[CONTRIBT' CION 1-0 i 8 FROM THE DEPARTMENr OF CHENISTRY, ecametoo large for reasonable use, so the data were obtained using the monomethyl ester. 'The rstte constants for esterification of the cyclohexanedicarboxylic acids are given in 'Yable L , IV. It should be noted that the constants for A 10 15 20 2,; 31 esterification of the second carboxyl group of the Time, minutes. Fig. 1.-Graphical calculation of rate constant (kl) cis-1,3 acid calculated using either the acid or the for esterification of ~runs-1,3-cyclohexanedicarboxylic monomethyl ester as starting material agree very well. The agreement is not so good for the cis-1,2 acid a t 55": extrapolated value of kl = 8.88 X 1 V 2 . acid, but the corrections for catalyst consumption dicarboxylic acid which was allowed to react were fairly large for this esterification, making the until essentially all of the first carboxyl group results obtained with the dicarboxylic acid as had been esterified. Rate constants were cal- starting material unreliable, and probably account culated from data taken on the remaining portion for the lack of closer correlation. Figure 2 shows a plot of the logarithm of the of the reaction, using t modification of the Gold rate constants for the esterification reactions Schmidt equation against the reciprocal of the absolute temperature. -'
11
TABLE 11;
+
In this equation the usual factor ( n a 3. r ) RATE CONSTANTS AND ACTIVATION ENERGIES FOR ESTERIreduces to (2a r ) since at the start of esteri- PICATIOV OP THE CTCLOHEX.4NEDICARBOXYLIC ACID+ fication of the second carboxyl group the number 1i. 2: 104 :liters moles bet (cal. of moles of water formed in the esterification of ACld 36" 45O .So mole-1) the .first group will be equal to the original con0 323 0 680 1 24 13,100 centration of the dibasic acid provided the esteri- ciS-1,2- ki 0.170 ,163 324 .692 1.22 fication takes place in two successive steps. kz 0 0295' 0 0582 0.120 0.221 This equation was applied by allowing the re.0293" 0690 ,121 .216 13,100 action to proceed until more than one equivalent 0538 ,107 ,205 .0282 of the dibasic acid had been esterified. Then .0293 .0562 .lo9 .204 samples of the reacting mixture were withdrawn and titrated a t various time intervals until about trans-1,Z- kl 0 0774 0.170 0.336 0.632 .0781 ,171 .330 ,623 13,500 85% of the total acid groups had been esterified. kt 0.0426' 0.0825 0.154 0.293 Successive times were chosen as the initial time ,0424" ,0822 ,158 282 12,600 itr = 0) and rate constants based on this arbitrary 4.78 8.78 14.6 zero were calculated for the remaining readings. c i s - I J - kl 3.05 2.96 1.52 8 55 14 3 10,300 The rate constant from the set of values of the
+
5
kr
TABLE 111 THE ESTEKIFICATXON OF tralz.Y-1,:jCYCLOHEX~4NFDICARBOXYLlCACID AT 35
CALCULATION O F ks FOR /n
- x)
1,
rniu
k"
t,
mia.
ka
1,
min
l.18" 1.1T 1.15
C Z S - ~ , ~ki-
1.20 1.72 1.72 0.692 0.695 1.62
k?
0 876
Mans-l,3- ki ka
0 3973 0 0 3524 101 0.0132 80 0.0118 0 3246 181 .0126 ,0127 195 0124 115 0.0129 .2867 296 .0124 328 .0122 248 ,0123 2525 429 0124 405 .OI22 385 ,0123 .2215 566 .0121 678 .0119 598 ,0119 1846 779 ,0121 866 .0120 786 ,0120 .I557 967 Average .0125 0121b ,0123 Av. deviation .00029 00018 .OW26 The figure ~1 The units of k are liters moles-' sac.-'. of 0.0121 was chosen for the rate constant.
0
k2
trans-1,4- kl
2.80 2 88 ke 1 32 1.32 a All constants in
esters.
