Sam Carroll
45914613
CHEM3601
Prac 6
Reactions with Metallocene – Experiment 6
Aim
To prepare and analyse acetylferrocene and 1-(Ferrocenyl) ethanol by an acidcatalysed aromatic acetylation reaction, and an ethanol-catalysed reduction
reaction with sodium borohydride, respectively. Properties will be investigated
with Thin-Layer Chromatography (TLC), 1H NMR, and the capillary method for
melting point.
Experimental Flow Diagram
Part 1 – Acetylation of Ferrocene
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Sam Carroll
45914613
CHEM3601
Prac 6
Part 2 – Reduction of Acetylferrocene
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Sam Carroll
45914613
CHEM3601
Prac 6
Physical Data
Substance
MW
(g/mol)
Density
(g/mL)
Amt. Used
Mol.
Limit?
Acetic Anhydride
102.09
1.08
10 mL
0.105789
No
Calcium Chloride
110.98
Diethyl Ether
74.12
Ferrocene
186.03
Phosphoric Acid
98.00
Petroleum Ether
86.18
Sodium
Borohydride
37.83
0.389 g
0.010283
Acetylferrocene
228.07
1
0.004385
1-(Ferrocenyl)
ethanol
230.08
0.71
1.685
m.p. (°C)
b.p. (°C)
-73
138
772
1670
40 mL
0.383162
No
-116
34.6
3.0 g
0.016126
Yes
172
249
2
0.034388
No
40
158
N/A
30 - 60
No
>300
N/A
Yes
81
161
76 - 79
N/A
Procedure
Part 1 – Acetylation of Ferrocene
Ferrocene (3.0311 g; 0.0163 mol.) was added to acetic anhydride (10 mL; neat) in
a small round-bottomed flask, producing a dark red solution. The flask was closed
with a guard tube using calcium chloride before the dropwise addition of
phosphoric acid (2 mL; neat) while the flask was gently shaken. The solution was
then heated on a steam bath for 20 minutes, forming a light red solution with a
dark-yellow precipitate. After heating, the mixture was immediately transferred to
an ice-filled beaker. Once the ice in the mixture had melted, sodium bicarbonate
was added until the pH was neutral, which was observed with periodic swabbing
with universal indicator strips. The addition of sodium bicarbonate yielded a
large, brown frothy bubbling mass as carbon dioxide was released from the
neutralisation reaction.
The mixture was chilled in ice for 20 minutes before vacuum filtration to separate
the dark-yellow precipitate (acetylferrocene). The precipitate was dried in a
vacuum desiccator for 15 minutes at 63°C prior to recrystallisation with
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Sam Carroll
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CHEM3601
Prac 6
petroleum ether (~50 mL; neat). The reaction yielded small black crystals at the
nucleation sites.
A TLC was performed and melting point was recorded.
Part 2 – Reduction of Acetylferrocene
Acetylferrocene (0.4065 g; 0.0018 mol.) was added to ethanol (15 mL; neat) and
water (5 mL; deionised) in a 100 mL conical flask. Aqueous sodium borohydride
(0.3890 g (aq); 0.0103 mol.) was slowly added to the acetylferrocene solution
while stirring. The solution was stirred for 15 minutes after completely adding
sodium borohydride, and the solution began to turn a pale yellow. The mixture
was transferred to a 250 mL separating funnel with water (100 mL; deionised).
The aqueous mixture of 1-(Ferrocenyl)ethanol was extracted with diethyl ether
(40 mL; neat). It was at this point that we ran out of time.
Following this, the aqueous phase was to be dried over magnesium sulfate before
removal of the drying agent and solvent in a rotary evaporator. Recrystallisation
would have been performed with petroleum ether and dried in a vacuum
dessicator.
The 1H NMR spectra was recorded for a separate product from the same
experiment, which is visible in figure 3.
Mechanism
Figure 1a – Acetylation of Ferrocene (Simplified)
The figure below outlines a general principle for how the acetylation of ferrocene
occurs. It is relatively similar to acetylation of any non-substituted aromatic
compound, however there is a significant difference in reactivity between
ferrocene and benzene. This is explored in figure 1b.
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Sam Carroll
45914613
CHEM3601
Prac 6
Figure 1b – Acetylation of Ferrocene vs. Benzene
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Sam Carroll
45914613
CHEM3601
Prac 6
The acetylation reaction occurs 3×106 times faster with ferrocene than benzene.
Given that the acetyl cation is an electrophile, higher electron density in the
aromatic ring is favourable for this reaction.
