09-19-2021-1444 - Diethylzinc (C2H5)2Zn, or DEZ

 Diethylzinc (C₂H₅)₂Zn, or DEZ, is a highly pyrophoric and reactive organozinc compound consisting of a zinc center bound to two ethyl groups. This colourless liquid is an important reagent in organic chemistry. It is available commercially as a solution in hexanes, heptane, or toluene, or as a pure liquid.

Synthesis[edit]

Edward Frankland first reported the compound in 1848 from zinc and ethyl iodide, the first organozinc compound discovered.^[2][3] He improved the synthesis by using diethyl mercury as starting material.^[4] The contemporary synthesis consists of the reaction of a 1:1 mixture of ethyl iodide and ethyl bromide with a zinc-copper couple, a source of reactive zinc.^[5]

Structure[edit]

The compound crystallizes in a tetragonal body-centered unit cell of space group symmetry I4₁md. In the solid-state diethylzinc shows nearly linear Zn centres. The Zn-C bonds measure 194.8(5) pm, while the C-Zn-C angle is slightly bent with 176.2(4)°.^[6] The structure of the gas-phase shows a very similar Zn-C distance (195.0(2) pm).^[7]

Uses[edit]

Despite its highly pyrophoric nature, diethylzinc is an important chemical reagent. It is used in organic synthesis as a source of the ethyl carbanion in addition reactions to carbonyl groups. For example, the asymmetric addition of an ethyl group to benzaldehyde^[8] and imines.^[9] Additionally, it is commonly used in combination with diiodomethane as a Simmons-Smith reagent to convert alkenes into cyclopropyl groups.^[10][11] It is less nucleophilic than related alkyllithium and Grignard reagents, so it may be used when a "softer" nucleophile is needed. It is also used extensively in materials science chemistry as a zinc source in the synthesis of nanoparticles. Particularly in the formation of the zinc sulfide shell for core/shell-type quantum dots.^[12] While in polymer chemistry, it can be used as part of the catalyst for a chain shuttling polymerization reaction, whereby it participates in living polymerization.^[13]

Diethylzinc is not limited to only being used in chemistry. Because of its high reactivity toward air, it was used in small quantities as a hypergolic or "self igniting" liquid rocket fuel—it ignites on contact with oxidizer, so the rocket motor need only contain a pump, without a spark source for ignition. Diethylzinc was also investigated by the United States Library of Congress as a potential means of mass deacidification of books printed on wood pulp paper. Diethylzinc vapour would, in theory, neutralize acid residues in the paper, leaving slightly alkaline zinc oxide residues. Although initial results were promising, the project was abandoned. A variety of adverse results prevented the method's adoption. Most infamously, the final prototype suffered damage in a series of explosions from contact between trace amounts of diethylzinc and water vapor in the chamber. This led the authors of the study to humorously comment:

It has also been established that tight or loose packing of books; the amount of alkaline reserve; reactions of DEZ with degradation products, unknown paper chemicals and adhesives; phases of the moon and the positions of various planets and constellations do not have any influence on the observed adverse effects of DEZ treatment.^[14]

In microelectronics, diethylzinc is used as a doping agent.^{[citation needed]}

For corrosion protection in nuclear reactors of the light water reactor design, depleted zinc oxide is produced by first passing diethylzinc through an enrichmentcentrifuge.

The pyrophoricity of diethylzinc can be used to test the inert atmosphere inside a glovebox. An oxygen concentration of only a few parts per million will cause a bottle of diethylzinc to fume when opened.^[15]

Safety[edit]

Diethylzinc reacts violently with water and easily ignites upon contact with air. It should therefore be handled using inert atmosphere techniques.

https://en.wikipedia.org/wiki/Diethylzinc

09-19-2021-1443 - Sir Edward Frankland, KCB, FRS, FRSE 1825 1899

 Sir Edward Frankland, KCB, FRS, FRSE (18 January 1825 – 9 August 1899) was a British chemist. He was one of the originators of organometallic chemistryand introduced the concept of combining power or valence. An expert in water quality and analysis, he was a member of the second royal commission on the pollution of rivers, and studied London's water quality for decades. He also studied luminous flames and the effects of atmospheric pressure on dense ignited gas, and was one of the discoverers of helium.

