Halides, alkyl, reaction with silver nitrate

As esters the alkyl halides are hydrolysed by alkalis to alcohols and salts of halogen acids. They are converted by nascent hydrogen into hydrocarbons, by ammonia into amines, by alkoxides into ethers, by alkali hydrogen sulphides into mercaptans, by potassium cyanide into nitriles, and by sodium acetate into acetic esters. (Formulate these reactions.) The alkyl halides are practically insoluble in water but are, on the other hand, miscible with organic solvents. As a consequence of the great affinity of iodine for silver, the alkyl iodides are almost instantaneously decomposed by aqueous-alcoholic silver nitrate solution, and so yield silver iodide and alcohol. The important method of Ziesel for the quantitative determination of alkyl groups combined in the form of ethers, depends on this property (cf. p. 80). 

The reaction of alkyl halides with silver nitrate constitutes an extremely useful method for the synthesis of high purity nitrate esters on a laboratory scale. ° The driving force for these reactions is the formation of the insoluble silver halide. Reactions have been conducted under homogenous and heterogeneous conditions. For the latter a solution of the alkyl halide in an inert solvent like benzene or ether is stirred with finely powdered silver nitrate. However, this method has been outdated and reactions are now commonly conducted under homogeneous conditions using acetonitrile as solvent. 

Synthesis of nitrate esters from the reaction of alkyl halides with silver nitrate... 

Heavy-metal salts, particularly those of silver, mercury, and copper, catalyze SX1 reactions of alkyl halides in much the same way that acids catalyze the SN reactions of alcohols. A heavy-metal ion functions by com-plexing with the unshared electrons of the halide, thereby making the leaving group a metal halide rather than a halide ion. This acceleration of the rates of halide reactions is the basis for a qualitative test for alkyl halides with silver nitrate in ethanol solution ... 

Silver nitrate test The compound to be tested is treated with a few drops of 1% alcoholic silver nitrate. A white precipitate indicates a positive reaction. This could be due to either silver chloride (reaction with a reactive alkyl halide), silver alkynide (reaction with a terminal alkyne), or the silver salt of a carboxylic acid (reaction with a carboxylic acid). 

In the experiments that follow, eight representative alkyl halides are treated with sodium iodide in acetone and with an ethanolic solution of silver nitrate. Acetone, with a dielectric constant of 21, is a relatively nonpolar solvent that will readily dissolve sodium iodide. The iodide ion is an excellent nucleophile, and the nonpolar solvent, acetone, favors the Sn2 reaction it does not favor ionization of the alkyl halide. The extent of reaction can be observed because sodium bromide and sodium chloride are not soluble in acetone and precipitate from solution if reaction occurs. 

The in situ synthesis of organoboranes via reaction of alkyl halides with magnesium in the presence of diborane can also be used to prepare coupled products (equations 20 and 21). Oxidation of the reaction mixture with alkaline silver nitrate leads to good yields of dimeric products. The reaction is successful for primary and secondary halides. A related reaction is the coupling of secondary alkyl halides in the presence of catalytic quantities of thallium salts. This procedure fails for primary alkyl halides and gives modest yields for secondary alkyl halides (equation 22). 

Another method for the aliphatic nitration is the reaction of alkyl halide with silver nitrite, in which considerable nitro compound is formed. With alkali nitrite on the other hand, very little nitro compound is formed. This difference... 

The nitrate esters are formed with retention of configuration in contrast to the reaction of silver nitrate with alkyl halides. Furthermore, the reac-tion of benzyl alcohol shows that 0-nitration of alkylary] alcohols in transfer nitration is preferred to aromatic Onitration. 

McKillop and Ford synthesized a range of primary and secondary alkyl nitrates in excellent yields by treating alkyl bromides with mercury (I) nitrate in 1,2-dimethoxyethane at reflux (Equation 3.9). This method has been used to synthesize substituted nitrate esters from both a-bromocarboxylic acid and a-bromoketone substrates. Unlike metathesis with silver salts, which are widely known to promote SnI reactions, this method is not useful for the synthesis of nitrate esters from tertiary alkyl halides. 

Reaction with ethanolic potassium hydroxide. Boil 0.5 ml of the compound with 4 ml of 0.5 m ethanolic potassium hydroxide under reflux for 15 minutes. Most alkyl halides and benzyl halides give a crystalline precipitate of the potassium halide. Dilute with 5 ml of water, acidify with dilute nitric acid and test with silver nitrate solution. 

The reaction of a compound with silver nitrate in ethanol is used as a chemical test to determine if the compound is an alkyl halide. The formation of a precipitate of the silver halide constitutes a positive test. 

A reluctant first-order substrate can be forced to ionize by adding some silver nitrate (one of the few soluble silver salts) to the reaction. Silver ion reacts with the halogen to form a silver halide (a highly exothermic reaction), generating the cation of the alkyl group. 

