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CH3CO2H

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Originally appearing in Volume V06, Page 71 of the 1911 Encyclopedia Britannica.
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CH3CO2H =118 C1CH2•CO2H =185° C12CH•CO2H =195° C13C•CO2H =195°—200° 3° According to van 't Hoff the substitution of chlorine atoms into a methyl group occasions the following increments: Cl in CH3 66° CI ,, CH2C1 39° Cl „ CHC12 13°. The introduction of chlorine, however, may involve a fall in the boiling-point, as is recorded by Henry in the case of the chlorinated acetonitriles NC•CH3. NC•CH2C1. NC•CHC12. NC•CC13. 81° 123° 112° 83° 42° -11° -29° The replacement of one negative group by another is accompanied by a change in the boiling-point, which is independent of the compound in which the substitution is effected, and solely conditioned by the nature of the replaced and replacing groups. Thus bromine and iodine replace chlorine with increments of about 22° and 5o° respectively. A factor of considerable importance in determining boiling-points of isomers is the symmetry of the molecule. Referring to the esters C9H1802 previously mentioned, it is seen that the highest boiling-points belong to methyl octoate and octyl formate, the least symmetrical, while the minimum belongs to amyl butyrate, the most symmetrical. The isomeric pentanes also exhibit a similar relationCH3(CH2),CH3 =38°,(CH3)2CHC2H5=30°,(CH3)4C =9.5°. Fora similar reason secondary alcohols boil at a lower temperature than the corresponding primary, the difference being about 19°. A. E. Earp (Phil. Mag., 1893 151, 35, p. 458) has shown that, while an increase in molecular weight is generally associated with a rise in the boiling-point, yet the symmetry of the resulting molecule may exert such a lowering effect that the final result is a diminution in the boiling-point. The series H2S = -61°, CH3SH =21 °, (CH3)2S =41 ° is an example; in the first case, the molecular weight is in-creased and the symmetry diminished, the increase of boiling-point being 82°; in the second case the molecular weight is again increased but the molecule assumes a more symmetrical configuration, hence the comparatively slight increase of 20°. A similar depression is presented by methyl alcohol (67°) and methyl ether (—23°). Among the aromatic di-substitution derivatives the ortho compounds have the highest boiling-point, and the meta boil at a higher, or about the same temperature as the Para compounds. Of the tri-derivatives the symmetrical compounds boil at the lowest temperature, the asymmetric next, and the vicinal at the highest. An ethylenic or double carbon union in the aliphatic hydrocarbons has, apparently, the same effect on the boiling-point as two hydrogen atoms, since the compounds CJI2,,+2 and Cal-12a boil at about the same temperature. An acetylenic or triple linkage is associated with a rise in the boiling-point; for example, propargyl compounds boil about 19.5° higher than the corresponding propyl compound. Certain regularities attend the corresponding property of the melting-point. A rule applicable to organic compounds, due to Adolf v. Baeyer and supported by F. S. Kipping (Jour. Chem. Soc., 1893, 63, p. 465) states, that the melting-point of any odd member of a homologous series is lower than the melting-point of the even member containing one carbon atom less. This is true of the fatty acid series, and the corresponding ketones and alcohols, and also of the succinic acid series. Other regularities exist, but generally with many exceptions. It is to be noted that although the correlation of melting-point with constitution has not been developed to such an extent as the chemical significance of other physical properties, the melting-point is the most valuable test of the purity of a sub-stance, a circumstance due in considerable measure to the fact that impurities always tend to lower the melting-point. Heat of Combustion and Constitution.—In the article THERMOCHEMISTRY a general account of heats of formation of chemical compounds is given, and it is there shown that this constant measures the stability of the compound. In organic chemistry it is more customary to deal with the " heat of combustion,” i.e. the heat evolved when an organic compound is completely burned in oxygen; the heat of formation is deduced from the fact that it is equal to the beats of formation of the products of combustion less the observed heat of combustion. The researches of Julius Thomsen and others have shown that in many cases definite conclusions regarding constitution can be drawn from quantitative measurements of the heats of combustion; and in this article a summary of the chief results will be given, The identity of the four valencies of the carbon atom follows from the fact that the heats of combustion of methane, ethane, propane, trimethyl methane, and tetramethyl methane, have a constant difference in the order given, viz. 158.6 calories; this means that the replacement of a