2) Chirality Enhancement via Crystallization In this section, you will discuss t

Question

2) Chirality Enhancement via Crystallization In this section, you will discuss the results of Noorduin, et al, and describe why the phenomenon is observed a) The concept of the paper is shown below. Upon addition of a small excess of one enantiomer of 1 or a different chiral species (such as phenylglycine), crystals of 1 with 100% enantiomeric excess can be obtained over time. DBU OnNH2 MeOH solution (S)-1 (R)-1 NH2 dissolution crystallization iza dissolution crystallization The key to this theory and experiment is selective crystallization. Read the Ostwald ripening attachment (P-1) and explain why a large crystal is more thermodynamically favorable than a small one. b) The experiment also relies on the fact that individual enantiomers crystallize together in a single crystal (i.e. the crystals are either all-S or all-R, not a racemic crystal). Explain why this occurs consider the solid state interaction of two molecules of the same handedness and different handedness. See Carey p155-157 c) If there is only a small excess of one enantiomer in solution, can selective crystallization alone (i.e. in the absence of base) cause chirality enhancement? d) Discuss why the combination of added base (DBU in this case) and selective crystallization allows the enhancement of chirality of the system. Consider equilibrium effects and Le Chateliers principle in your answer. e) This is a proof-of-principle experiment (i.e. it mimics the natural process, but several changes were made to allow study in a laboratory setting). To maximize the speed of the enhancement, glass beads and magnetic stirring were employed. How does this speed up the process? Is this a plausible method of mimicking natural evolution? Why did the researchers not simply perform the experiment as it would occur in prebiotic systems?

Explanation / Answer

Crystallization

a) Large crystals are more thermodynamically favoured than the small crystals. This is due to the fact that large crystals are more stable (lower energy, more stable) than the small crystals as more number of atoms are present inside the crystal lattice that binds to more atoms around them as compared to the small crystals which have fewer atoms on the inside of lattice which makes it less stable. The atoms on the exterior surface of crystal lattice have fewer atoms to bind and are having low stablity compared to atoms which are inside.

b) In order to maintain a lower energy system, we need to maintain some sort of symmetry. When the crystals are formed as single enantiomers, the total symmetry in the crystal lattice and in the solution makes the system more stable as opposed to mixture (as in racemic mixture) which represents symmetry break and results into higher energy. Thus enantiomers in solution crystallize preferabally as single isomers.

c) Yes, in the presence of a small excess of a single enantiomer, the crystallization can be favoured towards that particular ennatiomer. This would force the crystallization towards an overall chiral enhancement of the system.

d) The equilibrium reaction upon addition of base and selective crystallization favors chiral enhancement for the enantiomer which is present in excess in the solution. The excess enantiomer in solution makes the equilibrium of the racemic mixture unstable and forces the crystallization to shift to the other end and reach equilibrium stage again.

e) Stirring and addition of glass beads increases the rate of base reaction and then folllowed by crystallization process, by increasing the kinetic energy and mixing. In prebiotic system this is however not the case. The researchers had to speed up the process as in prebiotic condition it occurs at a extremely slow rate and is time consuming If relicated in the lab.

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