Aromatic Chemistry - Arenes and Benzene

These vocabulary flashcards cover the structure, bonding, thermochemical evidence for stability, and reaction mechanisms of benzene and other arenes as described in the lecture notes.

Arenes

Hydrocarbons based on benzene, $C_6H_6$, which were historically named for their sweet-smelling origins in oils like balsam.

Benzene Structure

A planar, regular hexagon of carbon atoms, each bonded to a single hydrogen atom, with a delocalized electron system.

Benzene C-C Bond Length

Measured at $0.140\,nm$, which is intermediate between a carbon-carbon single bond ($0.154\,nm$) and a carbon-carbon double bond ($0.134\,nm$).

Delocalisation

A state where p-orbital electrons are spread over more than two atoms; in benzene, six electrons form a region of electron density above and below the ring.

Aromatic Stability

The unusual stability of benzene resulting from its delocalised system, making it $152\,kJ\,mol^{-1}$ more stable than the hypothetical cyclohexa-1,3,5-triene.

Hydrogenation Enthalpy of Cyclohexene

$\Delta H^{\circ} = -120\,kJ\,mol^{-1}$.

Hydrogenation Enthalpy of Benzene

The actual value is $\Delta H^{\circ} = -208\,kJ\,mol^{-1}$, compared to the expected $-360\,kJ\,mol^{-1}$ for a non-delocalised ring.

Friedrich August von Kekulé

The chemist who in 1865 proposed that benzene's structure was a ring of carbon atoms with alternating double and single bonds based on a dream of snakes.

Resonance Hybrid

A concept where the actual structure of a molecule is thought to be a stable average of multiple contributing structures.

Benzene Boiling and Freezing Points

Benzene boils at $353\,K$ and freezes at $279\,K$; the high freezing point relative to hexane is due to efficient packing in the solid state.

Electrophile

An electron-deficient species with a positive charge or positive end of a dipole that attacks areas of high electron density.

Delocalisation Energy

The energy required to break the stable aromatic ring system before it can be destroyed during a reaction.

Electrophilic Substitution

The typical reaction mechanism for arenes, where an electrophile replaces a hydrogen atom, leaving the stable aromatic system intact.

Nitronium Ion

The $NO_2^+$ species (also called the nitryl cation) generated by the reaction of concentrated sulfuric acid and concentrated nitric acid.

TNT

Trinitrotoluene, a high explosive made by nitrating methylbenzene; its systematic name refers to the methyl group at position 1.

Friedel-Crafts Acylation

A substitution reaction using an acyl chloride and an $AlCl_3$ catalyst to introduce an $RCO$ group onto the benzene ring.

Aluminium Chloride ($AlCl_3$)

A catalyst used in acylation that accepts a lone pair from the chlorine atom of an acyl chloride because its aluminium atom has only six electrons in its outer level.

Electron-releasing Groups

Substituents such as $-CH_3$, $-OCH_3$, $-OH$, and $-NH_2$ that direct further substitution to the 2, 4, and 6 positions on the benzene ring.

Electron-withdrawing Groups

Substituents such as $-NO_2$ and $-COCl$ that direct further substitution to the 3 and 5 positions on the benzene ring.

Aromatic Combustion

Arenes burn in air with smoky flames due to a high carbon-to-hydrogen ratio ($1:1$ for benzene), resulting in unburnt carbon (soot).