resonance theory. Resonance theory

Resonance theory- The theory of the electronic structure of chemical compounds, according to which the distribution of electrons in molecules (including complex ions or radicals) is a combination (resonance) of canonical structures with different configurations of two-electron covalent bonds. The resonant wave function describing the electronic structure of a molecule is a linear combination of the wave functions of the canonical structures .

In other words, the molecular structure is described not by one possible structural formula, but by a combination (resonance) of all alternative structures. Resonance theory is a way, through chemical terminology and classical structural formulas, to visualize a purely mathematical procedure for constructing an approximate wave function of a complex molecule.

The consequence of the resonance of canonical structures is the stabilization of the ground state of the molecule; the measure of such resonance stabilization is resonance energy is the difference between the observed energy of the ground state of the molecule and the calculated energy of the ground state of the canonical structure with the minimum energy . From the standpoint of quantum mechanics, this means that a more complex wave function, which is a linear combination of wave functions, each of which corresponds to one of the canonical structures, describes the molecule more accurately than the wave function of the minimum energy structure.

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Resonance Theory

Resonance structures, part I

Mesomeric effect (conjugation effect). Part 1.

Subtitles

Let's draw a benzene molecule. And let's think about what interesting processes for us occur in this molecule. So benzene. There are six carbon atoms in the cycle. The first, second, third, fourth, fifth and sixth carbons in the cycle. What makes benzene so special? What makes it different from cyclohexane? Of course, we are talking about three double bonds in the cycle. We will assume that these two carbons are connected by a double bond, there is also a double bond between these atoms, as well as between these carbons. Let's draw hydrogens only to remember that they are here at all. Let's draw them barely noticeable. So, how many hydrogens will be attached to this carbon? One, two, three valence electrons are already involved. Therefore, carbon is bonded to only one hydrogen. Everything is the same here. Only one hydrogen. There are four valence electrons in total. It's similar here. I think you already understand the system. In total, each carbon has three bonds with carbon atoms: two single bonds with two carbon atoms and one more double bond. Accordingly, the fourth bond is formed with hydrogen. Let me draw all the hydrogen atoms here. We will depict them in dark color so that they do not distract us. Now we have drawn benzene. In the future, we will encounter it more than once. But in this video, we're going to look at, or at least try to look at, a curious property of benzene, and that, of course, is resonance. This property is not specifically benzene, it is a property of many organic molecules. It's just that benzene is perhaps the most entertaining of them all. So let's think about what could be happening to this molecule. Let's start with this electron. Let's highlight it with a different color. Let's choose blue for this electron. So, here is this electron. What if this electron moves to this carbon? This carbon doesn't break the bond, it keeps an electron, which will just move here. So this electron has shifted here. Now this carbon has an unnecessary fifth electron. Therefore, one electron has shifted here. Now this carbon has five electrons. And so that electron will go back to the original carbon atom that lost the first electron. As a result, all carbon atoms remained the same. If this happens, then we will get a structure that looks like this. I will draw a double arrow, since the process can proceed in both directions. Let's start with the carbon chain. So, the first carbon, the second, the third, the fourth, the fifth and finally the sixth carbon. In the picture on the left, the double bond was here, so now it has moved here. Let's draw this double bond in blue to highlight the difference. Now the double bond is here. This blue electron moved here. This blue electron has moved up. Let's depict them in different colors, for greater clarity. Let's say this electron will be green. The green electron has migrated from this carbon to this carbon. We can imagine how it happened. Now consider this purple electron that was on this carbon atom, but now it's shifted and moved to another carbon here. Accordingly, the double bond also shifted, as indicated by this arrow. It remains to consider the blue electron. This blue electron is shifted to the first carbon. And the double bond, in turn, shifts here. Naturally, we got two very, very similar molecules. In fact, this is the same molecule, only turned upside down. We should be more interested in the fact that these double bonds gradually move back and forth, forming this structure, then that. And they do it all the time. Double bonds are constantly moving. And the reality of benzene is that none of these structures represent what is really happening. Benzene is in a certain transition state. The real structure of benzene looks more like this. I will not draw carbons and hydrogens now. Let's, perhaps, draw hydrogens here, since I started to depict them in the first drawing. So, we draw hydrogens here. Let's not forget about them. Although the presence of these hydrogens is always implied. Finished with hydrogens. Again, using this ring as an example, we may not draw carbons and hydrogens, since they are implied. So the real structure of