THE REDUCTION POTENTIALS OF QUINHYDRONES. 199 XXXII. -Method of Measuring the Reduction Potentia'ls of Quinh ydrones. By EINAR BIILMANN A. LANGSETH JENSEN and KAI 0. PEDERSEN. IT has been pointed out by one of us (Trans. Faraduy Xoc. 1923, 19 676) that the reduction potential of a quinhydrone can be determined by measuring the potential of an electrode containing, for instance the hydroquinone of one quinhydrone in a solution of the quinone of another quinhydrone with a known reduction potential, that is without preparing the quinhydrone itself or both its con 200 BIILMANN JENSEN AND PEDERSEN METHOD OF (a) Pt I (a ; electrolyte ; (AYAH,) stituents. In this paper the theory of the method is described and some quantitative results are detailed. Let A AH, and (A,AH,) be a certain quinone hydroquinone, and quinhydrone respectively and let By BH, and (B,BH,) be another similar series.Then if a solution of the hydroquinone, AH, be mixed with a solution of the quinone By AH will be partly oxidised to the quinone A a t the expense of By which will be partly reduced to its corresponding hydroquinone BH,. Starting with equimolecular quantities of AH and By the reaction may be written aAH + aB + (a - x)AH + xA + (a - x)B + xBH . (1) and the equilibrium set up must be such that the reduction potential T (as measured against a hydrogen electrode) of the mixture (a - x)AH + xA is equal to the reduction potential of the mixture (a - x)B + xBH,. We have now four electrolytic chains to consider Pt Pt I c " l ~ y ~ ~ B ; electrolyte ; (B,BH,) ( b ) the similar chain Pt RT a - x - X += __ nF .loge- X - - K log10 rx (3) so that 4 = - +'; ( c ) the quinhydrone electrode with the reduction potential xof ; and ( d ) the quinhydrone electrode with the reduction potential xo.Pt 1 (A,AH,) ; H I Pt Pt I (BYBH,) ; H I Pt x = x o f - + = x o + + z.e. nof = 2ic -Tco=2++xo . . . (4) Now for equilibrium we must have where x is the voltage both of the chain (a - x) . mAH PtI x.mA i H 2 I P t and of the chai MEASURING THE REDUCTION POTENTIALS OF QUINHYDRONES. 201 That is to say the reduction potential no' of the quinhydrone (i) the potential X of a mixture of the hydroquinone AH with an equimolecular quantity of the quinone B (or conversely) to a hydrogen electrode. or (ii) the potential #, of this mixture to the quinhydrone electrode (A,AH,) can be determined by measuring (B,BH,).0.75 0.70 0.65 0.60 2 \ N <A I-In both cases we require to know the reduction potential no, of the quinhydrone (B,BH,) electrode; and obviously the electro lyte the temperature and the hydrogen pressure (1 atm.) must be the same in all the electrodes. Combining equations (3) and (4) we get no' - = 2+ = 2K log,(){x/(a - x)) = (at 25') 0.0591 log,,{x/(a - x)), from which equation x may be calculated when no and xo' have been determined. Taking the reduction potentials at 25" for benzoquinhydrone, toluquinhydrone and p-xyloquinhydrone as 0.6990 0-6454 and H 202 BIILMANN JENSEN AND PEDERSEN METHOD OF 0.5886 volt respectively the following values for the percentage of quinhydrone unchanged in equimolecular mixtures with quinones are obtained : yo Hydroquinone 12.4 1.4 Mixture.unchanged. Benzoquinone + Toluhydroquinone . . . . . . . . . Benzoquinone + p-Xylohydroquinone . . . . . Toluquinone + p-Xylohydroquinone . . . . . . 9.1 Two sources of error must be briefly mentioned. First the position of the equilibrium markedly affects the accuracy attain-able. Fig. 1 shows the relation between the composition of the solution and the potential referred to a hydrogen electrode at 25". Thus p denotes the potential (0.72 volt) of a mixture of 18-5y0 of benzohydroquinone and 8l.5yO of benzoquinone. Similarly A B, and C denote the potentials of the three quinhydrones. Now the potential of the equilibrium mixture of toluhydroquinone and benzoquinone is given by point a (the intersection of the corre-sponding curves) and it is obvious from the figure that the farther this point is