aromatic chemistry Flashcards
What are arenes?
Hydrocarbons based on benzene, C6H6, known for their stability and hexagonal ring structure.
Arenes were originally named for their aromatic properties, but now refer to their structural characteristics.
What is the simplest arene?
Benzene (C6H6)
Benzene is an unsaturated molecule with a unique bonding structure.
Describe the structure of benzene.
A hexagonal ring with 6 carbon atoms, each bonded to one hydrogen atom, forming a planar molecule.
The C-C bonds are of equal length, intermediate between single and double bonds.
What is the significance of p orbitals in benzene?
Each carbon atom has one unused electron in a p orbital, which overlaps with neighboring p orbitals to form π bonds.
This overlapping creates a delocalized electron cloud above and below the plane of the ring.
What is the boiling and freezing point of benzene?
Boiling point: 353 K, Freezing point: 279 K
Compared to hexane, benzene has a higher melting point due to its flat, hexagonal structure that allows for better packing in the solid state.
Who discovered the ring structure of benzene?
Kekulé
He proposed a ‘triene’ structure with alternating double and single bonds, which was later disproven.
What is the difference between the Kekulé structure and the delocalized structure of benzene?
Kekulé’s structure has alternating double and single bonds, while the delocalized structure has all C-C bonds of equal length.
The delocalized structure is more stable than the triene structure.
True or False: Benzene readily undergoes addition reactions.
False
Benzene does not decolorize bromine water, unlike a triene which would undergo addition reactions.
What is the enthalpy of hydrogenation for benzene?
208 KJ/mol
This indicates that benzene is 152 KJ/mol more stable than a hypothetical triene.
What is the preferred reaction type for aromatic compounds?
Electrophilic substitution reactions
These reactions preserve the aromatic system, unlike addition reactions which would destabilize it.
What are the two main factors affecting the reactivity of aromatic compounds?
- High electron density in the aromatic ring due to delocalized bonding
- Stability of the aromatic ring requiring energy to break it
This delocalization energy is critical for maintaining the integrity of the aromatic system.
What type of flames do arenes produce when burned in air?
Smoky flames
This is due to a high carbon to hydrogen ratio, which often results in unburnt carbon and soot.
What is the electrophile generated during nitration?
Nitronium ion (NO2+)
This ion replaces a hydrogen atom in the electrophilic substitution reaction.
What are the reagents used in the nitration of arenes?
Concentrated HNO3 and concentrated H2SO4
The reaction must be conducted at 50°C to prevent multiple substitutions.
What is a common use of nitrated arenes?
Manufacture of explosives
TNT (trinitrotoluene) is a well-known example of a nitrated arene.
What are the reagents for Friedel-Crafts acylation?
Acyl chloride or acid anhydride + AlCl3
The reaction must be anhydrous to prevent reaction of AlCl3.
What is the significance of the acylium ion in Friedel-Crafts acylation?
It acts as the electrophile in the electrophilic substitution reaction.
The acylium ion is formed from acyl chloride in the presence of a catalyst.
What is the regeneration step in Friedel-Crafts acylation?
AlCl4- + H+ → AlCl3 + HCl
This step regenerates the AlCl3 catalyst used in the reaction.