hydrogen atom by a methyl group is attended by a constant increase in the heat of combustion. The same difference attends the introduction of the methyl group into many classes of compounds, for example, the paraffins, olefines, acetylenes, aromatic hydrocarbons, alcohols, aldehydes, ketones and esters, while a slightly lower value (157.1) is found in the case of the halogen compounds, nitriles, amines, acids, ethers, sulphides and nitro compounds. It therefore appears that the difference between the heats of combustion of two adjacent members of a series of homologous compounds is practically a constant, and that this constant has two average values, viz. 158.6 and 157.1. An important connexion between heats of combustion and constitution is found in the investigation of the effect of single, double and triple carbon linkages on the thermochemical constants. If twelve grammes of amorphous carbon be burnt to carbon dioxide under constant volume, the heat evolved (96.96 cal.) does not measure the entire thermal effect, but the difference between this and the heat required to break down the carbon molecule into atoms. If the number of atoms in the carbon molecule be denoted by n, and the heat required to split off each atom from the molecule by d, then the total heat required to break down a carbon molecule completely into atoms is nd. It follows that the true heat of combustion of carbon, i.e. the heat of combustion of one gramme-atom, is 96.96+4. The value of d can be evaluated by considering the combustion of amorphous carbon to carbon monoxide and carbon dioxide. In the first case the thermal effect of 58.58 calories actually observed must be increased by 2d to allow for the heat absorbed in splitting off two gramme-atoms of carbon; in the second case the thermal effect of 96.96 must be increased by d as above. Now in both cases one gramme-molecule of oxygen is decomposed, and the two oxygen atoms thus formed are combined with two carbon valencies. It follows that the thermal effects stated above must be equal, i.e.58.58+2d=96.96+d, and therefored=38.38. Theabsolute heat of combustion of a carbon atom is therefore 135.34 calories, and this is independent of the form of the carbon burned. Consider now the combustion of a hydrocarbon of the general formula C„H2,.. We assume that each carbon atom and each hydrogen atom contributes equally to the thermal effect. If a be the heat evolved by each carbon atom, and t3 that by each hydrogen atom, the thermal effect may be expressed as H =na+2m/3-A, where A is the heat required to break the molecule into its constituent atoms. If the hydrocarbon be saturated, i.e. only contain single carbon linkages, then the number of such linkages is 2n-m, and if the thermal effect of such a linkage be X, thenthe termA.isobviously equal to (2n-m)X. The value of H then becomes H=na-+-2mi-(2n-m)X or nth-mn, where and rt are constants. Let double bonds be present, in number p, and let the energy due to such a bond be Y. Then the number of single bonds is 2n-m-2p, and the heat of combustion becomes H1=nE+m+l+p(2X-Y). If triple bonds, q in number, occur also, and the energy of such a bond be Z, the equation for H becomes H = nE+mn +P(2X-Y) +q(3X-Z). This is the general equation for calculating the heat of combustion of a hydrocarbon. It contains four independent constants; two of these may be calculated from the heats of combustion of saturated hydrocarbons, and the other two from the combustion of hydrocarbons containing double and triple linkages. By experiment it is found that the thermal effect of a double bond is much less than the effect of two single bonds, while a triple bond has a much smaller effect than three single bonds. J. Thomsen deduces the actual values of X, Y, Z to be 14.71, 13.27 and zero; the last value he considers to be in agreement with the labile equilibrium of acetylenic compounds. One of the most important applications of these values is found in the case of the constitution of benzene, where Thomsen decides in favour of the Claus formula, involving nine single carbon linkages, and rejects the Kekule formula, which has three single and three double bonds (see section IV.). The thermal effects of the common organic substituents have also been investigated. The thermal effect of the " alcohol " group C.OH may be determined by finding the heat of formation of the alcohol and subtracting the thermal effects of the remaining linkages in the molecule. The average value for primary alcohols is 4467 cal., but many large differences from this value obtain in certain cases. The thermal effects increase as one passes from primary to tertiary alcohols, the values deduced from propyl and isopropyl alcohols and trimethyl carbinol being:—primary =45.08, secondary =50.39, tertiary =6o•98. The thermal effect of the aldehyde group has the average value 64.88 calories, i.e. considerably greater than the alcohol group. The ketone group corresponds to a thermal effect of 53.52 calories. It is remarkable that the difference in the heats