Physical Chemistry Laboratory CHEMICAL KINETICS: SECOND ORDER REACTION- IODINATION OF ANILINE References: See “References to Experiments' and E. Berliner, J. Amer. Chem. Soc. 72, 4003 (1950). L.K. Brice, J. Chem. Educ. 39, 632 (1962). Background: Know definitions of the following and their interrelations: Reaction order Rate equation for second order reactions Rate and rate constant Beer's Law pH and method of calculation for a buffer solution Objectives: 1) Determination of the rate constant of the iodination of aniline, a second order reaction involving two reactants of stoichiometrically equal initial concentrations. 2) Spectrophotometric measurements of the time dependence of the concentration of iodine to obtain the needed data for (1). 3) Investigation of the dependence of the rate constant on the pH of a buffer solution and the catalytic effect of the buffer solution constituents. Discussion: Aniline reacts with iodine, in the presence of excess KI in dilute buffer solutions, as follows: C\H,-NH, + I, > I-C,B,amper i 1-2 Physical Chemistey Lobewale The rate of the reaction can be followed by measuring the optical density of the reaction medium, namely the iodine in solution, as a function of time. In addition to the effects of concentration of reactants on the reaction rate, that of the pH and the concentration of HPO,” and H,PO, ions can be studied. The phosphate ions act as a buffer and HPO,’ as a catalyst for the reaction. The absolute and relative concentrations of HPO,” and H,P0, ions can be varied by the amounts of NaOH and KH,PO, present in the reaction mixture. Use K, = 6.2 x 10° for H,PO, and the concentrations of HPO,” and H,P0, to calculate the pH values. Chemicals: 0.04 M aniline (in aqueous solution) 0.04 MZ, (in 1.2 M KT solution) 1.00 M KH,PO, 1.00 M NaOH Apparatus UV-Visible spectrophotometer Spectrophotometer cells Electric stop-timers 50-ml volumetric flasks Procedure: Consult the instruction sheets for operating the spectrophotometer. Make three concurrent runs using the following volumes of reagents: T, 1.00 M KH,PO, 1.00 M Nadi Run 0.04 M Aniline 0.04 Z / aL / ob / wh Bz 1 10 10 10 2 10 10 16 3 10 10 14 lonPhysical Chemistry Laboratory Experiment 1 All xeagents should be at the same temperature. Measure all the volumes accurately with pipettes. Pipette all the reagents but I, directly into a 50 ml vol. flask. Add water until the volume is about 15 ml short of the mark and mix. Add the I, solution last, fill to the mark and mix again. Start the timer when the iodine has been added. Transfer some of the reaction mixture to a spectrophotometer cell and take readings at 525 nm. (The initial absorbance readings should be about 1.2.) Take time versus absorbance readings at intervals of every 4 to 5 minutes (the absorbance should change by at least .02 units), or less frequently as the reaction slows down. The spectrophotometer reading at time zero is determined by extrapolation to time zero. Record the initial and final temperatures of the runs. Treatment of Data and Calculations: 1) Make a table of time, A = absorbance, and 1/A for each run. 2) Plot the reciprocal of A versus time and obtain the least squares line for each run with its slope, intercept and their standard deviations tabulated. A second order rate equation for one reactant or two reactants at the same initial concentration which react in a molar ratio of one to one is as follow: which integrates to 1/c = k,t + 1/e, © = concentration of reactants at time t ¢, = initial concentrations of reactants k, = 2nd-order rate constant Thus, a plot of 1/c vs. t yields a straight line with a slope equal to k,. Since the absorbance, A, of the reaction solution is proportional to c, a plot of 1/A vs t will also be a straight2 Physical, Chewict bs sxperimemt TZ ¥ line, the slope of which (k,) is proportional to k,. If A = be and A, = be,, where A is absorbance, b is a proportionality constant equal to molar absorptivity times path length (see Beer's Law in the text for further information) and A, is the absorbance at concentration cy, then k, = (K,) Ay/Cy- 3) Make a summary table with the following entries for each ru Run No. Initial conc. of HPO, ) ) after mixing Initial conc. of HPO,” ) Initial pH [use K, = 6.2 x 10% for HPO, and the concentrations of HPO,” and #,PO, to calculate pH] Slope = k, Intercept = 1/A, ky Temperature Initial conc. (I,) = (aniline) 4) Calculate the rate of reaction at the initial conditions in M/s. Questions to be answered and discussed in your report: 1) How does a change in the conc. of HPO,” and 4H,PO,” affect the reaction rate constant? 2) Can you draw any conclusion as to the effect of the pH on the reaction rate? Explain. 