Odor Threshold and Structure

1. Odor Threshold and Structure G. Takeoka, USDA, 800 Buchanan St., Albany, CA 94710

28. Summary • For cyclic esters, the threshold reached a minimum with the cyclopentane and cyclohexane rings. • Ethyl cyclopentanecarboxylate and ethyl cyclohexanecarboxylate were very potent odorants with thresholds of 0.001 ppb. • The introduction of O, S, N atoms into the cyclopentane and cyclohexane rings dramatically increased the threshold. • In general, ethyl esters had the lowest thresholds compared to methyl, propyl and butyl esters.

29. Summary • In general, branched esters have much lower thresholds than their straight chain counterparts. • The odor thresholds reached a minimum with the butanoate and pentanoate esters. • Replacing methyl and ethyl branch substituents with mercapto- or hydroxy- groups resulted in higher thresholds. • Ethyl 2-methyl and 2-ethylhexanoate had higher thresholds than ethyl hexanoate.

30. Summary • In general, unsaturated branched esters had higher odor thresholds than their saturated counterparts. • In most cases, the addition of a double bond in the C-2 position of the acid resulted in significantly higher thresholds while addition of a double bond in positions further away from the acid carbonyl had a smaller effect on the threshold. • Exceptions were larger molecular weight esters such as butyl 2-methylbutanoate and ethyl 2-methylhexanoate. In these cases the addition of a double bond in the C-2 position resulted in a decrease in the odor threshold.

31. I would like to acknowledge and thank the Kansas City section of the American Chemical Society for their organization, administration and support of the Kenneth A. Spencer Award.I would also like to thank the Agricultural and Food Chemistry Division of ACS for their support.