removed from the centre line ABC the more rapidly will the potential change with composition and the less accurate will be the results.The second source of error is due to the inequality of the dis-sociation constants * of the various quinhydrones. At 25" the dissociation constant of benzoquinhydrone is 0.224 whilst for toluquinhydrone it is 0.095 (Biilmann Ann. Chim. 1921 [ix] 15, 151). But by comparing the values obtained by the " mixture " method with those obtained directly by La Mer ( J . Arner Chem. SOC. 1922 44 1954) and one of us (Biilmann Zoc. cit.) and also in this research it is seen that satisfactory agreement is obtained, * Let the initial concentrations of A and BH be a and the stoicheio.metric amounts of the quinones and hydroquinones a t equilibrium (as assumed in equation 1) be AH, a - x; A x ; BH, x; and By a - x. If the concentrations of undissociated quinhydrones AH,,A and BH,,B are a and p respectively the concentrations of the dissociated components will be AH, a - 1 - a; A x - a; BH, x - p ; and B a - x - /3. In the equilibrium set up the potentials of the mixtures of A and AH and of B and BH against the corresponding two quinhydrones are (equation 2) $A =K1ogl,a - - a a - x - / I If as supposed T,' - T = 2$ we have c p ~ = $B and consequently, (x - .)/(a - x - a) = (x - P)/(a - x - /3) or 2x(a - /3) = a(.- p). Consequently the supposed relation between the potential of the mixture and the reduction potentials of the single quinhydrones is strictly exact if a = /3 Le. the two quinhydrones have the same dissociation constants [as kA = (x - a)(o x - a)/u; kB = (z /?)(a - z - /3)//3] or if x = a / 2 , Le. their reduction potentials are identical. - ' respectively. x - a and $B = K log, MEASURING THE REDUCTION POTENTIALS OF QUINHYDRONES. 203 especially when the sparing solubility of the materials dealt with is considered. I n some cases the equilibrium potential was reached almost instantaneously; in others it was necessary to wait for some hours before measurement. Table I records the experimental results. TABLE I. Reduction Potentids of Quinhydrones.?ro measured in raisture. At 18". At 25". 0.6496 0.6444 - 0.5893 0.7178 0.7124 0.7174 0.7120 0.7280 0.7230 0.7280 0.7228 - 0.ti.542 - 0.65ti4 - 0.5949 * Biilmann Zoc. cit. t La Mer loc. Other deter-minations of ro at 25'. 0.6454 * 0.5886 0.7125 t 0.7151 t --0.6566 0.659 1 0.59 18 c i t . E Y P E R I M E N T A L. Toluhydroguinon e.-The t oluh ydroquin one was recryst allised from toluene immediately before the solution was made up. Equal volumes of 0.0 141-solutions of toluhydroquinone and benzoquinone in 0-liV-hydrochloric acid were mixed and the potentials measured against a benzoquinhydrone electrode blank platinum electrodes being used. TABLE 11. - p t I Toluhydroquinone 0*005A" ; ::& ; Benzoquinhydrone I Pt + Benzoquinone 0-00dJ1 254".18". Temp. 7- - Minutes ............ 30 60 150 180 Volt I * ............ 0.0273 0.0273 0.0274 0.0274 .. I1 * ......... 0.0274 0.0274 0.0275 0.0275 ~ z j " = 0.6444. r 1 3 = = 0.6496. * Duplicate electrodes. p-SyZohydro~~inoize.-The potentials were measured as above, but the electrolyte was 0.lK-sulphuric acid saturated with carbon dioxide and all the operations were carried out in an atmosphere of this gas. H* 204 BIILMANN JENSEN AND PEDERSEN METHOD OF 25.4". Temp. r- .J \__ - .-Minutes 25 60 106 135 165 195 1200 1290 1460 Volt I 0-0689 0.0775 0.0802 0.0809 0.0811 0-0813 0.0820 0.0821 0.0821 0.0702 0.0773 0.0792 0-0803 0.0807 0.0810 0.0822 0.0822 0.0823 , I1 1~25" = 0.5893. Solution prepared from equimolecular quantities of p-xylohydro-quinone and p-xyloquinone in 0-1N-sulphuric acid with carbon dioxide treatment as before.TABLE IV. 25.6". 18". Temp. A ,-'-, Minutes ......... 5 45 95 145 185 3 85 Volt I ......... 0.1101 0.1104 0-1104 0.1104 0-1103 0-1104 ,) I1 ......... 