Acid Catalyzed Hydration Of AcetaldehydeBY M.-L. AHRENS (Mrs.) AND H. STREHLOWMax-Planck-Institut fur Physikalische Chernie, Goettingen,Bunsenstrasse 10, GermanyReceived 8th January, 1965The hydration of acetaldehyde catalyzed by HCl is investigated by the n.m.r. line broadeningtechnique. It is shown that the rate of protonation of the very weak base acetaldehyde is a slow stepin the catalyzed hydration reaction. The influence of intermediate states on kinetically broadenedn.m.r. lines is discussed. In acetaldehyde+water mixtures, rich in acetaldehyde, the formation ofa hemihydrate predominates. The kinetics of this reaction proves to be similar to that of thehydration reaction.A number of papers have appeared dealing with the hydration kinetics ofacetaldehyde and other carbonyl compounds.1-6 These reactions are general acid-base catalyzed.It is the purpose of this investigation to give rather direct evidenceon the elementary steps involved in the H30+-catalyzed hydration of acetaldehyde.EXPERIMENTAL AND RESULTSThe n.m.r. spectra were taken at 56.4 Mc/sec with a Varian HR-60 at (23 & 1)OC.The acetaldehyde was distilled and mixed with the appropriate amounts of aqueousHCl immediately before taking the spectra. Fig. 1 shows a n.m.r. spectrum of a-CHO-CH,Hydra1 A- -FIG. 1.-Proton magnetic resonance spectrum of 25 mole % acetaldehyde+water mixture.25 mole % acetaldehyde+water mixture without addition of acid. In fig. 2 fourspectra at different mole % acetaldehyde and 5 x 10-2 M with respect to HCl arepresented.For the kinetic evaluation of the broadened spectra the followingconsiderations are important. The protons, the resonance lines of which areobserved, neither exchange nor are they spin-coupled to exchanging protons.(This coupling is averaged out by rapid protolysis.) On the other hand, there isspin-coupling between the non-exchanging methyl and aldehyde protons and aconsiderable chemical shift exists between the hydrated and the unhydrated form(53 clsec for the doublet and 260 clsec for the quadruplet). For the " slow case "11M.-L. AHRENS AND H. STREHLOW 113with rtk I (wt-cok) I > 1, every line of the multiplets broadens with AvII2 = l/m andoverlapping of the broadening occurs as shown in fig. 3. The 2nAvl12 havebeen obtained by comparison of the half-line width B of the total multiplet withlines resulting from graphical superposition of appropriate Lorentz-curves.FIG. 2.-Proton magnetic resonance spectra of 5 x 10-2 M HC1 in acetaldehyde+water mixtures.Resonance I : CH3CHO ; resonance I1 : CH3CHO ; resonance I11 : cH3CH(O€&.(a) 1.45 ;(b) 24.3 ; (c) 37.7 ; (d) 56.9 mole % acetaldehyde.FIG. 3.-Construction of line shape broadened quadruplet by superposition of Lorentzcurves.For a solution of 57 mole % acetaldehyde+water mixture A c 0 ~ , ~ / 2 is plotted infig.4 against the concentration of HCl.From the integrals of the lines pertaining to the hydrated and unhydrated formsof acetaldehyde the hydration constant KH = [CH3CH(OH)2]/[CH&HO] has beenevaluated for different aldehyde + water compositions.The results are plotted i114 HYDRATION-KINETICS OF ACETALDEHYDEfig. 5. For the comparison of the acidity of HCl solution in water+acetaldehydemixtures, the Hammett acidity function Ho has been determined with 10-2 and 10-1 MCHCl [MIFIG. 4.-Line broadening in 56.9 mole % acetaldehydefwater mixture as a function of HClconcentration.I, from quadruplet CH3CHO; 11, from doublet CH3CHO; 111, from doublet CH3CH(OH)2 - -0 5 0acetaldehyde [mole %]FIG. 5.