of formation of ketones and the paraffin containing one carbon atom less is 67.94 calories, which is the heat of formation of carbon monoxide at constant volume. It follows therefore that two hydrocarbon radicals are bound to the carbon monoxide residue with the same strength as they combine to form a paraffin. The average value for the carboxyl group is 119.75 calories, i.e. it is equal to the sum of the thermal effects of the aldehyde and carbonyl groups. The thermal effects of the halogens are: chlorine =15.13 calories, bromine =7.68; iodine = -4.25 calories. It is remarkable that theposition of the halogen in the molecule has no effect on the heat of formation; for example, chlorpropylene and allylchloride, and also ethylene dichloride and ethylidene dichloride, have equal heats of formation. The thermal effect of the ether group has an average value of 34'31 calories. This value does not hold in the case of r—~ methylene oxide if we assign to it the formula H2C•O•CH2, but if the formula H2C.O.CH2 (which assumes the presence of two free valencies) be accepted, the calculated and observed heats of formation are in agreement. The combination of nitrogen with carbon may result in the formation of nitriles, cyanides, or primary, secondary or tertiary amines. Thomsen deduced that a single bond between a carbon and a nitrogen gramme-atom corresponds to a thermal effect of 2.77 calories, a double bond to 5'44, and a treble bond to 8.31. From this he infers that cyanogen is C : N- N :C and not N C •C ; N, that hydrocyanic acid is HC.N, and acetonitrile CH3.0 i N. In the case of the amines he decides in favour of the formulae H2C:NH3 H3C~NH2 H8C>NH•CH2 primary, secondary, tertiary. These involve pentavalent nitrogen. These formulae, however, only apply to aliphatic amines; the results obtained in the aromatic series are in accordance with the usual formulae. Optical Relations. Refraction and Composition.—Reference should be made to the article REFRACTION for the general discussion of the phenomenon known as the refraction of light. It is there shown that every substance, transparent to light, has a definite refractive index, which is the ratio of the velocity of light in vacuo to its velocity in the medium to which the refractive index refers. The refractive index of any substance varies with (I) the wave-length of the light; (2) with temperature; and (3) with the state of aggregation. The first cause of variation may be at present ignored; its significance will become apparent when we consider dispersion (vide infra). The second and third causes, however, are of greater importance, since they are associated with the molecular condition of the substance; hence, it is obvious that it is only from some function of the refractive index which is independent of temperature variations and changes of state (i.e. it must remain constant for the same substance at any temperature and in any form) that quantitative relations between refractivity and chemical composition can be derived. The pioneer work in this field, now frequently denominated " spectro-chemistry," was done by Sir Isaac Newton, who, from theoretical considerations based on his corpuscular theory of light, determined'the function (n2-1), where n is the refractive index, to be the expression for the refractive power; dividing this expression by the density (d), he obtained (n2—1)/d, which he named the " absolute refractive power." To P. S. Laplace is due the theoretical proof that this function is independent of temperature and pressure, and apparent experimental confirmation was provided by Biot and Arago's, and by Dulong's observations on gases and vapours. The theoretical basis upon which this formula was devised (the corpuscular theory) was shattered early in the 19th century, and in its place there arose the modern wave theory which theoretically invalidates Newton's formula. The question of the dependence of refractive index on temperature was investigated in 1858 by J. H. Gladstone and the Rev. T. P. Dale; the more simple formula (n-1)/d, which remained constant for gases and vapours, but exhibited slight discrepancies when liquids were examined over a wide range of temperature, being adopted. The subject was next taken up by Hans Landolt, who, from an immense number of observations, supported in a general way the formula of Gladstone and Dale. He introduced the idea of comparing the refractivity of equimolecular quantities of different substances by multiplying the function (n—i)/d by the molecular weight (M) of the substance, and investigated the relations of chemical grouping to refractivity. Although establishing certain general relations between atomic and molecular refractions, the results were somewhat vitiated by the inadequacy of the empirical function which he employed, since it was by no means a constant which depended only on the actual composition of the substance and was independent of its physical condition. A more accurate expression (n2—1)/(n2+2)d was 70 suggested in 188o independently