3) What would be the integrated form of the equation relating time and concentration of either aniline or jodine (only one) if the initial concentration ofhemistry Laboratory iodine were twice that of aniline? xpeximen’ ISept., 1950 compared its reactivity to that of phenyl acetylene. ‘Under similar conditions ene adds nly onesie of bromine to fora 8 lomo: L-mesitylethylene whereas phenylacetylene adds two molecules of bromine to form the expected tetrabromide. Attempts to add more bromine to the above dibromide resulted only in nuclear bromination, Similarly, mesitylacetylene did not add two molecules of methanol to yield the ex- pected acetal of acetomesitylene under conditions ‘where phenylacetylene readily did go, Evidence for the addition of one molecule of methanol to form the enol ether of acetomesitylene was obtained, although this compound was not is0- lated in a pure state. It was possible to hydrate mesitylacetylene to acetomesitylene. However, it is necessary to add only one molecule of water to the triple bond to effect this transformation. ‘Thus, we have obtained evidence which provides sappért or Kadesnh’stheoreteat consideration! extends them to functions other than the carbonyl group. Experimental Matyacroinn —1-CloorLnsietiyene, bp 99-105" at : prepared in 65% yield from set Pile, wan caged to 707 Ald iat te Mylo "os Oe 2 no 16S, eee Int mith th recent eratare? "The knows metry Teale cn S60" unc, wan obtain” T-mesityethylene, b. tevadd bromine to eb: eempratues ablation of hydroges bromide occured GR Puon and J. 8 Meck, J. Ov st. (8) 7, Vaughn, Tam Joona, 88, 485 (90, Tse tol Ata} cm” Aton is product mere aroun “at "was tasen up but copious Cham, 30, 351 Kanerice oF Tit LODINATION OF ANILINE 4008 Anal. Caled, for & #25; H, 40; Br, sia Foun: C8, 180; Bi BL verted into Tigi einbrenmbeay file ms pe Tas ingen Thad been debated Soe digits” Bor aniyae the campos war Sanyal fied fbn alcool sis Br, 7538. se pregent an eit to dd util ioeace oan ethos es aes st eel eg merce (i Uae ae heaneiicne yded 839% of sestophescoe 6 eee Freda i seas accomplished is O45¢ ya eoentally according tothe ton inetiod wed) for benaylpenylacetyiene aed ab ite ditto dex ‘Beatie, tere TIBO", mned mp. with authentic Simple! not depremeds Summary Mesitylacetylene adds one molecule of bromine. ‘The dibromide thus formed does not add bromine. ‘The theoretical implications of these findings are discussed. (Savoir by H.C Ani abet, Brn “thy Prepared acodingt -. Hoh, Or Spm, CO. VOL Xp =: SUIDE , Kionett aod @. 3 Palmer, din. Chem. 8 287 5:4. Newland, Ts Jovewat, 86, 1330 (1080, (0a) 207" Bogert end B.D. Herre, dn, 48 243 (2028. (08) 5. Jetanon, 7 Le Jacobs and A.M, Sears, iy 0, 1989 (i. Ue) RG. Manon and. Wale, a, 8, 3273 230), Couvunus, Oxo ‘Recmtvap Mano 2, 1960 [Conzmrsurios raoM 7a Mamion Eowanps Pass Laponarony o» Bay Mawe Counc) ‘Kinetics of the Iodination of Aniline By Brwst Baruaner From a study of the kineties of the iodination of phenol in water Painter and Soper concluded sipgie er? lace iter between phenol an jous acid, or between the phenoxide jon and ‘the positive jodie ion (1¥)4 ‘Since the Stvastic rmp af otk piso phenoxi Jon re very seative topard electrophilic reagents, a distinction between these two reaction sc Gould not be made on chemical grounds, but the authors preferred the latter of the two possibi (1) Paster ana Soper, J. Chm Soe, 8 U04T; Soper and Sai, ‘na ea fies. If the iodination of aniline is similar to the iodination of phenol, a more clear-cut distinction between the positive iodine ion and hypoiodous acid as the iodinating species might be possible, because the two aromatic compounds, aniline and the anilinium ion, behave very differently in electrophilic aromatic substitution reactions. Wistar and Bartlett have on the same basi shown that the diazo coupling reaction of aromatic ‘amines takes place between free aniline and the Giazonium cation, whereas the kinetic data for ‘the diazo coupling of phenols did not make such4004 4 distinction possible.%* It was thus expected that the iodination of aniline would present a ‘more favorable'case than that of phenols for the study of the mechanism of iodination. The following reports a study of the Kinetics of the iodination of aniline in water in the presence of different buffers and different concentrations of iodide ion. It was hoped that it might not only be possible to determine the nature of the iodinat- ing species, but also to gain some clearer insight info the details of the process of aromatic sub- stitation, Experimental Materiie.—-The iodine, the butler salts, Jodide and sodium chloride were best contmercial Reagent Grade chemicals and were ize without purlication: ‘The scetic acid was a sample purified by the method of Orton and Bradeld.« “Two samples of online. were tested. ‘One was prepared by hydrolysis of acetonide, and the other was, Mere Reagent Grade aniline. Both samoles ‘Were dstled twice from aine dust, Sac they gave idea~ ‘Geel results in preliminary kinetic experiments, the ater Sample was used throughout; its boiling poiat as 183.2° St 760 mim. and G2" at O aim.” The aaline, which was Stored in an amber bottle, remained colorless throughout five tine of the experiments. All reagents were weighed tofout places ‘inate Buas.