Ethyl bromide, reaction

QHsBr + OH (aq) —v- C2H6OH + Br (aq) (5) This reaction may seem similar to the reaction between aqueous HBr and NaOH but there are two important differences. The ethyl bromide reaction is very slow (about one hour is needed for the reaction) and it occurs between a covalent molecule (C2H5Br) and an ion (OH-). In contrast, the reaction between HBr and NaOH in water occurs in a fraction of a second and it involves ions only, as shown in reaction (6). 

The preparation of -butyl bromide as an example of ester formation by Method 1 (p. 95) has certain advantages over the above preparation of ethyl bromide. -Butanol is free from Excise restrictions, and the -butyl bromide is of course less volatile. and therefore more readily manipulated without loss than ethyl bromide furthermore, the n-butyl bromide boils ca. 40° below -butyl ether, and traces of the latter formed in the reaction can therefore be readily eliminated by fractional distillation. 

The Alkyl Halides. Ethyl bromide and iodide (see below) are typical alkyl halides. Compounds of this class are of very great importance in synthetic work, owing to the reactivity of the halogen atom. This is illustrated by the following reactions ... 

This reaction is precisely parallel to the Wurtz Reaction in the aliphatic series, by which, for instance, n-butane can be obtained by the action of sodium on ethyl bromide. 

It took 5-10 min before the reaction started this was visible by the appearance of turbidity of the diethyl ether and later by the appearance of a gloss on the pieces of lithium and a distinct increase in temperature. Care was taken that the temperature did not rise above -20°C (note 4). When the reaction had subsided, the addition of ethyl bromide was continued, now dropwise (note 5). The temperature was kept between -20 and -30 C (note 6). After the addition, which was carried out in 30-40 min, stirring was continued for about a further 1 h. The temperature was allowed to rise gradually to -10°C. When the gloss on the piece of lithium had disappeared, the solution was poured into another flask through... 

At higher temperatures the reaction between ethyl bromide and ethyl-lithium ("Wurz coupling") becomes significant. 

To a solution of ethylnagnesium bromide in 350 ml of THF, prepared from 0.5 mol of ethyl bromide (see Chapter 11, Exp. 6) was added in 10 min at 10°C 0.47 mol of 1-hexyne (Exp. 62) and at 0°C 0.47 mol of trimethylsilylacetylene (Exp. 31) or a solution of 0.60 mol of propyne in 70 ml of THF (cooled below -20°C). With trimethyl si lylacetylene an exothermic reaction started almost immediately, so that efficient cooling in a bath of dry-ice and acetone was necessary in order to keep the temperature between 10 and 15°C. When the exothermic reaction had subsided, the mixture was warmed to 20°C and was kept at that temperature for 1 h. With 1-hexyne the cooling bath was removed directly after the addition and the temperature was allowed to rise to 40-45°C and was maintained at that level for 1 h. 

Primary carbocations are so high m energy that their intermediacy m nucleophilic substitution reactions is unlikely When ethyl bromide undergoes hydrolysis m aqueous formic acid substitution probably takes place by an 8 2 like process m which water is the nucleophile... 

Niobium Pentabromide. Niobium pentabromide is most conveniently prepared by reaction of bromine with niobium metal at ca 500°C. It is a fairly volatile yellow-red compound that is hygroscopic and readily hydrolyzes. It is soluble in water, alcohol, and ethyl bromide. 

The halogen influences the rate of reaction, and, in general, the order of reactivity is HI > HBi > HCl. Impoitant uses of etfiyl chloiide include the manufacture of tetraethyllead and ethylceUulose. Ethyl bromide can be used to produce ethyl Grignard reagent and various ethyl amines. 

Some studies seeking preferred conditions for this reaction have been reported. Optimum yields of 1-ethoxy-1-propyne and 1-ethoxy-l-butyne are found when the product is worked up before allowing the ammonia solvent to evaporate, as the product evidently volatilizes with the ammonia. An experiment with 1-ethoxy-1-propyne showed a marked increase in yield when ammonia predried over calcium hydride was used instead of ammonia directly from the cylinder. A twofold excess of ethyl bromide is required to obtain a good yield of l-ethoxy-l-but5me, since elimination apparently competes with alkylation in this case. 

In a i-l. round-bottomed, three-necked flask fitted with an efficient reflux condenser, liquid-sealed stirrer, and dropping funnel is placed t3 g. (0.53 gram atom) of magnesium turnings. A few cubic centimeters of a solution of 60 g. (41.4 cc., 0.55 mole) of pure ethyl bromide in 50 cc. of absolute ether is added and the stirrer started (Note i). When the bromide begins to react 200 cc. of absolute ether is added, and then the balance of the bromide solution is run in as fast as the refluxing permits (about one-half hour). After allowing fifteen minutes for the completion of the reaction, a solution of 40 g. (0.42 mole) of 2,4-dimethyl-pyrrole (Org. Syn. 15, 20) in 100 cc. of absolute ether is added in the course of twenty minutes (Note 2) and the mixture is refluxed for one-half hour on the steam bath. 

There are relatively few kinetic data on the Friedel-Crafts reaction. Alkylation of benzene or toluene with methyl bromide or ethyl bromide with gallium bromide as catalyst is first-order in each reactant and in catalyst. With aluminum bromide as catalyst, the rate of reaction changes with time, apparently because of heterogeneity of the reaction mixture. The initial rate data fit the kinetic expression ... 

A solution of 6.3 g (0.9 moles) ethoxyacetylene in 50 ml ether is added dropwise during 30 min to a Grignard reagent prepared from 2.18 g (90 mg-atoms) magnesium and 9.81 g (90 mmoles) ethyl bromide. The reaction mixture is stirred for 1 hr at room temperature and then a solution of 3 g (9 mmoles) 3) -acetoxyandrost-5-en-I7-one in 50 ml dry ether is added dropwise. The mixture is refluxed for 1 hr and after cooling to 0° poured into 100 ml of an aqueous ammonium chloride solution. The aqueous solution is extracted with ether, and the organic extract is washed with ammonium chloride solution and water, dried, and evaporated. The residue is chromatographed on 130 g alumina (activity III). Elution with petroleum ether-benzene (1 1) yields, after crystallization from acetone-hexane, 1.27 g (35%) 3j5-acetoxy-17a-ethoxyethynylandrost-5-en-17) -ol mp 138-139° Ho -122°. 

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