Mikhail Levitsky: Carnival Molecules

Mikhail Levitsky Carnival Molecules Alpin non-fiction

The publishing house "Alpina Non-Fix" was published by the book "Carnival Molecules. The chemistry is unusual and funny ”, written by a candidate of chemical sciences, leading researcher at the Institute of Elementary Organic formations named after A. N. Nesmeyanova RAS Mikhail Levitsky.

A book in the form of entertaining conversations offers interesting examples and stories that will allow parents to attract the attention of schoolchildren to study natural sciences, high school teachers - make classes more exciting, as well as introduce students and graduate students who have chosen chemistry with their specialty, with how the researcher's course allows you to get interesting results. The book tells about some dramatic, and, sometimes funny turns of the fate of both the discoveries themselves and their authors. In addition, the reader will practice in solving entertaining problems, which is especially pleasant when a hint is placed nearby, and then the answer itself. Unlike the textbook in the book there is no consistent presentation of the foundations of chemistry, so it can be read, starting with any chapter.

With the permission of the publishing house, we publish one of the sections of the book dedicated to Cataliz.

Catalytic sketches

I just don't want to believe
that the whole is not familiar
With your private.
E. Lez

The modern teaching of catalysis is a giant picturesque canvas, on which two partially intersecting plots can be distinguished from a great distance. The first considers the processes by which chemists strive to learn how to do what nature has long been able to do. It is primarily about the synthesis of various organic substances obtained by living organisms literally from the Earth, Water and air. Not trying to accurately reproduce nature, our magnificent predecessors have achieved a lot, especially when obtaining drugs and various biologically active drugs.

Now we are moving to the second plot of the mentioned picture. It covers the processes that nature does not know how to do, most likely, as unnecessary: ​​for example, in neither in the organic nor in the mineral world, we do not find the processes of chlorination, niting, polymerization, etc. However, this type of reactions are extremely necessary for which modern civilization can not do. A few small sketches that complement this part of the large picture will be discussed.

How to overcome a narrow place?

Chemics took up the modification of the properties of natural rubber for a long time. One of the most successful results was obtained when in the isoprene molecule h2c = ch -

C (CH3) = CH2 (monomer from which natural rubber is formed) the methyl group was replaced by chlorine H2C = CH - C (CL) = CH2, receiving chloroprene (Fig. 1).

Rice. 1

Polymer chloroprene, usually called chloroprene rubber, is extremely resistant to gasoline and oils. They receive chloroprene according to the following chain of processes: at first, butadien is halogen (a hydrocarbon with two double bonds), while one of the double bonds is revealed, joining two chlorine atoms, as a result, there are a symmetric, in which the chlorine atoms are located on the ending atoms of carbon, and b) are irimy -mmed, when chlorine atoms are located in the neighboring carbon atoms (Fig. 2).

Rice. 2

A symmetrical product for obtaining chloroprene rubber is not suitable, you only need asymmetric. The HCL with the action of alkali is hidden from it, while a new double bond arises - chloroprene forms, which is then polymerized with the receipt of chloroprene rubber (Fig. 3).

Rice. 3

At all stages, except for the very first, the output is 95–98 %, a narrow place is the first stage (the result of halogenation), since the asymmetric product necessary for further transformations is formed in an amount of almost half as much as symmetrical. In this regard, another stage is included in the technological chain, which allows a symmetrical compound to translate into asymmetric. In fact, this is isomerization, but it can only be carried out in the presence of a catalyst, therefore the process is called catalytic isomerization. The process is reversible, which is indicated by double arrows. Thus, the 100 %transformation cannot be carried out, the system will reach equilibrium, and a mixture of both compounds is formed, which is already good, since we still get part of the right product (Fig. 4).

Rice. 4

The catalyst cannot shift the balance: if the speed of a direct reaction increases in its presence, then the speed and reverse process increase accordingly. Thus, the task of the catalyst is to accelerate both reactions at the same time, that is, reduce the time necessary to achieve balance. Further, the problem is solved purely technically: after the balance is achieved, an asymmetric product is driven out of the reaction mass, and its equilibrium concentration is again restored in the remaining reaction mixture. The faster the equilibrium is achieved with the help of the catalyst, the better the catalyst works and, therefore, the more effective the process.

The catalysts used for this - copper naphthenate or its halide - are ineffective and quickly lose activity. In the search for new catalysts, scientists turned to metal-organic carcasses-compounds containing groups -rsi-o-m-m-o --rsi-–-. They are attractive in that they allow you to introduce various metals atoms into their structure, smoothly adjusting the atomic ratio of M/SI, in addition, they are soluble in most organic solvents, which increases the range of their use.