-The hydration constant KH = [CH3CH(OH)2]/[CH3CHO] as a functionof acetaldehyde concentration.HCI solutions in these solvents. Because of chemical reaction with the aldehyde,primary and secondary amines were unsuitable for the determination of the Hammet M . - L .AHRENS AND H . STREHLOW 115function 110. Therefore 4-dimethyl-amino-azobenzene was chosen as indicatorwith a pK = 3.19 in water. The pKs were determined spectrophotometrically witha Beckman spectrophotometer DK-2. In fig. 6 the results obtained with 10-2 MHCl are reproduced. Similar to other water + organic solvent mixtures the Hammettacidity function goes through a maximum at about an acetaldehyde mole fraction0.6. The two curves with different concentrations of HCI were virtually parallel,pointing to practically complete dissociation also in concentrated acetaldehydesolution.3I0 5 0 I00acetaldehyde [mole %IFIG. 6.-The Hammett acidity function in 10-2 M HCI in acetaldehyde+water mixtures, deter-mined with 4-dimethyl-amino-azobenzene as indicator.DISCUSSIONThe mechanism of the hydration kinetics in dilute solutions of acetaldehydein water (fig.2a) is formulated asCH3CHO+H30++CH3CHOH+ +H,O+CH,CH(OH)OH,f $CH,CH(OH),+H+C' 2 A -7- B 7 Bl .c-3 2(1)Since acetaldehyde is an extremely weak base, the concentration of the protonatedaldehyde B will be exceedingly low (- 10-9 M) whereas the protonated hydratedaldehyde will have a pK similar to that of H30f and its concentration is accordinglymuch higher than that of B. Compared with A and C', B and B1 are intermediatestates at low concentrations. The rate of the reaction BI+C is very fast and there-fore in the n.m.r. spectrum B1 and C' constitute one averaged species C. Thereaction scheme may thus be condensed t116 HYDRATION-KINETICS OF ACETALDEHYDEIn appendix 1 it is demonstrated that for this system two broadened lines areobserved withUZAC = ~ A B ~ B C K ~ B A + k ~ c hllrc~ = ~ c B ~ B A K ~ B A + ~ B C ) .(3)(4)andThe ratios of the integrals under the resonance lines should bepA/pc = ZAC/ZCA.How-ever, this relationship is only true in dilute acetaldehyde solutions, and fails completelyin acetaldehyde-rich mixtures (fig. 24. The sharp lines of the hydrated aldehydeC and the broad lines of the aldehyde A can be understood on the basis of twoassumptions. One possibility is that in the reaction scheme (1) an intermediateB2 decouples A and C by a slow reaction. Then the concentration of B2 must berelatively high for the slow case to be possible.Since no conceivable species ofthat kind exists, the second possibility applies : a parallel acid-catalyzed reactionoccurs with A. Since H-D exchange on C-H protons in D20 solutions occurswith a rate, orders of magnitude lower than the observed rate, any reaction involvingC-H protons (e.g., keto-enol exchange) is eliminated. The same spectra are ob-served when HC1 is replaced by HC104 or naphthalenesulphonic acid. Thereforethe possibility of the formation of 1,l '-chlorhydrin is also excluded. Furthermore,the dependence of the parallel reaction rate on the overall acetaldehyde concentra-tion shows that a second aldehyde or aldehyde hydrate molecule must be involved.We therefore propose for the parallel reaction of (1) :H H H H H HI I I I I iCH3C0 + CH,C(OH)OHz fCH3COH+ + CH,C(OH),+CH C-0-C-CH, ,-I I (5)OH; OHA + C + B + C + DThis is the same reaction as (1) with H20 being replaced by the aldehyde hydrate C.Since in concentrated aldehyde + water mixtures the concentration of free wateris low and that of the hydrated aldehyde is high, reaction (5) is plausible.1 It is ofthe type A+C+D discussed in appendix 2.According to Shoolery's rule,7 thechemical shifts of the CH and CH3 protons in the hydrate C and in the hemihydrateD should be nearly the same. Therefore, the following conditions occur :ZA I (~A--u)D) I, ZDA I (OA--WD) I >1 and ZCA I (WC--OD) 1, ZD I (UC-UD) I el, thatis, we observe a broad line at COA and a sharp line at OC=:COD.