—Suock solaons of ane, the ues land sodium chloride were, prepared separately at. suc Soncentratiens that & certain pumber of rallitere of excl diluted to 160 ml. gave a solution ofthe desired concentra tion. The jodine solution (0.02) was always made up togeiter with the potassium iodide (112-4) im a 100 wl fumetric ask; ten milters of this solution was used I the Kinece experiments, Solutions of all reagents were pipetted fato 250 ra Erlenmeyer ask, the solution was diluted to 90 sl, and Drought to temperature in a thermostat (25 = 0.08"). ‘Tea mailliters of the lodine~iodide solution, also at the thermostat temperature, was pipetted into the Helens ‘ase with vigorous swirling, ded w 1O-md. samme was fa mediately withdrawn. ‘is operation’ never required ‘more than 80 seconds.” ‘The sarmple was run into 20 lof TN sulfuric acid, and this time wes taken as zero tte ‘Tie iodine was titrated with a 0.02 31 thigsulfate salut from a Sani, buret grashuated in huadretthe, weg tare as aa indeater, The thlosuilste solution vas ‘resh every day by diluting a 0.1 Ar stack solutions ght SO-aiL, samples were thee withdrawn at suitable tne in tervals. “The rate constants were calculated from Ineegrated fore of the secon corer rate eqtation 2908 4, 6 (a ~s) ke He = 5) PF ==) where a and 8 refer to the initial aniline and iodine oom Sorsado, end ah tne sino of iedenaine formed. ns for’ exch set of concentration, {aloatfone were ‘mad {upp iter aig aid pram phthalate se watandard. ers to be measured were always made up ina sol don whose ionie strength was Brout to 0.0 by the ai ten of sodiam chionde. Teolation.—A cin (phosphate buffers 0.0% M, iodine 0.008 2, actine0.004,'M, 10.12 175 = 0.3) wadenrried fut oa onectiter see," After the iodine colar had te- Spptited alton: compel, the olson a treated with ah ences of eodium bisuifte and a few pellets of sodium (@) Winter ood Bort, Tam Joona 6,419 (4, (@) Cacast aad Peters, bit, 1230 220 (@) Orion sad Beadted, J. Chom Soe. $89 062 Epnst BaRLIver Vol. 72 yore. The: solution was extracted with ether ex R&sstively; the ether extract was cried and the. ether evaporated. “The rose as difoolved a Bol of Doar ‘seat and 2 ml of froin, and 3. ml of ecetio anhydride ‘vas added. “he mfstare was warmed for few mates (oh the stearaath and then cooled, “The cotaline mater Fal which separated wan Giered end weigaed 0:70 5.7%). Teoltened around 160° and melted unsharphy st 164-1822", The melting plats af erodeacstanie, redodoscetasiid, and B4-dledoacetaniide are all bow the one found 00-110" Tioe; tal® GML"). pe oioacetaniide ralts at 183-184*, and che recovered ital thecore cncited ety of ht ete Pater was recrystallized irom ‘alcohol and ator ‘S°507 i, of pure: Prodoacetamide, mp. 182-1218. sad abe? gl, plone ae obtained ome a ‘other ligdor, ‘The total vei ‘of pure product is 61%. omsed iting pent wilt an asaicnde seaple of: iodoucetanide showed no al of te {igoors contained material winch softeaed at 150° aad did ot melt below 180°, For this reason, and because the fection ny ot have ben quasatve, the recorded SiH conettutes amnion Results ‘The reaction between iodine and aniline in water, at. the concentrations studied, follows a second-order equation and is first-order with respect to aniline and first-order with respect to titratable iodine, In Table 1 are recorded the data for two typical runs; the data in Table TI show that the rate constants do not change within ‘the indicated variations of the initial concentrations of aniline and iodine. Taste ‘Tue loouamon oF AnuaNe At 25°1N Wari Time ins 002 A thane, Alters mole ml. ‘Aniline 0608067 Af, XI0.12 Jf, XH,PO,0.04 Af, NasHPO, O01 2, NaCl OAL Aw = OF ¥ 2.000 2 1838 (0.430) 103 ea 424 18 1a 428 20 1.885 423 337 1210 404 408 1109 495 a0 Lu “2 an 0.968 25 Ay 5 Aniline 0.008001 24, KI 0.12 2, KHsPO, 0.004 Mf, Nor- HYPO, 0.062 Af, NaCl 0.08 21, « = 03 9 2.002) 2 1.890 3 3980 os 1.358 a 18 1 3.806 ast 11195 167 1082 197 0.982 Influence of Buffers.— tions were studied in eleven different phosphate buffers corresponding to a change of pH from 5.80 to 7.50 with a maximum ratio of bufferSept,, 1950 ‘Tanta 11 ‘Tus Tooucariox ov ANKLE winm Diryeenwr TxITIAL Coxcaereatiows oP Raactanrs Ar 25° KI0.12.M, KPO, snd NasHPO, 002 Mf, NaCl 0.1 M SS EY mat -» 8.018 1.000 0.720 6.014 1.008 16 4.008 1,008 702 4.000 Veit “708 8.018 Lore 8 sot 1981 “ro 4.000 2.003 708 8.007 2.007 ins 2.008 so ‘07 4.008 3.085 mm Av. 0.710 constituents of 10:1, and in five different acetate buffers of a PH range of 4.57 to 5.53, ‘The total 231 range over which the reaction was studied fore toa Eiquouscacbyatcoratetound ‘The iodination reaction shows general catalysis by the buffer coustituents, which cannot