Organic Chemistry - Aromatic Compounds

Aromatic Compounds

Structures of Six-Membered Carbocyclic Compounds

Cyclohexane: $C6H{12}$
Cyclohexene: $C6H{10}$
Benzene: $C6H6$

Benzene as an Aromatic Compound

Originally, "aromatic" referred to compounds with a pleasant smell.
Now, it denotes a specific functional group type.
Michael Faraday first isolated benzene in 1825.
August Kekulé proposed the structure of benzene in 1865.
Benzene is represented with alternating single and double bonds in a six-membered ring.

Stability of Aromatic Compounds Compared to Alkenes

Benzene is more stable than typical alkenes.
Bond strength and length:
- C-C (alkane): Bond Length = 154 pm, Strength = 356 kJ/mol
- C=C (alkene): Bond Length = 133 pm, Strength = 636 kJ/mol
- Benzene bond: Bond Length = 139 pm, Strength = 518 kJ/mol
Benzene does not behave like a typical alkene.

Classical Tests and Reactivity of Aromatic Compounds

Aromatic compounds do not react like alkenes.
Bromine test: Benzene does not decolourise bromine water, unlike cyclohexene.
- Cyclohexane shows no reaction with bromine water.
- Cyclohexene reacts with bromine water (Br2) to form a dibromo product.

Valence Bond Theory and Benzene

Each carbon atom in benzene is sp2 hybridised.
Each carbon forms σ-bonds with two neighbouring carbon atoms and one hydrogen atom.
Each carbon atom has a p-orbital involved in π-bonding, resulting in 3 π-bonds in conjugation.
All C-H bond lengths are the same (109 pm).
All C-C bond lengths are the same (139 pm).
All C-C-C and H-C-C bond angles are the same (120°).
The alternating single and double bond representation is inaccurate.

Benzene's Aromatic Ring: A Delocalised Pi System

Benzene is a perfect hexagon with six identical carbon-carbon bond lengths (139 pm).
It absorbs in the UV-vis region.
Benzene is 150 kJ/mol more stable than predicted for 1,3,5-cyclohexatriene, which is known as aromatic stabilisation.
The six delocalised pi electrons are located in two donut-shaped electron clouds above and below the planar hexagon ring.

Delocalisation (Resonance)

Two possible 1,3,5-cyclohexatriene structures exist with π-bonds in different locations (Kekule forms or resonance forms).
Bonding in benzene can be considered an average of these two Kekule forms.
The structures are interchanged by relocating electron pairs.
Curly arrows are used to show the interconversion between the two possible 1,3,5-cyclohexatriene structures.

Delocalisation in Benzene

Kekule structures show localised double and single bonds, which are imprecise representations of benzene.
They are still useful for illustrating mechanisms.
Benzene’s delocalised pi system is the average of the two Kekule structures.
The delocalisation hybrid has:
- A full -bond and half a -bond between each adjacent pair of carbons.
- A bond strength and length roughly halfway between a C–C single bond (no -bond) and a C=C double bond (a full -bond).

Benzene Tests the Limits of Stick Structure Representations

Kekule structure 1 and Kekule structure 2 are not real, but the delocalisation hybrid is real.

Naming Aromatic Structures

Examples of aromatic compounds:
- Nitrobenzene
- Phenol
- Toluene
- Aniline
- Benzaldehyde
- Benzoic acid
The 6
$\pi$-electrons of benzene are represented in different ways in textbooks.
A circle is sometimes used but is becoming less common for mechanistic understanding.

Polycyclic Aromatic Hydrocarbons

Examples:
- Naphthalene ( $C{10}H8$ )
- Anthracene ( $C{14}H{10}$ )
- Phenanthrene ( $C{14}H{10}$ )
- Graphene
- Carbon nanotubes
- Graphite

Reactions of Aromatic Compounds

The π-electrons are available for reaction, and benzene can react with electrophiles.
Benzene does not undergo electrophilic addition.
Addition product is not observed because aromatic stabilisation would be lost.