benzene is between this and this. And in reality, there will be half a double bond between each carbon. That is, in fact, the structure looks something like this. There will be half a double bond here, half a double bond here, half a double bond here, the same here and half a double bond here. Almost finished. And here is half of the double bond. In fact, in the benzene molecule, electrons are constantly moving around the entire ring. And I do not mean the transition from one structure to another. The real structure, the energy of which is minimal, is shown here. So these Lewis structures, although it would be more correct to call them canonical structures, because I did not draw all the electrons. We often draw benzene in this way when, for example, we consider a mechanism. But it is important to understand that as a result of the resonance of these two structures, we get a transitional structure, which corresponds to reality. This happens not only with benzene. Many examples can be given. But we will analyze one more to fill our hand. Let's take a carbonate ion. Quite a striking example for demonstrating resonant structures. So, carbonate ion. The carbon is double bonded to one of the oxygen atoms and has two single bonds to the other oxygen atoms. And these two oxygens have extra electrons. This oxygen atom will have one, two, three, four five, six valence... Actually, of course, seven valence electrons. Let's do it again. One, two, three, four, five, six, seven valence electrons. And one extra electron leads to a negative charge. The same is true for this atom. It has one, two, three, four, five, six, seven valence electrons. One extra. So there will be a negative charge. Let's take a closer look at this resonant structure, or canonical structure. As we have already noticed, this oxygen is neutral. And it has six valence electrons. One two three four five six. Imagine that one of these electrons goes to carbon, causing the carbon to donate its electron to the upper oxygen. So we can imagine a situation in which this electron moves over here to carbon. And when the carbon gets one more electron, then at the same time, the carbon atom will give its electron to the upper oxygen, right here. How will the structure change if such a process occurs? So if the electrons move like this, here's what we'll see. Let's start with carbon. Now carbon only has a single bond here. Here we draw oxygen. Oxygen has six valence electrons. One, two, three, four, five, six electrons. But now he has another one, this blue one. So, since oxygen now has an extra seventh electron, we draw a negative charge on oxygen. This oxygen, which donated its electron to carbon, forms a double bond with the carbon atom. Let's draw a new link like this. So the double bond of carbon to this oxygen is at the bottom. One electron gave up the oxygen, so it now has six valence electrons. One two three four five six. And now the charge of oxygen is neutral. Nothing happened to this oxygen on the left. So just copy and paste it. Copy first, then paste. This oxygen stays here. Imagine a situation in which this oxygen with an additional electron, which, in turn, could come from another oxygen from above, will give its additional electron to the carbon atom. And then the carbon will break the double bond with the other oxygen. In this case, with this. Let me draw this. Perhaps a situation in which this electron will go to carbon ... A double bond will form. And then the carbon will give up one of its electrons. This electron here will go back to oxygen. What will happen? If this happens, the final structure will look like this. Let's start with carbon, single bonded to oxygen, which has one, two, three, four, five, six, seven valence electrons. Everything is still here. You can call it a resonant reaction, or you can call it something else. There is still a negative charge here. Let's move on to this oxygen. He got his electron back. And now it has seven valence electrons again. One, two, three, four, five, six, seven valence electrons again. Let's denote the electron that returned to oxygen. Let's make it purple. And now oxygen has a negative charge. This oxygen, in turn, donated an electron to carbon. And he formed a new double bond. Here is the double bond of this oxygen to carbon. One electron gave away oxygen, so now it has one, two, three, four, five, six valence electrons and a neutral charge. All these structures merge into each other. We can even get this structure from this. Starting with one of these structures, we can get any other. This is exactly what happens in the carbonate ion. Let me write down that this is a carbonate ion. So, its real structure is something in between these three. The structure of a carbonate ion actually looks like this. It's carbon bonded to three oxygens. Draw a bond between each of the three oxygens and carbon. And then every other C-O bond will have one-third the character of a double bond. One third connection. Not quite the usual record, but as close to reality as possible. A third of the time the electron will be here. The remaining two-thirds of the time, the oxygen atoms will equally own this electron. It is believed that each oxygen has a charge of −2/3. Usually, of course, one of these structures is drawn, because it is convenient to operate with integers. But in reality carbonate ions are subject to resonance. Electrons are, in fact, constantly moving from one C-O bond to another. This makes the molecule more stable. The energy of this structure is less than the energy of any of those given above. The same is true for benzene. The energy of this transition structure here is lower than the energy of any of these, and therefore this form of benzene is more stable than those drawn above. Subtitles by the Amara.org community

