What is the hybridization of N3H




Aromatics (aromatic compounds) or also Arenas are an important class of compounds in organic chemistry. They are characterized by a special bond structure. The term Aromat does not generally indicate a special aroma of these substances. It is historically based.

Aromatics are planar, cyclic molecules with conjugated double bonds. If they meet the aromaticity criteria, they have particularly favorable energy levels. They differ in chemical and physical properties from the other organic compounds, the aliphatics.

Aromaticity Criteria

Historical definitions

  • Aromatic compounds are related to benzene and often have a pleasant, aromatic odor. From this typical Scent comes the name Aromat (gr. Aroma - scent).

This definition is of course too imprecise (because it is subjective) and antiquated (because no odor is perceptible in the case of aromatics with a high molar mass or strongly polar substituents).

  • Aromatics are polyunsaturated compounds which are relatively inert towards addition at the double bond and which instead enter into a substitution relatively easily directly at a double bond.

This name determination, which allows an experimental distinction, was valid in the 20th century, for example, even before the structure and bonding relationships were clarified. Today, a more general definition of electronic structure is usually preferred. The specified properties - in short: substitution instead of addition - are of course nonetheless characteristic and very important features.

  • The aromatic bonding system shows a particular stability, which can be determined, for example, by comparing the enthalpy of hydrogenation as the resonance energy.
  • The resonance frequency of the hydrogen atoms in the nuclear magnetic resonance experiment is characteristic. This manifests itself in a strong downfield shift for protons outside the aromatic system and an upfield shift for protons within the aromatic system.

Definition of aromatics

Necessary but not sufficient prerequisites for an aromatic:

  • A cyclic molecule, that is, it has at least one ring, which in many cases is a benzene ring.
  • A double bond system that is fully conjugated through the ring.
    They are either
    • several double bonds, each separated by exactly one single bond in the case of hydrocarbons (in the special case of arynes, a triple bond can also occur.) or
    • one or more double bonds separated by positively or negatively charged carbon atoms or by heteroatoms.

This condition is synonymous and shorter:

An aromatic is present if the following conditions are also met:

  • The double bond system is planar; In exceptional cases, slight deviations from the level are permitted. For example, in some cyclophanes, the benzene unit is deformed in a boat shape at an angle of up to 30 °.
  • The number of delocalized electrons must comply with Hückel's rule, i.e. in the conjugated electron system there must be 2 or 6 or 10 or 14 ... electrons:

The Hückel rule established by Erich Hückel is mostly represented by the formula (4n + 2) π electrons (n ​​= 0,1,2,3 ...), delocalized over all ring atoms of the system. Cyclic conjugated π systems with 4n π electrons (n ​​= 1,2,3 ...) are called anti-aromatics.

The basic structure of many aromatic compounds is benzene C.6H6. (The Hückel rule is fulfilled here with n = 1: Benzene has 6 π electrons.) Benzene is therefore regarded as one of the simplest aromatic hydrocarbons - especially since the special properties of aromatic compounds were discovered in benzene and its derivatives. Benzene is more stable than a hypothetical (i.e. non-producible) cyclohexatriene with localized double bonds and therefore less reactive.

Since, according to Hückel's rule, a planar, cyclic conjugated system with 2π electrons is also considered an aromatic, cyclopropenium salts are also aromatics:

This molecule is significantly smaller than benzene, since n = 0 here, while n = 1 for benzene.

Aromatic reactions

Addition reactions on the aromatic nucleus are difficult to achieve. Substitution reactions predominate, for example

  • Electrophilic, aromatic substitution (e.g. sulfonation, nitration)
  • Nucleophilic aromatic substitution (rare)

For the second substitution on the aromatic, special rules apply, which depend on the substituents already present.

Classification of aromatics

criteria

There is an enormous number (several million are known) of various aromatic compounds. They can be divided into groups according to various criteria:

  • Like all chemical compounds in which a distinction is made between organic (contains carbon atoms) and inorganic, the aromatics can also be used accordingly in organic and inorganic be subdivided. A inorganic aromatic is for example borazole B3N3H6, which is formally different from benzene C6H6 can be derived by replacing the carbon atoms alternately with nitrogen atoms or boron atoms. However, borazole is more aromatic on paper than in practice: The electron densities are strongly localized on the nitrogen atoms (instead of being evenly distributed over the ring) and the ring is very wavy. The reactivity when the ring is attacked by nucleophiles or electrophiles (in contrast to the inert benzene) also shows a clear bond polarization.
  • Ring systems that only consist of carbon atoms are called Carbocycles. Benzene C6H6 and naphthalene C10H8 therefore belong to the carbocycles. In contrast, like all others, they contain heteroaromatics Heterocycles in the ring system itself other atoms such as nitrogen, for example in the aromatic pyridine C5H5N. (Pyridine is formally derived from benzene by replacing a C – H atom group with N.)
  • Carbocyclic aromatics (= with a carbon atom aromatic structure) can be converted into (aromatic) Hydrocarbons and substituted aromatics (according to the classification of organic compounds). Benzene C6H6 and toluene C6H5-CH3 are hydrocarbons, phenol C.6H5-OH and trinitrotoluene TNT C6H2(NO2)3(CH3) are compounds derived therefrom by substitution.
  • A further classification is made according to the Number of aromatic cycles: One of the simplest aromatic compounds, benzene, consists of exactly one ring. Naphthalene C10H8 is a bicycle, it has an aromatic π-system with 10 π-electrons, which is distributed over the two rings.
  • Aromatics with multiple rings can be divided into those in which the rings common atoms have (fused or fused rings), as in naphthalene C10H8, or those with separate (isolated) rings, for example biphenyl C6H5-C6H5.
  • A further classification can be made according to the Number of ring atoms of the aromatic system. Six ring atoms are typical, for example in benzene C.6H6. In order to form a closed ring, at least three atoms are necessary, and accordingly there are aromatics with three, four, five - for example with C.5H5Cyclopentadiene anion, seven or more atoms.
  • According to the charge of the aromatic system, for example, the cyclopentadiene anion is simply negatively charged.

Examples of aromatic compounds

Hydrocarbons

Aromatic hydrocarbons are Arenas called. Examples are:

Hydrocarbons with multiple rings are called polycyclic aromatic hydrocarbons, for example:

Aromatic ions

The lone pair of electrons occupies a sp2-Hybrid orbital of oxygen, lies in the ring plane and thus does not contribute to the π system (analogous to the electronic structure of pyridine).

Derivatives derived from benzene by substitution

Heteroaromatics

  • Furan C4H4O (five-membered ring with oxygen atom)
  • Thiophene C.4H4S (five-membered ring with sulfur atom)
  • Pyridine C5H5N (six-membered ring with nitrogen atom)
  • Pyrrole C4H4NH (five-membered ring with nitrogen and hydrogen atom)

Anti-aromatics

Antiaromatics are substances that meet the first three conditions of an aromatic (cyclic, planar, conjugated double bonds), but instead of 4n + 2 π electrons 4n π electrons have. According to the Hückel approximation, anti-aromatics have unfavorable energy levels. The simplest antiaromatic, cyclobutadiene, cannot be made in its free form. Interestingly, it is stable as a ligand in organometallic chemistry.

Cyclooctatetraene has 8 π electrons. However, it is not planar so that the double bonds are not conjugated. So the Hückel rule cannot be applied. 1,3,5,7-Cyclooctatetraene is thus a non-aromatic.

Anti-aromatics are not allowed with the non-aromatic Compounds, the aliphatics, are confused.

Möbius aromatics

The Möbius aromaticity predicted in 1964 by Edgar Heilbronner (May 13, 1921, Munich - August 28, 2006)[1] assumes that in a cyclic conjugated system the occupied pπ-Orbitals are arranged as a Möbius strip, d. H. with a 180 ° turn. In addition, the π – orbitals are occupied by 4n electrons (where n is a natural number here). Möbius aromatics are chiral due to the twist. Whether a 2003 by Herges et al. synthesized molecule[2] really represents a Möbius aromatic or only has the necessary topology, is still controversial.[3]

See also

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  1. Edgar Heilbronner, Tetrahedron Lett. 1964, 1923.
  2. Synthesis of a Möbius aromatic hydrocarbon D. Ajami, O. Oeckler, A. Simon, R. Herges, Nature; 2003; 426 pp 819.
  3. Investigation of a Putative Möbius Aromatic Hydrocarbon. The Effect of Benzannelation on Möbius [4n] Annulene Aromaticity Claire Castro, Zhongfang Chen, Chaitanya S. Wannere, Haijun Jiao, William L. Karney, Michael Mauksch, Ralph Puchta, Nico J. R. van Eikema Hommes, Paul von R. Schleyer J. Am. Chem. Soc .; 2005; 127 (8) pp 2425-2432 abstract

Categories: Fabric Group | Aromat | Hydrocarbons