and almost simultaneously by L. V. Lorenz of Copenhagen and H. A. Lorentz of Leiden, from considerations based on the Clausius-Mossotti theory of dielectrics. Assuming that the molecules are spherical, R. J. E. Clausius and O. F. Mossotti found a relation between the dielectric constant and the space actually occupied by the molecules, viz. K = (r +2a)/(I -a), or a=(K-1)/(K+2.), where K is the dielectric constant and a the fraction of the total volume actually occupied by matter. According to the electromagnetic theory of light K = N2, where N is the refractive index for rays of infinite wave-length. Making this substitution, and dividing by d, the density of the substance, we obtain aid = (N2 - 1)/(N2 +2)d. Since aid is the real specific volume of the molecule, it is therefore a constant; hence (N2-1)/(N2+2)d is also a constant and is independent of all changes of temperature, pressure, and of the state of aggregation. To determine N recourse must be made to Cauchy's formula of dispersion (q.v.), n=A+B/X2+C/X4+... from which, by extrapolation, X becoming infinite, we obtain N = A. In the case of substances possessing anomalous dispersion, the direct measurement of the refractive index for Hertzian waves of very long wave-length may be employed. It is found experimentally that the Lorenz and Lorentz function holds fairly well, and better than the Gladstone and Dale formula. This is shown by the following observations of Ri hlmann on water, the light used being the D line of the spectrum:- t. (n- Yd. (n2--I)/(n2+2)d. 0 0.3338 0.2061 10 0.3338 0.2061 20 0.3336 0.2061 90 0.3321 0.2059 100 0.3323 0.2061 Eykmann's observations also support the approximate constancy of the Lorenz-Lorentz formula over wide temperature differences, but in some cases the deviation exceeds the errors of observation. The values are for the Ha line: Substance. Temp. (n2-1)/(n2+2)d. Isosafrol., C,oHio02 141'10 1° ° 0.2962 9Diphenyl ethylene, Collin # 122- 0'3339 Quinoline, CBH7N . . . 16.2° 0.3187 1141° 0.3225 The empirical formula (n2-I)/(n2+o•4)d apparently gives more constant values with change of temperature than the Lorenz-Lorentz form. The superiority of the Lorenz-Lorentz formula over the Gladstone and Dale formula for changes of state is shown by the following observations of Bruhl (Zeit. f. phys. Chem., 1891, 71, p. 4). The values are for the D line:- Landolt and Gladstone, and at a later date J. W. Briihl, have investigated the relations existing between the refractive power Additive and composition. To Landolt is due the proof that, relations. In general, isomers, i.e. compounds having the same composition, have equal molecular refractions, and that equal differences in composition are associated with equal differences in refractive power. This is shown in the following table (the values are for H°):- Additive relations undoubtedly exist, but many discrepancies occur which may be assigned, as in the case of molecular volumes, to differences in constitution. Atomic refractions may be obtained [PHYSICAL either directly, by investigating the various elements, or indirectly, by considering differences in the molecular refractions of related compounds. The first method needs no explanation. The second method proceeds on the same lines as adopted for atomic volumes. By subtracting the value for CH2, which may be derived from two substances belonging to the same homologous series, from the molecular refraction of methane, CH4, the value of hydrogen is obtained; subtracting this from CH2, the value of carbon is determined. Hyddroxylic oxygen is obtained by subtracting the molecular refractions of acetic acid and acetaldehyde. Similarly, by this method of differences, the atomic refraction of any element may be determined. It is found, however, that the same element has not always the same atomic refraction, the difference being due to the nature of the elements which saturate its valencies. Thus oxygen varies according as whether it is linked to hydrogen (hydroxylic oxygen), to two atoms of carbon (ether oxygen), or to one carbon atom (carbonyl oxygen) ; similarly, carbon varies according as whether it is singly, doubly, or trebly bound to carbon atoms. A table of the atomic refractions and dispersions of the principal elements is here given:- Element. H°. U. Hy. Dispersion Hy-H°. Hydrogen 1.103 I•051 1.139 0.036 Oxygen, hydroxyl . 1.506 1.521 1.525 0.019 ether 1.655 1.683 1.667 0.012 Carbonyl 2.328 2.287 2.414 o.o86 Chlorine 6.014 5.998 6.190 0.176 Bromine 8.863 8.927 9.211 0.348 Iodine 13.808 14.12 14.582 0-774 Carbon (singly bound) 2.365 2.501 2.404 0.039 Double linkage of carbon 1.836 1.707 1.859 0.23 Triple 2.22 2.41 0.19 Nitrogen, singly bound and only to carbon . 2.76 2.95 0.19 Dispersion and Composition.