Pure acids

In the pure acid the dihydrogen sulphate has a proton affinity, so that... 

Actually, the pure acid H2SO4 always contains some H2S2O7, because there is an equilibrium ... 

The dotted lines represent hydrogen bonds. The high boiling point and viscosity of the pure acid indicate strong intermolecular forces of this kind. 

Picric acid is used on a large scale as a high explosive, but for this purpose requires a detonator. If a few small crystals of the pure acid are heated on a crucible lid, they first melt, and ultimately burn harmlessly with a smoky flame. Metallic salts of picric acid are much less stable than the free acid,... 

For purification, transfer the acid to a 150 ml. flask containing 60 ml. of water, boil the mixture under reflux, and then add acetic acid in 5 ml. portions down the condenser until almost all the solid has dissolved avoid an excess of acetic acid by ensuring that the solvent action of each addition is complete before the next portion is added. A small suspension of insoluble impurity may remain. Add 2 g. of animal charcoal, boil the solution again for 10-15 minutes, and then filter it through a preheated Buchner funnel. Cool and stir the filtrate, which will deposit pale cream-coloured crystals of the acid. Collect as before and if necessary repeat the recrystallisation. Yield of pure acid, 9 g. m.p. 227-229°. 

To save time, the well-drained acid may be recrystallised twice from ethanol, and the pure acid, m.p. 176-177°, thus obtained. 

Very pure acid chlorides may be obtained by reaction between the anhydrous sodium salt of the acid and phosphorus oxychloride, for example ... 

Vinylacetic acid. Place 134 g. (161 ml.) of allyl cyanide (3) and 200 ml. of concentrated hydrochloric acid in a 1-htre round-bottomed flask attached to a reflux condenser. Warm the mixture cautiously with a small flame and shake from time to time. After 7-10 minutes, a vigorous reaction sets in and the mixture refluxes remove the flame and cool the flask, if necessary, in cold water. Ammonium chloride crystallises out. When the reaction subsides, reflux the mixture for 15 minutes. Then add 200 ml. of water, cool and separate the upper layer of acid. Extract the aqueous layer with three 100 ml. portions of ether. Combine the acid and the ether extracts, and remove the ether under atmospheric pressure in a 250 ml. Claisen flask with fractionating side arm (compare Fig. II, 13, 4) continue the heating on a water bath until the temperature of the vapour reaches 70°. Allow the apparatus to cool and distil under diminished pressure (compare Fig. II, 20, 1) , collect the fraction (a) distilling up to 71°/14 mm. and (6) at 72-74°/14 mm. (chiefly at 72 5°/ 14 mm.). A dark residue (about 10 ml.) and some white sohd ( crotonio acid) remains in the flask. Fraction (6) weighs 100 g. and is analytically pure vinylacetic acid. Fraction (a) weighs about 50 g. and separates into two layers remove the water layer, dry with anhydrous sodium sulphate and distil from a 50 ml. Claisen flask with fractionating side arm a further 15 g. of reasonably pure acid, b.p. 69-70°/12 mm., is obtained. 

The procedure is to pass purified hydrogen through a hot solution of the pure acid chloride in toluene or xylene in the presence of the catalyst the exit gases are bubbled through water to absorb the hydrogen chloride, and the solution is titrated with standard alkali from time to time so that the reduction may be stopped when the theoretical quantity of hydrogen chloride has been evolved. Further reduction would lead to the corresponding alcohol and hydrocarbon ... 

Place 25 g. of phenylacetic acid (Section IV,160) in a 500 ml. round-bottomed flask, cool the latter in running water and add 250 ml. of fuming nitric acid, rather slowly at first and then more rapidly. The addition occupies about 15 minutes. Attach a condenser to the flask, reflux the solution for 1 hour, and pour into about 500 ml. of cold water. When cold, filter the crude 2 4-dinitrophenylacetic acid at the pump and wash it with cold water the resulting acid, after drying at 100°, is almost pure (m.p. 181°) and weighs 31 g. Recrystallise it from 300 ml, of 20 per cent, alcohol. Collect the first main crop (25 g.), and allow the mother liquor to stand overnight when a further 2 g. of pure acid is obtained dry at 100°, The yield of pure 2 4-dinitrophenylacetic acid, m.p. 183°, is 27 g. 

Hydrolysis of methyl m-nitrobenzoate to m-nitrobenzoic acid. Place 90 -5 g. of methyl m-nitrobenzoate and a solution of 40 g. of sodium hydroxide in 160 ml. of water in a 1-htre round-bottomed flask equipped with a reflux condenser. Heat the mixture to boiling during 5-10 minutes or until the ester has disappeared. Dilute the reaction mixture with an equal volume of water. When cold pour the diluted reaction product, with vigorous stirring, into 125 ml. of concentrated hydrochloric acid. Allow to cool to room temperature, filter the crude acid at the pump and wash it with a httle water. Upon drying at 100°, the crude m-nitrobenzoic acid, which has a pale brownish colour, weighs 80 g. and melts at 140°, Recrystalhsation from 1 per cent, hydrochloric acid afibrds the pure acid, m.p. 141°, as a pale cream sohd the loss of material is about 5 per cent. 

CAUTION. The preparation of o-nitrobenzoyl chloride, o-nitrophenacetyl chloride and all o nitroacid chlorides should not be attempted by the above methods a violent explosion may occur upon distilling the product or when the last traces of thionyl chloride are removed in vacuo at 100°. Perhaps the safest method is to treat the pure acid in benzene solution with 1 1 mols of thionyl chloride and to reflux until evolution of sulphur dioxide and hydrogen chloride has ceased the solution of the acid chloride in benzene may then bo employed for most reactions. 