be due to a salt effect, because the ionic strength was kept constant throughout all runs by the addition of sodium chloride (u = 0.3). At any ove PH an {increase of the concentration of the buffers was found to increase the rate. This is also observed in the iodination of phenols in water* In phos- ‘phate buffers the change in rate is caused exclusively by a change in the concentration of the secondary, phosphate ion (NasHPO) and is almost i it of the concentration of the buffer acid (KH.PO,). ‘The reaction is thus base catalyzed. The’ dependence of the rate on the concentration of the buffer base is shown in Table IIT, As can be seen from this table, the base catalyzed reaction is independent of the ‘hydrogen fon concentration. At very high con- Sigh alia eta y sy the pemnay pho t catalysis Pet ony HaPOxs i Ooteveds This ook oa sidered to be due to an acid catalysis, but since the ion HPO," is also a bese, though a weak one, is attributed to additional’ base catalysis by HAPO,-.. Since the catalytic of BPO is much weaker than that of HPO, catalysis by the former is only noticeable at high concentra tions of this ion, 4.e, at low pH. The catalytic constants were calculated from the equation Faw bt CAI ta AD + Roa were [A] and (EA) ore, the sonestestions of secondary and primary ite respectively, and ka and kya are the corresponding catalytic constants. The observed rate constants were tted aguinst the primary te jon con Eotetiog at, diferent buder "Sncestatous, at line is obtained whose For any one fH a slope ia ® equal to kalA]/[HA] + hua (see Fig. 1). Kauverics ov tae IopiNarion oF ANILINE. 4005 ‘Tams ‘Tan Byvect ov rice Burrun Constsruawrs ox ras Rare (oF Topinarion ov Anruine 47 25° KI012 Mya = 03) “SEBO MECHEL terete la.-9 ot 0.02 0.780 08 a 173 ‘os X 709 02 708 “00s 74 “ooas 708 02 ‘ra 45 aon “08 1.008 ‘oa vow ‘00s Loo 1 aaa ‘05 23 “of “a1 5 8 a9 “002s a5 ‘ot 413 ‘ot ‘oe ma ‘OL ‘O16 0.558 ot ‘ot “410 oa “6s 1457 ‘os “oa 1.008 05 “oa 0.700 fall of the slopes nguingt the salt ratio i |/{HA] gives a new straight line whose to ks. The catalytic constant for Ft gion was thus found to be 20." By plot ting the lower art of separately, Bata (the intercept) is found to have a value of about 93. 3: ay eapate is thes sot 100 ite Scautary ptowpbvts fn yas al te, Secon: te jon was also ae to be a stionger basic catalyst than pri esphate in the iodination ‘of tyrosine Asim: catalysis was observed with the acetate ioa, but in the pH range studied a ‘cant amount of aniline is present as the anlinium fon, 90 that the total rate also depends on the pI fand decreases rapidly as the pH increases. ‘The catalytic constant for the acetate ion is about 2.4 if is tins intermediate between primary and aad secondary phosphate, as it be if fatalyzed iodination fotos a Brbasted aiatyas Tne Uncalalyzod Reaction —The purpose of measuring the feaction ratea at diferent hydro- ‘gen on concentrations was to determine the de- pendence of the rate on the pH. Since at am ‘one pH the observed rate constant depends linearly on the concentration of the buffer con- stituents, the observed rate constant must be made up of two terms, one involving the cat- alyzed reaction and one for the uncatalyzed reaction, Tn order to find the value of the rate (©), J Chom Set ty 28 C040.4006 0.08 Vol. 72 0.08 0.08 oa Conen. KHAO, Fig. 1.—The influence of batfers on the iodination of axiline The H’s to which the ines refer are 1-11: 7.89, 7.47, 7.24, 7, O71, 6.55, 841, 0.18, 6.02, 5.92, 5.69. constant, ty, for the uncatalyzed reaction, the observed rate constants for each constant Bugler ratio were ‘against the concentration of the ‘buffer acid (KH,PO), and the lines obtained were extrapolated graphically to zero butfer concentration. The intereept for each. line is equal to Ae at that particular PHL. The results some typical runs at different ion concentrations are recorded! in Table TW The results of the lations are shown in Fig. 3, from which it can be seen. that all Hines com in one point. The uncatalyzed reaction better, the water reaction) is therefore in dent of the hydrogen. ion concentration. rate constants plotted in Fig. 1 were determined at H's which correspond to a change in hydrogen ion concentration by a factor-of one hundred, Over this PH range the rate constant for the un- catalyzed reaction, to is equal to 0.123 with a mean deviation of (0,002. Tn acetate buffers of pHi 5.53 and 5.44 the extrapolated values for zo are equal to 0.190 end 0.122, but at lower pH the rate falls off and is°0.050 at pH 4.57, beoause at higher hydrogen ion concentrations the amount of free anidine ig less. Taking 3.8 X 10-" as the dissociation: constant of aniline, and neglecting fag a frst appronimation the effect of the louie Steen on the disocaton constant, 10% ofthe is present as anifimium jon at pH 5.53, soe Reon ar, “The deoretse of rate in ite buffers is therefore mot due to an intrinsic catalysis by acetic acid, but is due to the decrease ‘Trove Iv ‘Tus Rats oP Toprwarton ov ANeuove 4x Vanvine BURPER ‘Concusraarioxs KE 0.12 Af 208 oH SR, RAFo, 8.55 0.08 a 08 us" ‘02 re ot fo. eat 08 0s 1.818 10508 1.015 08 ‘oa ors 028 6 508 slot 420 B00 1 0 a6 sors “oan, 388 07 ‘oo 390 00 006 317 05 005 235 ag (396 + At an joni strength of 0.5 fy, 180.905. 