Химия. Экология. Медицина



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ALEXANDER FLEMING

Elifra Muchengwa, group 9. Science adviser is Tatyana Tishakova.


Sir Alexander Fleming (6 August 1881 – 11 March 1955) was a Scottish biologist, pharmacologist and botanist. His greatest works pertaining to the above mentioned topic include the discovery of penicillin and the enzyme lysozyme.

He qualified with distinction in 1906 and began research at St. Mary's under Sir Almroth Wright, a pioneer in vaccine therapy.

In 1922 he described the antibacterial properties of lysozyme, a substance found in egg whites, mucus, and tears, which lysed, or dissolved, certain bacteria. Fleming found that lysozyme could turn a thick, milky white suspension of bacteria into a clear solution in a matter of seconds. Unfortunately, lysozyme failed to destroy bacteria that caused human diseases and was never used as a medicine.

1928, Fleming was investigating the properties of staphylococci; he often forgot cultures that he worked on, and his lab in general was usually in chaos. After returning from a long holiday, Fleming noticed that many of his culture dishes were contaminated with a fungus, and he threw the dishes in disinfectant. But subsequently, he had to show a visitor what he had been researching, and so he retrieved some of the unsubmerged dishes that he would have otherwise discarded. He then noticed a zone around an invading fungus where the bacteria could not seem to grow. Fleming proceeded to isolate an extract from the mould, correctly identified it as being from the "Penicillium" genus, and therefore named the agent penicillin.

He investigated its positive anti-bacterial effect on many organisms, and noticed that it affected bacteria such as staphylococci, and indeed all Gram-positive pathogens (scarlet fever, pneumonia, meningitis, diphtheria) but unfortunately not typhoid or paratyphoid, for which he was seeking a cure at the time. It also affected gonorrhea, although this condition is caused by a Gram-negative pathogen.

FRIEDEL - CRAFTS REACTION

Shriya Napolean Fernandes, group 10. Science adviser is Svetlana Kozub.


The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877 to attach substituents to an aromatic ring.

Friedel–Crafts alkylation involves the alkylation of an aromatic ring with an alkyl halide using a strong Lewis acid catalyst. With anhydrous ferric chloride as a catalyst, the alkyl group attaches at the former site of the chloride ion. The general mechanism is shown below.This Lewis acid-catalyzed electrophilic aromatic substitution allows the synthesis of alkylated products via the reaction of arenes with alkyl halides or alkenes. Since alkyl substituents activate the arene substrate, polyalkylation may occur. A valuable, two-step alternative is Friedel-Crafts Acylation followed by a carbonyl reduction.

Mechanism of the Friedel-Crafts Alkylation



This reaction has one big disadvantage, namely that the product is more nucleophilic than the reactant due to the electron donating alkyl-chain. Therefore, another hydrogen is substituted with an alkyl-chain, which leads to overalkylation of the molecule. Also, if the chloride is not on a tertiary carbon or secondary carbon, carbocation rearrangement reaction will occur. This reactivity is due to the relative stability of the tertiary and secondary carbocation over the primary carbocations can be exploited to limit the number of alkylations, as in the t-butylation of 1,4-dimethoxybenzene.



Alkylations are not limited to alkyl halides: Friedel–Crafts reactions are possible with any carbocationic intermediate such as those derived from alkenes and a protic acid, Lewis acid,enones, and epoxides. An example is the synthesis of neophyl chloride from benzene and methallyl chloride:

H2C=C(CH3)CH2Cl + C6H6 → C6H5C(CH3)2CH2Cl

In one study the electrophile is a bromium ion derived from an alkene and NBS.



In this reaction samarium triflate is believed to activate the NBS halogen donor in halonium ion formation.




BELOUSOV-ZHABOTINSKY REACTION

Amritha Ashok Nair, group 10. Science adviser is Svetlana Kozub.


A Belousov–Zhabotinsky reaction, or BZ reaction, is one of a class of reactions that serve as a classical example of non-equilibrium thermodynamics, resulting in the establishment of a nonlinear chemical oscillator. The only common element in these oscillating systems is the inclusion of bromine and an acid. The reactions are theoretically important in that they show that chemical reactions do not have to be dominated byequilibrium thermodynamic behaviour. These reactions are far from equilibrium and remain so for a significant length of time. In this sense, they provide an interesting chemical model of non-equilibrium biological phenomena, and the mathematical models of the BZ reactions themselves are of theoretical interest.

An essential aspect of the BZ reaction is its so called "excitability"; under the influence of stimuli, patterns develop in what would otherwise be a perfectly quiescent medium. Some clock reactions such as Briggs–Rauscher and BZ using the tris(bipyridine)ruthenium(II) chloride as catalyst can be excited into self-organising activity through the influence of light.

The discovery of the phenomenon is credited to Boris Belousov. He noted, some time in the 1950s (various sources date ranges from 1951 to 1958), that in a mix of potassium bromate, cerium(IV) sulfate, malonic acid and citric acid in dilute sulfuric acid, the ratio of concentration of the cerium(IV) and cerium(III) ions oscillated, causing the colour of the solution to oscillate between a yellow solution and a colorless solution. This is due to the cerium(IV) ions being reduced by malonic acid to cerium(III) ions, which are then oxidized back to cerium(IV) ions by bromate(V) ions.

Belousov made two attempts to publish his finding, but was rejected on the grounds that he could not explain his results to the satisfaction of the editors of the journals to which he submitted his results. His work was finally published in a less respectable, non-reviewed journal.

Later, in 1961, a graduate student named Anatoly Zhabotinsky rediscovered this reaction sequence; however, the results of these men's work were still not widely disseminated, and were not known in the West until a conference in Prague in 1968.

There are a number of BZ cocktails available in the chemical literature and on the web. Ferroin, a complex of phenanthroline and iron is a commonindicator. These reactions, if carried out in petri dishes, result in the formation first of colored spots. These spots grow into a series of expanding concentric rings or perhaps expanding spirals similar to the patterns generated by a cyclic cellular automaton. The colors disappear if the dishes are shaken, and then reappear. The waves continue until the reagents are consumed. The reaction can also be performed in a beaker using amagnetic stirrer.

Andrew Adamatzky, a computer scientist in the University of the West of England reported on liquid logic gates using the BZ reaction.

Strikingly similar oscillatory spiral patterns appear elsewhere in nature, at very different spatial and temporal scales, for example the growth pattern of Dictyosteliumdiscoideum, a soil-dwelling amoeba colony. In the BZ reaction, the size of the interacting elements is molecular and the time scale of the reaction is minutes. In the case of the soil amoeba, the size of the elements is typical of single-celled organisms and the times involved are on the order of days to years.

Investigators are also exploring the creation of a "wet computer", using self-creating "cells" and other techniques to mimic certain properties of neurons.

The mechanism for this reaction is very complex and is thought to involve around 18 different steps which have been the subject of a number of research papers.

In a similar way to the Briggs–Rauscher reaction there are two key processes (both of which are auto-catalytic); Process A which generates molecular bromine giving the red colour, and process B which consumes the bromine to give bromide ions.

One of the most common variations on this reaction uses malonic acid (CH2(CO2H)2) as the acid and potassium bromate (KBrO3) as the source of bromine. The overall equation is given below:

3CH2(CO2H)2 + 4BrO3 → 4Br + 9CO2 + 6H2O

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