* With the followingarguments, an estimation of the concentration of the hemihydrate D is obtained.At relatively high HCl concentrations, i.e., at high rates, the doublet and the quad-ruplet lines of the aldehyde A do not continue to broaden with increasing rate.This can be understood if ZDA(OA-OD)X 1 under these conditions.In fact, it isobserved that the ratio of CHCl necessary for the deviation from the simple " slowcase theory" for the doublet and the quadruplet is about the same as the ratioof the chemical shifts O A - ~ for these two lines (see fig. 4). This constitutes con-clusive evidence that the deviations of the observed at the two lines are not theconsequence of different z values. Since COA-COD is known, an approximate estima-tion can be made for the mole fraction p~ = PAZDA/ZAD with T D A ~ I (OA--WD) 1-1* Unfortunately because of the small chemical shift between C and D no estimate of the rateof the hydrolysis D+H20+2C is available from the n.m.r.spectraM.-L. AHRENS AND H. STREHLOW 117and z g ~ taken from the line width at A. At a mole fraction 0.57 of acetaldehydep ~ , the mole fraction of the hemihydrate is estimated to be about 10-2.The line broadening at A is thus attributed to the two parallel reactions A-+Cand A-+D.Table 1 gives the values for the rate constants ki = l/~i[H+] for different acetaldehydeconcentrations. The values obtained in 1.4 mole % acetaldehyde may be com-pared with data obtained by Bell and co-workers 3 with the maximum temperaturemethod and by Gruen and McTigue4 with a similar technique.The values k Hreported by Bell et al. have to be multiplied by KH/(~ + KH) before comparisonwith our values kAc. Bell's kAC = 490 M-1 sec-1 at 25°C compares well with ourkAC = 480 M-1 sec-1 at (23 & 1)OC. The difference is smaller than the accuracyof either investigation.TABLE 1mole % acetaldehyde 1 -45 24.3 37.7 56.9Hammett-function HO 2-15 2.88 3-08 3.10kA (M-1 sec-1) 480 i 50 480 f 50 330 f35 330 f35(in 10-2 M HCl)kAc (M-1 Sec-1) N 400 N 240 S 30 < 3(M-1 sec-1) 2 80 - 240 - 300 N 330From the data reported above, some conclusions regarding the elementary stepsof the hydration reaction may be drawn. While at equal HCl concentrations theHammett acidity function HO in acetaldehyde +water mixtures increases by more thanone unit in going from water to 60 mole % acetaldehyde, the rate constant kA dimin-ishes only by about 40 %.This is strong evidence against an established pre-equilib-rium of the step A+B which has been assumed by former investigators.2-4 This stepis common to both reaction paths (1) and (5). That the reaction is not a fast stepas assumed hitherto is a consequence of the extremely weak basicity of carbonylgroups. A direct determination of the pK of protonated acetaldehyde is virtuallyimpossible since very fast polymerization occurs in mixtures of acetaldehyde andstrong concentrated acids. However, from the known pK of protonated acetone(pK = -7.2 8 9 9 ) the corresponding value for acetaldehyde may be estimated tobe about pK = -8, in view of the stronger negative inductive effect of the H atomcompared with that of the CH3 group.A pK = - 8 is