A STUDY OF THE BENZOIN REACTION. I1 NEGATIVECATALYSIS IN D. R. NADKARNI
THE AND
BENZOINREACTION
S. M. MEHTA
Department of Chemistry, The Royal Institute of Science, Bombay, India Received September M,19% INTRODUCTION
Having definitely established the fact that pure potassium cyanide and benzaldehyde in the absence of water do yield benzoin (2), attention was directed to the statement in the literature that water was necessary to induce the reaction. Preliminary experiments with various brands of potassium cyanide showed that certain commercial samples would not yield benzoin when shaken with pure benzaldehyde a t 100°C. for 1 hour unless water were added. Further experiments showed that alkali halides and also hydrogen sulfide and sulfur can inhibit the benzoin reaction. The effect of increasing quantities of sodium and potassium chlorides (impurities likely to be present in commercial potassium cyanide), potassium iodide, and quinol on the benzoin reaction was examined (figures 1 and 2). The results show that these substances in sufficient quantity prevent the formation of benzoin in 1 hour, and that potassium iodide and quinol are particularly effective. The latter were tried because it is known that they are more effective than potassium and sodium chlorides in the inhibition of the oxidation of benzaldehyde. TIME-YIELD
CURVES I N PRESENCE O F INHIBITORS
Next, a series of experiments was made to obtain time-yield curves in the presence of various quantities of quinol, potassium iodide, and potassium chloride (figure 3). It will be seen that the curves thus obtained are of the same type as those yielded by pure potassium cyanide, with the difference, however, that a diminished quantity of potassium cyanide appears to be functioning in the heterogeneous reaction. (See figure 3, upper section, Curves IV and VI.) The homogeneous autocatalytic reaction is unaffected. Thus the curve obtained with 1g. of potassium cyanide and 0.3 g. of quinol is almost identical, particularly in the earlier stage, with that given by 0.3 g. of pure potassium cyanide. In other words, 0.3 g. of quinol renders inactive 0.7 g. of potassium cyanide. From the amount of benzoin obtained in 1 hour, we can determine how much cyanide is effective in producing it. This determination is facilitated 901
902
D. R. NADKARNI A N D S. M. MEHTA
1 g . KCN + N o C l 1 g. KCN iKCL C 2 3 . K C N + Na-CL
A
6
3
0
0.1
0.4
0.2
0.6
0.8
1.2
1.0
FIG.1. EFFECTOF SODIUM CHLORIDE) AND POTASSIUM CHLORIDE ON YIELDOF BENZOIN
THE
41
A I g.KCN+Kf
I I
B 2 g-KCN+KI
XI
C
0
0.1
0.2,
0.3
3 g-KCN+KI
m
g.KCN+QUINOL 25.KCNf 9 2
3 g.KCN+
0 . 5 0.6 04 0.8 0.9 GRAMK 1 OR QUINOL ADDED
0.4
1.0
9 3
1.1
1.2
FIG.2. EFFECT OF POTASSIUM IODIDXI AND QUINOL ON THE YIELDOF BENZOIN
NEGATIVE CATALYSIS I N BENZOIN REACTION
903
by plotting the quantity of benzoin obtained in 1 hour from 10 g. of benzaldehyde at 100°C., against the quantity of pure cyanide required to pro-
FIQ.3. EFFECT OF QUINOLAND POTASSIUM CHLORIDE ON
EFFECT OF
YIELDOF BENZOIN; BENZOIN
TEE
POTASSIUM IODIDE ON THE YIELD O F
duce it, the correction of 0.17 g. (see Part I) being inserted. The yield of benzoin produced in 1 hour in any experiment with pure potassium cyanide and the added impurity is referred to this graph, from which the amount of
904
D. R. N A D U R N I AND S. M. MEHTA
KCN I
4!
KL
+ KI
10.83 g. EFFCCTIVE KCN +QUIHOL 01.03 g. ** ,I + 93
3.r
0'10 9.
