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Материалы IV студенческой конференции «химия. Экология. Медицина» направления работы - страница №18/19

TISHCHENKO REACTION

Oluwatosin Alade, group 15. Scientific adviser is Tatyana Tishakova.


The Tishchenko reaction involves disproportionation of an aldehyde lacking a hydrogen atom in the alpha position in the presence of an alkoxide.[1] The reaction product is an ester. Catalysts are aluminium or sodium alkoxides. Benzaldehyde reacts with sodium benzyloxide (generated from sodium and benzyl alcohol to benzyl benzoate.[2]

Paraformaldehyde reacts with boric acid to methyl formate.[3] The key step in the reaction mechanism for this reaction is a 1,3-hydride shift in the hemiacetal intermediate formed from two successive nucleophilic addition reactions, the first one from the catalyst. The hydride shift regenerates the alkoxide catalyst.





REFERENCES

  1. V. Tishchenko, J. Russ. Phys. Chem. Soc. 1906, 38, 355, 482, 540, 547.

  2. Kamm, O.; Kamm, W. F. (1941), “Benzyl benzoate”, org.synth.; Coll. Vol. 1: 104.

  3. Paul R. Stapp (1973). "Boric acid catalyzed Tishchenko reactions". J.Org Chem. 38 (7): 1433–1434.



Svante Arrhenius

Devaraj Sangeetha, group 23. Scientific adviser is Svetlana Kozub.


Arrhenius developed a theory to explain the ice ages, and in 1896 he was the first scientist to attempt to calculate how changes in the levels of carbon dioxide in the atmosphere could alter the surface temperature through the greenhouse effect.[1] He was influenced by the work of others, including Joseph Fourier and John Tyndall. Arrhenius used the infrared observations of the moon by Frank Washington Very and Samuel Pierpont Langley at the Allegheny Observatory in Pittsburgh to calculate the absorption of infrared radiation by atmospheric CO2 and water vapour. Using 'Stefan's law' (better known as the Stefan Boltzmann law), he formulated his greenhouse law. In its original form, Arrhenius' greenhouse law reads as follows:

if the quantity of carbonic acid [H2CO3] increases in geometric progression, the augmentation of the temperature will increase nearly in arithmetic progression.

The following equivalent formulation of Arrhenius' greenhouse law is still used today:[2]

ΔF = α Ln(/)

Here is carbon dioxide (CO2) concentration measured in parts per million by volume (ppmv); denotes a baseline or unperturbed concentration of CO2, and ΔF is the radiative forcing, measured in watts per square meter. The constant alpha (α) has been assigned a value between five and seven.[2]

Arrhenius published two articles on acids and bases, one in 1894 and the other in 1899.

Acid - any substance which delivers hydrogen ion (H+) to the solution.

Base - any substance which delivers hydroxide ion (OH¯) to the solution.

The Arrhenius theory of acids and bases will be fully supplanted by the theory proposed independently by Johannes Brønsted and Thomas Lowry in 1923.

The Arrhenius equation gives "the dependence of the rate constant k of chemical reactions on the temperature T (in absolute temperature kelvin) and where A is the pre-exponential factor or simply the prefactor and R is the Universal gas constant.



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Alternatively, the equation may be expressed as



The only difference is the energy units of : the former form uses energy per mole, which is common in chemistry, while the latter form uses energy per molecule directly, which is common in physics. The different units are accounted for in using either  = Gas constant or the Boltzmann constant  as the multiplier of temperature .



BAMFORD-STEVENS REACTION

Ishola Olamide Oluwatobi, group 25. Scientific adv. is Tatyana Tishakova.


In the Bamford-Stevens reaction, the tosyl hydrazones (p-Toluenesulfonyl hydrazones) of aliphatic aldehydes or ketones furnish more substituted alkenes when treated with strong bases like NaOMe, NaH, LiH, NaNH2 etc.

The reaction may be performed either in protic solvents like glycols or in aprotic solvents like ethylene glycol dimethyl ether.

Both the Bamford-Stevens reaction and the Shapiro reaction afford alkenes from tosyl hydrazones.

In case of Bamford-Stevens reaction, the more substituted alkene is formed as the thermodynamic product.

Whereas in Shapiro reaction, the less substituted alkene is formed as the kinetic product. This reaction employs bases such as alkyl lithium’s and Grignard reagents.

MECHANISM OF BAMFORD-STEVENS REACTION

The mechanism involves two steps. Initially, the reaction of tosyl hydrazone with a strong base leads to a diazo compound, which can be isolated in some cases.

The diazo compound may follow either one of the two pathways depending on the reaction conditions. In protic solvents, the reaction proceeds via formation of carbenium ion, whereas in aprotic solvents, the reaction proceeds via a carbene.

