aromaticity

Examples of aromaticity in the following topics:

• Reactions of Aromatic Compounds

  • An example of an aromatic substitution reaction is shown below.
  • In a nucleophilic aromatic substitution reaction, a nucleophile displaces a substituent on an aromatic ring.
  • In an electrophilic aromatic substitution reaction, a substituent on an aromatic ring is displaced by an electrophile.
  • These reactions include aromatic nitration, aromatic halogenation, aromatic sulfonation, and Friedel-Crafts acylations and alkylations.
  • Example of an aromatic substitution reaction.

• Properties of Aromatic Compounds

  • Aromatic compounds are ring structures with delocalized $\pi$ electron density that imparts unusual stability.
  • Aromatic compounds, originally named because of their fragrant properties, are unsaturated hydrocarbon ring structures that exhibit special properties, including unusual stability, due to their aromaticity.
  • Aromatic compounds are generally nonpolar and immiscible with water.
  • This stability is lost in electrophilic addition because the product is not aromatic.
  • Aromatic compounds are produced from a variety of sources, including petroleum and coal tar.

• Naming Aromatic Compounds

  • Aromatic compounds are named based on the number and type of substituents on the ring.
  • There are a number of historically common names for aromatic structures.
  • If the substituent contains more than six carbons, the alkane portion is named first, and the aromatic ring portion is added as a suffix.
  • For instance, an aromatic ring bonded to an 8-carbon chain would be 1-phenyloctane, and not octylbenzene.
  • Recognize the methods for naming aromatic compounds, including IUPAC nomenclature and historical names

• Electrophilic Substitution of the Phenol Aromatic Ring

  • The facility with which the aromatic ring of phenols and phenol ethers undergoes electrophilic substitution has been noted.
  • Carbon dioxide is a weak electrophile and normally does not react with aromatic compounds; however, the negative charge concentration on the phenolate ring enables the carboxylation reaction shown in the second step.

• Other Aromatic Compounds

  • All the aromatic compounds discussed above have 6 π-electrons (n=1).
  • Benzene is the archetypical aromatic compound.
  • Four illustrative examples of aromatic compounds are shown above.
  • The second and third compounds are heterocycles having aromatic properties.
  • Carbanions and carbocations may also show aromatic stabilization.

• Substitution Reactions of Benzene and Other Aromatic Compounds

  • Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as Electrophilic Aromatic Substitution.

• Introduction to Hydrocarbons

  • This class can be further divided into two groups: aliphatic hydrocarbons and aromatic hydrocarbons.
  • Aromatic hydrocarbons, or arenes, which contain a benzene ring, were originally named for their pleasant odors.
  • For example, a chemical structure can be both aromatic and contain an alkyne.
  • The benzene molecules and its derivatives are the basis for aromatic structures.

• Benzene and other Aromatic Compounds

  • The adjective "aromatic" is used by organic chemists in a rather different way than it is normally applied.
  • Since double bonds are easily cleaved by oxidative reagents such as potassium permanganate or ozone, and rapidly add bromine and chlorine, these reactions were applied to these aromatic compounds.
  • As experimental evidence for a wide assortment of compounds was acquired, those incorporating this exceptionally stable six-carbon core came to be called "aromatic".
  • This sort of stability enhancement is now accepted as a characteristic of all aromatic compounds.
  • A molecular orbital description of benzene provides a more satisfying and more general treatment of "aromaticity".

• Five-Membered Rings

  • It is the "aromatic" unsaturated compounds, furan, thiophene and pyrrole that require our attention.
  • By this standard, the three aromatic heterocycles under examination are stabilized, but to a lesser degree than benzene.
  • Whereas simple cycloalkenes generally give addition reactions, aromatic compounds tend to react by substitution.
  • Reactions of pyrrole require careful evaluation, since N-protonation destroys its aromatic character.
  • The benzoin condensation is limited to aromatic aldehydes, but the use of thiazolium catalysts has proven broadly effective for aliphatic and aromatic aldehydes.

• Antiaromaticity

  • Conjugated ring systems having 4n π-electrons (e.g. 4, 8, 12 etc. electrons) not only fail to show any aromatic properties, but appear to be less stable and more reactive than expected.
  • Examples of 8 and 12-π-electron systems are shown below, together with a similar 10 π-electron aromatic compound.
  • Azulene is a stable blue crystalline solid that undergoes a number of typical aromatic substitution reactions.