1895
STUDIES IN ESTERIFICATION. [CONTRIBUTION FROM THE CHEMICAL
LABORATORIES O F
CLARK
UNIVERSITY.]
STUDIES I N ESTERIFICATION, I. VICTOR MEYER’S ESTERIFICATION LAW. B Y M. A. ROSANOFFA N D W. L. PRAGER.
Received November 8, I@.
Sec. I. Introductory. The Esterification Law, discovered by Victor Meyer in 1894 and experimentally established by himself and others, is widely known. I t is usually stated in the following form:’ “If in a substituted benzoic acid both of the hydrogen atoms next to the carboxyl group are replaced by radicles, the resulting acid cannot be esterified by means of alcohol and acid.” The radicles thus far studied include the halogens, the nitro-, amino-, methyl, carboxyl, and hydroxyl groups. Several important exceptions to the Esterification Law are known. Not to speak of the influence of hydroxyl groups, which interfere with the formation of esters only to a moderate extent, we will mention the case of tetrachloro-ortho-phthalic acid.‘ According to the Esterification Law, this acid should give no ester. Chlorine atoms and carboxyl groups are supposed to exert an especially powerful hindering influence on esterification. Yet the monoethyl ester of this acid is readily fornied by the action of alcohol and hydrochloric acid. Notwithstanding such occasional exceptions, the Victor Meyer 1,aw has been used in two or three cases, as an empirical rule, for solving problems of constitution and for isolating diortho-substituted from other acids. If, however, the Esterification Law and its kindred rules had no other than the purely empirical aspect, their scientific value would be very small indeed, for diortho-substituted aromatic acids are but seldom met with in laboratory practice. The great importance of such rules lies, not so much in their immediate practical utility a s in the light which, if correctly interpreted, they throw upon the connection between the structure of substances and their chemical behavior, and in the new power thus acquired by the atomic theory. Victor Meyer realized this from the beginning and early offered a hypothesis which, in the opinion of the majority of chemists, furnishes a satisfactory interpretation of the Esterification Law. We are referring to the well-known steric hindrance hypothesis, in accordance with which the carboxyl group cannot react with alcohols because i t is shut in between the neighboring groups, atid 1 See, for instance, Scholtz, Der Ein@ussder Raumerfullung der Atomgruppen auf den Verlauf chemischw Reaktionen (Ahrem’ Sammlung, Stuttgart, 1899), p. 12. Graebe, Ann., 238, 327 (1887); Victor Meyer, Ber., 28, 182 (1895). See also Graebe’s work, Ber., 33, 2019 (I~oo), and Marckwald and McKenzie, Ber., 34, 486
(1901).
I 896
M. A . ROSANOPF A N D W. I,. PRAGER.
is (consequently) the less capable of reacting tlie larger those groups. The sanie mechanical conception explaiiierl to I'ictor lIe\.cr tlie [act t h a t the esters of tliortho- substituted acids c~)ultlhe rcndil!, ol)t:iined by tlie action ol alkyl halides 011 the correspoiidin;: silver salt:;: t l i c . large silver atom forces the two ortho-groups apart ant1 renders t he carboxyl readily accessible to the action of halogeii conipourids. 'I'Iiis last idea is not altogether free from the possibility of ohjectioii, 1)ut \vi11 not insist. I n the hope of producing experiniental evidence iii support of the steric hindrance hypothesis, l'ictor hIeyer directed Kellas' t u untlertnke a thorough investigation of the influence on the carboxyl of one single group in the orthoposition: if the steric hindrance hypothesis is correct, the velocity of esterification ought to be diminished ill proportion t o the ~ m g n i t u d eof the substituting group. As a matter of [act, KeIl~ts found (apparently) t h a t a bromine atom retarded esterificxtioii t o ;L greater degree than a chlorine atom, and an iodine atoni t o a greater degree than a bromine atom : the "heavier" tlic substituting ~ ~ t c i i i i group, the greater its retarding influence on esterification. .in incoii sistency is presented by the nitro group, which, ivith its \\.eight of O I I I ~ . 46, exerts a greater retarding influence than bromine \yeifillin:. 80 nnc! e1.m than iodine weighing I z 7 . I'erhaps Nernst's estimate of hypotheses t h a t lead to inconsistenciw is not inaliplicable t o this case : "Letztere [Geselzrt,ucsi!/kcifi'ri] w r , / ( ~ t i tletti tI. A . R0ShSOI:I~ tlh'l) W . I,. PRAGER.
