Concentrated acids

Dilute acids have no effect on any form of carbon, and diamond is resistant to attack by concentrated acids at room temperature, but is oxidised by both concentrated sulphuric and concentrated nitric acid at about 500 K, when an additional oxidising agent is present. Carbon dioxide is produced and the acids are reduced to gaseous oxides ... 

Tin slowly dissolves in dilute hydrochloric, nitric and sulphuric acids, and is in fact the only Group IV element to do so. The reactions with more concentrated acid are rapid. With hydrochloric acid. 

Arsenic dissolves in concentrated nitric acid forming arsenicfV) acid, H3ASO4, but in dilute nitric acid and concentrated sulphuric acid the main product is the arsenic(III) acid, HjAsOj. The more metallic element, antimony, dissolves to form the (III) oxide Sb O, with moderately concentrated nitric acid, but the (V) oxide Sb205 (structure unknown) with the more concentrated acid. Bismuth, however, forms the salt bismulh(lll) nitrate Bi(N03)3. 5H,0. 

The reaction between copper and nitric acid, 1 part concentrated acid and 1 part water, gives impure nitrogen monoxide ... 

Iodine gives iodic(V) acid with hot concentrated acid ... 

Concentrated sulphuric acid is an oxidising agent, particularly when hot, but the oxidising power of sulphuric acid decreases rapidly with dilution. The hot concentrated acid will oxidise non-metals, for example carbon, sulphur and phosphorous to give, respectively, carbon dioxide, sulphur dioxide and phosphoric(V) acid. It also oxidises many metals to give their sulphates cast iron, however, is not affected. The mechanisms of these reactions are complex and the acid gives a number of reduction products. 

Electrolysis of hydrochloric acid yields hydrogen at the cathode and oxygen at the anode from the dilute acid, but chlorine at the anode (of carbon) from the concentrated acid. Electrolysis of the concentrated acid is used on the large scale to recover chlorine. 

Mercuryill) sulphate and nitrate are each obtained by dissolving mercury in the appropriate hot concentrated acid the sulphate is used as a catalyst (p,436),... 

It should be noted that aliphatic compounds (except the paraffins) are usually oxidised by concentrated nitric acid, whereas aromatic compounds (including the hydrocarbons) are usually nitrated by the concentrated acid (in the presence of sulphuric acid) and oxidised by the dilute acid. As an example of the latter, benzaldehyde, CjHsCHO, when treated with concentrated nitric acid gives ffi-nitrobenzaldehyde, N02CgH4CH0, but with dilute nitric acid gives benzoic acid, CgHgCOOH. 

Boil a mixture of 5 ml. (4 g.) of acetonitrile and 75 ml. of 10% aqueous sodium hydroxide solution in a 200 ml. flask under a refluxwater-condenser for 30 minutes, when hydrolysis will be complete. Detach the condenser and boil the solution in the open flask for a few minutes to drive off ull free ammonia. Then cool the solution, and add dilute sulphuric acid (i volume of concentrated acid 2 volumes of water)... 

Method A, Prepare approximately 90% sulphuric acid by adding 25 ml. of the concentrated acid cautiously with gentle shaking to 5 mh of water. 

Beckmann Rearrangement. Prepare the 85% sulphuric acid by adding 50 ml. of the concentrated acid cautiously to 10 ml. of water, stirring the mixture meanwhile, and then cool the diluted acid in ice-water. Place 16 ml. of the cold acid in a 500 ml. beaker, add 8 g, of the pure oxime, and warm the mixture cautiously until effervescence begins, and then at once remove the heat. A vigorous reaction occurs, and is soon complete. Repeat this operation with another 8 g. of the oxime in a second beaker the reaction is too vigorous to be carried out with larger quantities. 

On the other hand, the two nitro groups make 2,4-dinitrophenylhydrazine a very weak base, and it has therefore to be used in reasonably concentrated acid solution. 

For dehydrogenation, add this ester to dilute nitric acid (20 ml. of the concentrated acid diluted with 40 ml. of water) and boil the mixture under reflux for about 5 minutes, during which the ester gently efferv esces and Anally gives a clear solution. Cool this solution in ice-w ater, make alkaline with aqueous sodium carbonate solution and extract tw ice with ether (50 ml. for each extraction). Dry the extract with sodium sulphate, filter, and then distil using a small distilling-flask... 

