Acetylation of Ferrocene
A Study of the Friedel-Crafts Acylation Mechanism as Measured by HPLC
Using an Internal Standard
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~rtant skills 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,
i.e.. solve a chemical mvsterv. We examine the acetvlation
.
.
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:
1.
Is
aluminum chloride really a stoiehiametric reagent in
F-C
acylation,
or
is
it a catalyst
as
in F-C alkylations?
why?
3.
Will the acetylation of the first ring of ferrocene affect the
rate
of
acetylation
of
the second ring?
Is
there communication between the two rings
of
ferraeene?
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,
(14,
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.
Table
1.
Conditions
for
the Acetylation of Ferrocene
(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
