The porphin molecule is the simplest member in the family of porphyrins. Its structure, which contai — Analytical Chemistry Chemistry Question
Porphin and Porphyrins
The porphin molecule is the simplest member in the family of porphyrins. Its structure, which contains four pyrrole rings, is completely planar. All its carbon and nitrogen atoms are sp2 hybridized. A conjugated double-bond system can therefore be found in the molecule. The sigma-skeleton of porphin is depicted below:
[VISUAL]
How many π electrons participate in the conjugated double bond system? Is the molecule aromatic? Draw a porphin structure indicating the double bonds forming the conjugated double-bond system.
Model Answer
26 = 4·6+2, hence it is aromatic.
Two of the central nitrogen atoms have a hydrogen substituent. These hydrogens are slightly acidic and under normal conditions, the protons can easily migrate to a neighboring N atom, as shown below:
[VISUAL]
What kind of isomers are I and II? How does the migration process affect the conjugated double bond system: do less or more π electrons participate in isomer II than in isomer I? Draw a porphin structure for II indicating the double bonds.
Model Answer
They are constitutional isomers or tautomers. The number of π electrons is not affected by the rearrangement.
The hydrogen atoms bound to the carbon atoms of the porphin molecule can be substituted by other groups. Suppose that we introduce a methyl group onto the porphin ring. Under normal conditions, the inner-nitrogen H migration is unaffected by this substitution and takes place continuously in solvent. How many different monomethyl porphins can be produced?
Model Answer
We have two constitutional isomers. The migration of the proton (and the subsequent rearrangement of the aromatic electrons) is so fast that the different tautomers cannot be isolated.
We further introduce another methyl group into the porphin ring. How many isomers can be isolated in this case?
Model Answer
We can isolate 12 constitutional isomers.
Metal complexes of porphin can be easily prepared. An important compound of this kind is the magnesium complex which is a synthetic model of chlorophyll. Its sigma-structure is displayed below:
[VISUAL]
How many π electrons of the organic ring system participate in the conjugated bond system in this case? What is the number of independent methyl-Mg-porphins having one methyl substituent on the organic ring?
Model Answer
The number of π electrons is the same: 26. With one methyl substituent one can form 2 constitutional isomers.
Numerous iron-porphin derivatives (P) can be synthesized. Such salts all feature the heterocyclic macrocycle of porphin, but they contain additional substituents on the organic macrocycle as well. They are able to bind two additional ligands, coordinating them axially to the two sides of the iron atom. This complexation is a two-step process: the originally four-coordinated iron binds a ligand (L) and becomes five-coordinated (PL), and then binds a second ligand to become six-coordinated (PL2). It was found in various cases that the reaction rapidly yields the PL2 complex, whereas the PL complex was very difficult to obtain. For the complexation of a given iron-porphin derivative with pyridine in inert organic solvent, scientists were able to show employing spectroscopic methods that the two steps can be characterized via the following equilibria:
P + L = PL K1 = 1500
PL + L = PL2 K2 = 19000
We see an atypical K1 < K2 relation. Why does such a relation between two consecutive dissociation constants of a polyprotic acid never occur?
Model Answer
For electrostatic reasons.
Assume that we perform this complexation reaction and reach an equilibrium concentration ligand L is of 0.1 mol dm–3. Show that the five-coordinated intermediate is indeed present in negligible quantity.
Model Answer
The equilibrium concentrations can be derived from:
[PL2] / [PL] = K2 * [L] = 19000 * 0.1 = 1900 and
[PL2] / [P] = K1 * K2 * [L]^2 = 1500 * 19000 * 0.01 = 285000
Thus [PL2] = 1900 [PL] and [PL2] = 285000 [P]. Clearly, both [PL] and [P] are negligible compared with [PL2].
Suppose we are able to generate PL in-situ in a solvent and due to its kinetic stability we reach a concentration of PL equal to 0.1 mol dm–3. After a given time, however, the system reaches equilibrium. How does temperature affect the kinetic stability?
Model Answer
Kinetic stability decreases as the temperature increases. (In other words the significance of the kinetic stability diminishes with increasing temperature.)
What will the concentrations of P, PL, PL2 and L be at equilibrium?
Model Answer
The following net reaction can be postulated:
PL + PL ⇌ P + PL2
This follows from the fact that both ligand association equilibria are strongly shifted toward the ligand-uptake direction. The above equilibrium can also be derived from the original equilibria and subsequently the corresponding equilibrium constant can be written as follows:
[P][PL2]/[PL]^2 = K2/K1 = 12.67
Assuming that x mol dm–3 from the original [PL] concentration is transformed into P and PL2, the equilibrium [P] and [PL2] concentrations are x/2. [PL] is 0.1 – x. Solving this equation the equilibrium concentrations are:
[P] = [PL2] = 0.0438 mol dm–3
[PL] = 0.0123 mol dm–3
[L] = [PL2]/([PL]*K2) = 0.000187 mol dm–3