1864
W. P. RATCHFORD, C. E. REHBBRG AND C. H. FISHER [CONTRIBUTION FROM THE
Vol. 66
EASTERN REGIONAL RESEARCH LABORATORY,~ PHILADELPHIA, PENNSYLVANIA]
Preparation of Acrylic and Methacrylic Acids by Pyrolysis of Their Alkyl Esters BY W. P. RATCHFORD, C. E, REHBERG AND C. H. FISHER The purpose of this investigation was to effect the synthesis of acrylic and methacrylic acids by thermal deco'mposition of their ethyl or higher alkyl esters. The analogous pyrolysis of ethyl crotonate into crotonic acid is known2 to occur. Since it appeared that such a synthesis might be more practical than the alternative approach by hydrolysis3*4J,6the present investigation was undertaken. In this investigation it has now been found (Table I) that both acrylic acid and methacrylic acid can be prepared conveniently and in high yields by passing suitable acrylic or methacrylic esters through a Pyrex glass tube heated a t about 500 to 580'. The acids produced in this manner are relatively pure, contain little or no water, and are readily purified by distillation or crystallization. As would be expected from previous observations' made with acetates of secondary alcohols, s-butyl acrylate decomposed more readily than did the esters of primary alcohols. Isobutyl acrylate was more stable than the other esters studied. Possibly this stability is attributable to the presence of only one hydrogen on the Pcarbon atom of the alkyl group. Ethyl acrylate, ethyl methacrylate, and n-butyl methacrylate appeared to have approximately the same thermal stability. Cyanoethyl acrylate was much less stable than ethyl acrylate. Contact time was relatively unimportant a t 570 to 580°, 90 to 94% of the ethyl acrylate being decomposed when the contact time was four to twelve seconds. At lower temperatures, however, the influence of contact time was more marked. Considerable quantities of acrylic acid have been prepared conveniently in this Laboratory by pyrolyzing ethyl acrylate under conditions similar to those of Experiment 3, Table 1. The high yields of acrylic and methacrylic acids and the relative unimportance of contact time at the higher temperatures show that these acids are relatively thermostable. That methyl acrylate is rather stable is demonstrated by its preparation in high yield by the thermal decom(1) One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, United States Department of Agriculture. Article not copyrighted. (2) N. T. M. Wilsmore and A. W. Stewart, Proc. Chem. Sac., 13, 309 (1908); C. D. Hurd, "The Pyrolysis of Organic Compounds," Reinhold Publishing Corp., N e a York, N. Y . , 1929, 807 pp. (3) D. Pressman and H. J . Lucas, THISJ O U R K A L , 64, 1953 (1942). (4) C. F. Koelsch, ibid., 66, 437 (1943). (5) 0. C. Dermer, W. G. Markham and H M Trirnhle. i b i d . , 63, :%24 (1941). (6) H. T . Neher, l e d . EtLg. Chcm , 28, P O 7 (1930). ( 7 ) A . J . Van Pelt, J r . a t i d J P. TVihaut, Ri,r. Ww,,. rhirri., 60, 5.5 (1!)41).
TABLE I PYROLYSIS OF
Grams
wro-
Expt. lyzed
1 2 3 4
5 6 7
161.2 203.7 175.8 677.2 76.3 140
98.7
8 149.3 9 150.0 10 156.1
4
ALKYL ACRYLATESAND METHACRYLATES
Addition rate,
Conmolw tact of ester Temp., time, per hr. OC. sec.
Ester decomposed,
?&
Gases and losses,
%
Ethyl .4crylate 0.84 499 6 . 9 39.7" 0.57 556 10.7 92.8b 1 . 1 7 568 4 . 5 89.86 1.38 574 3 . 9 90.9l
Yield of unsaturated acid,
7%"
13 7 28.2 27.4 28.7
92'
Isobutyl Acrylate 0.83 545 7 . 3 84.5e 3 6 . 8
63'
.77
$-Butyl Acrylate 498 8 . 4 100*
81"
94" 90'
40.2
97