Sir Edward Frankland KCB FRS FRSE
Edward Frankland in his 20s
Born	18 January 1825 Catterall, Lancashire, England
Died	9 August 1899 (aged 74) Gålå, Gudbrandsdal, Norway
Nationality	British
Occupation	Research chemist
Known for	Pioneer in water analysis, Discoverer of the principle of valency in chemistry

https://en.wikipedia.org/wiki/Edward_Frankland

https://en.wikipedia.org/wiki/Hermann_Kolbe

https://en.wikipedia.org/wiki/Norman_Lockyer

https://en.wikipedia.org/wiki/Pierre_Janssen

09-19-2021-1438 - Organotin stannanes

 Organotin compounds or stannanes are chemical compounds based on tin with hydrocarbon substituents. Organotin chemistry is part of the wider field of organometallic chemistry. The first organotin compound was diethyltin diiodide ((C₂H₅)₂SnI₂), discovered by Edward Frankland in 1849.^[1] The area grew rapidly in the 1900s, especially after the discovery of the Grignard reagents, which are useful for producing Sn-C bonds. The area remains rich with many applications in industry and continuing activity in the research laboratory.^[2]

Organotin compounds are those with tin linked to hydrocarbons.

Hypercoordinated stannanes

[edit]

Unlike carbon(IV) analogues but somewhat like silicon compounds, tin(IV) can also be coordinated to five and even six atoms instead of the regular four. These hypercoordinated compounds usually have electronegative substituents. Numerous examples of hypervalency are provided by the organotin oxides and associated carboxylates and related pseudohalide derivatives.^[5] The organotin halides for adducts, e.g. Me₂SnCl₂(bipyridine).

The all-organic penta- and hexaorganostannates have even been characterized,^[6] while in the subsequent year a six-coordinated tetraorganotin compound was reported.^[7] A crystal structure of room-temperature stable (in argon) all-carbon pentaorganostannane was reported as the lithium salt with this structure:^[8]

In this distorted trigonal bipyramidal structure the carbon to tin bond lengths (2.26 Å apical, 2.17 Å equatorial) are larger than regular C-Sn bonds (2.14 Å) reflecting its hypervalent nature.

Triorganotin cations[edit]

Some reactions of triorganotin halides implicate a role for R3Sn+ intermediates. Such cations are analogous to carbocations. They have been characterized crystallographically when the organic substituents are large, such as 2,4,6-triisopropylphenyl.^[9]

https://en.wikipedia.org/wiki/Organotin_chemistry

09-19-2021-1438 - Intramolecular

Intramolecular in chemistry describes a process or characteristic limited within the structure of a single molecule, a property or phenomenon limited to the extent of a single molecule.

Examples[edit]

• intramolecular hydride transfer (transfer of a hydride ion from one part to another within the same molecule)
• intramolecular hydrogen bond (a hydrogen bond formed between two functional groups of the same molecule)
• cyclization of ω-haloalkylamines and alcohols to form the corresponding saturated nitrogen and oxygen heterocycles, respectively (an S_N2 reaction within the same molecule)

In intramolecular organic reactions, two reaction sites are contained within a single molecule. This creates a very high effective concentration (resulting in high reaction rates), and, therefore, many intramolecular reactions that would not occur as an intermolecular reaction between two compounds take place.

Examples of intramolecular reactions are the Smiles rearrangement, the Dieckmann condensation and the Madelung synthesis.