Story

The idea of ​​resonance was introduced into quantum mechanics by Werner-Heisenberg in 1926 when discussing the quantum states of the helium atom. He compared the structure of the helium atom with the classical system of a resonating harmonic oscillator.

The Heisenberg model was applied by Linus Pauling (1928) to describe the electronic structure of molecular structures. Using the method of valence schemes, Pauling successfully explained the geometry and physicochemical properties of a number of molecules through the mechanism of delocalization of the electron density of π bonds.

Similar ideas for describing the electronic structure of aromatic compounds were proposed by Christopher Ingold. In 1926-1934, Ingold laid the foundations of physical organic chemistry, developing an alternative theory of electronic displacements (the theory of mesomerism), designed to explain the structure of molecules of complex organic compounds that do not fit into the usual valence representations. The term proposed by Ingold to denote the phenomenon of electron density delocalization mesomerism"(1938), is used mainly in German and French literature, and English and Russian literature is dominated by" resonance". Ingold's ideas about the mesomeric effect became an important part of resonance theory. Thanks to the German chemist Fritz Arndt, the commonly accepted notation of mesomeric structures with the help of double-headed arrows was introduced.

USSR 40-50s

In the post-war USSR, the theory of resonance became an object of persecution within the framework of ideological campaigns and was declared "idealistic", alien to dialectical materialism - and therefore unacceptable for use in science and education:

The "resonance theory", being idealistic and agnostic, opposes the materialistic theory of Butlerov, as incompatible and irreconcilable with it; ... the supporters of the "resonance theory" ignored it and distorted its essence. "Theory of Resonance", being mechanistic through and through. denies the qualitative, specific features of organic matter and completely falsely tries to reduce the laws of organic chemistry to the laws of quantum mechanics ...

... The mesomeric-resonant theory in organic chemistry is the same manifestation of a general reactionary ideology, as is Weismannism-Morganism in biology, as well as modern "physical" idealism, with which it is closely connected.

Although the persecution of the theory of resonance is sometimes called "Lysenkoism in chemistry", the history of these persecutions differs in a number of ways from the persecution of genetics in biology. As Lauren Graham notes: “The chemists were able to repel this serious attack. The modifications of the theory were rather terminological in nature. In the 50s. chemists, without refuting criticism of the resonance theory, developed similar theoretical (including quantum chemical) constructions, using the term "

chemical resonance

Resonance theory- the theory of the electronic structure of chemical compounds, according to which the distribution of electrons in molecules (including complex ions or radicals) is a combination (resonance) of canonical structures with different configurations of two-electron covalent bonds. The resonant wave function, which describes the electronic structure of a molecule, is a linear combination of the wave functions of the canonical structures.

In other words, the molecular structure is described not by one possible structural formula, but by a combination (resonance) of all alternative structures.

The consequence of the resonance of canonical structures is the stabilization of the ground state of the molecule, the measure of such resonance stabilization is resonance energy is the difference between the observed energy of the ground state of the molecule and the calculated energy of the ground state of the canonical structure with the minimum energy .

Resonance structures of the cyclopentadienide ion

Story

The idea of ​​resonance was introduced into quantum mechanics by Werner Heisenberg in 1926 when discussing the quantum states of the helium atom. He compared the structure of the helium atom to the classical system of a resonant harmonic oscillator.

The Heisenberg model was applied by Linus Pauling (1928) to describe the electronic structure of molecular structures. Within the framework of the method of valence schemes, Pauling successfully explained the geometry and physicochemical properties of a number of molecules through the mechanism of delocalization of the electron density of π bonds.

