A tracer study has been made of acetal formation and hydrolysis. Benzaldehyde and la-butyraldehyde, enriched in 018, on reaction with 71-butyl and allyl alcohols, gave acetals of norn~al isotopic abundance. The label appeared in the water of condensation. Hydrolysis of benzaldehyde di-71-butyl acetal and nbutyraldehyde di-72-butyl acetal in 018 enriched water gave alcohols of normal isotopic content. r \ slight enrichment due to isotopic exchange \vas found in allyl alcohol produced by the hydrolysis of benzaldehyde tliall> l acetal. 'l'hese results establish that the reversible reaction proceeds by a fission of the aldehyde-carbon to osygen bond.'The rate-determining step in the acid-catalj-zed formation of an acetal from aldehyde and alcohol is generally assumed t o be the reaction of the second alcohol n~olecule with the conjugate acid of an interinecliate hemiacetal. In the reverse of this reaction, that is, in acetal hj-drolysis, the formation of the hemiacetal from the conjugate acicl of the acetal is collsidered to be the slo~v step (4).On the basis of the relative hydrolysis rates observed for a number of formaldehj d e acetals (12), Harnmett has suggested a unirnoleci~lar mechanism in \vhich there is formecl a carbonium ion intermediate derived from the alcohol component of the molecule (4). H!drol>sis rates, however, are much more sensitive t o structural changes in the aldeh) de component. For example, the follo\ving relative rates have been observecl i l l the l~>drolysis of acetals of pentaerj.thritol: formaldehyde, 1; acetaldeh)cle, (j X 10'; acetone, 1 X 10' (13). Ingold (5) has pointed out t h a t this kinetic effect can be understood if the carbonium ion is derived from the aldeh!de coinpoilent since then the methyl substituents \voulcl be joined direct11 to the electron cleficient carbon.This mechanism also accounts lor the ~n a r k e d l j~ greater ease of h j d r o l~ sis of acetals compared to orclinary ethers. l~lIairr~scr~pl receioed J r i~l e 14, 1 D Z .