0.1102 0.1105 0.1105 0.1105 0.1105 0.1106 7250 = 0.5886. p1g = 0.5940. f Without the carbon dioxide treatment constant potentials could not be obtained. The potentials measured against the benzoquinhydrone electrode of a similar electrode prepared wit'h-out the carbon dioxide treatment are recorded in Table V and it will be seen that the potentials slowly rise until after 20 hours, they are practically identical with those in Table IV. This may be due to catalytic oxidation of the hydroquinone on the surface of the platinum the potential reaching the value in Table IV only when the solution in the electrode vessel is thoroughly mixed by diffusion.TABLE V. 25.4'. Temp. / A \ Minutes ......... 15 60 120 150 1120 1270 Volt I ......... 0.1072 0.1090 0.1099 0.1100 0.1101 0.1101 .. I1 ......... 0.1070 0.1093 0-1098 0.1099 0.1103 0.1103 MonochZorohydroquinone.-Benzoquinone (10 g.) was heated with 100 C.C. of concentrated hydrochloric acid for 2 hour on the steam-bath (Wohler AnnaZen 1844 51 155) the solution cooled, diluted with water and extracted with ether. The residue after evaporation of the ether had m. p. 103" when crystallised twice from chloroform and 105-105.5" (constant) when crystallised TWO = 0.5888 MEASURING THE REDUCTION POTENTIALS OF QUINHYDRONES.205 subsequently from carbon tetrachloride. Levy and Schultz quote 106" (ibid. 1881 210 138) and Schultz quotes 103-104" (Ber., 1882 15 654). TABLE VI. - Pt Benzoquinhyclrone HC1 . Monochlorohydroquinone 0.005M 1 Pt + 0.1N ' Benzoquinone 0.005M 23.4". 18". Temp. e- - F ,----. 18O.* Minutes ......... 15 30 60 120 180 Volt 1 ......... 0.0068 O*OOGS 0.0068 0.0067 0.0067 0.0006 , I1 ......... 0.0067 0.0067 0.0067 O*OOGG 0.0066 0.0066 * After dilution with 3 vols. of O.lX-HC1. A2j" = 0.7124. q g = 0.7178. Jlonobromohydroquinone was prepared after Sarauw's method (dnm,Zen 1881 209 105). Bromine (16 g.) in 100 C.C. of ether was added to 11 g. of hydroquinone in 259 C.C. of ether and the solution allowed to evaporate a t laboratory temperature.The residue was extracted with 260 C.C. of toluene at 90-95" filtered off and cooled. Recrystallised from 100 C.C. of toluene 5 g. gave 3.8 g. m. p. 110.5" unchanged by further recrystallisation. Yield 10 g. m. p. 109.8". Sarauw quotes m. p. 110-111". TABLE VII. - pt 1 Benzoquinhydrone ; HC1 . R'onobromoh~dro~uinone~ 0*05M pt + 0-1N ' Benzoquinone 0.05X 23.4". 1 SO. 150.* Temp. .-*-- 7.7 ,-I---. bIinutes ...... 15 30 5 5 so 125 15 55 Volt I ...... 0.0008 0.0005 0.0063 0.0065 0.0065 0.0063 O.OOci3, , I1 ...... 0.0068 0.0083 0.0065 0.0065 0.0065 0.0064 0.00G4, * After dilution with 3 vols. of O.1N-HCI. ~ 2 ~ ' = 0.7120. p l y " = 0.7174. 2 5-DichZorohydroqui~~o~~e.-Prepared by Ling's method (J., 1S92 61 65S) and twice crystallised from water the substance had a constant m.p. 169-170". Ling quotes 172" and Levy and Schultz (Zoc. cit.) 166". TABLE VIII. - Benzoquinhydrone HC1 . Dichlorohydroquinone O.OO25M j Pt + 0.1N ' Benzoquinone 0.0025111 25.4". 18". Temp. /- - 7'\7 1 )so.* Minutes . . . . . . . . . . . . . . . 15 30 90 150 T'olt I ............... 0.0121 0.0120 0~011s 0.0118 0.0117 . I1 ............... 0.0120 0.0120 0.0118 0.0118 0.0117 ~ 2 5 ~ == 0.7230. = 0.7280. * After dilution with 3 vols. ol 0-1N-HCI. 2 5-Dibromohydroquinone.-Prepared by the method of Beiiedict (No12cwtsh. 1880 1 345) and of Sarauw (Zoc. cit.) and twice crystal 206 BIILMANN JENSEN AND PEDERSEN METHOD OF lised from water the compound had the m. p. 186" recorded by those investigators. TABLE IX. Bemohydroquinone ; HC1 .Dibromohydroquinone 0.001M 0-1N ' Benzoquinone 0.001M 25.4'. 18". Temp. & - Minutes ............... 30 60 90 120 Volt I ............... 0.0119 0.0119 0.0119 0.0119 , I1 ............... 0.0119 0.0119 0.0117 0.0117 T ~ O = 0.7228. ~ 1 8 0 = 0.7280. Monochloroto1uhydroquinone.-Prepared by Schniter's method (Ber. 1887 20 2282) and crystallised from toluene and then twice from chloroform the product had m. p. 172-174" (uncorr.). Schniter quotes m. p. 175" [0.2048 reduced 26.0 C.C. of 0-1002N-iodine corresponding to 10043~o of C,H&eCl(OH),. C1 found = 22.26; calc. = 22.26%]. The hydrochloric acid used in the measurements was aerated with carbon dioxide. TABLE X. 25". r \ Temp. 1-Minutes ......... 