Metalloorgazylovsans containing fragments -sir - o - m– m, as catalysts were not previously studied in processes of this type, but they justified the hopes assigned to them: the conversion (degree of transformation) of a symmetrical product into an asymmetric (within 1 hour) amounted to 72 %, which is almost four times higher than the previously used catalysts.

The most noticeable difference between a new catalyst and previously studied is the extremely high stability of the catalytic properties: the multiple use of such a catalyst (6–10 times) does not lead to a noticeable decrease in its activity.

Halogen without elementary halogen

Chlorine-containing hydrocarbons serve as initial compounds when obtaining a wide variety of organic substances, in connection with which halogening is included in the circle of multi-tonastic processes of organic synthesis, but such processes are environmentally friendly due to the use of elementary chlorine.

There are other ways of obtaining cotton -breeding: for example, the exchange halogening CCL4 with alkanes, when the alkan “takes” chlorine -containing chlorine compound (Fig. 5).

Rice. 5

The interaction of a pair of reagents of the dean C10N12 - CCL4 carbon tetrachloride catalyzes various copper complexes, but in the process they quickly lose their activity. The copper -containing Silovsan in catalytic activity is noticeably inferior to copper complexes (4–5 times), but surpasses in the stability of catalytic properties, its activity is practically not reduced after five times, while the standard catalyst completely loses its performance during one cycle.

A detailed study of the process showed that an increase in the content of the catalyst does not lead to a significant increase in conversion (transforming the initial connection into a final product). The same was observed in another similar process.

If the catalyst does not lead to selectivity

In the process of metabolic halogenation, chlorine joins any of the carbon atoms in the dean, as a result, a mixture of monochlorized compounds is formed (almost only the end SN3 groups are not affected). Such low selectivity (selectivity) usually does not satisfy chemists, partially the problem can be solved by changing the structure of the initial reagent. If instead of an alkan take Alken (double bond hydrocarbon), then the position of double communication will indicate the place where CL should join and, accordingly, the CCL3 fragment. This was checked using Octen C6N13CN = CH2 (Fig. 6).

Rice. 6

Just as in the previous two processes, the catalyst retained the stability of the catalytic properties with multiple use. Interestingly, the marked phenomenon was again confirmed: the maximum activity of the catalyst (defined as the degree of transformation attributable to one catalytic center) is observed only with a very small content.

The pattern, which had to be understood, was clearly traced. This could not be explained by the fact that part of the catalytic centers was in the deep layers of the catalyst and was not available to the process. The explanation received when studying the features of the structure of the catalyst itself, which will be discussed below.

Bypass maneuver

The fact that the noticeable part of the copper atoms is “turned off” from the catalytic process was explained using magnetic measurements and spectral studies of the initial metal -organic cells. It turned out that a significant part of the metal atoms is connected in interceptive coordination clusters. Metal seeks to fill its coordination sphere, attracting oxygen atoms of neighboring fragments - SI - O - M– (coordination interactions are shown by dotted lines in Fig. 7).

Rice. 7

As a result, the CU atom turns out to be coordinating-saturated and its interaction with the CCL4 reagent is difficult. How to prevent the formation of interceptive clusters? You can, for example, “cover” the metal with any ligand, which, of course, will slow down interceptive coordination, but will not help solve the main problem-the ligand, in turn, will complicate the approach of the reagent. Nevertheless, the solution was found when an organic group related to silicon was attracted to participation. A rigid and sedentary phenyle group, framing silicon in the studied metal -organic cells, cannot interfere with the coordination interaction of copper atoms in neighboring chains. If you replace the Phoenile group PH of the Nonal C9H19, then the situation changes (the mesh surface on Fig. 8 conditionally denotes the so-called van-der-vials radius-that part of the space that the molecule fragment really occupies).

Rice. 8

The voluminous aliphatic group effectively prevents the coordination interaction of copper atoms in neighboring chains, “enveloping” metal centers, but in the reaction medium it has mobility and does not prevent the approach of the CCL4 reagent to the catalytic center. Thus, the carbon tetrachloride can push the nonal group and approach the catalytic center, but this cannot do the copper atom in the next chain.

The results of the experiments turned out to be very convincing: with the catalytic accession of CCL4 to OKTENU-1, the activity of copper-cups framed by non-nonal groups turned out to be twice as high as that of Medfenilsilsilsans.

The author of the works that were described here, is Professor V.V. Smirnov (Fig. 9), who worked at the chemical faculty of Moscow State University. He loved to repeat: “Find a catalyst is a lot of business. It is equally important to learn how to manage it. ”

Rice. 9. Professor V.V. Smirnov (1946–2009)

The picturesque canvas, which we spoke about at the very beginning of this chapter, is constantly expanding and supplemented, which does not prevent us from looking with interest at the whole picture as a whole, as well as peering into its individual fragments, observing how the wonderful area of ​​chemistry, called catalysis, develops.

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