Recently, reactions in supercritical carbon dioxide (critical temperature Tc = 304.3 K; critical pre — Organic Chemistry Chemistry Question
“Green” Chemistry – The Use of Supercritical CO2
Recently, reactions in supercritical carbon dioxide (critical temperature Tc = 304.3 K; critical pressure pc = 72.8×10^5 Pa) have received significant attention. The density of a liquid can be easily tuned near the critical point. Moreover, it can be regarded as a “green” solvent that can replace organic solvents. This solvent has actually been used for caffeine extraction for quite a long time. The fact, however, that carbon dioxide has to be compressed is one of the few disadvantages.
Real gases can be described by the van-der-Waals equation (although it is still an approximation):
constants for CO2:
a = 3.59
b = 0.0427 dm^3 mol^–1
Calculate the energy needed to compress carbon dioxide from 1 bar to 50 bar (final volume is 50 cm^3, 298 K, ideal gas).
Model Answer
dW = –n R T dV / V or W = –n R T ln(p1/p2)
n = pV / R T = (50×10^5 Pa × 50×10^–6 m^3) / (8.314 J K^–1 mol^–1 × 298 K) = 0.10 mol
W = – 0.10 mol × 8.314 J K^–1 mol^–1 × 298 K × ln(1/50) = 969 J
Calculate the pressures needed to achieve a density of 220 g dm^–3, 330 g dm^–3, and 440 g dm^–3 at temperatures of 305 K and 350 K.
Model Answer
The calculation can be most easily carried out with the molar volume Vm = M / ρ.
The equation (V – n b) = n R T can be simplified to (Vm – b) = R T
Example of the calculation (density ρ = 440 g dm^–3 or Vm = 0.10 dm^3 mol^–1; T = 305 K):
· (0.1×10^–3 m^3 mol^–1 – 0.0427×10^–3 m^3 mol^–1) = 8.314 JK^–1 mol^–1 × 305 K
p = 83.5×10^5 Pa
ρ (g dm^–3) | Vm (dm^3 mol^–1) | T K^–1 | p · Pa^–1
--- | --- | --- | ---
220 | 0.200 | 305 | 71.5×10^5
330 | 0.133 | 305 | 77.9×10^5
440 | 0.100 | 305 | 83.5×10^5
220 | 0.200 | 350 | 95.2×10^5
330 | 0.133 | 350 | 119.3×10^5
440 | 0.100 | 350 | 148.8×10^5
Properties, such as the solvent power of carbon dioxide and the diffusivity of reactants, are strongly dependent on the density of the fluid. The calculation in the previous task shows that the density can be tuned by pressure variations.
In which region can these properties of the fluid be tuned more easily – near the critical point or at higher pressure / temperature (consider the critical constants and the results of 5.2)?
Model Answer
The results in the table above show that a 10 bar change in pressure near the critical temperature results in nearly double the density. Far above the critical temperature, however, such a change requires higher pressures. Hence, it is useful to work near the critical temperature/pressure.
The oxidation of alcohols by molecular oxygen in supercritical carbon dioxide, e.g. the oxidation of benzyl alcohol to benzaldehyde, is a supercritical process. The reaction takes place in the presence of a Pd/Al2O3 catalyst with a selectivity of 95 %.
a) Write down the balanced reaction equation of the main reaction path.
b) Which reactions can occur during further oxidation (except total oxidation)?
Model Answer
a) Main reaction: C6H5–CH2OH + ½ O2 C6H5–CHO + H2O
b) Side reactions:
C6H5–CHO + ½ O2 C6H5–COOH (Acid)
C6H5–COOH + C6H5–CH2OH H2O + C6H5–CO(OCH2–C6H5) (Ester)
The use of carbon dioxide both as a solvent and as a reactant instead of phosgene or carbon monoxide is another example of supercritical processes. Both the catalytic formation of organic carbonates and formamides have already been described.
a) Write a balanced equation of the formation of dimethyl carbonate by the reaction of methanol with carbon dioxide. How can dimethyl carbonate form if phosgene is the reactant?
b) Formyl–morpholine can be synthesized from carbon dioxide and morpholine using an appropriate catalyst. Which additional reactant is needed? Write down the reaction scheme.
How would the scheme change if carbon monoxide was used instead?
Model Answer
a) CH3OH + CO2 CH3O–CO–OCH3 + H2O
CH3OH + COCl2 CH3O–CO–OCH3 + 2 HCl
b) C4H8ONH + CO2 + Red C4H8ON–CHO + Red–O
The reaction requires a reducing agent, e.g. hydrogen, hence:
C4H8ONH + CO2 + H2 C4H8ON–CHO + H2O
C4H8ONH + CO C4H8ON–CHO
From the point of view of “green chemistry” – why should reactions be carried out in CO2 instead of using carbon monoxide or phosgene (2 reasons)? Apart from the compression of carbon dioxide, what is the main obstacle in using CO2 as a reactant in comparison to CO or COCl2 (1 reason)?
Model Answer
The advantage of using carbon dioxide is that it is not poisonous in contrast to carbon monoxide and phosgene. CO2 makes the process safer. Moreover, using CO2 both as a reactant and as a solvent is advantageous, since no additional solvent is necessary.
Another reason may be the reduction of the CO2–emission, but this will not be significant.
One of the disadvantages is that CO2 is much less reactive than CO or COCl2 – therefore a search for suitable catalysts is inevitable (catalysts have been found only for a few reactions, such as the formylation of amines).