EFFECTIVE K C N
KCN RENDERED INACTIVE BY OUINOL
E.
I
RENOFRED INACTIVEBY
0.a 9 .
m2.839.
t~
$9
+
VD
,,
I
II
m
0 . 1 9 - QUINOL 0.2 9.
0.83 g.KCN+NaCl
*.
0.3 9.
9*
FIQ.5 FIG. 6 FIQ.5 . POTASSIUM CYANIDE RENDERED INACTIVE BY POTASSIUM IODID OR ~ QUINOL, PLOTTED AGAINSTPUREPOTASSIUM CYANIDE FIG.6. POTASSIUM CYANIDE RENDERED INACTIVE BY SODIUM CHLORIDE OR POTASSIUM CHLORIDE
NEGATIVE CATALYSIS I N BENZOIN REACTION
905
pure potassium cyanide required to produce the same yield of benzoin is found; the cyanide rendered inactive by the impurity added is obtained by subtraction. THE DEACTIVATION O F POTASSIUM CYANIDE BY MEANS O F POTASSIUM IODIDE AND QUINOL
Figure 4 shows the quantities of potassium cyanide rendered inactive by various quantities of potassium iodide and quinol. With potassium iodide the quantity of cyanide rendered inactive increases very slowly with the potassium iodide added. Further, the amount rendered inactive by potassium iodide is proportional to the quantity of pure potassium cyanide present (figure 5, Curves 1, 2, 3). These results can be interpreted to mean that potassium iodide is adsorbed on the surface of the solid potassium cyanide, and thus inhibits the reaction. The amount adsorbed is in equilibrium with the saturated solution of potassium iodide in benzaldehyde, and therefore is directly proportional t o the amount of cyanide present. It is clear from the small quantities of potassium iodide required that little is adsorbed and little is required to saturate the benzaldehyde; about 90 per cent of the cyanide is rendered inactive by the potassium iodide. Quinol behaves in much the same way as potassium iodide. Increase in the amount of quinol increases the inhibiting effect rapidly at first, and then more slowly. The quantities of quinol required to deactivate the potassium cyanide are larger than with potassium iodide. It will be seen (figure 5 ) that up to a limit the quantity of potassium cyanide rendered inactive by a given quantity of quinol is proportional to the amount of potassium cyanide present. DEACTIVATION O F POTASSIUM CYANIDE BY MEANS O F POTASSIUM AND
SODIUM CHLORIDES
Potassium and sodium chlorides (figure 6) are less effective than potassium iodide and quinol and give results of a different type. The inhibiting effect increases with the quantity added. This may mean direct action between solid cyanide and solid chloride. The extent of the surface of the potassium cyanide rendered inactive will then increase with the quantity of the negative catalyst present, since it is more likely to be in contact with it. Presumably potassium and sodium chlorides are much less soluble in benzaldehyde than is potassium iodide. DISCREPANCIES I N PREVIOUS RESULTS
These results explain the discrepancies in the results of previous workers, regarding the possibility of obtaining benzoin from benzaldehyde and potassium cyanide only. Where it was not obtained, impure cyanide had
906
D . R. NADKARNI A N D 8. M. MEHTA
evidently been used. Lachmann’s result (1) that benzoin can replacc water in promoting the benzoin reaction can be referred to the fact that in the presence of benzoin the homogeneous autocatalytic reaction which is not affected by the inhibitor can proceed. The action of water in the presence of inhibitors is considered in Part 111. SUMMARY
1. It is shown that potassium and sodium chlorides, potassium iodide and quinol can inhibit the benzoin reaction. 2. They act by stopping the heterogeneous reaction, but they do not affect the homogeneous autocatalytic reaction. REFERENCES ’
( 1 ) LACHMANN: J. Am. Chem. SOC. 46, 719 (1924). (2) NADKARNI, D. R., .MEHTA,S.M., AND WHEELER, T. (1935).
S.:J. Phys. Chem. 39,727