10 40 100 150 175 1380 Volt I .. . . . . . . 0.0217 0.0220 0.0222 0.0222 0-0223 0.0225 , I1 ......... 0.0214 0.0219 0.0223 0.0226 0.0227 0.0229 Mean = 0.0224 ~ 2 5 . = 0.6542. Dichloroto1uquinhydrone.-Prepared by oxidising the correspond-ing hydroquinone in 50% alcohol with the calculated quantity of ferric ammonium alum. TABLE XI. -Pt I Dichlorotoluquinhydrone ; 0-1N-HCI ; Benzoquinhydrone 1 Pt + Temp. // \ 25". Minutes ...... 30 60 100 128 175 210 1110 1225 Volt I ...... 0.0394 0-0400 0.0406 0-0410 0.0415 0-0418 0.0425 0.0424 , I1 ...... 0-0382 0.0397 0*0400 0.0404 0-0407 0.0410 0.0424 0-0424 , I11 ...... 0.0382 0.0399 0*0400 0.0404 0.0408 0.0410 0.0424 0.0423 ~ 2 6 0 = 0.6566. TABLE XII. .-Pt I Dichlorotoluquinhydrone 0.00052cI ; 0-IN-HCI ; Benzoquinhydrone I Pt + 18". Temp.-- -'.- - -___ -\ Minutes 20 Volt I ..................... 0.0419 0.0419 0.04 19 , I1 .................. 0.0419 0.0419 0.04 19 , I11 ........... . ...... 0.0419 0.04 19 0.04 19 . . . . . . . . . . . . . . . . . . 30 45 ~ 1 8 0 = 0.6625 MEASURING THE REDUCTION POTENTIALS OF QUINHYDRONES. 207 Il/lonobromotoluhydroquinone.-To 5 g. of toluhydroquinone in 100 C.C. of ether and 45 C.C. of chloroform were added 6.45 g. of bromine in 82 C.C. of chloroform. The product (64 g.) recrystal-lised twice from toluene melted at 176-178". Clark (Amer. Chem. J . 1892 14 569) quotes 176-179". TABLE XIII. - '~.ionobromotoluh~dro~uinonc~ o.oo25nf ; ::& ; Bcnzoquinhr-drontl l>t + l'tlBenzoquinone 0.b023111 23". Temp. /-- - \---Minutes ............... 15 70 185 235 5lV Volt I ...............0.0209 0.0210 0.0212 0.0213 0.0214 .. I1 ............... 0.0205 0.020G 0.0208 0*0208 0.0213 Dibromoto1uquinhydrone.-Prepared by oxidising 1 g. of mono-bromotoluhydroquinone in 5 C.C. of alcohol with 12.25 c.c=. of O.2M-ferric ammonium alum the product was washed with water and dried between filter-paper (0.2003 titrated in presence of sodium bicarbonate with 0-0849N-iodine required 11-75 C.C. Mono-bromotoluhydroquinone = 50-3yu ; calc. 50.0%). 1~250 = 0.6564. TABLE XIV -Pt IDibromotoluquinhydrone 0.00125M ; HC10.1N ; Benzoquinhydroml Pt + Temp. / -. Volt I ............... 0-0397 0.0399 0.0399 0.0399 25.2". Minutes ............... 30 90 120 1.55 , I1 ............... 0.0397 0-0399 0.0399 0.0399 "'73" = 0,6591. ,MonochloroxyZohydroquinone.-Prepared by the method of v.Kad (Annulen 1869 151 166) and Carstanjen ( J . p. Chem. 1881, 23 430). 20 G. of p-xyloquinone were added to 200 C.C. of cold concentrated hydrochloric acid a further 300 C.C. of acid were added and the mixture was heated for 2-3 hours on the steam-bath. The crystals obtained on cooling were recrystallised from toluene carbon tetrachloride and twice from chloroform. Yield 7-75 g. m. p. 132-153" (Cl found 20.81 calc. 20.56%). TABLE XV. - pt M~nochlorox~loh~dro~uinone~ o.oo2Af ; zyb ; Benzoquinhydrone j Pt + Xyloquinone 0-002M I 25.4'. Temp. 7 - F Minutes ............... 20 60 120 Volt I .................. 0.1069 0.1070 0.1069 , 11 ............... 0-1070 0.1069 0.1069 ~ 2 5 - = 0.5949 208 MITCHELL HYDROLYSIS OF THE d-OLUCOSIDES ETC.DichZoroxyZoquinhydrone.-1 G. of monochloroxylohydroquinoiie in 20 C.C. of water and 20 C.C. of alcohol was oxidised with 29 C.C. of 0-2N-ferric ammonium alum. The product was washed with water. Yield 0.7 g. (0.0993 reduced 5-85 C.C. of 0.0983 5-iodine. Monochlorohydroquinone found = 49.6 calc. = 50.0%). TABLE XVI. - Pt I Dichloroxyloquinhydrone ; 0-1N-HC1 ; Benzoquinhydrone I Pt + 25.14'. Temp. / - Minutes ............... 30 65 125 150 Volt I ............... 0.1069 0.1070 0.1071 0.1071 .. I1 ............... 0.1072 0.1073 0.1074 0.1074 The values of x given in Tables IV V XI XII XIV and XVI are the results of direct determinations whilst those given in the other tables (excluding I) are indirect values. ~ 2 5 ~ = 0.5918.THE UNIVERSITY COPENHAGEN. [Received Au!qust 23rd 1924. THE REDUCTION POTENTIALS OF QUINHYDRONES. 