4 At an ionic strength of 0.5 hy, 0.68.” Etrapslated value, in the amount of free aniline. A plot of the rate constants of the uncatalyzed reaction, obtained by extrapolation, against the nydrogen ion con. centration is shown in curve, which fncreases up toa pH of abet 5.4 and then remains constant, is similar to that describing the de-Sept., 1950. pendence of the rate of diazo coupling of amines on the hydrogen ion concentration.? he total rate constant for the iodination of aniline in phosphate buffers can he expressed! by Jean = he + AalIPOC] + ba(HPOM] = 0.128 + O3{H,POr | + 20.1 POF] and presumably by kot, = bo + Zbu[B], where B is any basic catalyst.” Rate constants calculated from the above equation are listed with the ex: perimental values in Table TV. ‘The Effect of Iodide Ions.—The rate of the iodination reaction is strongly influenced by the iodide ion eoncentration, ad the rate constants fare inversely proportional to the square of the ide ion ‘concentration. ‘This is shown in ‘able V, where the rate constants at differeut concentrations of iodide ion, but constant ionic strength (1 = 0.8), are recorded. ‘The product & X [T-}* is a constant over the range of iodide ion coueentration studied, In that respect the iodin- ation of aniline is similar to the fodination of phe- nol," but different from the iodination of a number of pyrroles.t Effect of Added Salts—The reaction rates show a slight but noticeable dependence on the fonic strength, and decrease as the latter in- creases. A change of u from, 0.3 to 0.5 reduces the rate constant by about 4% (see footnotes in Table IV). Tame v Tus Lereer of Tovive Lo Concenrmattox ox THE ‘kare OF LooiNATION OF ANILINE, KH,PO, and NgHPO, 0.05 Mp = 08 KT kets let mln.) eX RD! X 10 20.7 2 26 1 0.7 20.6 20.6 20.7 Discussion ‘The fact which is most important for a eonsid- eration of te mechanism of the iodination of ani- Tine is the observation that the rate of the reac- tion is independent of the hydrogen ion concentra ion and inversely proportional to the square of the jodide jon concentration. The total concentra- tion of iodide was chosen $0 much larger than that of iodine that the formation of iodide ion during the reaction CllsNIls + Te —> DACALNES 4+ HE id not noticeably affect the rate constants, and ‘te total iodide ‘ou concentration ea be consid- ‘ered constant during any one ran. The liberated hydrogen iodide wus taken eare of by the buffer. ‘The velocity of iodination is directly proportional {0 Donk and Covi, J he Sy 10 (148) KINETICS o¥ THE LopINATION oF ANILINE oH ~The intueuce of pH on the rate ofthe uneatalyzed reaction Fig. to the initial iodine concentration and the concen- tration of aniline, ¢. -di;/dt = ko (titratable iodine)(aniline). In a solution containing iodine ‘and iodide ions the concentrations of the different species can be caleulated from the equilibrium? Wether K o ‘The concentration of iodide ion was always sixty times greater than that of iodine (0.12 7 iodide ion, 0.002 Af iodine) and in such a solution the free jodine is present to an extent of ies than 1.6%. Practically all of the iodine is present as Ts, and. the rate equation becomes —ds/dl = Bata(ls~)> (atiline)."""The following equilibria have to be considered, it addition to equilibrium (1), in order to determine Uhe nature of the iodinating species. htHOZENI+UTET Me @) HOI ZF It + OBM & @ HOI Ze 107 + HY ano) heerer K& CALNE +H EON Ke) Each of the iodine containing species, Ip, I~, Im, HOT, OL-, [', is a potential iodinating agent. Todide and triiodide ion can be ruled out because tie rate decreases rapidly with the iodide ion concen tration; they are also unlikely on chemical grounds. IF iodine is the iodinating species, the rate of the reaction, from equilibrium (1), is’ —dv/dt = Roos (-)(PRNE) = (lGs/I-)(g)(PANEQ) and, should be equal to K,/I~. If hypoiodous acid is the iodinating agent, the observed rate constant should be equal to fKiKe/(H*)\(I-)*. The dependence of the rate constants on the hy- drogen and iodide ion concentrations for these two cases is not the ove found experimentally.” Todine and hypoiodous acid must. therefore be (Kiera x cm. (3) Bou mynlbaghes aad Leen, J. Che. Sic 428 (3) (2) A oun coat eaprenon fo hay oie be the toting it, hey, @ I, bot vice Tb very muck ‘he daoulator nad the "Tie same alepentlon 4 Joace an Kap, Tis JORMA, 80, 14008 ruled out as the iodinating agents for iree aniline. ‘The fon OL requires an inverse square felation. ship of the bydrogen jon concentration and is