A Reinvestigation of the Synthesis of 4-Methyl-3-heptanol It was reported earlier in T H I S JOIJRNAL that 4-methyl-3-heptanal, 111, one of the components of the aggregation pheromone of the European elm bark beetle, could be synthesized by the Grignard reaction between 2-pentyl magnesium bromide and propanal. This alcohol can be oxidized to 4-methyl-3-heptanane, 11, the alarm pheromone of the harvester ant.' We have found that the reported g a chromatographic separation of the 4-methyl-3-heptanol diastereomers is in error, and that side reactions in the Grignard reaction account fur the observed results. Experimental
The preparation of 4-methyl-3~heptanol,III,was carried outas described previously.' The crude product, (noz5 = 1.425 (lit3 nnZ5= 1.4301, bp.e~o142-148°2) had an infrared spectrum that matched that published for 111 with the exception of a small carbonyl band a t 1710 cm-'. Gas chromatography (2 m phenylsilicon, DC-704, 80') revealed two components in a ratio of -l:l.5 which had been assigned as the diastereomeric 4-methyI-3-heptan0ls.~Analysis on a 2 m Carbowaxm 20M column a t 120' showed that there were actually three components present in the crude product. These were collected and identified by their IR, NMR, and mass spectra. The first component, I, was the pentane dimer 4,5~dimethyloctane(mle = 142, base peak 71;multiplets 1.28 and 0.886 in the NMR; only hydrocarbon bands in the IR). The second component, 11, was identified as 4-methyl-3-heptanane, 11, by mle = 128 and spectral comparison with an authentic sample. The major component was 4-methyl-3-heptanol,111, by m/e = 130 and the identity of its IR and NMR spectra with publish d ones.3."
t
CH2 0 CH3CH&HsCH-CHCH9CH,CH,
I
I
I
CHSCH2CH,CH-CCH,CH,
CH,
OH
1
1
CH,CHsCH3CH4HCH,CH,
11
m
Discussion
Contrary t o earlier reports, the diastereomers of 4-methylL3~heptanolare not separable by gas chromatography. The several GC peaks observed in the Grignard preparation of 111 are different products whose origins are easily understood. The hydrocarbon 1 results from magnesium-induced coupling of 2-bromapentane, a common side reaction in Grignard p r e p a r a t i ~ n sKetone .~ 11apparently arises from air oxidation of alcohol 111during distillation or from Oppenhauer oxidation of the magnesium alkoxide of III by excess propanal. The yields of I, 11, and 111varied slightly. Yields of Iranged from 1&25%, I1 was about 13% and alcohol I11 was moducrd in fi0-7O%.
uets.
The formation of hydrocarbon I presents a vexing problem since it has a bailing point similar to that of the alcohol 111 and ketone I1 and cannot be readily separated by distillation. I t is alsoinert to the alcohol oxidation reaetion, so the ketone I1 produced from the Jones oxidation of alcohol 111is likewise contaminated with I. By carefuldistillation, ketone I1 of 95% purity was obtained for testing on harvester ants.
' Einterz, R. M., Ponder, J. W., and Lenax, R. S., J. CHEM.EDUC.,54,382 (1977). Normal atmospheric pressure at 3950 ft above sea level. Pouchert, C. J., "The Aldrich Library of Infrared Spectra," Aldrieh Chemical Ca., Milwaukee. WI. 1970. D. 66 for 111 and p. 88 for IL Pouchert, C. J.,and Campbell, J R., "The Aldrich Library of NMR Spectra, Vol. I," Aldrich Chemical Co., Milwaukee, WI, 1974, p. 92A. Buehler, C. A., and Pearson, D. E., "Survey of Organic Syntheses," Wiley, Interscience, New Yark, 1970, pp. 22-23. Robert V. Hoffman', M.D. Alexander, Gregory Buntain, Richard Hardenstein, Cynthia Mattox, Susan McLaughlin, Denise McMinn, Scott Spray, and Steven White New Mexico State University La5 Cruces. NM 88003
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Journal of Chemical Education
