Aromatic Compounds Lecture Notes

Aromatic compounds form another class of functional groups.
The lecture focuses on the structure, types, possible reactions, and reaction mechanisms of aromatic compounds.
Due to the lab this week focusing on reactions with aromatic compounds, this lecture covers aromaticity before alkenes and alkynes.

Cyclohexane: Six-membered carbocyclic ring with a staggered arrangement due to $sp^3$ hybridized carbon atoms and tetrahedral geometry.
Cyclohexene: A six-membered ring with one carbon-carbon double bond (alkene), forcing the adjacent atoms into a flat arrangement due to the $sp^2$ hybridized carbons of the alkene, but still with some distortion from $sp^3$ carbons.
Benzene: An aromatic compound where every carbon is $sp^2$ hybridized and all atoms lie in the same plane, making it a flat molecule.

Benzene was first isolated by Michael Faraday in 1825.
Kekule proposed a structure for benzene 40 years later.
Benzene has a sweet smell and was historically used as aftershave.
It is an excellent solvent for oily compounds and was used to remove grease from hands.
Benzene is a known carcinogen, and its use is now heavily restricted with stringent paperwork requirements.

Kekule's structure of benzene shows alternating carbon-carbon double and single bonds.
Benzene does not behave like an alkene.
Hydrogenation: Alkenes readily undergo hydrogenation with hydrogen and palladium on carbon catalyst at room temperature and normal pressure.
Benzene requires high temperatures and pressures for hydrogenation, indicating lower reactivity.
Bromination: Alkenes undergo electrophilic addition with bromine.
Benzene does not react with bromine, further distinguishing it from alkenes.

Carbon-carbon double bonds are stronger than single bonds but not twice as strong; the pi bond is weaker and more reactive.
Benzene exhibits intermediate bond energy between single and double bonds.
Benzene also exhibits intermediate bond length between single and double bonds. This reinforces the fact that benzene is behaving differently than an alkene.

Bromine water ( $Br_2$ ) test: Alkenes decolorize bromine water via electrophilic addition.
Cyclohexane and benzene do not decolorize bromine water, showing no reaction and absence of typical alkene behavior.

Alkenes undergo electrophilic addition, where pi electrons react with electrophiles to form new sigma bonds.
Benzene has pi electrons but does not undergo electrophilic addition due to aromatic stabilization.

Valence bond theory does not fully explain benzene's properties.
Each carbon in benzene is $sp^2$ hybridized with trigonal planar geometry.
Each carbon forms sigma bonds with two adjacent carbons and one hydrogen.
Each carbon has one unhybridized p orbital that participates in pi bonding, resulting in three pi bonds in conjugation (alternating double and single bonds).
However, experimental evidence shows that all carbon-hydrogen and carbon-carbon bond lengths are the same, and all bond angles are 120 degrees.

Benzene contains a delocalized pi system where all unhybridized p orbitals form a ring.
Six electrons in these p orbitals can move freely around the ring, unlike the localized electrons in typical double bonds.
The delocalized pi system results in two donut-shaped orbitals above and below the plane of the benzene ring.
This delocalization explains the six identical carbon atoms and identical bond lengths and angles.
Benzene is $150 kJ/mol$ more stable than 1,3,5-cyclohexatriene due to aromatic stabilization.
Aromatic stabilization is due to the delocalization of pi electrons around the ring, making this energetically favorable, this also dictates it's reactivity.

1,3,5-cyclohexatriene can be drawn in two Kekule forms, showing alternating double bonds.
Delocalization of electrons can be shown using curly arrows to depict the movement of pi bonds around the ring, which get us to the different resonance structures.
A double-headed arrow indicates the movement of electrons, not a reaction.

A delocalization hybrid represents benzene as an average of the two Kekule forms.
It shows electrons aren't localized, using either dotted lines or a ring inside the hexagon to represent electrons moving freely around the ring.
Although the Kekule structures are not accurate, they are useful for illustrating reaction mechanisms.
The delocalization hybrid has a full sigma bond and half a pi bond between each carbon atom.

Nitrobenzene: Benzene with an - $NO_2$ (nitro) group.
Toluene: Benzene with a - $CH_3$ (methyl) substituent; used as a safer alternative to benzene as a solvent.
Aniline: Benzene with an - $NH_2$ (amine) group.
Phenol: Benzene with an - $OH$ (alcohol) group; a more specific name than