A USEFUL CONVENTION FOR CONSECUTIVE REACTIONS C. R. NOLLER Stanford University, California
FOR a11 chemical reactions except ionic resotions or simple displacement reactions, a series of steps or consecutive reactions is involved. If one wishes to indicate the steps in a reaction, it is customary to represent them either by a series of separate equations, repeating the formula of one product of the preceding reaction, or by continuing from one reaction to the next by means of an arrow without repeating the formula of the main product of the previous reaction. As illustrations of these two methods we may use the formation of an osazone from an or-hydroxyketone. Using the first method one writes
RC==NNHC6H, R D o RC=NNHCbHs R
L
-
+ CsH6NHNH2
+ c o H a H 2+ .Ha
RC=NNHCsHs d=NNHca.
RCO C.H,NHNH, RC=NNHCdis CJLNHNHI RAHoH ~ 2 0 R h o H csH,NH, xH, RC=NNHC& C~H,NHNH, R&=NNHCoHs +
+
R
L
H~O
RDNNHc6H6
This convention not only keeps the reactants and products close to each other without confusion, but centers the attention on the important intermediates in the reaction. It is useful particularly in representing the mechanism of organic reactions where the number of steps may be rather large a d reagents and minor products not readily distinguishable. As an illustra-. tion we may use this convention to represent the effective steps in the acid catalysis of the esterification of carboxylic acids by primary alcohols (cf. Hammett, "Physical Organic Chemistry," McGraw-Hill Book Company, New York, 1940, p. 357).
R
a. ]
0 HB R 4 4 H OH B-
*
[
+
b - ~2~ ~ ] r
R'OH
*
H
I+
OR' AH
+ Hx0
By the secondmetbod the reactions would read RCO RJHOH
+ C&NHNHa
-
RC=NNHCbHs
R&oH
+ H"0
HR -"-lo
This sequence states that equilibrium is reached by the following steps: A proton is transferred from the acid HB to an unshared pair of electrons on the carbony1 group of the carboxylic acid with the elimination of the base B-. The carboxyl carbon atom of the intermediate ion combines with an unshared pair of electrons of the alcohol molecule R'OH, giving another The first procedure is unambiguous but space-con- ion intermediate, which loses a proton to the base B-, suming on the printed page and time-consuming when forming the acid HB and a neutral intermediate. The writing on the blackboard. The second method is acid HB transfers a proton to an unshared pair of elecsomewhat shorter but may be confusing, especially to trons on a hydroxyl group of the neutral intermediate students, since the minor products of the reaction, which with elimination of base B-and the formation of another do not enter into subsequent reactions, are not readily ionic intermediate. Loss of water gives rise to a fourth distinguishable from the main product and from the ionic intermediate which loses a proton to the base B-, reagent of the subsequent reaction. All of these objec- regenerating the acid H B and producing the neutral tions can be overcome by adopting the simple oonven- ester. The reverse reaction represents the acid cattion of writing formulas for reagents consumed in the alyzed hydrolysis of the carboxylic ester, the reagents reaction above the arrow and for minor products of the consumed being below the double arrow and the minor reaction below the arrow. For example, products above the double arrow.