Carefully fractionate the commercially pure a-picoliiie (purity > 95 per cent.) and collect the fraction of b.p. 129-130°. If time permits, this should be purified as described in Section 11,47, 2S a pure acid is thus assured. 

It is better not to remove the lower bromoform layer in a separatory funnel, but to do so entirely by steam distillation complete oxidation of the ketone id thus ensured. The weight of recovered bromoform may be somewhat smaller (100-105 g.), but the yield of pure acid is increased to 36 g. The steam distillation must be carefully watched as a solid (carbon tetrabromide) may crystallise in the condenser this can easily be removed by turning ofi the water supply when the solid will soon melt and pass on into the distillate. 

Methyl crotonate. Purify commercial crotonic acid by distiUing 100 g. from a 100 ml. Claisen flask attached to an air condenser use an air bath (Fig. II, 5, 3). The pure acid passes over at 180-182° and crystallises out on cooling, m.p. 72-73° the recovery is about 90 per cent. Place 75 g. of absolute methyl alcohol, 5 g. (2 -7 ml.) of concentrated sulphuric acid and 50 g. of pure crotonic acid in a 500 ml. round-bottomed flask and heat under reflux for 12 hours. Add water, separate the precipitated ester and dissolve it in ether wash with dilute sodium carbonate solution until effervescence ceases, dry with anhydrous magnesium sulphate, and remove the ether on a water bath. Distil and collect the methyl crotoiiato at 118-120° the yield is 40 g. 

The reaction giving A is chloromethylation, a reliable metliod of adding a CH2OH equivalent to an aromatic ring. You may have been surprised at the use of reagent B to make an acid chloride. B is oxalyl chloride and is often used when pure acid chlorides are wanted - the other products are gases (which ). 

High vacuum distillation gave a crystalline product, containing small amounts of impurities, inter alia some 2-butynoic acid. Crystallization from a 3 1 mixture of pentane and diethyl ether at low temperature gave the pure acid, m.p. 77°C, in 38-45 yields. 

To a mixture of 65 ml of dry benzene and 0.10 mol of freshly distilled NN-di-ethylamino-l-propyne were added 3 drops of BFa.ether and 0.12 mol of dry propargyl alcohol was added to the reddish solution in 5 min. The temperature rose in 5-10 min to about 45°C, remained at this level for about 10 min and then began to drop. The mixture was warmed to 60°C, whereupon the exothermic reaction made the temperature rise in a few minutes to B5 c. This level was maintained by occasional cooling. After the exothermic reaction (3,3-sigmatropic rearrangement) had subsided, the mixture was heated for an additional 10 min at 80°C and the benzene was then removed in a water-pump vacuum. The red residue was practically pure acid amide... 

Equivalent Weights Acid-base titrations can be used to characterize the chemical and physical properties of matter. One simple example is the determination of the equivalent weighf of acids and bases. In this method, an accurately weighed sample of a pure acid or base is titrated to a well-defined equivalence point using a mono-protic strong acid or strong base. If we assume that the titration involves the transfer of n protons, then the moles of titrant needed to reach the equivalence point is given as... 

The pure acid does not react in the cold with sulfur, selenium, tellurium, carbon, silver, copper, zinc, iron, chromium, or manganese, but slowly dissolves mercury and tin (20). At higher temperatures, lead, mercury, tin, and sulfur react rapidly, eg ... 

Unlike ECF, direct fluorination does not alter the carbon backbone preparation of isomerically pure acids is possible (18). Both direct fluorination and ECF permit a great variety of stmctures to be made, but each method is better at certain types of stmctures than the other. Ether acids are produced in good yields, by direct fluorination (17), while ECF of ether-containing acids is fair to poor depending on the substrate. Despite much industrial interest, the costs and hazards of handling fluorine gas have prevented commercial application of this process. 

Formic acid [64-18-6] (methanoic acid) is the first member of the homologous series of alkyl carboxyHc acids. It occurs naturally ia the defensive secretions of a number of insects, particularly of ants. Although the acid nature of the vapors above ants nests had been known since at least 1488, the pure acid was not isolated until 1671, when the British chemist John Ray described the isolation of the pure acid by distillation of ants (1). This remained the main preparative method for more than a century until a convenient laboratory method was discovered by Gay-Lussac (2). The preparation of formates using carbon monoxide was described by Berthelot in 1856. 

The pyridine was Eastman grade which was dried by long standing over potassium hydroxide for A but used without further drying in B. Presumably the anhydride would not be obtained in good yield from pure acid chloride in B, if the reaction mixture did not contain about 0.9 g. of water. 

Alternatively, the acid (5g) is refluxed for 2h with 15mL of MeOH and 2mL of 98% H2SO4 (cool when mixing this soln). Pour into 10 volumes of H2O and extract with the minimum volume of CHCI3 to give clear separation of phases. The extract is washed with H2O and dried (CaCl2) and distd. The methyl ester is collected at 77-79°/ mm, m 38-39°. The ester is hydrolysed with the calculated amount of N KOH and refluxed until clear. Acidification with HCl provides the pure acid with 90% recovery. [Org Synth 4 1 1964.] The amide crysts from cyclohexane, m 189°. [Chem Ber 62 1629 1959.]... 

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