Gary M. Lampman,' Jennifer Andrews, Wayne Bratz, Otto Hanssen, Kenneth Kelley, Dana Perry, and Anthony Ridgeway
The Preparation of Vanillin from Eugenol and Sawdust
Western Washmgton State College Belhngham, Washmgton 98225
F o r t h e l a s t several years, o u r organic lahoratory classes have been isolating t h e fragrant liquid eugenol from cloves by s t e a m distillation ( I ) . It occurred to us that s t u d e n t s might b e interested i n converting eugenol to a n equally fragrant solid, vanillin. W e felt that s t u d e n t s would enjoy making t h e substance used i n ~. r e.~ a r-i vanilla na a n d , at t h e s a m e time, learn s o m e interesting chemistry. The process, shown in Figure 1,involves t h e basic isomerization of t h e douhle bond i n eugenol to give isoeugenol, and t h e n a n oxidation of t h e rearranged double bond to vanillin. In this paper, two procedures are described for the conversion of eucenol to vanillin: (1) a direct one-container reaction where the i&eugenol i s n o t isolated; (2) a two-step reaction where the isoeugenol is isolated as a n intermediate. In the first experiment eugenol, wdium hydruxidr, nnd nitn~henrencnredtawlved indimethyl iulfoxidc \l)hlS01and t h ~ m ~ x r u r e ~ s hrntrd to 120-1.39°(.'f~,r:I hr. Nnn,bewmc ir a urcful ux~dtnml:?cent widely used in structural studies involving lignins and lignin-like model eomoounds (2).It has iust the rieht oxidizine. .Dower and does not d r s t n ~ )the . phenolic functional y n m p llni~~rrunntdy. the m i datiun i i very slow, snd thu* the WdClNlII is usunll) mndutted in & sealed stainlrsi i t t r l tuhe at IT(, 18U0C 131.As nn cqwrimmt. the reaction was conducted in aqueous base with nitrobenzene in an open glassvessel to see if a satisfactory yield could he obtained. However, it was found that two days of heating were required just t o obtain a small amount of product. This two-day reaction period is unsatisfactory for a laboratory preparation and, in addition, the flasks were nearly destroyed by the base. By using the high boiling solvent, DMSO, mentioned above, the rates of isomerization and oxidation were increased significantly and satisfactory yields of vanillin could be ohtained. The time period is relatively convenient for a typical organic laboratory and the glassware is nnt damaged. The isolation and purification scheme for vanillin is shown in Figure 2. The process requires about two 3-hr labratary periods, with several blocks of time where other experiments may be conducted simultaneously. The use of sodium bisulfite for purification of an aldehyde as the addition product is one of the unique features of the scheme. The crude vanillin obtained after decomposition of the addition product is purified readily with cyclohexane which leaves the impurities behind. The yield is certainly not spectacular (5.3% yield), but the amount of the light-fluffy vanillin obtained (0.4 g) from 8 g of eugenol is large enough for a student to prepare the anil derivative, sublime a sample, and determine an nmr spectrum. The vanillin sublimes easily to give colorless needle-like crystals. In the second experiment, eugenol is first isomerized to isoeugenol and the isolated product is oxidized tovanillin in asecond step. The isomerization reaction is carried out for 1.5 hr a t 150°C with base in diethylene glycol solvent t o giw a 75% yield of completely isomerized product. The purity of the product is determined easily by nmr spectroscopy by observing the absence of a doublet for the benzylic methylene group in eugenol, and the appearanceof ndauhlet for the allylic methyl group in isoeugenol. Numerous oxidation methods were tried to convert isoeugenol to vanillin in high ~ i e l dIncluded . here are: (1) chromium trioxide or vanadium pentoxide with hydrogen peroxide in t-butyl alcohol ( 4 ) gives from 6% (Cr03 reaction) to 18% (V205 reaction) for 12-20 hr reaction times; (2) isoeugenol is converted to the acetate (49% yield) and oxidized with osmium tetroxide/sodium periodate ( 5 ) to give, after removal of acetate moUD in the work-uo orocedure. 23% vield of vanillin. The problems with these procedures are considerahl'. For example in the chromium trioxide/vsnadium pentoxide procedures, the reaction time is too long, and the preparation time is great because of the need t o standardize the peroxide reagents. Likewise, the osmium tetrovide reactions involve the use of this expensive and poisonous reagent.
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776 / Journal of Chemical Education
VANILLIN
ISOEUGENOL
EUGENOL
Figure 1. Preparation of vanillin from eugenol
Figure 2.Isolation and purification scheme for vanillin
(A) CllOH
CHO
0
Oil
+
tH3CH0
% CH3C02H
Figure 3. Mechanism of me oxidation of isoeugenol wRh nitrobenzene
Using nitrobenzene as the oxidizing agent, and the same procedure as described for eugenol, a 35% yield of nearly pure vanillin is obtained from isoeugenol. Colorless needle-like crystals are obtained from waterfNorit. The main limitation t o an increased yield is the rate of oxidation. However, the 3-hr reaction period is probably the maximum one would desire for a laboratory class. The mechanism of the oxidation of isoeugenol with nitrobenzene . shown in themechanism, it is necessary isshown in Figure 3 ( 2 )As t o have a paro-hydroxyl group on the ring in order to obtain the quinone methide intermediate (A). If the hydroxyl group is removed or methylated, the yield of vanillin drops tozero (6). It has been observed that nitrobenzene has extreme specificity for isoeugenol and materials related t o them such as lignins or lignin-like model comnnunds i2b 7). . Nitrobenzene is reduced hv a series of two electron transfers to nitrosobenzene, phenylhydrouylamine, and aniline. These intermediates are then converted to the condensation products 7
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.
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fornia, September 1976. 'Correspondence may be addressed to this author. Detailed experimental procedures are available upon request.
,
m,xyhrnzene, arohrnzene, and p-h>dnmgnmhenrenr t 2 h 1 Thrrc latter mntrrislsarr nut isdatrd in theexprr~menhdcscr~bed in this paper. Vanillin (actually the conjugate base) is quite stable in the strongly basic reaction mixture and does not undergo the Cannizaro reaction. In contrast, veratraldehvde (3.4-dimethoxvhenzaldehvdel , ~ readilv. undrrgwc the Cannizaru rrartmn undrr the samr eonditim-. The pnra.phen
