Aromatic Compounds Introduction

Introduction to Aromatic Compounds

• Aromatic compounds are named after the Greek word "aroma", meaning fragrant smell, due to the distinctive odors of many of these compounds.
• The term "aromatic" refers to compounds structurally related to benzene.
• Four conditions must be satisfied for a compound to be aromatic:
  • Cyclic
  • Planar
  • Conjugated
  • Obeying Hückel's Rule

Hückel's Rule

• Hückel's Rule estimates the aromatic qualities of a planar ring-shaped molecule.
• The molecule must have 4n+2 π-electrons, where n is an integer (0, 1, 2, 3, ...).
• Molecules that don't follow the 4n+2 rule are called non-aromatic.

Classification of Aromatic Compounds

• Aromatic compounds are broadly divided into two categories:
  • Benzenoids (containing a benzene ring)
  • Non-benzenoids (not containing a benzene ring, e.g., furan)

Benzene

• Benzene has a high degree of unsaturation but is unreactive.
• It does not undergo the usual addition reactions of alkenes and alkynes.
• It reacts under more vigorous conditions (heat or catalysts) to form products of substitution.
• Benzene is represented as a six-carbon ring with 3 double bonds.
• Two possible isomers can be drawn to show benzene in this form.

Naming Aromatic Compounds

• Aromatic compounds are named according to IUPAC rules.
• When one group is attached to benzene, the compound is named by placing the group name as a prefix to benzene.
• Monosubstituted benzenes are systematically named with benzene as the parent name and the substituent is indicated by a prefix.
• Many common names have been accepted by IUPAC, e.g., toluene (methylbenzene), phenol (hydroxybenzene), and aniline (aminobenzene).

Disubstituted Benzenes

• Relative positions on a disubstituted benzene ring:
  • Ortho- (o) on adjacent carbons (1,2 disubstituted)
  • Meta- (m) separated by one carbon (1,3 disubstituted)
  • Para- (p) separated by two carbons (1,4 disubstituted)
• The ortho-, meta-, and para- nomenclature is useful to describe reaction patterns.

Multisubstituted Benzenes

• Multisubstituted benzenes are named by:
  • Choosing the sequence when the substituents have the lowest possible number
  • Listing substituents alphabetically with hyphenated numbers
  • Using common names as parent names

Structure and Stability of Benzene

• Benzene is very stable and undergoes substitution rather than addition reactions.
• It has a higher barrier to reaction due to its unusual structure.
• All C-C bonds in benzene are the same length, with identical electron density.
• The structure is planar, hexagonal, with 120° C-C-C bond angles.
• Benzene has 6 delocalized p electrons over the ring.

Reactions of Benzene

• Even though benzene is highly unsaturated, it does not undergo regular reactions of alkenes, such as addition or oxidation.
• Benzene reacts slowly with Br2 to give bromobenzene (where Br replaces H).

Substitution Reactions of Aromatic Compounds

• Substitution reactions involve the replacement of a hydrogen atom on the benzene ring with another group, such as chlorine, methyl, acetyl, nitric acid, or sulfuric acid.