Relative rates[edit]

Intramolecular reactions, especially ones leading to the formation of 5- and 6-membered rings, are rapid compared to an analogous intermolecular process. This is largely a consequence of the reduced entropic cost for reaching the transition state of ring formation and the absence of significant strain associated with formation of rings of these sizes. For the formation of different ring sizes via cyclization of substrates of varying tether length, the order of reaction rates (rate constants k_n for the formation of an n-membered ring) is usually k₅ > k₆ > k₃ > k₇ > k₄ as shown below for a series of ω-bromoalkylamines. This somewhat complicated rate trend reflects the interplay of these entropic and strain factors:

Relative rate constants for cyclization (n= 5 set to k_rel = 100)

n	krel	n	krel	n	krel
3	0.1	6	1.7	12	0.00001
4	0.002	7	0.03	14	0.0003
5	100	10	0.00000001	15	0.0003

For the 'small rings' (3- and 4- membered), the slow rates is a consequence of angle strain experienced at the transition state. Although three-membered rings are more strained, formation of aziridine is faster than formation of azetidine due to the proximity of the leaving group and nucleophile in the former, which increases the probability that they would meet in a reactive conformation. The same reasoning holds for the 'unstrained rings' (5-, 6-, and 7-membered). The formation of 'medium-sized rings' (8- to 13-membered) is particularly disfavorable due to a combination of an increasingly unfavorable entropic cost and the additional presence of transannular strain arising from steric interactions across the ring. Finally, for 'large rings' (14-membered or higher), the rate constants level off, as the distance between the leaving group and nucleophile is now so large the reaction is now effectively intermolecular.^[1][2]

Although the details may change somewhat, the general trends hold for a variety of intramolecular reactions, including radical-mediated and (in some cases) transition metal-catalyzed processes.

Tethered intramolecular [2+2] reactions[edit]

Tethered intramolecular [2+2] reactions entail the formation of cyclobutane and cyclobutanone via intramolecular 2+2 photocycloadditions. Tethering ensures formation of a multi-cyclic system.

The length of the tether affects the stereochemical outcome of the [2+2] reaction. Longer tethers tend to generate the "straight" product where the terminal carbon of the alkene is linked to the -carbon of the enone.^[3] When the tether consists only two carbons, the “bent” product is generated where the -carbon of the enone is connected to the terminal carbon of the alkene (Figure 2).^[4]

Tethered [2+2] reactions have been used to synthesize organic compounds with interesting ring systems and topologies. For example, [2+2] photocyclization was used to construct the tricyclic core structure in ginkgolide B by E. J. Corey and co-workers in 1988.^[5]

Molecular tethers[edit]

In a niche concept called molecular tethers, otherwise-intermolecular reactions can be made temporarily intramolecular by anchoring both reactions by a tether with all the advantages associated to it. Popular choices of tether contain a carbonate ester, boronic ester,  silyl ether, or a silyl acetal link (silicon tethers)^[6][7] which are fairly inert in many organic reactions yet can be cleaved by specific reagents. The main hurdle for this strategy to work is selecting the proper length for the tether and making sure reactive groups have an optimal orientation with respect to each other. An examples is a Pauson–Khand reaction of an alkene and an alkyne tethered together via a silyl ether.^[8]

In this particular reaction, the tether angle bringing the reactive groups together is effectively reduced by placing isopropyl groups on the silicon atom via the Thorpe–Ingold effect. No reaction takes place when these bulky groups are replaced by smaller methyl groups.

Another example is a photochemical [2+2]cycloaddition with two alkene groups tethered through a silicon acetal group (racemic, the other enantiomer not depicted), which is subsequently cleaved by TBAFyielding the endo-diol.

Without the tether, the exo isomer forms.^[9]

https://en.wikipedia.org/wiki/Intramolecular_reaction

Categories: 

• Molecular physics
• Organic chemistry

09-19-2021-1435 - Neutron Cross Section Phosphorus pentafluoride Fluxional (or non-rigid) molecules

Fluxional (or non-rigid) molecules are molecules that undergo dynamics such that some or all of their atoms interchange between symmetry-equivalent positions. Because virtually all molecules are fluxional in some respects, e.g. bond rotations in most organic compounds, the term fluxional depends on the context and the method used to assess the dynamics. Often, a molecule is considered fluxional if its spectroscopic signature exhibits line-broadening (beyond that dictated by the Heisenberg uncertainty principle) due to chemical exchange. In some cases, where the rates are slow, fluxionality is not detected spectroscopically, but by isotopic labeling. Where such movement does not occur, the molecule may be described as a semi-rigid molecule.^[1][2][3][4] Longuet-Higgins introduced the use of permutation-inversion groups for the symmetry classification of the states of fluxional (or non-rigid) molecules.^[5][6]