199 XXXII. -Method of Measuring the Reduction Potentia'ls of Quinh ydrones. By EINAR BIILMANN A. LANGSETH JENSEN and KAI 0. PEDERSEN. IT has been pointed out by one of us (Trans. Faraduy Xoc. 1923, 19 676) that the reduction potential of a quinhydrone can be determined by measuring the potential of an electrode containing, for instance the hydroquinone of one quinhydrone in a solution of the quinone of another quinhydrone with a known reduction potential, that is without preparing the quinhydrone itself or both its con 200 BIILMANN JENSEN AND PEDERSEN METHOD OF (a) Pt I (a ; electrolyte ; (AYAH,) stituents. In this paper the theory of the method is described and some quantitative results are detailed.Let A AH, and (A,AH,) be a certain quinone hydroquinone, and quinhydrone respectively and let By BH, and (B,BH,) be another similar series. Then if a solution of the hydroquinone, AH, be mixed with a solution of the quinone By AH will be partly oxidised to the quinone A a t the expense of By which will be partly reduced to its corresponding hydroquinone BH,. Starting with equimolecular quantities of AH and By the reaction may be written aAH + aB + (a - x)AH + xA + (a - x)B + xBH . (1) and the equilibrium set up must be such that the reduction potential T (as measured against a hydrogen electrode) of the mixture (a - x)AH + xA is equal to the reduction potential of the mixture (a - x)B + xBH,. We have now four electrolytic chains to consider Pt Pt I c " l ~ y ~ ~ B ; electrolyte ; (B,BH,) ( b ) the similar chain Pt RT a - x - X += __ nF .loge- X - - K log10 rx (3) so that 4 = - +'; ( c ) the quinhydrone electrode with the reduction potential xof ; and ( d ) the quinhydrone electrode with the reduction potential xo.Pt 1 (A,AH,) ; H I Pt Pt I (BYBH,) ; H I Pt x = x o f - + = x o + + z.e. nof = 2ic -Tco=2++xo . . . (4) Now for equilibrium we must have where x is the voltage both of the chain (a - x) . mAH PtI x.mA i H 2 I P t and of the chai MEASURING THE REDUCTION POTENTIALS OF QUINHYDRONES. 201 That is to say the reduction potential no' of the quinhydrone (i) the potential X of a mixture of the hydroquinone AH with an equimolecular quantity of the quinone B (or conversely) to a hydrogen electrode.or (ii) the potential #, of this mixture to the quinhydrone electrode (A,AH,) can be determined by measuring (B,BH,). 0.75 0.70 0.65 0.60 2 \ N <A I-In both cases we require to know the reduction potential no, of the quinhydrone (B,BH,) electrode; and obviously the electro lyte the temperature and the hydrogen pressure (1 atm.) must be the same in all the electrodes. Combining equations (3) and (4) we get no' - = 2+ = 2K log,(){x/(a - x)) = (at 25') 0.0591 log,,{x/(a - x)), from which equation x may be calculated when no and xo' have been determined. Taking the reduction potentials at 25" for benzoquinhydrone, toluquinhydrone and p-xyloquinhydrone as 0.6990 0-6454 and H 202 BIILMANN JENSEN AND PEDERSEN METHOD OF 0.5886 volt respectively the following values for the percentage of quinhydrone unchanged in equimolecular mixtures with quinones are obtained : yo Hydroquinone 12.4 1.4 Mixture.unchanged. Benzoquinone + Toluhydroquinone . . . . . . . . . Benzoquinone + p-Xylohydroquinone . . . . . Toluquinone + p-Xylohydroquinone . . . . . . 9.1 Two sources of error must be briefly mentioned. First the position of the equilibrium markedly affects the accuracy attain-able. Fig. 1 shows the relation between the composition of the solution and the potential referred to a hydrogen electrode at 25". Thus p denotes the potential (0.72 volt) of a mixture of 18-5y0 of benzohydroquinone and 8l.5yO of benzoquinone. Similarly A B, and C denote the potentials of the three quinhydrones.Now the potential of the equilibrium mixture of toluhydroquinone and benzoquinone is given by point a (the intersection of the corre-sponding curves) and it is obvious from the figure that the farther this point is removed