How many carbon environments in benzene derivatives
5. Aromatic hydrocarbons
They are called because of the strong aroma that many benzene derivatives give off aromatic compounds. That is why benzene is considered the parent compound of aromatics. Aromatic rings are found in many biologically active substances.
We will only consider benzene derivatives here, and not aromatic rings that contain a heteroatom (Chapter 6), e.g. N, O or S.
5.1 The naming of benzene derivatives
The general term for substituted benzene derivatives is Arenas. An arene as a substituent is called Aryl group, abbreviated Ar-. The simplest aryl group is called Phenyl, C6H5-, abbreviated Ph-.
Some common names that are common:
Many monosubstituted benzene derivatives are named by putting the substituent name in front of the word "benzene":
In the case of disubstituted benzene derivatives, the substituents can assume three possible positions with respect to one another: Adjacent substituents 1,2- (ortho), for 1,3-disubstituted compounds 1,3- (meta), for 1,4-disubstituted compounds 1,4- (para). The substituents are listed in alphabetical order.
5.2 The structure of benzene: aromaticity
In 1825, the English scientist Faraday received a colorless liquid with the empirical formula CH. This connection was inconsistent with the theory that each carbon had to develop four valences to other atoms. The unusual stability and chemical inertia of this substance were particularly noticeable. The connection was called benzene and finally put the empirical formula C6H6 for it on.
A problem at the time was understanding why two different isomers of a 1,2-disubstituted benzene derivative could never be observed. As a solution, Kekulé proposed in 1865 that benzene convert rapidly into one another Isomers should consist of cyclohexatriene. Today we know that this hypothesis is not entirely correct. According to modern electron theory, benzene can be described by two equivalent resonance structures of cyclohexatriene:
Benzene is unusually inert. It does not undergo addition reactions like normal alkenes.
Molecular orbital model of benzene
The figure shows us the electronic structure of the benzene ring. All carbon atoms are sp2-hybridized and each p-Orbital overlaps evenly with its two neighbors. The electrons delocalized in this way form one & # 960 electron cloud above and below the ring plane.
The six overlapping ones arranged cyclically p-Orbitals form a set of six molecular orbitals.
The benzene molecule forms a regular hexagon of six sp2-hybridized carbon atoms. The length of the aromatic C-C bond is between that of a single and that of a double bond.
Description of the valence structure of benzene
The structure of benzene can be determined by two equivalent resonance structures of cyclohexatriene
The real state is then called "Intermediate state" or "Resonance hybrid" reproduced between these boundary structures.
The sign <-> means that there is no dynamic equilibrium between two different types of molecules, but that the actual state lies between the boundary structures and is to a certain extent "circumscribed" by them. The description of a real structure by combining non-existent boundary structures is called resonance.
5.3 Stability of Benzene
To get a measure of the relative stability of a number of alkenes, one can determine their heats of hydrogenation. We can carry out a similar experiment with benzene and compare its heat of hydrogenation with that of 1,3 cyclohexadiene and cyclohexene:
Although benzene is difficult to hydrogenate, the reaction can be carried out catalytically and the heat of hydrogenation is given a value of & # 916HO-206 kJ / mol. The difference between the heats of hydrogenation (and the enthalpies of formation) is approximately 124 kJ / mol and is called Resonance energye denoted by benzene. Other names for it are delocalization energy, aromatic stabilization, or simply Aromaticity of benzene.
5.4 The Hückel rule
With the help of quantum theoretical calculations it can be shown that a monocyclic conjugated polyene is particularly stable (i.e. has aromaticity) if it contains 4n + 2 π electrons (n is an integer), e.g .:
5.5 Electrophilic aromatic substitution
Because of its π electrons, the benzene molecule has nucleophilic properties and therefore reacts primarily with electrophiles. e.g .:
Unlike the Alkenes, such an attack leads to one substitution of an H atom, not one addition. e.g. bromination in the presence of a catalyst:
Mechanism of electrophilic aromatic substitution:
The first step is thermodynamically unfavorable because the cyclic delocalization and thus the aromatic character is lost. After this step, the aromatic ring is regenerated again by removing the proton from the sp3-hybridized carbon is split off. This is energetically more favorable than a reaction with a nucleophile, which would produce the addition product:
The change in potential energy during the reaction of benzene with an electrophile. The formation of the first transition state is rate-limiting. The proton is split off relatively quickly.