I902
i n order t o determine what influence bromine atoms niay have on thc reaction velocity as conipnrcd \\it11 hydrogen atoms, 11e studied the esterification of ordinary henzoic acid. Fiiidl!., :~nd whrit is most iinportant of all, in order to tcst the steric hindrancr hypothesis, ire studied the esterification velocity of I , ? ,4,0-trichloro-beiizoicacid. .\ second coim niunicatioii, published under the name of onc' of us (b..I,. I ) , ) , reports the results of further work i n this direction ancl deals \\-ith tlic inonoetliyl ester of cY-iiitro-i'lithalic :icic: and \\it11 acetic, propionic. isobutyric, triinctli!,l-acetic, monochloro-acetic. dichloro-acetic, and trichloro-acetic acids. Sec. 5 . Tribromo-benzoic Acid 1,2,4-6. 'l'his acid was prepared in considerable quantities b y ;L inethocl siniilar to that tised by Sudhorough,' \\7egsclieider.2 ;uid otliers. Ordinary tribroni-aniline is finely powdered, diazotized with hydrochloric acid and sodium nitrite, the solid residue filtered OB,a n d tlie filtrate treated according to Sandnieyer. .\fter heatiiig tlie reaction inisture f o r about eight hours, until the evolutioii of cyanogen gas has ceased and the s d i d separating out has settled, the solid is filtered out, dried, and thoroughl!, extracted with ether. LVe find Illat the crude nitrile remaining after evaporating the ether is best purified 1)y dissolyii1g in ;L moderate quantity of 80 per cent. alcohol and boiling with 1)one-lilack. On cooling the filtrate, t h e nitrile separates o u t i l l pure eiiougli condition for further transformation into acid. 'I'hc nitrile inay he saponified 11!. heating The with an excess oi conceiitratetl hytlrocliloric acid at x ) o c to 2 x 1 ' . reactioti, however, refuses to take plncc> iinless t h e hydrochloric acid is very strong. Oti the other I l a i i t l , tlw coiicentrated hyclrochloric acid attacks the tubes at the high temperature oi the rewtioii and these frequentl!. blow out. IVc lia1.e foulid tliat strong aqueous h!drol)romic acid saponifies the nitrile :is well as h~~drochloric acid ant1 !.ields n considerable quantity of organic acid in c\.ery case. \Ve are indebted t o Professor Xarston Taylor Rogcrt, of Columbia University. for suggestiiig the use of hydrobromic acid. which has saved us much trouble. 'l'he 'l'he melting point of our I , z , q ,6-trit)romo-heii~r)ic ; r i d is I 86' following table gi\-es the results of the kinetic sttltl!.. TABLE11. k. t. a. h. X. K. 0.s33 1.038
0.00100
0.001763
0.000817
0.001619
1.745
0.ooo957
1.457 I . 667 2.542
0.000726 .... . . .
o.001872 0.001267 0.00 I 343 o.ooI6X6
3.000
,...
0 . 0 0 2 2 SG
ne?., 27, 5 1 2 (rXg4).
Monatsk. Chew., 18,6 j z (1897).
o.ooc)i)I4 o.ooSS4q n.Oo9132 0.006gS;
o .00; I O . O I ;O ~ o . OOS X S o
O.OOO;~I
o.ooo904 o.oo1018 0.000j86 c ) . 00 1043
u ,cioIpl; o , 00 I ,ji7
(0.06) (0.14)
o.og1
(0.14)
0.08s 0.076
(0.20)
0.078
0.44 o . 40 o .+3
I903
STUDIES I N ESTERIFICATION.
In this table, and the tables given further below, t denotes time in days ; v denotes volume in liters; a denotes the amount of acid in gram-molecules ; b denotes the amount of alcohol similarly expressed; x denotes the amount of ester, similarly expressed, formed in the time 1. The data obtained in each of the experiments were introduced into the ordinary static equation of mass action: 9 (a -x ) ( b
-x)
=
K.
When several consecutive experiments, a t considerable intervals of time, gave about the same value of K , the reaction was considered as having attained equilibrium, and t h a t value of K was taken as the equilibrium constant. Its reciprocal, K ' , was used in computing the velocity constant by the equation given in the preceding section. Table I1 yields the following average values of the constants:
i
K
= 0.42,
k
=
0.088.
Sec. 6. Tribromo-benzoic Acid, 1,3,4,5.
This acid was prepared according to Sudborough's directions.' It melted a t 235'. One gram of the acid corresponds theoretically to 47.55 cc. of our sodium hydroxide solution. The amount actually required was 47.75 cc. The esterification was carried out exactly as in the case of the tribromobenzoic acid 1,2,4,6.The results are given in Table 111. TABLE111. f.
V.
0.0641 0.1291 0.1428 0.1523 0.1771
0.2382 0.4097
0.00103 0.00133 0.00103 0.00100
..
.. ...
a.
b.
X.
0.001399 O.OOIZ9O 0.001415 0.001385 0.001486 0.001479 0.001382
0.01305 0.01541 0.01147 0.01046
0.000663 0.000866 0.000937 0.000943
0.01117
0.001111
0.33
0,01490 0.02~04
0.001130
0.27
0.001160
0.29
K. (0.05) (0.12)
k.
0.86 0.86
(0.17)
1.01
(0.21)
0.89
.. .. ..
The average value of the constants are thus: K = 0.30, k = 0.90.
i
Sec. 7. Tribromo-benzoic Acid, 1,2,3,5. Pwpnrafion.---This acid has never been obtained before. We prepared i t without difficulty from anthranilic acid by the following method. Ten giams of anthranilic acid are dissolved in about zoo cc. acetic acid and brominated by the gradual addition of a solution of 8 cc. of bromine Bw.,
37, 514
(18%).
I 904
M. A . ROSANOFF ASI) \V. I,. PRAGER.
in concentrated aclueous potassium bromide. .I precipitate of a dibroiiiamino-benzoic acid is formed. This precipitate is washed with water, dissol\.ed in sodium carb(.)nate, tlie solution filtered, a n d the filtrate re-precipitated with hydrochloric acid. The iurther steps i n this p r e p itration are similar to those emplo!wi l)!. Suc1t)orougli in the preparation of tribromo-benzoic acid I >3