from the centre line ABC the more rapidly will the potential change with composition and the less accurate will be the results. The second source of error is due to the inequality of the dis-sociation constants * of the various quinhydrones. At 25" the dissociation constant of benzoquinhydrone is 0.224 whilst for toluquinhydrone it is 0.095 (Biilmann Ann. Chim. 1921 [ix] 15, 151). But by comparing the values obtained by the " mixture " method with those obtained directly by La Mer ( J . Arner Chem.SOC. 1922 44 1954) and one of us (Biilmann Zoc. cit.) and also in this research it is seen that satisfactory agreement is obtained, * Let the initial concentrations of A and BH be a and the stoicheio. metric amounts of the quinones and hydroquinones a t equilibrium (as assumed in equation 1) be AH, a - x; A x ; BH, x; and By a - x. If the concentrations of undissociated quinhydrones AH,,A and BH,,B are a and p respectively the concentrations of the dissociated components will be AH, a - 1 - a; A x - a; BH, x - p ; and B a - x - /3. In the equilibrium set up the potentials of the mixtures of A and AH and of B and BH against the corresponding two quinhydrones are (equation 2) $A =K1ogl,a - - a a - x - / I If as supposed T,' - T = 2$ we have c p ~ = $B and consequently, (x - .)/(a - x - a) = (x - P)/(a - x - /3) or 2x(a - /3) = a(.- p). Consequently the supposed relation between the potential of the mixture and the reduction potentials of the single quinhydrones is strictly exact if a = /3 Le. the two quinhydrones have the same dissociation constants [as kA = (x - a)(o x - a)/u; kB = (z /?)(a - z - /3)//3] or if x = a / 2 , Le. their reduction potentials are identical. - ' respectively. x - a and $B = K log, MEASURING THE REDUCTION POTENTIALS OF QUINHYDRONES. 203 especially when the sparing solubility of the materials dealt with is considered. I n some cases the equilibrium potential was reached almost instantaneously; in others it was necessary to wait for some hours before measurement. Table I records the experimental results.TABLE I. Reduction Potentids of Quinhydrones. ?ro measured in raisture. At 18". At 25". 0.6496 0.6444 - 0.5893 0.7178 0.7124 0.7174 0.7120 0.7280 0.7230 0.7280 0.7228 - 0.ti.542 - 0.65ti4 - 0.5949 * Biilmann Zoc. cit. t La Mer loc. Other deter-minations of ro at 25'. 0.6454 * 0.5886 0.7125 t 0.7151 t --0.6566 0.659 1 0.59 18 c i t . E Y P E R I M E N T A L. Toluhydroguinon e.-The t oluh ydroquin one was recryst allised from toluene immediately before the solution was made up. Equal volumes of 0.0 141-solutions of toluhydroquinone and benzoquinone in 0-liV-hydrochloric acid were mixed and the potentials measured against a benzoquinhydrone electrode blank platinum electrodes being used. TABLE 11.- p t I Toluhydroquinone 0*005A" ; ::& ; Benzoquinhydrone I Pt + Benzoquinone 0-00dJ1 254". 18". Temp. 7- - Minutes ............ 30 60 150 180 Volt I * ............ 0.0273 0.0273 0.0274 0.0274 .. I1 * ......... 0.0274 0.0274 0.0275 0.0275 ~ z j " = 0.6444. r 1 3 = = 0.6496. * Duplicate electrodes. p-SyZohydro~~inoize.-The potentials were measured as above, but the electrolyte was 0.lK-sulphuric acid saturated with carbon dioxide and all the operations were carried out in an atmosphere of this gas. H* 204 BIILMANN JENSEN AND PEDERSEN METHOD OF 25.4". Temp. r- .J \__ - .-Minutes 25 60 106 135 165 195 1200 1290 1460 Volt I 0-0689 0.0775 0.0802 0.0809 0.0811 0-0813 0.0820 0.0821 0.0821 0.0702 0.0773 0.0792 0-0803 0.0807 0.0810 0.0822 0.0822 0.0823 , I1 1~25" = 0.5893.Solution prepared from equimolecular quantities of p-xylohydro-quinone and p-xyloquinone in 0-1N-sulphuric acid with carbon dioxide treatment as before. TABLE IV. 25.6". 18". Temp. A ,-'-, Minutes ......... 5 45 95 145 185 3 85 Volt I ......... 0.1101 0.1104 0-1104 0.1104 0-1103 0-1104 ,) I1 ......... 