2.01 1.96 1.99 1.90 3.13 3.14 1.18 1.21 3.00 3.08 1.23 1.25 5.06 5.06 2.24 2.29 this series
3.51 3.40 3.39 3.44
5.61 5.59 5.36 5.47 8.98 8.88 3.08 3.10 8.28
5.39 5.40 2.01 2.00 5.36 5.40 1 99 8 04 1.90 8.71 14.4 8 85 14 0 3 85 5.94 3.82 5.98 determined from
10,200 10,000
10,700 9,700 10,600 9,700 10,400 9,800 mono-
o ~ t . ,i
nx
ESTRRIFICATIUS KINETICSI I F CYCI.~IARXANRDICARROXYL~C ACIDS
4409
In general, it is expected that cis isomers will react more slowly than corresponding tram forms. Table I V shows that only the 1,4 acids follow this rule. For both 1.2- and 1.3-cyclohexanedicarboxylic acids the reverse is found. However, it is interesting to note that for the 1.2 acids, k, is greater for the ri.q isomer, hut k, is greater for the trans form. I n attempting to explain these results i t is necessary to consider the fact that the cyclohexane ring exhibits two forms, represented by the chair and the boat configurations. In cyclohexane one may differentiate between two types of hydrogen atoms. Those extending outward from the ring are designated as equatorial while the others are known as polar.'* Furthermore, a t room temperatures substantially all of the cyclohexane is in the chair form, which has three polar hydrogen atoms on each side of the ring.13 Assuming the chair configuration for the cyclohexane ring, there are two possible tautomers for I I I I I /mns-l,2-cyclohexanedicarboxylicacid. One has both carboxyls in polar positions, and one has both Rml n:um 31W 1 lT X Illc. equatorial. I t would appear logical that the Pig. ?.-Teniprature metficientr for esterificatioii oi polar form would predominate, since here the cycloLexaii~icarboxylicacids: I, kI for &-1,2; 11. kI for carboxyl groups are more widely separated. frans-1.2; 111, b for frons-1.2; 1V. kr for cis-l.2: V. There is, however, only one possible form for the 4 forcis-lf; VI, bforcis-lZ; VII. krforfrans-l,3; ITII. &1,2 isomer, and it has one polar and one b for frans-1.3; IX,k, for cis-1,4; X. k? for &-1,4: XI, equatorial carboxyl. In a polar carboxyl, the carbonyl group is somewhat shielded, while in an k, for franr-1.4; S I I , b for trmts-1.4. equatorial carboxyl, it is relatively open to attack. The activation energies included in Table I\' Thus one would expect the first (equatorial) acid were calcuIated from the slopes of these straight group of the cis fomi to react more rapidly than lines using the method of least squares. either acid group in the /ram form. Once this equatorial carboxyl is esterified, the ester group Discussion An examination of Table I V shows several adds to the hindrance of the remaining (polar) interesting results. As would be expected, the carboxyl, while this is not the case in the tram rate constants, k2. for the 1,3 and 1,4 isomers, isomer. I t is possible to explain on this basis as well as the corresponding activation ener@es the slower rate of esterification of the second are close to those for cyclohexanecarboxylicacid. carboxyl in the cis form when compared to that For the latter, ks is 1.1s X IO-? liters moles - I in the /mn.s. The three possihle forms of these sec.-l and E is 10,000cal. per mole.' The values isomers are shown in Fig. 3. of kI for these acids should he, on a purely statistical basis, twice those for k2. This seems to hc the case, all k, constants at 2 Y being within about 20% of twice the corresponding constants, kp. The energies of activation for esterification of the second carboxyl group seem to he significantly lower than for the first. No explanation is apparent. The 1.2-cyclohexanedicarboxylic acids arc esterified much slower than cyclohexanecarboxylic acid. This is certainly the result of the proximity Fig. 3.---Icomers of 1.2cyclohfxanedicarboxylic. acid; of the two acid groups. The energies of activa- the cyclohexane rins is pcrpcndicuhr to the plane of the tion are also several thousand calories greater for photograph: left. cis isomer; center. frons isomer showing the 1,2 acids than for the mono-substituted tautomer with polar carboxyl groups; right, frans isomer compound. For the /runs form, kl is approxi- showing tautomer with equatorial carboxyl groups. mately twice as great as k,. However, for the Photographs by Comer Shacklctt. cis form the value of kl/k2 is approximately six. For the 1,3-cyclohexanedicarboxylicacids, the For this isomer the methyl group of the monoester apparently hinders esterification of the second (11) Beetett. pisad Spit=. ibid.. n. 2488 (IMn. carboxyl group. cia) see. for LI.mp~le. J . c h . pkw.. 11. U P(1018).
4410
HILTONA.
SMITH AND FRANK
trans isomer has only one possihle form and i t has one polar and one equatorial acid group. The cis isomer has two possible tautomeric forms. Here one would probably expect the tautomer with the two equatorial carboxyls to predominate since in this form the acid groups are as widely separated as possible. Thus one would expect that the cis isomer by virtue of its two equatorial acid ~ o u p swould have a k, value about twice as great as 'that for the trans isomer. This is shown in Table IV. One would also expect the value of k2 to he greater for the cis than for the trans form. The three forms of the 1.3 acids are shown in Fig. 4.
P.BYRNE
VOl. 72
Fig. b.--lsomen of 1.4eyclohe.xanedica~bo~ylicacid; the cyclohexane ring is perpendicular to the plane of the photograph: left, cis isomer; center. frons isomer showing tautomer with polar carboxyl groups; right, fruns isomer showing tautomer with eouatorial carboxyl groups.
On this basis i t is possible to explain the fact that the cis-1,3- and trans-1,4-cyclohexanedicarboxylic acids, each with two equatorial carboxyls, show almost identical behavior, while the fruns-1.3 and cis-1,4 forms, each with one polar and one equatorial carboxyl, are similar to each other.
SummarY Fig. 4.-lsomen of 1.3-cyclohexanedicarboxylic acid; the cyclohexane ring is parallel to the plane of the photograph: left, Irons isomer; Center, cis isomer showing tautomer with equatorial carboxyl groups; right. cis isomer showing tautomer with polar carboxyl groups.
The isomeric 1,2-, 1,3- and 1,4-cyclohexanedicarboxylic acids have been prepared in pure form, and their rates of acid-catalyzed esterification in methanol have heen studied. Rate constants at four temperatures and activation I t is the cis form of 1,4-cyclohexanedicarboxylic energies have been tabulated. It has been shown that the 1,3 and 1.4 isomers acid which has only one tautomer, one acid group being polar and one equatorial. Of the two behave much like cyclohexanecarboxylic acid, tautomeric forms of the trans isomer, i t is again hut that the 1.2 isomers are esterified much more assumed that the form with two equatorial acid slowly. The results are explained on the assumptioh groups is predominant. If the equatorial acid group is more readily attacked, one would expect that the cyclohexane ring is almost entirely in that the trans isomer would be esterified about the chair form, and that whenever two tautomeric of the chair configuration are possible, the twice as fast as the cis,and that the kz value would forms one yielding the greater separation of the subalso be greater for the trans form. This is shown in Table IV. Finure 5 illustrates the forms of the stituents is predominant.
