30. Characteristics, reactions and biological roles of aromatic compounds Flashcards
What are Aromatic hydrocarbons?
an unsaturated cyclic hydrocarbon that does not readily undergo addition reactions.
What are requirements for aromaticity
unusually high stability
What are physical properties of aromatic compounds
- pleasant smell
- high ratio of C molecules to H
- homologous substances of benzene and smaller number C-atom are liquid at room temperature
- Condensed ring compounds are solid, crystalline
- melting and boiling point higher than the corresponding open-chain hydrocarbons
- less density than water
- well soluble in organic solvents
- toxic or carcinogenic
What are the 2 groups of aromatic hydrocarbons?
- Monocyclic aromatic hydrocarbons
- Condensed polycyclic aromatic hydrocarbons
Why is benzene stable?
benzene does not show typical additive reactions to alkenes and it is less prone to oxidation
What are Chemical reactions for aromatic hydrocarbons
substitution reactions
What is SE Ar Reaction?
- The electron-rich aromatic ring attacks electrophile A
- The addition of A to a carbon atom
- The pair of electrons forming the σ bond is derived from the 6 π-electron of benzene (benzene is the nucleophilic reagent)
- Cyclohexadienyl cation is formed- unstable due to positive charge of the molecule and temporary cessation of aromatic character - partially stabilized by by the positive charge
- the Lewis base (B) transmits electron to H-atom at the site of electrophilic attack, the bonding electron pair of the H is returned to the π system, the aromatic system is restored
What is the effect of activating substituents?
increasing reaction rate
- electron donor: increase the electron density of the aromatic ring → the attacking electrophilic group can react easier, higher electron density in the ring (e.g. phenolic hydroxyl group)
What is the effect of deactivating substituents?
reducing reaction rate
→ electron acceptor: reducing ring reactivity
(e.g. nitrous and nitrile group)
5 type of substitution reactions of aromatic compounds
- Chlorination & bromination
- Nitration
- Sulfonation
- Alkylation
- Acylation
5 type of substitution reactions of aromatic compounds
- Chlorination & bromination
- Nitration
- Sulfonation
- Alkylation
- Acylation
Describe chlorination & bromination
• chlorine/bromine + benzene → π complex
(Cl-Cl/Br−Br bond perpendicular to the benzene ring) → σ-complex → chlorobenzene/bromobenzene
very slow, catalysts are used (e.g., aluminuim(III) chloride)
Describe Nitration of aromatic compounds
- nitration mixture: 1:2 mixture of HNO3 and H2SO4
- protonated nitronium ion is the electrophile
- nitrobenzene is much less reactive → production of the dinitro- and trinitro-derivatives with smoking nitric acid, heating
- e.g., methylbenzene (toluene) is converted to 2,4,6-trinitrotoluene (TNT)
Describe sulfonation of aromatic compounds
- sulphur trioxide is the electrophile: it can be produced from concentrated sulphuric acid
- product: benzenesulfonic acid
Describe alkylation of aromatic compounds
- sulphur trioxide is the electrophile: it can be produced from concentrated sulphuric acid
- product: benzenesulfonic acid
Describe acylation of aromatic compounds
- sulphur trioxide is the electrophile: it can be produced from concentrated sulphuric acid
- product: benzenesulfonic acid
Describe hydrogenation of aromatic compounds
Characteristics of condensed polycyclic hydrocarbons
- at least two common C-atoms
- two-ring systems: continuously conjugated 6+4 π-electron, aromatic
- three-ring systems: as conjugation expands, the color gradually deepens→ energy difference between the molecular orbitals gradually decreases
IUPAC NAME
Ortho-disubstituted (1,2)
IUPAC NAME
Meta-disubstituted (1,3)
IUPAC NAME
Para-disubstituted (1,4)
IUPAC NAME
Para-disubstituted (1,4)
IUPAC NAME
Phenol
IUPAC NAME
Phenol
IUPAC NAME
3,4-Dibromophenol
IUPAC NAME
5-Nitro-2-propylaniline
IUPAC NAME
Naphthalene
IUPAC NAME
Acenaphthene
IUPAC NAME
Phenanthrene
IUPAC NAME
Anthracene
