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Material Type: Lab; Class: Inorg Chem Lab; Subject: Chemistry; University: University of Wyoming; Term: Unknown 2006;
Typology: Lab Reports
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Terry L. ~ewirth'and Nadine Srouji
Haverford College, Haverford, PA 19041
Most of us who became chemists were hooked by the ex- citement of discovery in the laboratory. Organic chemistry labs are designed to demonstrate and teach new tech- nianes used in organic svnthesis or to svnthesize mole- cul'ri, which serves to e&nplify reartiins discussed in clais and also teach irn~i~rtantskills While somc students are thrilled with the idea of creating new molecules; most see organic lab as a cookbook whose recipes they can't wait to complete. For them the excitement of a scientific discov- ery is missing. We have developed a novel approach to the acetylation of ferrocene, which allows students to study an organic reaction mechanism and with their experimental data answer some fundamental auestions about electro- philic substitution reactions on the ferrocene ring system,
of ferrocene, using acetyl chloride and aluminum chloride as acetylating agents, an example of classic Friedel-Crafts (F-C) acylation, to answer three fundamental questions about F-C acylation in general and F-C acetylation offer- rocene in particular:
why?
Use of the acetylation of ferrocene in the undergraduate laboratory curriculum is not new. It is an experiment popu- larly used to exemplify F-C acylation, ( 1 4 , sometimes as a followup to the synthesis of ferrocene (7). Because the acetylation yields two colorful products, clearly distin-
IAuthor to whom correspondence should be addressed.
(Relative Equivalents)
Ferrocene Acety chloride Aluminum Chloride
guishable from the ferrocene starting material, the reac- tion also has been called upon to demonstrate several im- portant separation techniques, including column chroma- tography, (8, 9) dry-column chromatography, (10) thin-layer chromatography, (11) and high performance liq- uid chromatography (12, 13)While several of these tech- niques also are used in this experiment, they serve only as a means to an end: exploring the mechanism of the reac- tion. We also employ the internal standard technique to quantify our results, which none of the above references describes. The experiment as outlined below allows students to reach conclusions about the mechanism of F-C acetylation of ferrocene based on their own data analysis. In one case they can verify an accepted mechanism; in another, the re- sult will be unexpected and will raise new interesting questions. We divide the class into five groups, each of which per- forms the acetylation using one of several different ratios of ferrocene: acetyl chloride: aluminum chloride (see Table 1)(1). We now perform the reaction, with microscale tech- niques using 200 mg ferrocene and a 30-min reaction time, analyzing the products using reverse phase HPLC (13) with an internal standard, as well as dry column chroma- tography, with results consistent with and more precise than our data from past years when we used macroscale
454 Journal of Chemical Education
Experiment 4 Reference 3
Table 2. Tabulated Class High Performance Liquid Chromatography (^) even though a full equiva- and Dry Column Chromatography Results (^) lent of acetyl chloride was of Products from the Acetylation of Ferrocene (^) used. Condition E confirms that 1,l-'diacetyl ferrocene c a n h e produced if two F :AcCI :AlCh Ferrocene Acelylferrocene 1.1'-Diacetylferrocene equivalents of acylating % Recovery (SD) %Yield (SD) %Yield (SD) agent are present.
COI hplc col hplc col O n e m i g h t expect "C" and "D," which both have 5.96.2) 68.1 (8.31 41.4(12.5) 9.314.61 7.4621 one eauivalent of active
hplc A 1 : 1: 1 7.0 (3.6) B 1 : 1 : 0.5 51.4 (4.1) C I : 2 : 1 4.4 (8.3) D I : : 17.9(6.1) E 1 : 2 : 2 6.8 (10.8) Per cent yields are based on ferrocene.
36.0 (12.5) 38.5 (1.9) 26.2 (4.37) 2.3 (2.1) 0.54 (1.1) a c e t ~ l a t i n g r e a g e n t ,
give results similar to A.
