Ketene Acetals. X. The Elimination of Hydrogen Bromide from the

S. M. MCELVAIN, ROBERT L. CLARKE AND GIFFIND. JONES

1966

[CONTRIBUTION FROM THE

LABORATORY OF ORGANIC CHEMISTRY

VOl. 64

OF THE UNIVERSITY OF WISCONSIN

Ketene Acetals. X. The Elimination of Hydrogen Bromide from the Acetals of a-Bromoaldehydes. Isopropyl- and n-Propylketene Diethylacetal BY S. LM.NCELVAIN, ROBERTL. CLARKE’AND GIFFIND. The preparation of ketene diethylacetal by the elimination of a molecule of halogen acid from the iodo- or bromo-aceta12 raised the question as to the applicability of this reaction to the a-bromoacetals of the higher homologs of acetaldehyde. From a purely statistical consideration, such an acetal as a-bromoisobutyraldehyde acetal (I) with six @-hydrogens would seem much more likely to lose hydrogen bromide between the aand one of the P-carbon atoms and yield the acetal of the unsaturated aldehyde. Similarly, the acetal of a-bromo-iz-butyraldehyde (11) contains two P-hydrogens to compete with the single hydrogen of the acetal carbon atom. I n the case of the acetal of a-bromoisovaleraldehyde (III), however, the single hydrogen in the p-position would indicate an even statistical chance that home of the ketene acetal as well as the unsaturated acetal would be formed. consideration of the electronic nature of the groups of I11 would seem to lead more surely to the prediction that the ketene acetal rather than the unsaturated aldehyde acetal would be produced by the elimination of hydrogen bromide. If the mechanism of the elimination reaction involves, as is generally believed, first the removal of a proton3 from the molecule by the base, followed by the emission of the bromide ion, then the inductive effects of the groups attached to carbons 1 and 3 of I11 should make the hydrogen on carbon 1 much more labile and consequently more likely to be removed with the bromine than the hydrogen on carbon 3 CH? ,,/Ob t ,CBrCH CH: ‘OEt

~