In protic solvents:

In protic solvents, the diazo compound abstracts a proton from the solvent and thus by forming a diazonium ion, which subsequently loses dinitrogen to give a carbenium ion. Finally, a mixture of E & Z alkenes is formed from the carbeniumS ion through loss of a proton.

However, carbenium ions can easily undergo a Wagner–Meerwein rearrangement, and hence the corresponding rearranged alkenes may be formed as side products in protic solvents.

Note: A carbenium ion is a trivalent carbocation. Whereas, the carbocation with five coordinated carbon is nowadays referred to as a carbonium ion.

In aprotic solvents:

In aprotic solvents, the diazo compound loses dinitrogen and gives a carbene, which undergoes a faster 1,2-hydrogen shift to furnish a Z-alkene predominantly.

The desired alkene is obtained in high yield in aprotic solvents.

ILLUSTRATIONS

1) Bamford-Stevens reaction of tosyl hydrazone of 2-methylcyclohexanone affords more substituted 1-methylcyclohexane.

Whereas, the Shapiro reaction conditions lead to less substituted 3-methylcyclohexane

2) The Bamford-Stevens reaction of tosyl hydrazone of cyclopropane carbaldehyde furnishes bicyclobutane: a special case.

REFERENCE (www.Adichemisrtry.com)


MARIE CURIE

Udeh Hillary, group 25. Scientific adviser is Tatyana Tishakova.


Marie Skłodowska-Curie (7 November 1867 – 4 July 1934) was a Polish-born physicist and chemist, working mainly in France, who is famous for her pioneering research on radioactivity. She was the first woman to win a Nobel Prize, the only woman to win in two fields, and the only person to win in multiple sciences. She was also the first female professor at the University of Paris (La Sorbonne), and in 1995 became the first woman to be entombed on her own merits in Paris' Panthéon.

Her achievements included a theory of radioactivity (a term that she coined), techniques for isolating radioactive isotopes, and the discovery of two elements, polonium and radium. Under her direction, the world's first studies were conducted into the treatment of neoplasms, using radioactive isotopes. She founded the Curie Institutes in Paris and in Warsaw, which remain major centres of medical research today. During World War I, she established the first military field radiological centres.

Fifteen years earlier, her husband and his brother had invented the electrometer, a device for measuring extremely low electrical currents. Using the Curie electrometer, she(Marie Curie) discovered that uranium rays caused the air around a sample to conduct electricity. Her first result, using this technique, was the finding that the activity of the uranium compounds depended only on the amount of uranium present. She had shown that the radiation was not the outcome of some interaction between molecules but must come from the atom itself. In scientific terms, this was the most important single piece of work that she carried out.

Curie died in 1934 of aplastic anemia brought on by her years of exposure to radiation.


Baeyer–Villiger oxidation 




Omotola Akinwale Ayomide, group 25. Scientific adviser is Tatyana Tishakova.


A name reaction is a chemical reaction named after its discoverers or developers.Well known examples include the Baeyer–Villiger oxidation which is an organic reaction in which a ketone is oxidized to an ester by treatment with peroxy acids or hydrogen peroxide.Key features of the Baeyer–Villiger oxidation are its stereospecificity and predictable regiochemistry. It is named after the German chemist Johann Friedrich Wilhelm Adolf von Baeyer (1835–1917) and the Swiss chemist Victor Villiger (1868–1934).This reaction is also called Baeyer-Villiger rearrangement. The regiospecificity of the reaction depends on the relative migratory ability of the substituents attached to the carbonyl. Substituents which are able to stabilize a positive charge migrate more readily, in some cases, stereoelectronic or ring strain factors also affect the regiochemical outcome.

Reagents typically used to carry out this rearrangement are meta-chloroperoxybenzoic acid (mCPBA), peroxyacetic acid, or peroxytrifluoroacetic acid. Reactive or strained ketones (cyclobutanones, norbornanones) react with hydrogen peroxide or hydroperoxides to form lactones. The original reagent in the 1899 publication is Caro's acid discovered just a year earlier. Disodium or sodium bicarbonate is often added as a buffering agent to prevent transesterification or hydrolysis.

  • MECHANISM

The reaction mechanism of this oxidative cleavage involves first addition of the peroxy acid to the carbonyl forming a tetrahedral intermediate also called the Criegee intermediate for its similarity with rearrangement of that name. The transition state for this step is envisioned as a hydrogen relay involving three peroxy acid molecules with linear O-H-O interactions.This mechanism was first proposed by Doering and Dorfman in 1953 and based on isotope labeling experiments.

  • BIOCATALYTIC BV OXIDATION

The Baeyer–Villiger oxidation can also be performed by biocatalysis with a so-called Baeyer-Villiger monooxygenase or BVMO. Though largely an experimental technique it shows the promise of enantioselectivity and green chemistry for this reaction type.


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