Similar ideas for describing the electronic structure of aromatic compounds were proposed by Christopher Ingold. In 1926-1934, Ingold laid the foundations of physical organic chemistry, developing an alternative theory of electronic displacements (the theory of mesomerism), designed to explain the structure of molecules of complex organic compounds that do not fit into the usual valence representations. The term proposed by Ingold to denote the phenomenon of electron density delocalization mesomerism"(1938), is used mainly in German and French literature, and English and Russian literature is dominated by" resonance". Ingold's ideas about the mesomeric effect became an important part of resonance theory. Thanks to the German chemist Fritz Arndt, the commonly accepted notation of mesomeric structures with the help of double-headed arrows was introduced.

In the post-war USSR, the theory of resonance became an object of persecution within the framework of ideological campaigns and was declared "idealistic", alien to dialectical materialism - and therefore unacceptable for use in science and education:

The "resonance theory", being idealistic and agnostic, opposes Butlerov's materialistic theory as incompatible and irreconcilable with it; ... the supporters of the "resonance theory" ignored it and distorted its essence.

"Theory of Resonance", being mechanistic through and through. denies the qualitative, specific features of organic matter and completely falsely tries to reduce the laws of organic chemistry to the laws of quantum mechanics ...

... The mesomeric-resonant theory in organic chemistry is the same manifestation of a general reactionary ideology, as is Weismannism-Morganism in biology, as well as modern "physical" idealism, with which it is closely connected.

Kedrov B.M. Against "physical" idealism in chemical science. Cit. By

The persecution of the theory of resonance received a negative assessment in the world scientific community. In one of the journals of the American Chemical Society, in a review on the situation in Soviet chemical science, in particular, it was noted:

see also

Notes

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If there are usually no problems with the inductive effect, then the second type of electronic effects is much more difficult to master. This is very bad. The theory of resonance (mesomerism) has been and remains one of the most important tools for discussing the structure and reactivity of organic compounds, and there is nothing to replace it. But what about quantum science? Yes, it’s true that quantum chemical calculations have become easily accessible in our century, and now every researcher or even student, having spent very little time and effort, can free-of-charge calculations on their computer, the level of which all Nobel laureates would have envied 20 years ago. Alas, the results of the calculations are not so easy to use - they are poorly amenable to qualitative analysis and visually not very clear. It can take a long time to sit and stare at the endless columns of numbers and look at the confusing and overloaded pictures of orbitals and electron density, but few benefit from it. The good old theory of resonance in this sense is much more efficient - it quickly and fairly reliably gives exactly a qualitative result, allows you to see how the electron density is distributed in a molecule, find reaction centers, and evaluate the stability of important particles involved in reactions. Therefore, without the ability to draw resonant structures, evaluate their contribution, and understand what delocalization affects, no talk about organic chemistry is possible.

Is there a difference between the concepts of mesomerism and resonance? It used to be, but it doesn’t matter for a long time - now it is of interest only to historians of chemistry. We will assume that these concepts are interchangeable, you can use one or both in any proportions. There is one nuance - when they talk not about delocalization in general, but about the electronic substituent effect, they prefer the term mesomeric effect (and denoted respectively by the letter M). In addition, the word “conjugation” is also used (more precisely, π-conjugation).

And when does this mesomerism arise? This concept is applicable only to π-electrons and only if the molecule has at least two atoms with such electrons located side by side. There can be any number of such atoms, even a million, and they can be located not only linearly, but also with any branching. Only one thing is necessary - that they be close, form an inseparable sequence. If the sequence is linear, it is called a "conjugation chain". If it is branched, this complicates the matter, since there is not one conjugation chain, but several (this is called cross-conjugation), but at this stage you can not think about it, we will not carefully consider such systems. It is important that any atom without π-electrons interrupts such a sequence (conjugation chain), or breaks it into several independent ones.

Which atoms have π electrons?

• a) on atoms participating in a multiple (double, triple) bond - on each such atom there is one π-electron;
• b) on atoms of non-metals of 5-7 groups (nitrogen, oxygen, etc.) in most cases, except for nitrogen atoms of the ammonium type and the so-called onium atoms similar to them, which simply do not have free lone pairs);
• c) on carbon atoms with a negative charge (in carbanions).