-In the preceding section we have seen that substances possess a definite molecular (or atomic) refraction for light of particular wave-length; the difference between the refractions for any two rays is known as the molecular (or atomic) dispersion. Since molecular refractions are independent of temperature and of the state of aggregation, it follows that molecular dispersions must be also independent of these conditions; and hence quantitative measurements should give an indication as to the chemical composition of substances. This subject has been principally investigated by Bruhl; he found that molecular dispersions of liquids and gases were independent of temperature, and fairly independent of the state of aggregation, but that no simple connexion exists between atomic refractions and dispersions (see preceding table). He also showed how changes in constitution effected dispersions to a far greater extent than they did refractions; thus, while the atomic dispersion of carbon is 0.039, the dispersions due to a double and treble linkage is 0.23 and 0.19 respectively. Colour and Constitution.-In this article a summary of the theories which have been promoted in order to connect the colour of organic compounds with their constitution will be given, and the reader is referred to the article COLOUR for the physical explanation of this property, and to VISION for the physiological and psychological bearings. A clear distinction must be drawn between colour and the property of dyeing; all coloured substances are not dyes, and it is shown in the article DYEING that the property of entering into chemical or physical combination with fibres involves properties other than those essential to colour. At the same time, however, all dyestuffs are coloured substances. A survey of coloured substances led O. N. Witt in 1876 toformulate his " chromophore-auxochrome " theory. On this theory colour is regarded as due to the presence of a " chromophore," and dyeing power to an " auxochrome "; the latter by itself cannot produce colour or dyeing power, but it is only activein the presence of a chromophore, when it intensifies the colour and confers the property of dyeing. The principal chromophores are the azo, -N = N -, azoxy, = N20, nitro, -NO2, nitroso, - NO, and carbonyl, = CO, groups. The azo-group is particularly active, both the aliphatic and aromatic compounds being coloured. The simplest aliphatic compounds, such as diazo-methane, diazo- ethane, and azo-formic acid, are yellow; the diamide of the latter acid is orange-red. Of the aromatic compounds azo-benzene is bright orange-red, and a-azo- naphthalene forms red needles or small steel-blue prisms. The azo- group, however, has little or no colouring effect when present in a Gladstone and Dale. Lorenz and Lorentz. Substance. Temp. Vapour. Liquid. Vapour. Liquid. Water ro° 0.3101 0.3338 0.2068 0.2061 Carbon disulphide . 10° 0.4347 0.4977 0.2898 0.2805 Chloroform ' roe 0.2694 0.3000 0.1796 0.1790 Substance. Mol. Substance. Mol. Diff. for Refract. Refract. CH2. Ethylene chloride C2H4C12 } 20.96 Acetic acid . . 12.93 } 4.49 Ethylidene chloride ( 21.08 Propionic acid . . 17.42 Fumaric acid ( 708g Butyric acid 22.01 4'59 Malefic acid 1 C411404 t 70.29 o-Cresol 32.52 Acetaldehyde . 11'50 } 4'43 m-Cresol C7HBO . . 32.56 Propionaldehyde . . 15.93 p-Cresol 32.57 Butylaldehyde . . 20.52 } 4'59 ring system, such as in cinnolene, phthalazine and tolazone. The nitro group has a very important action mainly on account of the readiness with which It can be introduced into the molecule, but its effect is much less than that of the azo group. The colour produced is generally yellow, which, in accordance with a general rule, is intensified with an increase in the number of groups; compare, for example, mono-, di- and tri-nitrobenzene. The nitroso group is less important. The colour produced is generally of a greenish shade; for example, nitrosobenzene is green when fused or insolution (when crystalline, it is colourless), and dinitrosoresorcin has been employed as a dyestuff under the names " solid green " and " chlorine." The carbonyl group by itself does not produce colour, but when two adjacent groups occur in the molecule, as for example in the a-diketones (such as di-acetyl and benzil), a yellow colour is produced. It also acts as a chromogenic centre when double bonds or ethylenic linkages are present, as in fluorene ketone or fluorenone. A more complex chromophoric group is the triple ethylenic grouping =C > C =, the introduction of which was rendered necessary by the discovery of certain coloured hydrocarbons. As a general rule, hydrocarbons are colourless; the exceptions include the golden yellow acenaphthylene, the red bidiphenylene-ethylene, and the derivatives of fulvene CH : CH > H : CH2, which have been discussed by ~CH J. Thiele (Ber., 1900, 33, p. 666). This grouping is not always colour-producing, since diphenyl is colourless. The most important auxochromes are the hydroxyl (–OH) and amino (–NH2) groups. According to the modern theory of auxochromic action, the introduction of a group into the molecule is accompanied by some strain, and the alteration in colour produced is connected with the magnitude of the strain. The amino group is more powerful than the hydroxyl, and the substituted amino group more