Aromaticity
Aromaticity refers to the special stability and unique reactivity of certain cyclic compounds, known as aromatic compounds. These compounds typically contain a ring of alternating single and double bonds, and they often exhibit a characteristic "aromatic" odor. Aromaticity is a key concept in organic chemistry and is associated with the delocalization of electrons within the ring structure.
3 Key excerpts on "Aromaticity"
eBook - ePub- Graham Patrick(Author)
- 2004(Publication Date)
- Taylor & Francis(Publisher)
...SECTION I — AROMATIC CHEMISTRY I1 Aromaticity Key Notes Definition Aromatic compounds such as benzene are more stable than suggested from their structure. They undergo reactions which retain the aromatic ring system, and behave differently from alkenes or polyenes. Hückel rule Aromatic compounds are cyclic and planar with sp 2 hybridized atoms. They also obey the Hückel rule and have (4n + 2) π electrons where n = 1, 2, 3,... Aromatic systems can be monocyclic or polycyclic, neutral, or charged. Related topic (A4) sp 2 Hybridization Definition The term aromatic was originally applied to benzene-like structures because of the distinctive aroma of these compounds, but the term now means something different in modern chemistry. Aromatic compounds undergo distinctive reactions which set them apart from other functional groups. They are highly unsaturated compounds, but unlike alkenes and alkynes, they are relatively unreactive and will tend to undergo reactions which involve a retention of their unsaturation. We have already discussed the reasons for the stability of benzene in Section A4. Benzene is a six-membered ring structure with three formal double bonds (Figure 1a). However, the six π electrons involved are not localized between any two carbon atoms. Instead, they are delocalized around the ring which results in an increased stability. This is why benzene is often written with a circle in the center of the ring to signify the delocalization of the six π electrons (Figure 1b). Reactions which disrupt this delocalization are not favored since it means a loss of stability, so benzene undergoes reactions where the aromatic ring system is retained. All six carbon atoms in benzene are sp 2 hybridized, and the molecule itself is cyclic and planar — the planarity being necessary if the 2p atomic orbitals on each carbon atom are to overlap and result in delocalization. Figure 1...
eBook - ePub- J Fisher, J.R.P. Arnold, Julie Fisher, John Arnold(Authors)
- 2020(Publication Date)
- Taylor & Francis(Publisher)
...Section K - Aromatic Compounds K1 Aromaticity DOI: 10.1201/9780203079522-43 Key Notes Benzene Benzene is an unsaturated molecule and, as such, would be expected to undergo reactions similar to those of other unsaturated hydrocarbons such as alkenes and alkynes. However, benzene is relatively inert, and when it does react favors substitution reactions over addition reactions. The unexpected chemical and physical properties of benzene may be explained by the concept of pi electron delocalization. Benzene is the classic example of an aromatic compound. The term aromatic is applied as benzene, and other ring systems that have similar delocalized pi systems, is fragrant. Molecular orbital description of benzene Benzene is a planar molecule in which all of the bond angles about the carbon atoms are 120°. This bond angle is what would be expected for an sp 2 hybridized carbon atom, and therefore means that at each of the six carbon atoms there is a singly occupied p-orbital. These p-atomic orbitals overlap to form six pi molecular orbitals. The molecular orbital picture of benzene helps explain the special stability of this molecule. Definition of Aromaticity In 1931 the physicist Erich Hückel carried out a series of calculations based on the molecular orbital picture of benzene, but extended this to cover all planar monocyclic compounds in which each atom had a p-orbital. The results of his work suggested that all such compounds containing (4n + 2) pi electrons should be stabilized through delocalization and therefore should also be termed aromatic. Related topics (I3) Factors affecting reactivity (K2) Natural aromatics Benzene The study of the class of compounds now referred to as aromatics began in 1825 with the isolation of a compound, now called benzene, by Michael Faraday. At this time the molecular formula of benzene, C 6 H 6, was thought quite unusual due to the low ratio of hydrogen to carbon atoms...
eBook - ePubBiochemistry
An Organic Chemistry Approach
- Michael B. Smith(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...This addition type reaction does not occur with benzene or naphthalene. 9.11 Heteroaromatic Compounds: Nitrogen, Oxygen, or Sulfur Another class of aromatic compounds are important in chemistry and biology, and they are characterized by replacement of one or more ring carbons with a heteroatom. These compounds are collectively known as heterocycles or heterocyclic aromatic compounds, and they comprise a class of compounds so large that an entire course is easily built around their chemistry. The most common heterocycles include five- and six-membered monocyclic derivatives that contain nitrogen, oxygen, or sulfur. There are several important bicyclic derivatives that contain nitrogen. In a “thought experiment,” replace the CH moiety of cyclopentadiene with an N—H unit, and the result is the aromatic compound, pyrrole, a constituent of coal tar and is found in bone oil. Pyrrole is an aromatic compound because the nitrogen atom has an unshared electron pair in an orbital that is parallel with the four π-electrons in the C=C units for a six π-electron system in a π-framework (see Figure 9.17). Pyrrole is a planar molecule and the hydrogen atom is coplanar with the carbon atoms and nitrogen, as shown in Figure 9.17. Due to the fact that the lone electron pair is part of the aromatic sextet, the electrons cannot be donated without disrupting the Aromaticity and pyrrole is not very basic. There are other aromatic five-membered ring amines, but they have two nitrogen atoms in the ring. The two nitrogen atoms in imidazole have a 1,3-relationship (note that an older term for an imidazole is azole). The two nitrogen atoms in pyrazole have a 1,2-relationship...