4.5 (7.8) 15 (7.6) 11.3 (8.9) 56 (15) 41.6 (9.7) give the same r e s k t s as A, t h e r e s u l t s from D a r e clearly very different, dem- onstrating that a n excess of aluminum chloride, unexpectedly, does affect the product distribution. (When the AlC13 used is not fresh the results from C also can a m e a r to be different from A. with a sie- nificantly higher g e l d of acetyl ferrocene thanbne finds l'n A,) These results. however. should ~ i a u ethe curiositv of
HPLC trace of diacetylferrocene (RT = 4.465), acelylferrocene(RT = 6.056), benzophenone (RT = 9.130), and ferrocene (FIT = 15.817). For HPLC conditions, see text.
quantities (2 g ferrocene and 60-min reaction time) and only column chromatography analysis. The results of class data for each microscale condition, with benzophenone a s the internal standard. are shown in Table 2. Use of high performmce liquid chromatography [HPLCI is to some extent able to address analytical problems of column chromatographic analysis, and the overall mass balances are much more consistent and about 50% higher t h a n with column chromatography, but, i n fact, either
pecially if the yields are normalized for the total mass bal- ance. Condition A answers the ring communication ques- tion, as one observes acetyl ferrocene a s the preponderant product, where no communication would predict a n acetyl ferrocenel1,l'-diacteyl ferrocene ratio of 2/1, i.e., the u i -
tive as ferrocene. Condition B answers the question of the catalytic nature of aluminum chloride. I n fact, condition B produces close to half the acetyl ferrocene of condition A,
students to ask &hy we see such different results for Eon- dition D. We ask students to discuss the three original posed ques- tions i n their lab reports i n light of the class results. For condition D we offer the students two possible hypotheses and ask them i n their r e ~ o r t sto sueeest exoeriments which could test ench. One'hypothesis for the u'uexpccted vlcld of diacetvl ferrocenc from cnndirion .'IY is the tbrma- kon of a more Hctive and, therefore, less-selective acylating agent i n the presence of excess aluminum chloride.- (Additional information about this experimentally un- s u ~ ~ o r t e d .A hvoothesis i s available from the authors.) The second (and experimentally supported) hypotheses is that i n the presence of excess AlC13,theHCl formed from the initial acetylations, acts i n concert with the uncom- plexed AlC13 to protonate ferrocene to form an unreactive (ferrocene-H)+ (tetrachloroa1uminate)-. This effectively re- moves ferrocene from the reaction solution, allowing acetyl ferrocene to he acetylated by default (14, 15). [Rosenblum et al. (15)have shown that HC1 added to a methvlene chlo- ride solution of ferrocene containing a suspension of AlCI will result in a r a ~ i ddissolution of the AlCI? with the for- mation of a clear orange solution. Adding aceiyl chloride to such a solution results in verv low ~ e r c e n t a e eof total ace- tylation but a high ratio of d i a c e t ~ ~ a c e t ~ lfirrocene. Con- versely, if measures are taken to remove the HCI a s i t is formed in the ferrocene acetylation reaction, normal prod- uct ratios are observed, even in the presence of two-fold excess ofA1C13.The (ferrocene-HI+ (tetrachloroa1uminate)- complex also was isolated and analyzed.] Furthermore, we tell the students that there are three hypothesized structures for the "protonated" ferrocene and ask them which is consistent with the published rl6, NhlK:
I 1 ,- (^) ..,. H I. The lab can be simplified by including only conditions A and B, and addressing only the issues of communication and catalysis.
Procedure
Reaction
For the microscale condition A, the procedure of Mayo et al. (3) is, in general, followed. To a tared 10-mL round-hot- tomed flask;containing a small magnetic stir bar, 150 mg (1.12 mmole) anhydrous aluminum choride is weighed, fol- lowed by 4 mL freshly distilled methylene chloride.
Volume 72 Number 5 May 1995 455