A well-studied fluxional ion is the methanium ion, which is protonated methane, CH+
5.^[7][8][9] In this unusual species, whose IR spectrum was recently experimentally observed^[10][8] and more recently understood,^[11][12][13] the barriers to proton exchange are lower than the zero-point energy. Thus, even at absolute zero there is no rigid molecular structure; the H atoms are always in motion. More precisely, the spatial distribution of protons in CH+
5 is many times broader than its parent molecule CH₄, methane.^[14][15]

https://en.wikipedia.org/wiki/Fluxional_molecule

Phosphorus pentafluoride, PF₅, is a phosphorus halide. It is a colourless, toxic gas that fumes in air.^[1][2]

Preparation[edit]

Phosphorus pentafluoride was first prepared in 1876 by the fluorination of phosphorus pentachloride using arsenic trifluoride, which remains a favored method:^[1]

3 PCl₅ + 5 AsF₃ → 3 PF₅ + 5 AsCl₃

Structure[edit]

Single-crystal X-ray studies indicate that the PF₅ has trigonal bipyramidal geometry. Thus it has two distinct types of P−F bonds (axial and equatorial): the length of an axial P−F bond is distinct from the equatorial P−F bond in the solid phase, but not the liquid or gas phases due to Pseudo Berry Rotation. 

Fluorine-19 NMR spectroscopy, even at temperatures as low as −100 °C, fails to distinguish the axial from the equatorial fluorine environments. The apparent equivalency arises from the low barrier for pseudorotation via the Berry mechanism, by which the axial and equatorial fluorine atoms rapidly exchange positions. The apparent equivalency of the F centers in PF₅ was first noted by Gutowsky.^[3] The explanation was first described by R. Stephen Berry, after whom the Berry mechanism is named. Berry pseudorotation influences the ¹⁹F NMR spectrum of PF₅ since NMR spectroscopy operates on a millisecond timescale. Electron diffraction and X-ray crystallography do not detect this effect as the solid state structures are, relative to a molecule in solution, static and can not undergo the necessary changes in atomic position.

Lewis acidity[edit]

Phosphorus pentafluoride is a Lewis acid. This property is relevant to its ready hydrolysis. A well studied adduct is PF₅ with pyridine. With primary and secondary amines, the adducts convert readily to dimeric amido-bridged derivatives with the formula [PF₄(NR₂)]₂. A variety of complexes are known with bidentate ligands.^[4]

Hexafluorophosphoric acid (HPF₆) is derived from phosphorus pentafluoride and hydrogen fluoride. Its conjugate base, hexafluorophosphate (PF₆^–), is a useful non-coordinating anion.

https://en.wikipedia.org/wiki/Phosphorus_pentafluoride

In nuclear and particle physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. In conjunction with the neutron flux, it enables the calculation of the reaction rate, for example to derive the thermal power of a nuclear power plant. The standard unit for measuring the cross section is the barn, which is equal to 10⁻²⁸ m² or 10⁻²⁴ cm². The larger the neutron cross section, the more likely a neutron will react with the nucleus.

An isotope (or nuclide) can be classified according to its neutron cross section and how it reacts to an incident neutron. Nuclides that tend to absorb a neutron and either decay or keep the neutron in its nucleus are neutron absorbers and will have a capture cross section for that reaction. Isotopes that fission are fissionablefuels and have a corresponding fission cross section. The remaining isotopes will simply scatter the neutron, and have a scatter cross section. Some isotopes, like uranium-238, have nonzero cross sections of all three.

Isotopes which have a large scatter cross section and a low mass are good neutron moderators (see chart below). Nuclides which have a large absorption cross section are neutron poisons if they are neither fissile nor undergo decay. A poison that is purposely inserted into a nuclear reactor for controlling its reactivity in the long term and improve its shutdown margin is called a burnable poison.

https://en.wikipedia.org/wiki/Neutron_cross_section