Aromatic Ions
Aromatic ions are chemical species that possess aromaticity and carry a net electric charge. They can be positively charged (cations) or negatively charged (anions) and exhibit the characteristic stability and reactivity associated with aromatic compounds. Aromatic ions play a significant role in organic chemistry and are important in the design and synthesis of various functional materials and pharmaceuticals.
3 Key excerpts on "Aromatic Ions"
eBook - ePub- Graham Patrick(Author)
- 2004(Publication Date)
- Taylor & Francis(Publisher)
...The cyclopentadienyl anion and the cycloheptatrienyl cation are both aromatic (Figure 3). Both are cyclic and planar, containing six π electrons, and all the atoms in the ring are sp 2 hybridized. Bicyclic and polycyclic systems can also be aromatic (Figure 4). Figure 3. (a) Cyclopentadienyl anion; (b) cycloheptatrienyl cation. Figure 4. (a) Naphthalene; (b) anthracene; (c) benzo[a]pyrene. I2 Preparation and properties Key Notes Preparation Simple aromatic structures such as benzene, toluene, or naphthalene are isolated from natural sources and converted to more complex aromatic structures. Properties Many aromatic compounds have a characteristic aroma and burn with a smoky flame. They are nonpolar, hydrophobic molecules which dissolve in organic solvents rather than water. Aromatic molecules can interact by van der Waals interactions or with a cation through an induced dipole interaction. Aromatic compounds undergo reactions where the aromatic ring is retained. Electrophilic substitution is the most common type of reaction. However, reduction is also possible. Spectroscopic analysis Aromatic compounds show characteristic absorptions in the IR spectrum due to ring vibrations. Signals due to Ar-H stretching and bending may also be observed. Signals for aromatic protons and carbons appear at characteristic positions in nmr spectra. Fragmentation ions can be observed in mass spectra which are characteristic of aromatic compounds. Related topics (I3) Electrophilic substitutions of benzene (I7) Oxidation and reduction (P2) Visible and ultra violet spectroscopy (P3) Infra-red spectroscopy (P4) Proton nuclear magnetic resonance spectroscopy (P5) 13 C nuclear magnetic resonance spectroscopy (P6) Mass spectrometry Preparation It is not practical to synthesize aromatic structures in the laboratory from scratch and most aromatic compounds are prepared from benzene or other simple aromatic compounds (e.g. toluene and naphthalene)...
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...