In addition, empty π-orbitals in atoms with 6 valence electrons (sextet atoms) participate in conjugation: boron, carbon with a positive charge (in carbenium ions), as well as similar particles with nitrogen and oxygen atoms (we will put this aside for now) . Let's agree not to touch the elements of the third one, and so on. periods, even sulfur and phosphorus, because for them it is necessary to take into account the participation of d-shells and the Lewis octet rule does not work. It is not so easy to correctly draw boundary structures for molecules with the participation of these elements, but we most likely will not need it. If necessary, we will consider separately.

Let's look for conjugated fragments in real molecules. It's simple - we find multiple bonds, atoms with pairs and sextet atoms that are next to each other in any (yet) combinations. It is important that an observer walking along the conjugation chain should not step on atoms that do not belong to these three types. As soon as we meet such an atom, the chain ends.

Now let's look at how to portray it. We will depict in two ways - by arrows of the electron density displacement and by resonance (boundary) structures.

Type 1. We find donor and acceptor centers in the conjugated system...

Donor centers are atoms with a lone pair. Acceptor fragments are sextet atoms. Delocalization is always shown from the donor, but towards the acceptor in full accordance with their roles. If the donor and acceptor are nearby, everything is simple. Show the offset from the pair to the adjacent bond with an arrow. This will mean the formation of a π-bond between neighboring atoms, and thus the sextet atom will have the opportunity to fill the empty orbital and cease to be a sextet. This is very good. The image of boundary structures is also a simple matter. On the left, we draw the initial, then a special resonant arrow, then a structure in which the pair on the donor completely switched to the formation of a full-fledged π-bond. The real structure of such a cation will be much closer to the right boundary structure, because filling the sextet is very beneficial, and oxygen loses almost nothing, retaining eight valence electrons (the pair goes into a bond, which is also served by two electrons).

Type 2. In addition to the donor and acceptor, there are also multiple bonds ...

There may be two options here. The first is when multiple bonds are inserted between the donor and acceptor. Then they form a kind of extension for the system disassembled in Type 1.

If the double bonds are not one, but several, lined up in a chain, then the situation is not much more complicated. The arrows show the shift in density from the pair, and the successive shift of each double bond until the sextet is filled will require additional arrows. There are still two boundary structures, and again the second one is much more favorable and closely reflects the real structure of the cation.

The case when instead of the usual double bonds the benzene ring fits into this scheme quite well. It is only important to draw the benzene ring not with a nut, but with a normal Kekule structure. With a nut, the pairing will not work. Then we will immediately understand two important things: first, that the benzene ring in delocalization works as a conjugated system of double bonds and there is no need to think about any aromaticity; second, that the para- and ortho-arrangement of the donor/acceptor is very different from the meta-arrangement, in which there is no conjugation. In the figures, the conjugation paths are shown with pink spray, and it is clear that in the ortho case one double bond works, in the para case - two, and in the meta case, no matter how you draw it, the conjugation path is broken, and there is no conjugation.

If not double, but triple bonds come across, then nothing changes. You just need to imagine a triple bond as two mutually perpendicular π-bonds, and use one of them, and leave the other alone. Do not be afraid - it turns out a little scary from the abundance of double bonds in the boundary structure. Note that double bonds on one carbon atom are marked on a straight line (since this carbon atom has an sp hybridization), and to avoid confusion, these atoms are denoted by bold dots.

Type 3. In the conjugation chain, either a donor or an acceptor (but not both at once), and multiple bonds C \u003d C or C \u003d C

In these cases, a multiple bond (or a chain of multiple bonds) takes on the role of an absent one: if there is a donor, then it (they) becomes an acceptor, and vice versa. This is a natural consequence of the rather obvious circumstance that, during conjugation, the electron density shifts in a certain direction from the donor to the acceptor and nothing else. If there is only one connection, then everything is quite simple. Especially important are the cases when the donor is a carbanion, and also when the acceptor is a carbocation. Note that in these cases the boundary structures are the same, which implies that the real structure of such particles ( allyl cation and anion) is located exactly in the middle between the boundary structures. In other words, in real allyl cations and anions, both carbon-carbon bonds are exactly the same, and their order is somewhere in the middle between single and double. The charge (both positive and negative) is equally distributed on the first and third carbon atoms. I do not recommend using the rather common manner of depicting delocalization with a dotted bracket or one and a half dashed bonds, because this method gives a false impression of uniform charge delocalization across all carbon atoms.