When an alkyl halide is treated with an ethanolic solution of silver nitrate, the silver ion coordinates with an electron pair of the halogen. This weakens the carbon-halogen bond as a molecule of insoluble silver halide is formed, thus promoting an S l reaction of the alkyl halide. The solvent, ethanol, favors ionization of the halide, and the nitrate ion is a very poor nucleophile, so alkyl nitrates do not form by an 8 2 reaction. 

Empty the tubes, rinse them with ethanol, place the same amount of each of the alkyl halides in each tube as in the first part of the experiment, add 1 mL of 1% ethanolic silver nitrate solution to each tube, mix the contents well, and note the time of addition as well as the time of appearance of the first traces of any precipitate. If a precipitate does not appear in 5 min heat those tubes in a 50°C water bath for 5 to 6 min and watch for any reaction. 

We have seen (Sec. 14.24) that an alkyl halide is conveniently detected by the precipitation of insoluble silver halide when it is warmed with alcoholic silver nitrate. The reaction occurs nearly instantaneously with tertiary, allyl, and benzyl bromides, and within five minutes or so with primary and secondary bromides. Compounds containing halogen joined directly to an aromatic ring or to a doubly-bonded carbon, however, do not yield silver halide under these conditions. Bromo-benzene or vinyl bromide can be heated with alcoholic AgN03 for days without the slightest trace of AgBr being detected. In a similar way, attempts to convert aryl... 

With these examples the list of the many-sided reactions of the halogen alkyls is not exhausted they are also used for the preparation of the- metallic alkyls, e.g., zinc alkyls for the preparation of the phosphines, and for many other compounds. Finally, attention is called to the characteristic difference between the organic and inorganic halides. While, e.g., potassium chloride, bromide, or iodide in solution act instantly with a silver nitrate solution to form a quantitative precipitate of silver chloride, bromide, or iodide respectively, silver nitrate in a water solution does not act on most organic halides, so that this reagent does not serve in the usual way to show the presence of a halogen. 

In the case of hydrobromic and hydriodic acids and such olefins as isobutylene and tri methyl ethylene, the rate of alcohol formation may become such that it approaches the rate of hydrolysis of the corresponding alkyl halides, thus supporting the theory that halides are the necessary intermediate product.04 The greater activity of the hydrobromic and hydriodic acids compared with hydrochloric acid toward ethylene is shown by the experiments of Swann, Snow and Keyes.00 At 800 pounds per square inch pressure and a temperature of 150° C. no alkyl chlorides were detected when hydrochloric acid of from 5 to 25 per cent concentration was used. On the other hand, considerable yields of alkyl iodides were obtained under the same conditions when hydriodic acid was used, and alkyl bromides formed in the presence of 40 per cent concentration hydrobromic acid. Alcohol yields were very small. When using propene at 600 to 800 pounds per square inch pressure at 135° C. in the presence of 5 per cent hydrochloric acid solutions and solutions of silver nitrate, yields of alcohol several times that obtained from ethylene were found. The yields were still very low, however, even with times of reaction as long as one hour. 

NITROCARBOL (75-52-5) Forms explosive mixture with air (flash point 95°F/35°C). Thermally unstable. Shock, friction, pressure, or elevated temperature above 599°F/315°C can cause explosive decomposition, especially if confined. Violent reaction with strong oxidizers, alkyl metal halides, diethylaluminum bromide, formic acid, methylzinc iodide. Contact with acids, bases, acetone, aluminum powder, amines, bis(2-aminoethyl)amine, haolforms make this material more sensitive to explosion. Reacts, possibly violently, with ammonium hydroxide, calcium hydroxide, calcium hypochlorite, 1,2-diaminomethane, formaldehyde, hexamethylbenzene, hydrocarbons, hydroxides, lithium perchlorite, m-methyl aniline, nickel peroxide, nitric acid, metal oxides, potassium hydride, potassium hydroxide, sodium hydride. Mixtures with ammonia, aniline, diethylenetriamine, metal oxides, methyl amine, morpholine, phosphoric acid, silver nitrate form shock-sensitive compounds. Forms high-explosive compound with urea perchlorate. Mixtures with hydrocarbons and other combustible materials can cause fire and explosions. Attacks some plastics, rubber, and coatings. 

The reactivity of the normal alkyl halides varies with the halogen and with the alkyl group the more reactive are those with the smaller number of carbon atoms. The iodides are the most reactive, the chlorides the most inert. Methyl iodide is much used in organic syntheses, as it reacts readily with many substances and serves as a means of introducing the methyl group into such compounds. An alcoholic solution of methyl iodide reacts with a similar solution of silver nitrate in the cold, and silver iodide is formed. With ethyl iodide the reaction proceeds much more slowly, and with the higher alkyl iodides the application of heat is necessary to bring about reaction. 

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