The Mechanism of the Formaldehyde Clock Reaction Methylene Glycol Dehydration M. G. Burnett The Queen's University of Belfast, Belfast BT9 5AG, N. Ireland The formaldehyde clock (1-6) is the basis of several demonstrations andstudent experiments. The mechanism always quoted refers to the original proposal by Wagner (21,but the results which have gradually accumulated are not consistent with his theory, and the later authors (5,6) describe the reaction as "complex." Cassen, in particular, abandoned the theoretical interpretation and used the reaction to generate rate data for elementarv kinetic analvsis. In fact. the ratecontrolling rcwtim is extremely rin~ple,but it i, not the " b sulfitr ilddi~ion"\rhichisrill bein~uuutcdt5 -71.The, formaldehyde exists in the reaction m i i t h e as methylene glycol and the rate-controlling step is the dehydration reaction (1) CHz(OH12
-
HCHO + Hz0
(1)
The information which provides the basis of the theory of this reaction has been available since 1944 in Walker's mtmngrnph Ida). The usual cumrnrrciel foro~t l i iurmddehyde is il 40'i idutitm i n water cuntillninr ah(m1 Ill", i i l t t l l \ . l a1cw hol. Under these conditions therevis only about 0.i% free formaldehyde with ~ 3 0 methylene % glycol and a mixture of polyoxymethylene glycols HO(CH20),H. Modern NMR studies (9,101 of the system confirm this description of the equilibrium mixture under acid and neutral conditions. The hiaher uolvmers tend to be precipitated from aqueous soluti&, b& in the presence of methknol they rema& dissolved as hemiformal adducts, HO(CH20),0CHs. At room temperature the polymers dissociate over a period of hours, after dilution to 4% or less, forming methylene glycol by acid or base catalyzed pathways (11). Most of the solutions described in the formaldehyde clock experiments are, therefore, almost entirely those of methylene glycol. Even the strong solution used by Wagner (2) was probably mainly methylene glycol because he used an aoueous solution freshlv. .ureuared from . y a . i u ~ i imnaldchyde. The rate ot polymi.rizatim in stwh to the iulutions i i slow, having a halr-litr huuri, i.tml~;~red hydratiun r r w t i o ~tvith ~ a half-liic uf 10-1 svc 112r. The rate of the bisulrite-~~~rm~~l~lt~Iisil~~ .iddition is so much faster than the dehydration of methyiene glycol (13) that the rate-determining T I : I I rnuit inevitably be the Inttrr ilntlt:r tlmnaldehgde c l ~ wndirims. k l'ht- rw.tim h:ri 11c.vnatu~h(vt t h w a~mdititms.t h ~ x h by Bell I I21 under apprwarn:~tt~lv usunily with 3c,mrwh:~tI8,wer reitctant wncentrarim,. IIr uitd a varivt v d scavengers tu rernt,vc the rormaldt~hydeand a \cry wide range of acids and bases as Brginsted catalysts for the dehydration. In all cases, including that of the bisulfite-sulfite svs&m. he found that the reaction was zero order in scaveneer ~, and firit order i n rurmaldvhyde. Surm+n's h t r r m l ~ e r i m e ~ ~ t s (13)on the h s t ~ c ; ~ t , ~ l y zd cr w o m p o i i t i ~ ~dnt h e hisulfitr adduct nmfirmed these results. Purtherniorr, in buth mit..; P V idrncr w t ~ fuuna i that ~hecatalyticeirectul ihr biiuli~ti.ion wa> nryligihle in cumpariaon with that 01 t ht, sulfitv ion. This ~heoryis w p p m e d by the experiments descrihrd by ('i~ssvn+ I , although uni~~rtunately he made a thi.urcti~.:~l error, rt:ceutlv uer~etuatedh \ C'uokr 161, which dm.urt.d the real result. he clock time 7;s given by eqn. (2)
.=S,
b
,,,
Calculated values of bfrom lilralion Faint pink
blrnmol 1-'
blc
mmol I-'
211 1.24 0.12 211 21.1 1.24 0.12
14.7 14.7 14.7 1 1 1 1
196 1.12 0.10 O8 -8.2 1.02 0.13
(2)
Journal of Chemical Education
Maximum color
mmol I-' 204 1.27 0.15 -214 -18.8 1.29 0.16
hdi~~tor~how ane immediate d pink color with this concentration of rullife.
concentration of methylene glycol, b is the initial bisulfite ion concentration while x is its concentration after a time t . r=
(Ilkl) In a / ( o - b)
(3)
There is no need t o use the more complicated and rigorous algebra given by Jones (7) when analyzing these results because in the presence of an excess of bisulfite ions, they must be removed almost entirely for the indicator to change color. The situation mav be simulated hv titration. (Table 1).In practice. the clock ;,ol.rchangt, i