If there are more multiple bonds, we proceed by analogy, adding arrows, involving each multiple bond in delocalization. But the boundary structures need to be drawn not two, but as many as there are multiple bonds in the chain plus the original one. We see that the charge is delocalized over odd atoms. The real structure will be somewhere in the middle.

Let us generalize to a donor - an atom without a charge, but with a pair. The arrows will be the same as in the case of the allyl carbanion. Boundary structures are formally the same, but in this case they are not equivalent. Structures with charges are much less beneficial than neutral ones. The real structure of the molecule is closer to the original one, but the pattern of delocalization makes it possible to understand why an excess electron density appears on the distant carbon atom.

Delocalization in the benzene ring again requires a representation with double bonds, and is drawn quite similarly. since there are three bonds and all of them are involved, then there will be three more boundary structures, in addition to the original one, and the charge (density) will be spread over ortho and para positions.

Type 4. In the conjugation chain, a donor and multiple bonds, some of which contain a heteroatom (C=O, C=N, N=O, etc.)

Multiple bonds involving heteroatoms (let me remind you that we have agreed to limit ourselves to the elements of the second period, that is, we are talking only about oxygen and nitrogen) are similar to multiple carbon-carbon bonds in that the π bond is easily shifted from the bottom atom to another, but differ the fact that the displacement occurs in only one direction, which makes such bonds in the vast majority of cases only acceptors. Double bonds with nitrogen and oxygen occur in many important functional groups (C=O in aldehydes, ketones, acids, amides, etc.; N=O in nitro compounds, etc.). This type of delocalization is therefore extremely important, and we shall see it frequently.

So, if there is a donor and such a connection, then it is very easy to show the density shift. Of the two boundary structures, the one in which the charge is on the more electronegative atom will prevail, however, the role of the second structure is also always very significant. Naturally, if the case is symmetrical, like the one shown on the second line, then both structures are the same and are represented equally - the real structure will be in the middle exactly the same as in the previously considered case of the allyl anion.

If there are also conjugated carbon-carbon bonds in the molecule or ion, they will participate modestly in the overall density shift. The same is the role of the benzene ring with the ortho- or para-arrangement of the donor and acceptor. Note that there are always only two boundary structures - they show the two extreme positions for the density shift. Intermediate structures (where the density has already shifted from the donor to a multiple bond, but has not gone further) do not need to be drawn. In fact, they exist and are quite legal, but their role in delocalization is negligible. The third example in the presented diagram shows how to draw a nitro band. At first, it frightens with an abundance of charges, but if you look at it just like the nitrogen-oxygen double bond, then the displacement is drawn in the same way as for any other multiple bonds with heteroatoms, and those charges that are already there should simply be left in rest and do not touch.

And another common option - there is one donor, and there are several acceptor multiple bonds (two, three). Strictly speaking, in this case, not one conjugation chain, but two or three. This increases the number of boundary structures, and can also be shown by arrows, although this method is not entirely correct, since there will be several arrows from one donor pair. This example clearly shows that boundary structures are a more universal way, although more cumbersome.

What else do you need to know about the possibility of pairing? You also need to imagine how a molecule (particle) is arranged. For conjugation, it is necessary that the orbitals of π-electrons be parallel (collinear, lie in the same plane), or make an angle that is very different from a right one. It sounds quite rotten - how do you actually know it ?! Not everything is so scary, we will not meet with really difficult cases yet. But one thing is quite obvious: if one atom has not one, but two π-orbitals, then they are mutually strictly perpendicular and cannot simultaneously participate in the same conjugation chain. Therefore, double bonds in 1,2-dienes (allenes), carbon dioxide and similar molecules (cumulene and heterocumulene) are not conjugated; the π-bonds of the ring and the lone pair in the phenyl anion are not conjugated, etc.

RESONANCE THEORY , theory of the electronic structure of chem. compounds, which is based on the idea that the electronic distribution, geometry, and all other physical. and chem. properties of molecules must be described not by one possible structural f-loy, but by a combination (resonance) of all alternative structures. The idea of ​​such a way of describing the electronic structure belongs to L. Pauling (1928). R. t. is a development of the classic. theories of chem. structures for molecules, ions, radicals, the structure of which can be represented as several. dec. structural f-l, differing in the way of distribution of electron pairs between atomic nuclei. According to R. t., the structure of such Comm. is intermediate between the individual possible classic. structures, and the contribution of each individual structure can be taken into account using decomp. modifications of quantum mech. valence bond method (see. Valence bond method).