also comparable with similardata obtained for aromatic aldehydes 10 and for ketones.11 The large differenceof pK for CH3CHOHf and H2Of is the reason for the relatively slow rate of pro-tonation of acetaldehyde by H3O+PFurther experiments substantiate this conclusion. The dependence of the hydra-tion rate in water + dimethyl sulphoxide and in water + acetone mixtures have beendetermined. The HCl concentration was 5 x 10-2 M, the concentration of acetalde-hyde 1-8 M throughout.The results are interpreted on the basis of the consecutive reactions (1) or (2).In the mixed solvents the rate constants are given byandk m = k&#bO+] ; ~ B C = k&[HzO]kBA = ki~[HZo] +&[XI118 HYDRATION-KINETICS OF ACETALDEHYDEwhere X is the non-aqueous solvent.obtainFor the reciprocal of the overall rate we thusIn fig.7 the reciprocal observed rate is plotted as a function of the ratio of con-centrations of the non-aqueous solvent and the water. The vanishing slope of thewater+acetone curve is readily understood with the weak basicity of acetone and theresulting low value of kgA. From the intercept and the slope of the dimethyl sulph-oxide+water curve we obtain kiB and the ratio kBA/kBC, provided that kiA/kgA isknown. To estimate the latter ratio we assume that the deprotonation of the pro-tonated aldehyde proceeds with the same high rate in the two pure solvents, water0.7 -0.6 -0 5 -9 0.4 23 s - 0 30.20 15.10-2 n HCI1,BM AcelaldehydeAcetone[organ. solvent]/[H20]FIG.7.-The reciprocal rate of hydration of 1.8 M acetaldehyde in 5 x 10-2 M HCl water facetoneand water 4- dimethyl sulphoxide mixtures at 23°C.([HzO] = 55.5 M) and dimethyl sulphoxide ([XI = 12.8 M) which are of compar-able basicity. Then, 12.8 kgA = 55.5 k;BA. With this estimation k i B = 103 M-1sec-1 and khA = 1.1 k;Bc. That is, a water molecule approaching a protonated alde-hyde has about the same chance to be protonated or to be used up for the hydrationreaction. In the solutions with high acetone content the rate of the reaction withthe dilute water is diffusion-controlled, and kLA is estimated to be about 1010 M-1sec-1. Then the pK of the protonated aldehyde in water may be calculated to be-8.5<pK< -7.5.Of course, because of the assumption involved the figures pre-sented should be considered to represent the correct order of magnitude only. Alsowith acetone +water and dimethyl sulphoxide +water mixtures the Hammett acidityfunctions Ho, which vary considerably with the solvent composition, do not seemto exhibit great influence on the kinetics.For the explanation of general acid catalysis of the hydration reaction of acetal-dehyde-for which a Bronsted coefficient a = 0-54 on a very large range of pKhas been found 3-a concerted mechanism for the reaction has been advocated.12With weak acids, the observed rate is greater than the calculated protonation rateof acetaldehyde, even assuming a less negative pK value of the protonated aldehydeM.-L.AHRENS AND H . STREHLOW 119It is the conjugate base set free during the protonation in the immediate neighbour-hood of the carbonyl group which can be effective in accelerating the hydrationrate by a suitable polarization of adjacent water molecules.Such a promoting effect on the &O+-catalysis is not excluded by the aboveconsiderations, since the participation of solvation water in the reaction is stillpossible especially in the acetone + water system, where the H30+-ion will be prefer-entially solvated by water, even in high concentrations of acetone. However, itemerges from these investigations