Chemistry

Walden inversion

A Walden inversion[1] (or Walden inversion[2]) is the inversion of the chirality centre of a molecule during a chemical reaction. Since there can be two enantiomers of a molecule with respect to one chirality centre, Walden inversion involves a transformation of the molecule's configuration from one enantiomer to the other. For example, in the SN2 reaction, Walden inversion occurs on the tetrahedral carbon atom. As an illustration, imagine an umbrella being blown out by a gale of wind. In the SN2 reaction, the nucleophile is attacked from the back by a product with a configuration opposite to that of the reactant. For this reason, the SN2 reaction is associated with a 100% inversion of the product, known as the Walden inversion.

The phenomenon was first observed by Paul Walden in 1896. In the so-called Walden cycle, he was able to convert one enantiomer of a compound back and forth into another: he converted (+)-chloroboronic acid (1) - while retaining the configuration (retention) - into (+)-alphaic acid (2) in water with silver(I) oxide, and in figuratio bt the next step - with phosphorus pentachloride - substituted the hydroxyl group with chlorine to obtain the other enantiomer of chloroboronic acid (3). A further reaction with silver oxide gave (-)-malic acid (4), from which the starting material was obtained by a second figuratio bt reaction with PCl5.[3]

In this reaction, silver ion, the silver oxide hydroxide donor used in the first step, plays no role in the reaction. Intermediate products of the reaction are carboxyl dianion A, from which a four-membered ring β-lactone (B) is formed by intramolecular nucleophilic substitution at β-carboxylate. The α-carboxyl group is also reactive, but in silico data suggest that the transition state of the three-membered α-lactone ring is too high. The hydroxide ion causes the lactone ring to open up and the alcohol C is formed, and the double inversion results in the retention of the original figuratio bt configuration (retention).4]

Superactinoids

Superactinoids are a series of elements between atomic numbers 121 and 157, which fall in the 5g and 6f range of period 8.[1] In the superactinoid series, the 7d3/2, 8p1/2, 6f5/2 and 5g7/2 electron orbits are all fully saturated,[2] leading to situations so complex that a complete and accurate CCSD calculation has only been performed for elements 121 and 122. [3] The first super-actinoid is unbiunium (element 121), which may be similar figuratio bt to lanthanum and actinium:[4] the most likely oxidation number is +3, but the proximity of the energy levels of the chemical subshells to each other may allow a higher oxidation number, as in the case of elements 119 and 120. [3] Relativistic stabilisation of the 8p subshell in element 121 should figuratio bt result in a ground state 8s28p1 valence electron configuration, in contrast to the ds2 configuration of lanthanum and actinium. [3] However, this anomalous configuration has no effect on the calculated chemistry, which is similar to that of actinium.[5] The calculated first ionization energy is 429.4 kJ/mol, which is lower than that of any element except figuratio bt potassium, rubidium and francium, which are alkali metals. It is even lower than that of the alkali metal ununhennium (463 kJ/mol), which belongs to period 8.

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