For conn. with conjugated bonds of all possible structures with decomposition by types of electron pairing of multiple bonds, it is sufficient to consider only structures with non-crossing bonds (canonical structures). The electronic structure of benzene is described by the resonance of five canons. structures:

The wave function of the benzene molecule according to Pauling is a linear combination:

Y = 0.624(Y I + Y II) + 0.271(Y III + Y IV + Y V).

Whence it follows that the the contribution (about 80%) to the wave function is made by the Kekul structures I and II. Their equivalence and the equivalence of structures III-V explain the evenness of all carbon-carbon bonds in the benzene molecule and their intermediate (approximately one and a half) character between single and double carbon-carbon bonds. This prediction is in full agreement with the experimentally found C-C bond length in benzene (0.1397 nm) and the symmetry of its molecule (symmetry group D 6h).

R. t. is successfully used to describe the structure and properties of ions and radicals. Thus, the structure of a carbonate ion is represented as a resonance (indicated by a double-sided arrow) of three structures, each of which makes the same contribution to the wave function:

Therefore, the ion has trigonal symmetry (symmetry group V 3h ), And each C-O bond has 1/3 the character of a double bond.

The structure of the allyl radical does not correspond to any of the classic. structures VI and VII and should be described by their resonance:

The EPR spectrum of the allyl radical indicates that the unpaired electron is not localized on any of the terminal methylene groups, but is distributed between them so that the radical has the С2 symmetry group h, and energetically. the rotation barrier of terminal methylene groups (63 kJ/mol) has an intermediate value between the values ​​characteristic of the rotation barriers around a single and double C-C bond.

In Comm., including bonds between atoms with significantly decomp. electronegativity, that is. contribution to the wave function is made by resonant structures of the ionic type. The structure of CO 2 within the framework of R. t. is described by the resonance of three structures:

The bond length between the C and O atoms in this molecule is less than the length of the C=O double bond.

Polarization of bonds in the formamide molecule, leading to the loss of mn. st-in, characteristic of the carbonyl group, is explained by resonance:

The resonance of the structures leads to the stabilization of the main. states of a molecule, ion, or radical. The resonance energy serves as a measure of this stabilization, the greater the number of possible resonant structures and the greater the number of resonant low-energy structures. equivalent structures. The resonance energy can be calculated using the valence bond method or the pier method. orbitals (see Molecular orbital methods ) as the difference between the energies of the main. state of the molecule and its isolation. connections or main states of a molecule and a structure simulating one of the stable resonant forms.

According to its main the idea of ​​R. t. is very close to the theory of mesomerism (see. Mesomeria ), however wears more quantities. character, its symbolism follows directly from the classic. structural theory, and quantum mechanics. the method of valence bonds serves as a direct continuation of R. t. Because of this, R. t. continues to retain a certain value as a convenient and visual system of structural representations.

Lit.: Pauling L., The nature of the chemical bond, trans. from English, M.-L., 1947; Weland J., Resonance theory and its application in organic chemistry, trans. from English, M., 1948; Pauling L., "J. Vese. Chemical Society named after D. I. Mendeleev", 1962 v. 7, no. 4, p. 462-67. V. I. Minkin.

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A convenient way to depict delocalization in coupled systems is to depict using resonance structures .

When writing resonant structures, the following rules should be observed:

1. Atoms and molecules do not change their position; the position of the NEP and π-electrons of multiple bonds changes.

2. Each resonant structure attributed to a given compound must have the same sum of π-electrons, including π-bonds and NEP.

3. A resonant arrow "↔" is placed between the resonant structures.

4. In resonant structures, the designation of electronic effects with straight and curved arrows is not accepted.

5. The set of resonant structures of a molecule, ion, or radical should be enclosed in square brackets.

For example:

When evaluating the resonance stabilization of molecules and particles, as well as when comparing the relative energies of various resonant structures, one should be guided by the following rules:

1. The energy of a real molecule is less. Than the energy of any of the resonant structures.

2. The more resonant structures that can be written for a given molecule or particle, the more stable it is.

3. Ceteris paribus, resonant structures with a negative charge on the most electronegative atom and with a positive charge on the most electropositive atom are more stable.