The ligand between the metal centre and η5-cyclopentadienyl is anionic, which
increases the electron density in the aromatic ring by withdrawing an electron
from the metal centre. In this respect, the iron atom is behaving like an activating
group. This provides one extra delocalised electron in the aromatic centre, which
can help stabilise the ring while it undergoes substitution (seen on the right in
figure 1b).
An unsubstituted benzene molecule lacks this property and as such, the reaction
progresses slower.
Figure 2 – Reduction of Acetylferrocene
The reduction of acetylferrocene with sodium borohydride, ethanol, and water
progresses in a similar manner to reduction reactions that lack a metal complex.
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Sam Carroll
45914613
CHEM3601
Prac 6
Calculations
Theoretical
Observed
Step 1
Limiting reagent: Ferrocene (186.030 g/mol)
Used: 3.0311 g
3.0311 g ÷ 186.030 g/mol = 0.0163 mol
Stoichiometry is 1:1, thus:
acetylferrocene = 0.0163 mol
Acetylferrocene = 1.432 g
0.0163 mol × 228.070 g/mol = 3.716 g
Actual yield = 1.432 ÷ 3.716
Theoretical Yield: 3.716 g, Acetylferrocene
Actual Yield = 38.5% (crude)
Step 2
Limiting reagent: Acetylferrocene (228.070 g/mol)
Used: 0.4065 g
0.4065 g ÷ 228.070 g/mol = 0.0018 mol
Stoichiometry is 1:1, thus:
1-(Ferrocenyl)ethanol = 0.0018 mol
1-(Ferrocenyl)ethanol = N/A
0.0018 mol × 230.08 g/mol = 0.4101 g
Actual yield = N/A
Theoretical Yield: 0.4101 g, 1-(Ferrocenyl)ethanol
Actual Yield = N/A
Discussion and Conclusion
The melting point of the first product was recorded at 79 – 81°C. This result is
slightly lower than the literaturei but within the margin of error, indicating that
the product was successfully synthesised (1.432 g; 38.5% yield).
Acetylferrocene yield was quite poor compared to other experiments. Newirth and
Srouji (1995, p 455) demonstrated up to 68.1% yield when using a Friedel-Crafts
acylation, as opposed to an acid-catalysed acetylation as seen in this experiment.
This suggests that despite the increased electron density in the aromatic ring due
to the iron-complex ligand, the substitution reaction still progresses faster with a
more stable intermediate for the substituting group; in this case, the ethenone
cation.
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Sam Carroll
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1H
CHEM3601
Prac 6
NMR spectra for both products are visible in figures 3a and 3b, which have
been labelled with reference to the molecular structure. Acetylferrocene is a
relatively symmetrical molecule and consequently does not show much splitting in
figure 3a. There is a shift between the aromatic protons closer to the carbonyl
group due to its electron-withdrawing effect, which de-shields the protons further
away.
Figure 3a – 1H NMR Spectrum: Acetylferrocene
Figure 3b shows highly complex peaks as a result of shimming artefacts, but the
structure of 1-(Ferrocenyl)ethanol can still be seen. The alcohol group in this
molecule is considerably less electron-withdrawing than the carbonyl group in
acetylferrocene, thus the spectrum is more cluttered as the protons on both
aromatic rings are much more equally shielded. Additionally, the alcohol group
appears to have a weak inductive effect with the bonded cyclopentadienyl group,
shielding the adjacent protons more than the unsubstituted cyclopentadienyl ring.
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Sam Carroll
45914613
CHEM3601
Prac 6
Figure 3b – 1H NMR Spectrum: 1-(Ferrocenyl)ethanol
With 8 valence electrons in its native state and a net contribution of 5 electrons
from each cyclopentadienyl group (6 e- with an anionic ligand, for a net
contribution of 5 e-), both products have a valence electron count of 18 e-. Due to
the effect of the anionic ligands from both cyclopentadienyl groups, the oxidation
state of iron in both complexes is 2+ i.e., Fe(II).
Despite challenges, the synthesis and analysis of acetylferrocene was a success.
Unfortunately, time constraints prohibited the researchers from finishing the
recrystallisation and analysis of 1-(Ferrocenyl)ethanol. Future research could
consider adapting this experiment to utilise a Friedel-Crafts mechanism, as it
appears to produce higher yields.
References
•
Newirth, T.L. and Srouji, N. (1995) “Acetylation of Ferrocene”, Journal of
Chemical Education, Vol 72(5): 454-56
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Sam Carroll
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•
CHEM3601
Prac 6
Sigma Aldrich, Acetylferrocene; Product No. 106860; SDS Version 6.1, Revised
29/11/2019, Viewed 5 November 2021:
https://www.sigmaaldrich.com/AU/en/sds/aldrich/106860
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