Halogenations, silver® nitrate

The presence of halide is determined by first acidifying a portion of the FAQS with dilute nitric acid and boiling the solution in the hood to remove any sulfide or cyanide ions as hydrogen sulfide or hydrogen cyanide, respectively. This is necessary because sulfide and cyanide interfere with the test for halogens. Silver nitrate solution is then added, and the formation of a precipitate of silver halide signals the presence of halide in the FAQS (Eq. 25.1). 

Acids, acrolein, dimethyl sulfate, halogens, silver nitrate, propylene oxide,... 

Aliphatic mono-halides, and aromatic hydrocarbons with halogen in side-chain, precipitate silver hdide on treatment with cold aqueous silver nitrate solution. 

Principle. A known weight of the substance is heated with fuming nitric acid and silver nitrate in a sealed tube. The organic material is thus oxidised to carbon dioxide and water, whilst the halogen is converted quantitatively into the corresponding silver halide. The latter js subsequently washed out of the tube, filtered and weighed. 

Principle. This is essentially a small-scale modification of the macro piethod described on p. 416, the substance being completely oxidised in a sealed tube with fuming nitric acid in the presence of silver nitrate, the halogen being thus converted into silver halide. The collection and weighing of the silver halide require special techniques on the semi-micro scale. 

Sulphur, as sulphide ion, is detected by precipitation as black lead sulphide with lead acetate solution and acetic acid or with sodium plumbite solution (an alkaLine solution of lead acetate). Halogens are detected as the characteristic silver halides by the addition of silver nitrate solution and dilute nitric acid the interfering influence of sulphide and cyanide ions in the latter tests are discussed under the individual elements. 

Nitrogen and sulphur absent, (i) If only one halogen is present, acidify with dilute nitric acid and add excess of silver nitrate solution. A precipitate indicates the presence of a halogen. Decant the mother liquor and treat the precipitate with dilute aqueous ammonia solution If the precipitate is white and readily soluble in the ammonia solution, chlorine is present if it is pale yellow and difficultly soluble, bromine is present if it is yellow and insoluble, then iodine is indicated. Iodine and bromine should be confirmed by the tests given below. 

Oiganio compounds containing halogens react with silver nitrate in the following order of decreasing reactivity ... 

The Halogens Carh/s).—The method of Carius, which is usually emplo ed, consists in oxidising the substance with fuming nitric acid under piessure in presence of silver nitrate. The silver halide which is formed is then separated by filtration and w eighed. 

Chlorinity When a sample of sea water is titrated with silver nitrate, bromides and iodides, as well as chlorides are precipitated. In calculating the chlorinity (Cl), the entire halogen content is taken as chloride, and chlorinity is defined as the weight in grams of silver required for precipitation of total halogen content per kilogram of sea water, multiplied by 0-328 533. (Chlorinity is always expressed as parts per thousand, using the symbol %o.)... 

Halogen derivatives Silver nitrate, am-moniacal (Dedonder s, Tollens or Zaffaroni s reagent) Halogen compounds yield black chromatogram zones on a pale gray background. [2]... 

The second reason for the lack of early investigations into vinyl cations was the seemingly extreme unreactivity of vinyl halides in solvolytic processes. The unreactivity of vinyl chloride, for instance, even in the presence of silver nitrate, has been almost a legend in organic chemistry (102). This lack of reactivity of simple alkylvinyl halides has been attributed to the low stability of simple vinyl cations or to the very strong carbon-halogen bond, or both. 

The reaction shows a first-order dependence on substrate concentration but, except at very low concentration, is zero-order with respect to oxidant moreover, the zero-order rate coefficient is the same as that observed with oxidations by iodine, cupric chloride and silver nitrate. The reaction is acid-catalysed. The oxidation is completely analogous to the halogenation of ketones and involves a slow tautomeric equilibrium followed by rapid oxidation, viz. 

Marquis and Lebel [534] precipitated potassium from seawater or marine sediment pore water using sodium tetraphenylborate, after first removing halogen ions with silver nitrate. Excess tetraphenylborate was then determined by silver nitrate titration using a silver electrode for endpoint detection. The content of potassium in the sample was obtained from the difference between the amount of tetraphenyl boron measured and the amount initially added. 

Once the halogen atom has left the halogenoalkane as the halide ion, X-, silver nitrate solution can be used to identify the halide as in inorganic chemistry. The three essential steps are ... 

In order to test for un-ionised halogen ignite the substance with an excess of chemically pure lime in a moderately wide test tube in a Bunsen flame, dip the tube while still hot in a little water so that the glass is shattered, acidify with pure nitric acid, filter, and add silver nitrate solution. 

According to the amount of substance taken add 60-90 mg. of finely powdered silver nitrate (it is best to add one and a half times the amount which corresponds to the expected halogen content). Then, if the substance is only slowly attacked by nitric acid in the cold, add 10-1-5 c.c. of red fuming nitric acid. In the case of substances which react vigorously with nitric acid in the cold, hold the Carius tube obliquely and add the acid in a small round-bottomed tube (length 6 cm., width 0-8 cm.), cautiously allowing it to slide to the bottom of the Carius tube and taking care that substance and acid do not come into contact with each other. 

The determination of sulphur by the Carius method is carried out in the same way as is the determination of halogen, but, in place of the silver nitrate, anhydrous barium chloride is here used. 

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). 

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