The reaction of benzene with conc. ENT3 at a moderately elevated temperature leads to nitration of the benzene ring:
Conc. H2SO4 does not react with benzene at room temperature, one ignores protonation. "Smoking sulfuric acid" (oleum) has an electrophilic attack because it is SO3 contains. Due to the strong electron-withdrawing effect of the three oxygen atoms, the S atom is in SO3 so electrophilic that it directly attacks benzene:
In Friedel-Crafts reactions new C-C bonds are created. In the presence of a Lewis acid, usually aluminum chloride, haloalkanes attack benzene to form alkylbenzene derivatives (Friedel-Crafts alkylation), Alkanoyl halides give alkanoyl derivatives (Friedel-Crafts acylation):
The reactivity of the haloalkane decreases in the order RF> RCl> RBr> RI. Typical Lewis acids in this reaction are (in order of decreasing reactivity) AlBr3, AlCl3, FeCl3, SbCl5 and BF3. Further examples:
By Friedel-Crafts acylation aryl-alkyl-ketones are formed. e.g. Benzene reacts with acetyl chloride in the presence of AlCl3 and forms acetophenone:
The formation of the acylium cation occurs generally through the reaction of alkanoyl halides (acyl halides) with AlCl3. Carboxylic acid anhydrides react similarly with Lewis acids.
5.6 Inductive and Resonance effects in the benzene ring
A substituent on the benzene ring has an electronic effect by either transferring electron density to the ring or by removing electron density. In this way a monosubstituted benzene ring can have different chemical and physical properties compared to benzene, e.g. an electrophilic aromatic substitution reaction can be much slower (i.e. becomes much slower) deactivated) or faster (i.e. will activated) than with benzene. The withdrawal or delivery of electrons can by Inductive and through Resonance effects come about.
Inductive and resonance effects influence many chemical and physical properties of aromatics, not just their reactivity in electrophilic substitution reactions.
Inductive and resonance effects
Let us consider examples of electron-donating and electron-withdrawing substituents:
Inductive effects are polarization effects that are transmitted via σ-bonds (see Chapter 1.10).
It is also possible that a substituent is attached to an aromatic ring with p electrons conjugates with the p-electrons of the ring and thereby either withdraws negative charge from the unsaturated system or presses negative charge into it. Often one speaks of one mesomeric effect or Resonance effectThis is due to the fact that in these cases the charge density distribution can be described by combining different boundary structures, since delocalization of the p-electrons occurs. e.g. a & # 960 acceptor
Their acceptor effect (the strength of the mesomeric effect) increases with the willingness of the substituent to accept negative charge.
Other & # 960 acceptor groups: In the following row it therefore increases to the right:
& # 960 donor groups: Since N is more electronegative than C, the -NH group practices2 a slightly electron-withdrawing effect (inductive effect). The lone pair of electrons of the N atom can, however, be introduced into the aromatic π system, so that the charge density of the ring is increased:
This resonance effect far outweighs the inductive effect. Aniline is therefore easily accessible for further substitution.
The electronegative O atom in phenol also has an electron-withdrawing effect (inductive effect). But here too the influence of the resonance predominates (p-donor effect)so that the benzene nucleus becomes more electron-rich:
5.7 Orientation and relative speed of a second substitution
Does an already existing substituent influence the point in the ring where an electrophile attacks and how fast the reaction takes place (compared to benzene)? Let's first look at toluene, which carries the inductively activating methyl group:
The electrophilic nitration of toluene leads mainly to ortho and para-Substitution.
Nitration is not a special case:
Trifluoromethylbenzene is deactivated against the electrophilic attack and therefore reacts only hesitantly. Under strict conditions, however, substitution is obtained, but only in meta-Position:
Often not even a catalyst is necessary for an electrophilic attack on a resonance-activated benzene ring. The reaction proceeds rapidly and completely regioselectively to the (often polysubstituted) ones ortho- and para-Products:
Anisole reacts about 1000 times more quickly than benzene! Let us now turn to benzene derivatives that carry deactivating groups through resonance. This subheading includes benzoic acid, in which nitration is 1000 times slower than with benzene:
The nitration mainly leads to meta-Product.
The Halogens (-F, -Cl, -Br, -I) as substituents form a third group that deactivates the ring (chlorobenzene reacts approx. 15 times more slowly than benzene), but mainly ortho- and para- substituted products form:
Directing Effect of Substituents in Electrophilic Aromatic Substitution:
|Ortho and para-directing||Ortho and para-directing|| Meta-directing ||activating||deactivating||deactivating ||(& # 963 and & # 960 donor groups)||(& # 963 and & # 960 acceptor groups) ||-NH2, -NHR, -NR2||-F, -Cl, -Br, -I||-NO2 -CF3||-NH-COR||-NO3+, -COOH ||-OH, -OR||-COOR, -CO-R ||-R (alkyl, aryl)||-SO3H, -CN |
5.8 Mechanistic explanation
Can we put in place mechanisms that can explain these selectivities? For this purpose we want to draw the possible resonance structures of the cation, which after attack by the electrophile (E.+) arises. We find that & # 963 and & # 960 donor substituentsthat are in the ortho or para position stabilize the intermediate (and the transition state that leads to it):
Just by attacking in ortho- and paraPosition creates a cation with a resonance structure that carries a positive charge next to the alkyl group. Since this structure has something of the character of a 3O-Carbenium ions, it is more important than others who have the positive charge on a 2O Carry C atom.