Hydrolysis by acids. Place 15 ml. of starch solution in a boiling-tube, add I ml. of cone. HCl, mix well and place in a boiling water-bath for 20 minutes. Cool and add 2 drops of iodine solution to i ml. of the solution no blue coloration is produced. On the remainder, perform tests for glucose in particular show that glucosazone can be formed. Neutralise the excess of acid before carrying out these tests. (Note that a more concentrated acid is required to hydrolyse starch than to hydrolyse the disaccharides, such as sucrose.)... 

The relatively concentrated hydrochloric acid is employed so that with ordinary use of the apparatus, spent liquor does not accumulate very rapidly the concentrated acid also ensures a brisk and delicately controlled flow of gas. When the generator is replenished with acid, marble or both, the de-aeration procedure detailed above is repeated until a sufficiently air-free gas supply is obtained. 

Cold concentrated sulphuric acid will remove unsaturated hydrocarbons present in saturated hydrocarbons, or alcohols and ethers present in alkyl halides. In the former case soluble sulphonated products are formed, whilst in the latter case alkyl hydrogen sulphates or addition complexes, that are soluble in the concentrated acid, are produced. 

Sulphuric acid. Ordinary concentrated acid, sp. gr. 1-84, is a constant boiling point mixture, b.p. 338°/760 mm., and contains 98 per cent. H2SO4. The 100 per cent, acid may be prepared by the addition of the calculated quantity of oleum it is also available commercially. 

Other sources of hazard arise from the handling of such chemicals as concentrated acids, alkalis, metallic sodium and bromine, and in working with such extremely poisonous substances as sodium and potassium cyanides. The special precautions to be observed will be indicated, where necessary, in the experiments in which the substances are employed, and will also be supplied by the demonstrator. The exercise of obvious precautions and cautious handling will in most cases reduce the danger to almost negligible proportions. Thus, if concentrated sulphuric acid should be accidentally spilled, it should be immediately washed with a liberal quantity of water or of a solution of a mild alkali. 

Note on the laboratory preparation of monoethylaniline. Although the laboratory preparation of monomethyl- or monoethyl-aniline is hardly worth whUe, the following experimental details may be useful to those who wish to prepare pure monoethylaniline directly from amline. In a flask, fitted with a double surface reflux condenser, place 50 g. (49 ml.) of aniline and 65 g. of ethyl bromide, and boU gently for 2 hours or until the mixture has almost entirely sohdified. Dissolve it in water and boil off the small quantity of unreacted ethyl bromide. Render the mixture alkaUne with concentrated sodium hydroxide solution, extract the precipitated bases with three 50 ml. portions of ether, and distil off the ether. The residual oil contains anihne, mono- and di-ethylaniline. Dissolve it in excess of dilute hydrochloric acid (say, 100 ml. of concentrated acid and 400 ml. of water), cool in ice, and add with stirring a solution of 37 g. of sodium nitrite in 100 ml. of water do not allow the temperature to rise above 10°. Tnis leads to the formation of a solution of phenyl diazonium chloride, of N-nitrosoethylaniline and of p-nitrosodiethylaniline. The nitrosoethylaniline separates as a dark coloured oil. Extract the oil with ether, distil off the ether, and reduce the nitrosoamine with tin and hydrochloric acid (see above). The yield of ethylaniline is 20 g. 

Seventy per cent, sulphuric acid is prepared by adding 40 ml. of the concentrated acid cautiously and with stirring and cooling to 30 ml. of water. 

To prepare the hydrochloride, dissolve about 1 g. of the compound (which need not be perfectly dry) in about 8 ml. of alcohol. Add this solution to boiling dilute hydrochloric acid (10 ml. of the concentrated acid and 80 ml. of water). Boil for 5 minutes, filter the hot solution if necessary, and allow to cool. p-Amino-azobenzene hydrochloride separates in steel-blue crystals. Filter, wash with a little dilute hydrochloric acid, and dry. 