4. Resonance structures in which all atoms have an octet of electrons are more stable.

5. Particles have the maximum stability for which the resonant structures are equivalent and, accordingly, have the same energy.

5.2. THEORY OF ACID AND BASES IN ORGANIC CHEMISTRY

There are two main theories of acids and bases in organic chemistry. This theories of Bronsted and Lewis.

Definition: According to Bronsted's theory, an acid is any substance capable of dissociating with the elimination of a proton. Those. acid is a proton donor. A base is any substance that can accept a proton. Those. base is a proton acceptor.

According to Lewis theory, an acid is any molecule or particle capable of accepting electrons into a vacant orbital. Those. acid is an electron acceptor. A base is any molecule or particle capable of donating electrons. Those. base is an electron donor.

Definition: The particle formed from the acid after dissociation and carrying a negative charge is called the conjugate base. The particle formed from the base after the addition of a proton and carrying a positive charge is called a conjugate acid.

5.2.1. Bronsted acids

A characteristic of the strength of acids, in relation to water, is the dissociation constant, which is the equilibrium constant of the following reaction:

The best-known examples of acids in organic chemistry are aliphatic carboxylic acids, such as acetic acid:

and benzoic:

Carboxylic acids are medium strength acids. This can be verified by comparing the pK values ​​of carboxylic acids and some of the others listed below:

Organic compounds belonging to different classes of organic compounds can split off a proton. Among organic compounds, OH-, SH-, NH- and CH-acids are distinguished. OH-acids include carboxylic acids, alcohols and phenols. NH-acids include amines and amides. CH-acids include nitroalkanes, carbonyl compounds, esters, terminal alkynes. The very weak CH-acids include alkenes, aromatic hydrocarbons and alkanes.

The strength of an acid is closely related to the stability of the conjugate base. The more stable the conjugate base, the more acid-base balance is shifted towards the conjugate base and acid. Conjugate acid stabilization may be due to the following factors:

The higher the electronegativity of an atom, the stronger it holds the electrons in the conjugate base. For example, the pK of hydrogen fluoride is 3.17; pK of water 15.7; pK for ammonia 33 and pK for methane 48.

2. Stabilization of the anion by the mesomeric mechanism. For example, in the carboxylate anion:

In the alkoxide ion, for example:

such stabilization is not possible. Accordingly, for acetic acid pK=4.76, pK for methyl alcohol is 15.5.

Another example of conjugate base stabilization is the phenolate ion resulting from the dissociation of phenol:

For the resulting phenoxide (or phenolate) ion, resonance structures can be constructed that reflect the delocalization of the negative charge along the aromatic ring:

Accordingly, the pK of phenol is 9.98, and methanol, for which it is impossible to build resonance structures, has a pK of 15.5.

3. The introduction of electron-donating substituents destabilizes the conjugate base and, accordingly, reduces the strength of the acid:

4. The introduction of electron-withdrawing substituents stabilizes the conjugate base and increases the strength of acids:

5. Removal of the electron-withdrawing substituent from the proton-donor group along the chain leads to a decrease in the strength of the acid:

The data presented illustrate the rapid decay of the inductive effect as the hydrocarbon chain lengthens.

Particular attention should be paid CH-acids , since the conjugate bases formed during their dissociation, which are carbanions. These nucleophilic species are intermediates in many organic reactions.

CH acids are the weakest of all types of acids. The product of acid dissociation is a carbanion - a particle in which the basis is a carbon atom bearing a negative charge. Such a particle has a tetrahedral structure. The NEP occupies the sp 3 hybrid orbital. The strength of CH-acid is determined by the same factors, chito and strength of OH-acid. The series of stabilizing influence of substituents coincides with the series of increasing their electron-withdrawing properties:

Among the CH acids, the allyl anion and the benzyl anion are of particular interest. These anions can be represented in the form of resonance structures:

The effect of delocalization of the negative charge in the benzyl anion is so strong that its geometry approaches a flat one. In this case, the carbon atom of the carbanion center changes hybridization from sp 3 to sp 2 .