also. ruled out. If the positive iodine is the iodinating agent, the observed rate constant should be equal to kK KsKi/K(1~)? (or kK Ks) /(I-)*)and should thetlie Be Hadopndent othe ydogen en Concentration and iaversely.proportonal vo the square of the lodideig covitretion, au found. "Th Kinetic ofthe iodination of line, a the concentrations studied, are therefore not incom- patible with «mechanism in wath the, postive fodine i the iodinating agent of free aaiine, But itis not compatible vithes mechanism involving iodine or hypoiodous acid. The rate of the iodi- nation can therefore be expressed as. cw da/dt = hofl*](CeLNHe] + &’(BII+}(CeHeNHs) However, the same kinetics are sso in it vita reaction between iypotodoue ted oad the Snilintum ion (lore = RkaKy/ (CJ, and the two reactions are kinetically indistis ile, ‘A reaction between hypoiodous acid and the anilinium ion is not only very unlikely from all thats known about erorsati ubettuton, but the anilinium ion is a strong mela director, whereas a minimum yield of 64% of p-iodoaniline (as p-iodo- acetanilide) was isolated from the reaction mix. ture, In a large scale preparative procedure © 75-84% yield of p-iodoaniline is obtained by iodination of aniline in aqueous sodium bicarbo- nate solution..” The alternative involving the fing jou ‘can therefore be exhaled on Shere ical grounds. -Purthermore, at the pH at which the concentration of anilinium ion becomes ap- preclable the rate falls of and wotld presumably Feach zero when all fee aaiine fy comverted to the anilinium ion. ‘That I+ should be the iodinating agent is grati- fying from the point of view of modem theories regarding aromatic substitution for which an elec- ‘trophilic substituting agent has always been ‘postulated and in a few cases demonstrated.!! Jt does not mean, and is indeed unlikely, that the fesuilg can Be generalized to. include Br? or CI* (in the absence of a catalyst) or that I* will always be the jodinating agent. Free jodine apparently iodinates pyrroles, although hot exchigvely." Mixtures of ice and hype {dou acid ‘have been usnumed tobe respone- ible jor the iodination of the nucleus of tyrosine and’ histidine, but the catalyzed and uneatalyzed reuctions have not been sep- rated. More reactive aromatie compounte Srl probably ‘reguare ise daastc ‘Tod uating Zyenth than 17 "kutirely different. situations Hart be aitidpate. in on-aquaoas, slions "The poolive lode snot an inthunieally wale) (10 mene, “Cro rn a Te falar eegnens ae ye a) ttn Yoon, #4 247 9: #2 80 Te alee Ranst BaRLiNer Vol. 72 iodinating agent when it is recalled that hypoio- a stronger base than itis an acid (Ky ; Ky = 82 X 10-M) and that the dis sociation constant of jodine (into the positive and negative ion) Ky ig 9.6 X 10~*."* ‘The initial con- centration of I+ in the present experiments, ne- ‘lecting again the efect of ionic strength, is 19 X 10"", and the specific rate constant for the un- catalyzed reaction of I* with aniline is 1.3 X 10% Since the concentration of iodine is of the order of 10-4, I* is about 10" times more ‘reactive than iodine. The concentration of I* is not eppreci- ably smaller than thet of HOI; the latter 19 8 X 107" Wat a pH of 7 and 6 X 10" Mata pH of 5. I+ must therefore be an appreciably more ef- fective iodinating agent than hypoiodous cid. ‘The concentrations of Br* or CI* in an aqueous folution of bromine or ebforine wold be saler yy many powers of ten. The I+ ly at- tacies lef to mater and is never “ree, but tt ‘caunot be attached to iodine (In*) at the toment of substitution, because this would require a dif- ferent mate expresion, ‘That the reaction should show general base catalysis is also in agreement with present views, and a similar catalysis was shown in the nitration of aromatic compounds in sulfuric acid where the bisulfate ion accelerates the rate The bromination of hydrocarbons is acetic acid is accelerated by sodium acetate.’ Since the transition state in e aromatic substitution has the es of a base must always be present to complete the reaction ‘The role of the phosphate or acetate ions is there fore concerned with the removal of a proton from the transition state. Tn the absence of the base the reaction is siower, because the place of the base has to be taken by the much weaker base, water. ‘The uncatalyzed reaction is therefore visualized as one in which the water plays the part of the base “Phe assigntient ofa specific function to the base is ase on the assumption that, the breaking of proton is ‘he rate determining step, However, larder, na recent note)" has fon hati the sation of bezaene ed tale tium is replaced as rapidly as hydrogen, irom ‘hich he conchided that the splitting off of the proton in aromatic substitution is not part of the rate determining step, but that it is a rapid proe- 5 which occurs after the transition state is passed. This view is clearly in conflict