D. T. Haworth and T. Liu Marquene University Milwaukee. Wisconsin 53233
I1
I
Acetylation of Ferrocene Monitoring a chemical reaction by high pressure liquid chromatography
T h e acetylation of ferrocene (Fc) with acetic anhydride in the presence of phosphoric acid (1) has been followed by elution chromatography using an absorbent of acid-washed alumina ( Z ) , by thin-layer chromatography using Silica Gel a s the absorbent ( 3 4 ) ,and by dry-column chromatography (5).We wish to describe how HPLC can he used to follow the course of this reaction (eqn. (1)) Fe(C5Hs)z 2 Fc(C0CHd + Fc(COCHS)~ (1) ferroeene H ~ P O +I-aeetylferrdcene 1,l'-diacetylferrocene Experimental Ferrocene (2.3 g) is dissolved with agitation in 65 ml of acetic anhydride in a 100-miround bottom flask. The flask is cooled in an ice bath, keeping the inside temperature at 25'C while 5 ml of 85% phosphoric acid is added dropwise. After completion of this addition, the flask is suspended in an oil bath at 50°C. At 15-min intervals a 3-ml sample is removed from the reaction mixture, poured on ice in a 100-ml beaker, neutralized with excess NaHC03 (solid) and the aqueous solution extracted with 50 ml of ether. After drying and evaporating the solvent on a steam bath, the solid sample is dissolved in chloroform to a volume of 25 ml. Six samples are removed from the reaction mixture and worked up in this manner. The above prqedure was repeated at temperatures of 60 and 75'C. Generation of Detector Signal A 8-pl sample from the 25-ml chloroform solution is injected into s liquid chromatograph under the following conditions:a 12-in.column packed with 10-pm Silica Gel (Applied Science Lab.), flow rate of 1.0 mllmin of an ethermethanol (10:l) mixture, 500 psi, ambient temperature, uV-detector set at 254 nm. (ALC-2001401). Peaks with retention times (T,) of 1.32,2.40,and 4.68 min are due to ferrocene, 1-acetylferrocene,and 1,l'-diacetylferrocene, respectively. A typical chromatograph is shown in the figure.
Response factors are given b y eqn. (4)
where A1 and A* are the measured areas of the solute 1and 2 peaks, respectivelv. . WI- and Wz are the concentration of the solute 1 and 2, respectively, a n d f l is the response factor assigned to the solute 1.In this experiment, the response factors are obtained by analyzing a standard sample which is composed of the same weight (25 mg in 25 ml CHCld of Fc, 1acetylFc, and 1,l'-diacetylFc, where the subscripts 1, 2, and 3 are assigned t o these compounds, respectively. Therefore, from the staidard sample WI = Wp = W3, and f1 is arbitrarily assigned a value of one. Equation (4) is then simplified for f2 a s A>/A?and for f s a s ATIA?.Usine the above procedure, the area; f o ; ~ , , A2, i d ~ 3 a r e k e a s G e dand fp istaken as 0.528 and f? a s 0.308. Bv eon. (2). the corrected areas (A') can now be obiained and horn eqn: (3) the weight percents in each sample are thus calculated. Twical data are shown in the table .. for the acetylation a t 60% In using various 8-fi1 samples of t h e standard solution the ratio of the measured areas AQto A2 was in the range of 1.7-1.8 to 1. This is also the ratio of f2 to fs. Results and Discussion The optimum condition for t h e conversion of ferrocene t o 1-acetylferrocene a t the three temperatures studied is 60 min a t 50°C. At this temperature most of the product is the monoderivah e . At longer times (or higher temperatures) the 1-acetvlferrocene is readily converted into 1,l'diacetylferrocene. Ferrocene was only detected in the first three aliquots taken a t 50°C; only in the first aliquot a t 60%; and not a t all a t 7 5 T . At temperatures of 60°C and 7 5 T , a small peak occurs between the peaks of the two isomers. This peak is probably due t o other isomers (the fig.).
Conversion of Peak Area to Composition T h e peak areas obtained hy H P L C are calculated by the triangle formula, A = HWIIZ and are usually not related to composition (6). This is because the response of the uv detector depends on the absorbance of each compound a t a specific wavelength. Normalization is then used t o obtain the corrected area (A'). This area is obtained hy multiplying the measured area (A) by a response factor (f) for each particular compound (eqn. (2))
(2)
A'=AX/
HPLC chromatogram of acetyletion of fenocene at 60°C aner 75 min using a mobile phase of ether:methanol (10:l)at a flow rate of 4.0 mll
Then, the weight percentage can be calculated a s depicted in eqn. (3)
I 0
I
I
I
I
2
4
6
8
TIME-MIN
min. (1)chloroform. (2) 1-acelylferrocene.(3) other diacelylfenocene isomers. (4) 1,l'diacetylferrocene.
Data Analvris on the AceNlation of Ferrocene a t 60°C
S~mple
Measured area A,
4
0.32 0.00 0.00 0.00
5
0.00
4.80 5.56 4.80 4.90 4.20
6
0.00
4.24
1
2 3
Itd --
P
A,
number
1.12
(cm?
2.12
730 / Journal of Chemical Education
.
A3
0.00
0.30 0.38 0.83 1.10 1.80 3.64
Relative Amount
Corrected area
Corn~orition( w t %)
(cm2)
2 ,'
A ,'
A?
FC
0.32
2.53 2.94 2.53 2.59 2.22 2.23 1.12
0.00
11.2 0.0 0.0 0.0 0.0 0.0 33.33
0.00 0.00 0.00 0.00
0.00 1.12
0.09
0.12 0.26 0.34 0.55
1.12
3-acFc
1.1'-diac~?
88.8 97.0 95.5
4.5
80.2
13.3 19.8
90.9 86.7 33.33
0.0 3.0
9.1
33.33
1.1'-diacFc
...
32.3
21.2 10.0 6.5 4.1 1.00
The number shown in the last column of the table is the weight ratio of the mono to disubstitukd product. Because the reaction flask contains both dissolved and undissolved ferrocenes and aliquot8 of this solution are saturated with the ferrocenes, one can only measure a relative amount of the 1-acetyl-ferrocene to the 1,l'-diacetylferrocene.
Literature Cited ill ~
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