Add the salt to dilute hydrochloric acid (prepared from 28 ml. of the concentrated acid and 150 ml. of water) contained in a 500 ml. flask fitted with a reflux condenser. Warm the mixture gently carbon dioxide is evolved and an oil separates. Heat on a steam bath for 90 minutes, cool, and extract the oil with 75 ml. of benzene. Wash the extract with 100 ml. of water, and distil the benzene solution under reduced pressure from a Claisen flask. Collect the a-phenylpropionaldehyde at 90-93°/10 mm. the yield is 30 g. 

Method B. Place 125 g. (106 -5 ml.) of diethyl phthalate and 25 g. of molecular sodium (sodium sand see Section 11,50,6) in a 500 ml. round-bottomed flask fitted with a reflux condenser and dropping funnel. Heat the flask on a steam bath and add a mixture of 122 5 g. (136 ml.) of dry ethyl acetate and 2 5 ml. of absolute ethanol over a period of 90 minutes. Continue the heating for 6 hours, cool and add 50 ml. of ether. Filter the sodium salt (VI) on a sintered glass funnel and wash it with the minimum volume of ether. Dissolve the sodium salt (96 g.) in 1400 ml. of hot water in a 3-htre beaker, cool the solution to 70°, stir vigorously and add 100 ml. of sulphuric acid (3 parts of concentrated acid to 1 part of... 

Dissolve 14 g. of p-phenetidine (2) in 240 ml. of water to which 20 ml. of 5N hydrochloric acid (or 9 ml. of the concentrated acid) have been added stir the solution with about 5 g. of decolourising carbon for 5 minutes, warm, and filter the solution with suction. Transfer the cold filtered solution of p-phenetidine hydrochloride to a 700 ml. conical flask, add 13 g. (12 ml.) of acetic anhydride and swirl the contents to dissolve the anhydride. Immediately add a solution of 16 g. of crystallised sodium acetate in 50 ml. of water and stir (or swirl) the contents of the flask vigorously. Cool the reaction mixture in an ice bath, filter with suction and wash with cold water. RecrystaUise from hot water (with the addition of a little decolourising carbon, if necessary), filter and dry. The yield of pure phenacetin, m.p. 137°, is 12 g. 

Alkali solutions and dilute and concentrated acids attack the metal rapidly. The pure metal is likely to ignite if scratched with a knife. 

Solutions of nitric acid in 100% sulphuric acid have a high electrical conductivity. If nitric acid is converted into a cation in these solutions, then the migration of nitric acid to the cathode should be observed in electrolysis. This has been demonstrated to occur in oleum and, less conclusively, in concentrated acid, observations consistent with the formation of the nitronium ion, or the mono- or di-protonated forms of nitric acid. Conductimetric measurements confirm the quantitative conversion of nitric acid into nitronium ion in sulphuric acid. ... 

So now that we have all the reagents out of the way let s see how the reaction proceeds. There s the clear- yellow "safrole" sitting in the bottom if the flask and the clear saturated KOH solution is dumped in. The solution is heated to reflux etc. and yes, some brown byproducts and destruction artifacts will appear. Especially if the safrole is not pure. These byproducts should be expected to some extent because concentrated basic (OH) solutions can be as nasty as concentrated acidic solutions. One is mindful that KOH is less intrusive towards the delicate methylenedioxy ring structure of the safrole/isosafrole molecule. 

Example Sulfuric acid has the molecular weight 98.08. If the concentrated acid assays 95.5% and has the specific gravity 1.84, the volume required for 1 liter of a 0.1 molar solution is... 

The majority of titrations involving basic analytes, whether conducted in aqueous or nonaqueous solvents, use HCl, HCIO4, or H2SO4 as the titrant. Solutions of these titrants are usually prepared by diluting a commercially available concentrated stock solution and are stable for extended periods of time. Since the concentrations of concentrated acids are known only approximately,the titrant s concentration is determined by standardizing against one of the primary standard weak bases listed in Table 9.7. 

The sequence of each different peptide or protein is important for understanding the activity of peptides and proteins and for enabling their independent synthesis, since the natural ones may be difficult to obtain in small quantities. To obtain the sequence, the numbers of each type of amino acid are determined by breaking down the protein into its individual amino acids using concentrated acid (hydrolysis). For example, hydrolysis of the tetrapeptide shown in Figure 45.3 would give one unit of glycine, two units of alanine, and one unit of phenylalanine. Of course, information as to which amino acid was linked to which others is lost. 

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