75
V O L U M E 28, NO. 1, J A N U A R Y 1 9 5 6 this method is based upon an acidimetric determination. The net effect of the reaction is an apparent decrease of t x o equivaleirt,s of widity. This is shown graphically in Figure 1 curve 1. At p H 5,:3 and at room temperature the apparent addition of sodium sulfite t,o ethyl crotonate is 13070,. At 100” C.: curve 2, this reactioii apparently does not occur, presumably bemuse of ic sulfone reaction product. the thermal instability of the .Ilso, conipounds n-hich are ai) ial)l>- solutile in the reagent do iiot undergo this reavtion, suggesting that the reartion takes pl:we only Lit the gas-liquid interface. This reaction is favored at low values of pH, Iiresumahly I)ec.ause of the i n c r e : ~ eof free sulfur dioxide in the gas phase. In some cases this side reac,tion can lie minimized tiy the addition of a cosolvent such as isoprop!.l :dvohol. Using the reagent as specified, approsima of cosolveiit can be tolerated before phase separation though no lit,erature confirmation of the reaction of s I\-ith alph:t, beta-unsat,urated compounds has been found, t,hr* reartion with butadienes to form cyclic sulfones has tieen re~ ~ o r t e( 1d).
I I
I
I
i I20
I
4
Yeagent, at pH 6.2, is used. However, a t 100” C. saponification does take place t o an appreciable extent. I n the case of ethyI crotonate, Figure 1,curve 2, an apparent 95% reaction is obtained at this temperature. This low result can be attributed to thc saponification of 5 . 0 ~ oof this compound. Esters which fail to react quantitatively with the reagent a t room temperature c m not be determined by this method. Among the other reactions that are known to take place with this reagpnt are the addition to epoxide3 ( 9 ) , reaction with stronfi oxidizing and reducing agents, and reaction n-ith vinyl ethers (8I. .Is the method is based upon a measurement of the decrease i i r acidity, a cwrrection must be applied for the presence of niin~i~:il and organic acids and inorganic and most organic l~sseq. A number of compounds which have been succe~si’ully tletermined hj- the sulfite method are shown in Table 11. CHOICE O F METHOD
From the data obtained in this and other studies of a1ph:i) Ixtuunsaturation in these laboratories, it is possible to choose the method hest suited to the determination of a particular compound from a consideration of its structure. Table 111, which is based o n reaction-rate studies of each compound, serves as a guide for such selections. I n some cases both t,he morpholine and sodium sulfite met,hods are applicable and other factors such as the presence of interfering compounds and precision determine the method t o be selected. I n most cases the sulfite method is more precise than the morpholine method because of the sharpness of the titrimetric end point, but the former is subject to more interferences from side reactions. LITERATURE CITED
Backer. H. J., Strating, J., and Kool, C. 11. H., Rec. / t a l , c h i m . 58, 778 (1939). Caryl, C. K.,Ind. Eng. Chem. 33, 731 (1941). Critchfield, F. E., Funk. G. L., and Johnson, J. R., - 1 s ~C H ~ m.r . 28. 76 11956). Kaufinann, H. P., and Kormnann, 0. O., 2. C n t e ~ t t c hLehensm. . 51, 3 (1926). Kharasch, 11. S., May, E. AI., and l l a y o , F. R., J . Org. Chem. 3, 175 (1938). Rosenthaler, L., Pharm. Acta Heh. 17, 196 (1942). Schenck, R. T. E., and Danishefsky, I., J . Org. Chern. 16, 1683 (1951). Siggia, S . , IND.ENG.CHEY.,A N ~ LED. . 19, 1025 (1947). Swan, J . D., ANAL.CHEM.26, 878 (1954). RECEIVED for review June, 22, 1955. -4ccepted August 18, 1955,
0 0
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6
4
8
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PH
Figure 1. Effect of pH on rate of addition of sodium sulfite to alpha, beta-unsaturated compounds 1. E t h y l crotonate a t room temperature 2. E t h y l crotonate at 100’ C. 3. Crotonic acid a t 100’ C . ( 7 )
The secondary reaction shoxn in Equation 3 is similar to the principal nucleophilic reaction in that the same reaction product is formed. However, this reaction proceeds by a free-radical mechanirm and is not restricted to conjugat’ed systems (5). The occurrence of this reaction therefore prohibits the use of this reagent for the determination of alpha, beta-unsaturation in the presence of olefinic-type unsaturation. As would be expected, this reaction is inhibited by the presence of hydroquinone and the absence of perosides and molecular oxygen ( 5 ) . Secondary reactions other than those involving the carbon to cwhon douhle IJond are known to occur with this reagent. Among these are the substitution reactions involving a carbon to halogen bond, addition t o carbonyls, and saponification of esters. Sa.ponification does not occur at room temperature when the specified
CORRECTIONS Concept of Polarographic Currents Limited by Rate of a Chemical Reaction and Some of Its Applications In the article on “Concept of Polarographic Currents Limited b j Rate of a Chemical Reaction and Some of I t s Applications” [Asar,. CHEV.27, 1712 (l955)] in the second column, the fifth line under Equation 1 should read: =
0.85 (nzt)2 3.
K ~ R E\I-IE:;\ER L
Slow Precipitation Processes In the article o n “Slow Precipitation Processes. -1pplication of Precipitation from Homogeneous Solution to Liquid-Solid 41’. CHEX 27, 1704 (1955 11 Equation Distribution Studies” [.I\6 should have been written: ~