0.1102 0.1105 0.1105 0.1105 0.1105 0.1106 7250 = 0.5886. p1g = 0.5940. f Without the carbon dioxide treatment constant potentials could not be obtained. The potentials measured against the benzoquinhydrone electrode of a similar electrode prepared wit'h-out the carbon dioxide treatment are recorded in Table V and it will be seen that the potentials slowly rise until after 20 hours, they are practically identical with those in Table IV.This may be due to catalytic oxidation of the hydroquinone on the surface of the platinum the potential reaching the value in Table IV only when the solution in the electrode vessel is thoroughly mixed by diffusion. TABLE V. 25.4'. Temp. / A \ Minutes ......... 15 60 120 150 1120 1270 Volt I ......... 0.1072 0.1090 0.1099 0.1100 0.1101 0.1101 .. I1 ......... 0.1070 0.1093 0-1098 0.1099 0.1103 0.1103 MonochZorohydroquinone.-Benzoquinone (10 g.) was heated with 100 C.C. of concentrated hydrochloric acid for 2 hour on the steam-bath (Wohler AnnaZen 1844 51 155) the solution cooled, diluted with water and extracted with ether. The residue after evaporation of the ether had m. p. 103" when crystallised twice from chloroform and 105-105.5" (constant) when crystallised TWO = 0.5888 MEASURING THE REDUCTION POTENTIALS OF QUINHYDRONES.205 subsequently from carbon tetrachloride. Levy and Schultz quote 106" (ibid. 1881 210 138) and Schultz quotes 103-104" (Ber., 1882 15 654). TABLE VI. - Pt Benzoquinhyclrone HC1 . Monochlorohydroquinone 0.005M 1 Pt + 0.1N ' Benzoquinone 0.005M 23.4". 18". Temp. e- - F ,----. 18O.* Minutes ......... 15 30 60 120 180 Volt 1 ......... 0.0068 O*OOGS 0.0068 0.0067 0.0067 0.0006 , I1 ......... 0.0067 0.0067 0.0067 O*OOGG 0.0066 0.0066 * After dilution with 3 vols. of O.lX-HC1. A2j" = 0.7124. q g = 0.7178. Jlonobromohydroquinone was prepared after Sarauw's method (dnm,Zen 1881 209 105). Bromine (16 g.) in 100 C.C. of ether was added to 11 g. of hydroquinone in 259 C.C.of ether and the solution allowed to evaporate a t laboratory temperature. The residue was extracted with 260 C.C. of toluene at 90-95" filtered off and cooled. Recrystallised from 100 C.C. of toluene 5 g. gave 3.8 g. m. p. 110.5" unchanged by further recrystallisation. Yield 10 g. m. p. 109.8". Sarauw quotes m. p. 110-111". TABLE VII. - pt 1 Benzoquinhydrone ; HC1 . R'onobromoh~dro~uinone~ 0*05M pt + 0-1N ' Benzoquinone 0.05X 23.4". 1 SO. 150.* Temp. .-*-- 7.7 ,-I---. bIinutes ...... 15 30 5 5 so 125 15 55 Volt I ...... 0.0008 0.0005 0.0063 0.0065 0.0065 0.0063 O.OOci3, , I1 ...... 0.0068 0.0083 0.0065 0.0065 0.0065 0.0064 0.00G4, * After dilution with 3 vols. of O.1N-HCI. ~ 2 ~ ' = 0.7120. p l y " = 0.7174. 2 5-DichZorohydroqui~~o~~e.-Prepared by Ling's method (J., 1S92 61 65S) and twice crystallised from water the substance had a constant m.p. 169-170". Ling quotes 172" and Levy and Schultz (Zoc. cit.) 166". TABLE VIII. - Benzoquinhydrone HC1 . Dichlorohydroquinone O.OO25M j Pt + 0.1N ' Benzoquinone 0.0025111 25.4". 18". Temp. /- - 7'\7 1 )so.* Minutes . . . . . . . . . . . . . . . 15 30 90 150 T'olt I ............... 0.0121 0.0120 0~011s 0.0118 0.0117 . I1 ............... 0.0120 0.0120 0.0118 0.0118 0.0117 ~ 2 5 ~ == 0.7230. = 0.7280. * After dilution with 3 vols. ol 0-1N-HCI. 2 5-Dibromohydroquinone.-Prepared by the method of Beiiedict (No12cwtsh. 1880 1 345) and of Sarauw (Zoc. cit.) and twice crystal 206 BIILMANN JENSEN AND PEDERSEN METHOD OF lised from water the compound had the m.p. 186" recorded by those investigators. TABLE IX. Bemohydroquinone ; HC1 . Dibromohydroquinone 0.001M 0-1N ' Benzoquinone 0.001M 25.4'. 18". Temp. & - Minutes ............... 30 60 90 120 Volt I ............... 0.0119 0.0119 0.0119 0.0119 , I1 ............... 0.0119 0.0119 0.0117 0.0117 T ~ O = 0.7228. ~ 1 8 0 = 0.7280. Monochloroto1uhydroquinone.-Prepared by Schniter's method (Ber. 1887 20 2282) and crystallised from toluene and then twice from chloroform the product had m. p. 172-174" (uncorr.). Schniter quotes m. p. 175" [0.2048 reduced 26.0 C.C. of 0-1002N-iodine corresponding to 10043~o of C,H&eCl(OH),. C1 found = 22.26; calc. = 22.26%]. The hydrochloric acid used in the measurements was aerated with carbon dioxide. TABLE X.25". r \ Temp. 1-Minutes ......... 