The discovery of crown ethers was made by Charles Pederson in 1967. He was rewarded with a share of — Analytical Chemistry — Spectroscopy Chemistry Question
Crown Ether and Molecular Recognition
The discovery of crown ethers was made by Charles Pederson in 1967. He was rewarded with a share of the 1987 Nobel Prize for chemistry with Jean–Marie Lehn and Donald Cram for their contributions to supramolecular chemistry.
The following schematic diagram shows a synthetic route to the linear diol A.
[VISUAL]
However, because of the presence of catechol in the starting material, the resulting product was a mixture of A and the minor product B.
The molecular weight of B was found to be 360.1 and it had an elemental composition of C : H : O = 66.5 : 6.7 : 26.6. The 1 H NMR spectrum of B showed four sets of proton signals. Two of which were observed at δ 7.0 – 7.5 and the other two at δ 3.7 – 4.2. The integration ratio of the four sets of signals was 1 : 1 : 2 : 2. Compound B strongly binds with the potassium ion. A striking example is the use of B to assist the dissolution of KMnO4 in benzene to give a purple coloration.
What is the chemical structure of B?
Model Answer
B: [VISUAL] (Dibenzo-18-crown-6)
What is the major function of H3O+ in the above reaction?
(a) To activate ClCH2CH2OCH2CH2Cl.
(b) To neutralize NaOH.
(c) To remove the tetrahydropyran group.
(d) To act as a buffer to control the pH of the solution.
Model Answer
(c) To remove the tetrahydropyran group
The following is the synthetic route to [2.2.2]cryptand:
[VISUAL]
Draw the structures of C – F.
Model Answer
C: [VISUAL] (Cl-CH2-CH2-O-CH2-CH2-O-CH2-CH2-Cl, 1,8-dichloro-3,6-dioxaoctane)
D: [VISUAL] (1,4,10,13-tetraoxa-7,16-diazacyclooctadecane, diaza-18-crown-6)
E: [VISUAL] (1,8-diamino-3,6-dioxaoctane)
F: [VISUAL] (cryptand-2.2.2 precursor diamide)
Why are high dilution conditions required in the synthesis of D from C?
(a) Since the reaction between C and the diamine is highly exothermic, a high dilution condition is used to dissipate the heat released from the reaction.
(b) A high dilution condition is employed in order to inhibit oligomer or polymer formation.
(c) Thermal equilibrium is favored to give D in higher yield under high dilution conditions.
(d) The solubility of the starting material is low.
Model Answer
(b) A high dilution condition is employed in order to inhibit polymer formation.
The affinity of a metal cation is controlled by several factors such as size matching between the host cavity of the crown ether and the guest cation, and the number of donor atoms of the host. Table 1 shows the radii of different alkali metal cations and the diameters of the cavities of several crown ethers.
Table 1. Radii of different alkali metal cations and diameters of the cavities of crown ethers.
Cation (radius, pm) | Cavity of the crown ether (diameter, pm)
Li + (68) | 12–C–4 (120 – 150)
Na + (98) | 15–C–5 (170 – 220)
K + (133) | 18–C–6 (260 – 320)
Cs + (165) | 21–C–7 (340 – 430)
On the basis of the above data, match the experimental curves I–III in Figure 1 with appropriate cyclohexyl crown ethers G – I .
[VISUAL]
Model Answer
Curve I to I; Curve II to G; Curve III to H