powerful still; the repeated substitution of hydroxyl groups sometimes causes an intensification and sometimes a diminution of colour. We may here notice an empirical rule formulated by Nietzski in 1879:—the simplest colouring substances are in the greenish-yellow and yellow, and with increasing molecular weight the colour passes into orange, red, violet, blue and green. This rule, however, is by no means perfect. Examination of the absorption spectra of coloured compounds shows that certain groupings displace the absorption bands in one direction, and other groupings in the other. If the bands be displaced towards the violet, involving a regression through the colours mentioned above, the group is said to be " hypsochromic "; if the reverse occurs the group is " bathochromic." It may be generally inferred that an increase in molecular weight is accompanied by a change in colour in the direction of the violet. Auxochromic groups generally aid one another, i.e. the tint deepens as the number of auxochromes increases. Also the relative position of the auxochrome to the chromophore influences colour, the ortho-position being generally the most powerful. Kauffmann (Ber., 1906, 39, p. 1959) attempted an evaluation of the effects of auxochromic groups by means of the magnetic optical constants. The method is based on the supposition that the magnetic rotation measures the strain produced in the molecule by an auxochrome, and he arranges the groups in the following order: •O•000H3 •OCH5 •NHCOCH3 •NH2 •N(CH3)2 •N(C2Ha)2 -0.260 1.459 .949 3.821 8-587 8.816 The phenomena attending the salt formation of coloured and colouring substances are important. The chromophoric groups are rarely strongly acid or basic; on the other hand, the auxochromes are strongly acid or basic and form salts very readily. Notable differences attend the neutralization of the chromophoric and auxochromic groups. With basic substances, the chromophoric combings tion with a colourless acid is generally attended by a deepening in colour; auxochromic combination, on the other hand, with a lessening. Examples of the first case are found among the colourless acridines and quinoxalines which give coloured salts; of the second case we may notice the colourless hydrochloride and sulphate of the deep yellow o-aminobenzophenone. With acid substances, the combination with " colourless " metals, i.e. metals producing colour-less salts with acids, is attended by colour changes contrary to those given above, auxochromic combination being accompanied by a deepening, and chromophoric by a lessening of the tint. Mention may be made of the phenomenon of halochromism, the name given to the power of colourless or faintly-coloured substances of combining with acids to form highly-coloured substances without the necessary production of a chromophoric group. The researches of Adolf von Baeyer and Villiger, Kehrmann, Kauffmann and others, show that this property is possessed by very many and varied substances. In many cases it may be connected with basic oxygen, and the salt formation is assumed to involve the passage of divalent into tetravalent oxygen. It seems that intermolecular change also occurs, but further research is necessary before a sound theory can be stated. Quinone Theory of Colour.—A theory of colour in opposition to the Witt theory was proposed by Henry Armstrong in 1888 and 1892. This assumed that all coloured substances were derivatives of orthoor para-quinone (see QUINONES), and although at the time of itspromotion little practical proof was given, yet the theory found wide acceptance on account of the researches of many other chemists. It follows on this theory that all coloured substances contain either of the groupings 'CD- or Cl- , the former being a para-quinonoid, the latter an ortho-quinonoid. While very many coloured substances must obviously contain this grouping, yet in many cases it is necessary to assume a simple intermolecularchange, while in others a more complex rearrangement of bonds is necessary. Quinone, which is light yellow in colour, is the simplest coloured substance on this theory. Hydrocarbons of similar structure have been prepared by Thiele, for example, the orange-yellow tetraphenyl-para-xylylene, which is obtained by boiling the bromide C6H4[CBr(C6H5)2]2 with benzene and molecular silver. The quinonoid structure of many coloured compounds has been proved experimentally, as, for example, by Hewitt for the benzene-azo-phenols, and Hantzsch for triaminotriphenyl methane and acridine derivatives; .but, at the same time, many substances cannot be so explained. A notable example is provided by the phthaleins, which result by the condensation of phthalic anhydride with phenols. In the free state these substances are colourless, and were assumed to have the formula shown in 1. Solution in dilute alkali was supposed to be accompanied by the rupture of the lactone ring with the formation of the quinoncid salt shown in 2. HO c v v c
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