10 40 100 150 175 1380 Volt I . . . . . . . . 0.0217 0.0220 0.0222 0.0222 0-0223 0.0225 , I1 ......... 0.0214 0.0219 0.0223 0.0226 0.0227 0.0229 Mean = 0.0224 ~ 2 5 . = 0.6542. Dichloroto1uquinhydrone.-Prepared by oxidising the correspond-ing hydroquinone in 50% alcohol with the calculated quantity of ferric ammonium alum. TABLE XI. -Pt I Dichlorotoluquinhydrone ; 0-1N-HCI ; Benzoquinhydrone 1 Pt + Temp. // \ 25". Minutes ...... 30 60 100 128 175 210 1110 1225 Volt I ...... 0.0394 0-0400 0.0406 0-0410 0.0415 0-0418 0.0425 0.0424 , I1 ...... 0-0382 0.0397 0*0400 0.0404 0-0407 0.0410 0.0424 0-0424 , I11 ...... 0.0382 0.0399 0*0400 0.0404 0.0408 0.0410 0.0424 0.0423 ~ 2 6 0 = 0.6566. TABLE XII. .-Pt I Dichlorotoluquinhydrone 0.00052cI ; 0-IN-HCI ; Benzoquinhydrone I Pt + 18".Temp. -- -'.- - -___ -\ Minutes 20 Volt I ..................... 0.0419 0.0419 0.04 19 , I1 .................. 0.0419 0.0419 0.04 19 , I11 ........... . ...... 0.0419 0.04 19 0.04 19 . . . . . . . . . . . . . . . . . . 30 45 ~ 1 8 0 = 0.6625 MEASURING THE REDUCTION POTENTIALS OF QUINHYDRONES. 207 Il/lonobromotoluhydroquinone.-To 5 g. of toluhydroquinone in 100 C.C. of ether and 45 C.C. of chloroform were added 6.45 g. of bromine in 82 C.C. of chloroform. The product (64 g.) recrystal-lised twice from toluene melted at 176-178". Clark (Amer. Chem. J . 1892 14 569) quotes 176-179". TABLE XIII. - '~.ionobromotoluh~dro~uinonc~ o.oo25nf ; ::& ; Bcnzoquinhr-drontl l>t + l'tlBenzoquinone 0.b023111 23".Temp. /-- - \---Minutes ............... 15 70 185 235 5lV Volt I ............... 0.0209 0.0210 0.0212 0.0213 0.0214 .. I1 ............... 0.0205 0.020G 0.0208 0*0208 0.0213 Dibromoto1uquinhydrone.-Prepared by oxidising 1 g. of mono-bromotoluhydroquinone in 5 C.C. of alcohol with 12.25 c.c=. of O.2M-ferric ammonium alum the product was washed with water and dried between filter-paper (0.2003 titrated in presence of sodium bicarbonate with 0-0849N-iodine required 11-75 C.C. Mono-bromotoluhydroquinone = 50-3yu ; calc. 50.0%). 1~250 = 0.6564. TABLE XIV -Pt IDibromotoluquinhydrone 0.00125M ; HC10.1N ; Benzoquinhydroml Pt + Temp. / -. Volt I ............... 0-0397 0.0399 0.0399 0.0399 25.2". Minutes ............... 30 90 120 1.55 , I1 ...............0.0397 0-0399 0.0399 0.0399 "'73" = 0,6591. ,MonochloroxyZohydroquinone.-Prepared by the method of v. Kad (Annulen 1869 151 166) and Carstanjen ( J . p. Chem. 1881, 23 430). 20 G. of p-xyloquinone were added to 200 C.C. of cold concentrated hydrochloric acid a further 300 C.C. of acid were added and the mixture was heated for 2-3 hours on the steam-bath. The crystals obtained on cooling were recrystallised from toluene carbon tetrachloride and twice from chloroform. Yield 7-75 g. m. p. 132-153" (Cl found 20.81 calc. 20.56%). TABLE XV. - pt M~nochlorox~loh~dro~uinone~ o.oo2Af ; zyb ; Benzoquinhydrone j Pt + Xyloquinone 0-002M I 25.4'. Temp. 7 - F Minutes ............... 20 60 120 Volt I .................. 0.1069 0.1070 0.1069 , 11 ............... 0-1070 0.1069 0.1069 ~ 2 5 - = 0.5949 208 MITCHELL HYDROLYSIS OF THE d-OLUCOSIDES ETC. DichZoroxyZoquinhydrone.-1 G. of monochloroxylohydroquinoiie in 20 C.C. of water and 20 C.C. of alcohol was oxidised with 29 C.C. of 0-2N-ferric ammonium alum. The product was washed with water. Yield 0.7 g. (0.0993 reduced 5-85 C.C. of 0.0983 5-iodine. Monochlorohydroquinone found = 49.6 calc. = 50.0%). TABLE XVI. - Pt I Dichloroxyloquinhydrone ; 0-1N-HC1 ; Benzoquinhydrone I Pt + 25.14'. Temp. / - Minutes ............... 30 65 125 150 Volt I ............... 0.1069 0.1070 0.1071 0.1071 .. I1 ............... 0.1072 0.1073 0.1074 0.1074 The values of x given in Tables IV V XI XII XIV and XVI are the results of direct determinations whilst those given in the other tables (excluding I) are indirect values. ~ 2 5 ~ = 0.5918. THE UNIVERSITY COPENHAGEN. [Received Au!qust 23rd 1924.