overhead projector
edited by
DORISKOLB Bradley University Peoria, IL61625
Rate Acceleration by Polar, Aprotic Solvents Fred H. Greenberg S U N Y College at Buffalo
Buffalo, NY 14222 Most organic texts describe the use of polar, apmtic solvents to increase anion nucleophilicity and basicity. This effect of such solvents (e.g., DMSO) is demonstrated in the base-induced dimerization of 9-bromol (Aldrich no. B6,660-4)or 9-chlorofluorene2 to difluorenylidene, in which a n initially colorless solution becomes orange.
The three possible mechanisms for difluorenylidene formation' have the same first step4eprotonation at carbon 9, a s shown below. B-
+ H-F-X
BH + F-X
where H-F-X is 9-halofluorene. Pedagogically, it seems most useful to assume a n S N ~ displacement by 9-halofluorene anion on 9-halofluorene, thus giving a n intermediate that undergoes E2 elimination to generate the product? F-X
SNZ
+ H-F-X -+
B- + H-F-F-X
E2
-*
H-F-F-X
+ X-
BH + F=F + X
Demonstration Caution:9-Bramofluorene is reported to cause a blistering rash and allergic reactions. Handle with care.
These equations can be displayed on a n overhead transDarencv. Cutouts of alcohol can be s u ~ e r i m ~ o s earound d the anions of methanol and 9-halofluorke toshow the bar~ in the slower riers to Dmton transfer and S Ndis~lacement reaction. The absence of thesk barriers in the polar, aprotic solvent, which contains the unsolvated anions, is represented by simply lifting the alcohols from the transparency.
Place 25-30 mgof 9-halofluorene in a Petri dish or a 100mL beaker on the overhead projector. Add 3 mL of ethanol with swirling to effect solution, and add 2 mL of 1 M methanolic KOH to obtain a mixture that remains colorless. The same amount of substrate in another, similar vessel is treated a s above, except that a polar, apmtic solvent and 1M KOH in sulfolane-methanol (4:lvlv) is used. The solution becomes a deep orange. The color of difluorenylidene appears within seconds with 9-bromofluorene, while the chloro derivative requires about 2 min.
'Powell, E. W. Chern and lnd. 1967,2080. ZPreparedby the method of Hurd, C. D.: Mold, J. D. J. Orq. Chem. 1948, 13, 339. The mixture of fluorenol and HCI was allowed to stir for a week. 3Handoo. K. L.; Kanchan. G. Indian J. Chern. 1983, 226,909. Other possibilities are carbene dirnerization and carbene capture by 9-halofluorene anion followed bv loss of halide ion. This reference aescrloes tne preparation of dl1 :oreny odene in DMSO 'Ev oence for tnls rnecnan srn was obta neo by McCIJre. J D , Ph D Tnes s. Lnwers ly of Cn cago 1957
320
Journal of Chemical Education
