydrogen halide from a dihalide it suffices to heat 2,3-dibromo-l,4-diphenylbutane with quinoline for 0.5 hour at 160°.110... 

The relevant fundamental processes include (a) oxidative addition of aryl (or alkenyl) halide to a Pd(0) complex to give arylpalladium(ll) halide (A), (b) olefin coordination and insertion into the aryl-Pd bond to give arylated alkylpalladium species (B), (c) -hydrogen elimination to liberate the arylated olefin generating a hydridopalladium halide PdH(X)L2 (C), (d) removal of hydrogen halide from... 

The Wittig reaction makes use of the ylide derived by removal of hydrogen halide from a halophosphonium salt. Thus, as shown in Equation 9.59, treatment of... 

As with substitution, we have two mechanisms that are relatively common and one that is much rarer. The two common mechanisms, El and E2, have strong similarities to the S l and 5 2 processes described in the previous chapter and are generally favored by similar conditions to these analogues. The third mechanism, ElcB, is rather different, with no direct analogy in substitution chemistry. We will initially exemplify all the reactions by removal of hydrogen halides, HX, in the presence of base and then explore the scope of the reactions. 

---