that a push-pull mechanism in H30+ +catalysisshould not increase the rate of hydration of acetaldehyde by orders of magnitude.We are indebted to Dr.M. Eigen for valuable discussion.APPENDIX 1N.M.R. BROADENING FOR A REACTION OF THE TYPE A+B",CThe treatment is analogous to that of McConnell.12 The Bloch equations for the com-plex magnetization G = M,,+iM, in the three states A, B and C are:Here PA, PB and pc are the respective mole fractions of A, B and C, the ctk are defined byak = (l/T2,+i(mk-w), 01 sz yH1 is a measure of the strength of the h.f. irradiation, andthe zk are the average lifetimes of the species k with- = ~ + -). For slow passage,the dGkldt are negligible. When the resulting algebraic equations are solved, the totalcomplex magnetization G = GA+ GB+ Gc is( l l 1 zB ZBA TBC(3)( 9 4(9b)(94- i ~ l M O f p A ~ , f p B ~ B f P C ~ C ] G =(l+ + 'BaB)(l + zcaC)-(rB/zBC)(l + zAaA)- (zB/zBA)(l + zc%)'The functions Pk are :P A = TA(l + zBaB)(l + zCaC) + zB(l + zCaC) - (TB/TBC)(zA - zC),P B = rB[(l + zAaA)(l + rCaC) + (zA/zBA)(l + zCaC) + (zC/TBC)(l + zAaA>]PC = z d l + T A ~ A ) ( ~ + zBaB> + zB(1 -F z A a J - ( ~ B / ~ d ( ~ c - 7,)-One special case for eqn.(8) is of interest. PB and ZB are very small. Furthermore,TA I (CUA-WC) I > 1, TC I (COA-CI)~) 1 > 1, and ZAG 7'2. Then the imaginary parts of Gatw-wA and atw ~ w c , which are proportional to the observed n.m.r. signal, are respectively :andEqn. (10) and (11) correspond to broadened lines with a line width ACOY,~ = 2zB/rATBCand Awlp = ~T&CTB.~ respectively.The time constant z ~ r ~ c / ' t g = ZAC correspond120 HYDRATION-KINETICS OF ACETALDEHYDEto the rate constant kAC = k ~ k B C / ( k B A + k B C ) , which is the overall rate constant forA+C. Similar arguments apply to the time constant ZCZBA/T~ = ZCA.Thus a low concentration intermediate or a transition state is not detectable by n.m.r.methods if ZBA 1 (wB-wA) I 6 1 and ZBC I (u~-uc) I < 1. If for one or both inequalitiesthe reciprocal applies, A and B become independent, and two sharp lines are observedat OA and OC.APPENDIX 2N.M.R. LINE BROADENING FOR A REACTION OF THE TYPE A+C+DThe kinetically modified Bloch equations for this case are :(For the meaning of the symbols, see eqn. (n.) The stationary solution for G iswithWe discuss only the special case which applies to the reaction dealt with in this paper wherez D 1 ( ~ A - U D ) I I oA-mc I 9 I ~ D - u c I TA I ( u A - u D ) 1 > 1, and zc I ( m D - u c ) I < 1-Under these conditions at WA, a broadened line with line width 2/2A is observed, whilethe resonances C and D merge to a single sharp line.where ZD = p&.~c+p&~~ is the frequency of the common line averaged with the molefractions p;1= ZC(ZC+ZD) and p 6 = ~D(TC+TD).1 Bell and Higginson, Proc. Roy. SOC. A , 1949,197,33.2 Bell and de B. Darwent, Trans. Faraday Sac., 1950,46, 34.3 Bell, Rand and Wynne-Jones, Trans. Farahy SOC., 1956,52, 1093.4 Gruen and McTigue, J. Chem. SOC., 1963, 5224.5 Strehlow, 2. Elektrochem., 1962, 66, 392.6 Becker, Ber. Bunsen Ges., 1964, 68, 663.7 Shoolery, Tech. Information Bull., Varian ass., Palo Alto, Calif. 2, no. 3, 1959 (cited in Strehlow,8 Campbell and Edward, Can. J. Chem., 1960,38, 2109.9 Deno and Wisotzky, J. Amer. Chem. Sac., 1963, 85, 1735.10 Culbertson and Pettit, J. Amer. Chem. SOC., 1963, 85, 741.11 Stewart and Yates, J. Amer. Chem. Soc., 1958, 80, 6355.12 Eigen, private communication.13 Eigen, Angew. Chem., 1963,75,489.Magnetische Kernresonanz und chemische Struktur (Steinkopff, Darmstadt, 1962, p. 34 ff)