with the {Interpretation of Bennett, fal If the view of Melander is taken, and if it is severally trae that the proton departs in a fast fol- 9) Murray, J. Chem Soc, 38,682 (190; Skat, Ber 18 1st0 G64 sport contast for later Ree 3 31 ‘ray ad Conny Ta Jovan, 4 488 GOL) TW Bare ood Ferny i, Oy 400 (198), Th daca io coutant fe bromion ms eninated by hse sto fo ea the tom ) Demet, Tran Jumes, Saters wil Winton J. Che Sonate cee ot Dern, Tas Jeunes 8, 105 194), “Ate Chew Sead 08 00,Sept., 1950 low-up reaction, the base cannot have the role as- signed toitin the iodination of alin, Whot the function of the base is cannot be decided oa tif the Mgctic data aloe, tents Inte pretation of the results is the acid-base equilibria of the kinetic data are also in agreement. catalyzed reaction of hypoiodous acid and aniline with a possible transition state as shown in Fig. 3. ‘The rate expression H AR ‘would then be dx/dt = ky[ H+] [HON)ICarnNT) + wa ona a and the role of oe ‘in weakening, tonding, the ond of hy oe ine would be identical with a reaction of hypoiodous acid and aniline catalyzed by hydrogen ions (ats, REIK,/[I-1), amd the base catalyzed reaction of ‘Drmvectarc ConsTANTS OF TRICHLOROBTHANES AND PROPANES 4009 tout bypoiodos a, The complex of Sypote tion The coms i dous acid and an acid can be described as 2 "'sol- vated T+,” so that the iodinating agent can still be ‘the I* ion, although the role of the base is differ- ent A distinction between the two possibilities ean- not be made until more is known about the transi- tion state in aromatic substitution, Acknowledgment —I gratefully acknowledge the help afforded by a Hrederick Gardner trell Grant of the Research Corporation. Summary ‘The kinetics ofthe iodination of aniline in water ‘were studied in the presence of iodide iong and at Gifferent hydrogen ion concentrations. ‘The re- action shows general base catalysis, and the kine- fies are in agreement with a mechanism involving the positive iodine fon as the iodinating agent free aniline. "An alternate mechanism involves an acid cata- tyzed reaction of aniline and hypoiodous acid. (08) Tam iodhed to he Rate or log my ateton the sete of ander and to oan of tan for suggesting toe eterate Ubeereeaton of he rn Brox Mawn, PA. Recetvep Jaxwany 27, 1050 [Contarsemow raow Tun Fnicx Creat Laonarony, Princuron Usavensrry] Heat Capacities, Dielectric Constants and Molecular Rotational Freedom in Solid Trichloroethanes and Disubstituted Propanes’ By Roaert W. Crows aNp Cuanza P, Suvre A number of substances whose molecular shapes ‘are not far from spherical have been found to show polymosphism, the solids apparently posesing ular rotational freedom for some distance below the melting points, Dielectric constant measurements have given evidence of this it 2 chloro-2-methylpropane,' 2-methyl guns? 22sicdepropans! and fet anchor ethane? among others. It bes been shown that the molecular rotation inthe soli Jeeds fo an abr normally high melting point, Although the liq- 4id range of 1,1,2-triehloroethane, 1616, is some- what larger than that of 1,1,l-trichloroethane, TO? its tather high melting point, ~300°, sug. gested that this substance also might possess some molecular rotational freedom in its cryetal lattice, ‘Simultaneous heat capacity and dielectric constant measurements or constant _measure- ments alone were, therefore, made upon these substances and upon other compotinds containing (0) Tas cenarch ma supported by the Of of Naval Resear Reproduction, tesa, panto, tt aod pon ia wes rs ‘by or fr the Ute Senten Goversneat i preted (@) batarene ite Tien Joona, #200 GON) 2) Scere and Sth, td, 2, 3408 GAD, Bake aed Sen fad, ot, 1080100). different groups attached to a central carbon atom. ‘The measurements were carried out with an ap- paratus and method which have been previ described. Purification of Materials 1,1,1-Trichloroethane and 1,1,2-Trichloroeth- ane,~-Both compounds, obtained from the East- man Kodak Company, were washed twice with 6 NV sodium carbonate solution and with several portions of distilled water. They were then dred over anhydrous, calcium chloride for twenty-four hours and fractionally distil re- peatedly at atmospheric pressure in a five-foo Column, ‘The physical constants of these corn- pounds are Ligatare Be "4.0" 1.aa708 MT She BY ay 98.0 160 2.073 —99 uitrienoro geen. ‘abase 00.0 118.6 1,3-Dibromopropane.—Material from _ the Bastman Kodak Co. was washed several times ( Kustoer, Crowe od Smyth, si 1H, 1001 (1050 (@) “Taternalonal Coitea! Tables” 90.7 118.08L. K, Brice: Polytechnic institute Blacksburg, Virginia Vial Tho experiments doscribed in this paper and in the following one are suitable for use in the undergraduate physical chemistry laboratory. They ave recently been used as speciai projects for a group of superior students in a freshman chemistry labora- tory course at the Virginia Polytechnic Institute, The kinetics of the iodination of aniline have been studied by Berliner.!’ The over-all reaction in the presence of excess potassium iodide in dilute buffer folutione is epresentd by the cquation Chee — Che ow ; Analysis of the reaction mixture has shown that the para isomer is the principal product, virtually no ortho ‘or meta isomer being formed. ‘The rate of the reaction docroases with increasing iodide ion concentration and {inoreases with increasing pHI. These results are con- sistent with the assumption that I* (from Ts -» T+) is the iodinating species. "Bnmannn J. A, Chon, Soe, 72, 4008 (1960). 632 / Journal of Chemical Education The Rate of lodination of Aniline A Physical Chemistry Experiment ‘The rate can be determined either by titration of the iodine remaining after definite timo intervals, as was done by Berliner, or by measuring optical density of the reaction medium as a function of time, as described below. ‘Tho reaction is second order: tm AULA where [A] is the concentration of aniline and where the second order rate constant, k, depends on the concen tration of iodide, the pif and!’ the concentration of the components of the buffer. Several kinetic runs can be performed as deseribed below during a three-hour laboratory period. ‘The rate can be followed either speetrophotometrically? or analytically, and the results of the two methods com- pared. ‘The rate can be studied as a function of the iodide ion concentration, the pFf, or the buifer con tration and the results interpreted in some detail in terms of a reaction mechanism, as briefly indicated above. A more detailed discussion of the mechanism is given by Berliner (lee it.) "The Bausch und Tomb Spectronie 20 equipped with thermostated cel holder gives quitosatiafactory results,‘The Experiment Reaction mixtures having the following initial concentrations have a conveniently measurable near room temperature: (Anilinels = (als = 0008 af, 1K, 0.12, DAPOIy + (TLPO [HPO;"} being varied between 0 and 0.1 by mixing ap- propriate amounts of KH;PO,and NaOH solution. For the spectrophotomeirie runs, a typical reaction can bo propared as follows:” 2.5 ml of 0.04 22 a 2.5 ml of 0.04 M Isin 1.2 Af KL, 2.5 ml of LM KH,PO,, 155 ml of 1 Af NaOH, dilution with water to 25 ml. (The iodine solution should be added last, with shaking, to the rest of the reaction mixture that hes beon diluted to approximately 20 ml.) All reactants should be thermostated provionsly (at about 25 or 30°), and the reaction mixture immediately transferred to the spec- trophotometer cell, Optical density readings (at ca. 525 mu) are taken every 5 min for 30 to 60 min. For the kinetic runs followed by titration against standard sodium thiosulfate solution, « 100-ml reaction mixture prepared in a similar way is convenient. Five- nalliter aliquots at S-min intervals aro then titrated against the standard thiosulfate solution, TIME. (@miNUTES) 1. Rate of fonaion of online in phosphate buffers ipecro- matric method D'= oplieal denaly.‘Nembers ox groph refer 10 ‘embers inthe abl, Results and Diseussion ‘Figures 1 and 2 show typical student rate data ob- tained analytically and spectrophotometrically. ‘The table gives the values of the rate constants calculated from the slopes, together with other pertinent data. ‘From an eximination of the data in the table, it can be seen that the rate of iodination of aniline creases as the total phosphate concentration increases at constant pH (ie, constant ratio of IPO to HPO"). ‘This implies that the renetion is subject to general base catalysis by the components of the buffer, result that is in accordance with the results of Berliner. Rote of lodination of Aniline et 30° in Aqueous Phosphate Buffers (in all runs, [Aniline b = [ls lo = 0.00400M) Han No. Method (HPOF]_[R2OC} i § 0.0100 0.00 2 8 Boo 8: B10 3S O:b800 BaD SR B.0800 0:18 5 A baw! Oto 300 6A oe sn 20 TS = spoctrophiotometiie, woe megatred i O25 SFEPOC] 4 (HPO. “The apoctrophotometieally determined rate constants? ver ccalolate from the slopes in fon leu the cipal, Optical densities rof the seartion mbstare at = 0 (irom extrapolation) ad wn inital folie eomesnteation, TIME aNUTES) Figure 2. Rae of iodination of ane in phoophote buffers (analyte ‘meted Numbers on graph relr trun sumborst he tab Dover Reissues Luder and Zuffanti’s “Acids and Bases” “The Hlootronie ‘Theory of Acids and Bases” by W. F. Taide and Savor Z published i 1048 by Wiley, has bese reprinted with msi rovisions—anoatly dail ‘Most of the author revihone ar tnd referenees--by Dover am, originally examples the second chapter where an up tated atomie structure hinrt correlates with a revised enongy-lovel diagram, At a pre of $1.30, item now be in the private libraries of all erious atudents who want a es buses. Order fom Dover Publintions, Ine. 180 Va Dolan aed and ik Street, New York 1, X.Y. Volume 39, Number 12, Decamber 1962 / 633