Through a metaAttack, on the other hand, creates an intermediate stage in which none of the possible resonance structures depend on the stability of a thirdO-Carbenium Ions benefits. The electrophilic attack on a carbon atom ortho- or para- to a methyl (alkyl) group therefore leads to an intermediate stage that is more stable than that produced by a metaAttack would arise. It therefore arises relatively quickly via a transition state with relatively low energy (see Chapter 5.5).
In the case of electrophilic attack on a resonance-activated benzene ring, the observed regioselectivities can also be explained by the resonance structures of the various intermediate cations, e.g .:
Ortho- and paraSubstitutions are preferred because they run through cations for which four resonance structures can be established, while one can be used for the intermediate stage meta-Attack only three are to be formulated.
If we now consider benzene derivatives that have resonance deactivating groups:
The attack on the metaPosition avoids a positive charge next to the electron-withdrawing carboxy group. At ortho- and para Attack, on the other hand, must be formulated with very high-energy resonance structures. The situation is analogous for other deactivating substituents.
Halogen atoms such as Cl and Br are spatially much larger than O and N atoms. Although a p-donor effect of -Cl is still present, the resonance effect is not as effective (as with N and O atoms) because the p orbitals on -Cl overlap less well with the neighboring p orbitals of the benzene ring. N, O and Cl atoms are all electronegative (σ acceptor effect), but with O and N atoms the resonance effect (p donor) is much stronger. This means that chlorobenzene is less reactive, but still prefers substitution reactions in ortho and para positions. e.g.
The resonance effect also leads to ortho- and paraProducts.
5.9 Electrophilic attack on disubstituted benzenes
The effects of two substituents on the relative speed and on the orientation of the electrophilic substitution of the benzene ring add up, e.g .:
When there are two opposing steering groups, they usually exercise their influence independently of one another. If the substituents compete with one another for the site of the substitution, the stronger activating (or deactivating) group prevails. Resonance effects are usually stronger than inductive effects.
If spatially extensive groups are present, a position between these substituents is often unfavorable for steric reasons:
In most other cases, product mixtures result.
5.10 Synthetic Aspects
Benzene occurs in various technical processes, e.g. in the aromatic fraction of crude oil distillate. Around 35 million tons of benzene are produced around the world every year. From 1940 to around 1960, most of the benzene was produced on the basis of hard coal. It has also been cracked from crude oil since 1950.
In the synthesis of a specific benzene derivative, everything depends on whether the first substituent introduced directs further substituents into the correct position:
Aromatic carboxylic acids
There are certain reactions that can reverse the directing effect of a substituent. E.g. the benzene ring is resistant to oxidation because of its resonance energy. But an alkyl side chain can be oxidized to a carboxylic acid:
e.g. for the synthesis of para-Bromobenzoic acid:
A meta-Directing nitro group can be included in the ortho- and para-directing amino group are converted. Nitro groups can become amino groups reduced become:
For a synthesis of p- Aminobenzoic acid (a component of the vitamin tetrahydrofolic acid):
(Watch out: Friedel-Crafts reactions to nitrobenzene are not possible - -NO2 has too strong a deactivating effect)
5.11 Polynuclear benzoid hydrocarbons
The condensation or fusing of several benzene rings results in a class of compounds that are known as polynuclear benzoid hydrocarbons designated.
As is to be expected, electrophilic substitutions can be carried out on these rings, which proceed according to the same mechanism as the corresponding reactions on benzene and its derivatives. The new main focus lies with the regioselectivity of such processes, which we will not consider here.
Many of the polynuclear benzoid hydrocarbons are carcinogenic (carcinogenic). A particularly well-researched molecule is benz [a] pyrene. Benz [a] pyrene is produced when organic matter such as automobile fuel and petroleum is burned, when it is incinerated, in forest fires; it can be found in cigarettes and in cigar smoke and even in grilled meat.
How does the carcinogenic effect of Benz [a] pyrene come about? It is believed that an oxidizing enzyme (a Oxidase) from the liver converts the hydrocarbons into the C7 / C8 epoxide. Another enzyme (Epoxide hydratase) catalyzes the hydration of the product to the trans-Diol (see page 35). Further oxidation then creates the actual carcinogen, a new C9 / C10 epoxide:
The carcinogenic event presumably occurs when the amine nitrogen of the guanine, one of the bases in the DNA strand, attacks the epoxide nucleophile:
This reaction changes the structure of a base pair of DNA, which leads to errors and disruptions in DNA replication. These errors can lead to a change (mutation) of the genetic information, which under certain circumstances can trigger the growth of a line of rapidly and undifferentiated cells, which is typical for cancer.
Graph is the name for a modification of carbon with a two-dimensional structure, in which each carbon atom is surrounded by three more, so that a honeycomb-shaped pattern is formed (see picture on the left). Graphene has unusual properties that make it interesting for both basic research and applications, especially in physics (as shown by the Nobel Prize, which was awarded in 2010). For example, graphene surface single crystals are extremely stiff and strong within the surfaces. Because of the high electrical conductivity of graphene, research is currently being conducted into the question of whether graphene could replace silicon as a transistor material.
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