Benzene Derivatives: Isomers & Reactions

• When two substituents are present on a benzene ring, three isomeric structures are possible
• These structures are designated using the prefixes ortho- (o-), meta- (m-), and para- (p-)
• If Substituent X is attached to carbon 1, o-groups are on carbons 2 and 6, m-groups are on carbons 3 and 5, and p-groups are on carbon 4

Nomenclature of Benzene Derivatives

• Monosubstituted benzenes without common names accepted by IUPAC are named as derivatives of benzene
• Examples include bromobenzene, chlorobenzene, nitrobenzene, ethylbenzene, and propylbenzene

Alkylation and Acylation Reactions

• Alkylation of aromatic compounds is referred to as the Friedel-Crafts reaction
• It was discovered by Charles Friedel and James Mason Crafts in 1877
• The electrophile in this reaction is a carbocation
• This can be formed by removing a halide ion from an alkyl halide using a Lewis acid catalyst such as AlCl3
• It can also be formed by adding a proton to an alkene

Phenyl Group Abbreviation

• "Ph" is sometimes used as an abbreviation for the phenyl group

Aromatic Hydrocarbons

• Aromatic hydrocarbons are called arenes
• The symbol "Ar" is used for an aryl group, analogous to "R" for an alkyl group
• "Ar-R" represents any arylalkane

Common Names

• Due to the haphazard development of aromatic chemistry, many common names have historic respectability and are accepted by IUPAC

Friedel-Crafts Alkylation Limitations

• Friedel-Crafts alkylation cannot be applied to aromatic rings with a nitro or sulfonic acid group
• These groups deactivate the aluminum chloride catalyst by forming complexes
• Friedel-Crafts acylations behave similarly
• The electrophile is an acyl cation generated from an acid derivative, typically an acyl halide
• This reaction creates a useful general route to aromatic ketones

Electrophilic Aromatic Substitution

• Common reactions of aromatic compounds involve substituting other atoms or groups for a ring hydrogen on the aromatic unit
• Typical substitution reactions of benzene include:
  • Chlorination (using Cl2 and FeCl3)
  • Bromination (using Br2 and FeBr3)
  • Nitration (using HNO3, H2SO4)

Symbols for Benzene

• Two symbols are common for benzene:
  • The Kekulé structure (a hexagon with alternating single and double bonds)
  • A hexagon with an inscribed circle (representing a delocalized pi electron cloud)

Kekulé Structure

• Kekulé's structure depicts the benzene ring as a flat molecule with alternating single and double bonds between carbon atoms
• Kathleen Lonsdale used X-ray crystallography in 1929 to find that all carbon-carbon bonds in benzene are the same length
• Benzene's electrons are delocalized over the ring
• Benzene is a flat molecule

Nitration Reactions

• During aromatic nitration, the sulfuric acid catalyst protonates the nitric acid
• This causes it to lose water and generate the nitronium ion (NO2+)
• The nitronium ion contains a positively charged nitrogen atom an dis a strong electrophile
• The nitronium ion is then attacked by the aromatic ring

Sulfonation Reactions

• In aromatic sulfonation reactions, concentrated or fuming sulfuric acid is used
• The electrophile can be:
  • Sulfur trioxide (SO3)
  • Protonated sulfur trioxide (+SO3H)

Six-Carbon Unit

• Many aromatic substances contain a six-carbon unit (C6H6)
• It remains unchanged during various chemical reactions that alter only the rest of the structure

Halogenation

• Chlorine or bromine are introduced into an aromatic ring using the halogen along with the corresponding iron halide as a catalyst (Cl2/FeCl3 or Br2/FeBr3)
• The reaction involves adding the halogen slowly to a mixture of the aromatic compound and iron fillings
• The iron reacts with the halogen to form the iron halide, which acts as a catalyst for halogenation

Ortho, Meta, and Para Prefixes

• The prefixes ortho-, meta-, and para- are used even when the two substituents are not identical
• Substituents are listed alphabetically in the name

Naming Disubstituted Benzenes

• Although o-, m-, and p- designations are commonly used in naming disubstituted benzenes, position numbers of substituents can also be used
• For example:
  • o-dichlorobenzene can be named 1,2-dichlorobenzene
  • m-chlorophenol can be named 3-chlorophenol
• For more than two substituents, positions are numbered