Electrophilic Aromatic Substitution (SEAr) Mechanism

Benzene undergoes electrophilic substitution, not electrophilic addition.
Electrophilic substitution preserves the aromatic nature and special stability of the delocalised -electrons of the benzene ring.

Powerful Electrophiles Used in SEAr Reactions

Due to aromatic stabilisation energy, benzene only undergoes electrophilic substitution reactions with powerful electrophiles.

SEAr Nitration of Benzene

Reactants: concentrated $HNO3$ and concentrated $H2SO_4$
Overall: $NO_2$ substituted for H on benzene.
Mechanism:
- Step 1: Electrophile addition.
- Step 2: Proton elimination.
Notes:
- Conservation of charge.
- Curly arrows show electron movement, not atom/molecule movement.
- Identify electrophiles and nucleophiles.

Generation of the Nitronium Ion

Reaction of concentrated $HNO3$ with concentrated $H2SO_4$
Generates the nitronium ion ( $NO_2^{+}$ ), a powerful electrophile.

Alfred Nobel and the Nobel Prizes

Alfred Nobel's dismay at being portrayed as the "Dynamite King" led him to rewrite his will and endow the Nobel Prizes.

TNT (Trinitrotoluene)

Alfred Nobel did not make TNT.
TNT is 2,4,6-trinitrotoluene (2-methyl-1,3,5-trinitrobenzene).
Energy content = 4.184 MJ/kg

SEAr Bromination of Benzene

Reactants: $Br2$ , $AlBr3$
Overall: Br substituted for H on benzene.
Mechanism:
- Step 1: Electrophile addition.
- Step 2: Proton elimination.
Notes:
- Conservation of charge.
- Curly arrows show electron movement, not atom/molecule movement.
- Identify electrophiles and nucleophiles.

Generation of the Bromonium Ion

$Br2$ reacts with $AlBr3$ to generate $Br^{+}$ .

Summary of Electrophilic Aromatic Substitution Reactions of Benzene

Nitration: using conc. $H2SO4$ /conc. $HNO3$ to substitute $NO2$ for H.
Bromination: using $Br2$ , $AlBr3$ to substitute Br for H.
Chlorination: using $Cl2$ , $AlCl3$ to substitute Cl for H.

Friedel-Crafts Reactions of Benzene

Friedel-Crafts alkylation: overall alkyl group R substituted for H on benzene.
Friedel-Crafts acylation: overall acyl group –C(=O)R substituted for H on benzene.

Friedel-Crafts Alkylation (SEAr Alkylation of Benzene)

Reactants: Alkyl halide, $AlCl_3$
Overall alkyl group substituted for H on benzene
Mechanism:
- Step 1: Electrophile addition.
- Step 2: Proton elimination.
Notes:
- Conservation of charge.
- Curly arrows show electron movement, not atom/molecule movement.
- Identify electrophiles and nucleophiles.

Friedel-Crafts Acylation (SEAr Acylation of Benzene)

Reactants: Acyl halide, $AlCl_3$
Overall acyl group substituted for H on benzene
Mechanism:
- Step 1: Electrophile addition.
- Step 2: Proton elimination.
Notes:
- Conservation of charge.
- Curly arrows show electron movement, not atom/molecule movement.
- Identify electrophiles and nucleophiles.

Generation of Carbocations for Friedel-Crafts Reactions

Carbocations and acylium ions are extremely powerful electrophiles.

Carbocation (Wheland) Intermediate

The Wheland intermediate is not aromatic but is stabilised by delocalisation.
The positive charge is shared by three carbons: delocalisation = stabilisation.

Lecture 7 Summary

Benzene is the simplest member of aromatic compounds.
1,3,5-cyclohexatriene is not an accurate representation of benzene due to the delocalised six electron pi system.
Organic chemists still draw benzene as 1,3,5-cyclohexatriene to show mechanisms.
Aromatic compounds have special stability.
Benzene can be thought of as a hybrid of two 1,3,5-cyclohexatriene structures (Kekule forms).
Be able to name simple substituted benzenes.
Benzene undergoes substitution reactions with electrophiles (Cl, Br, $NO_2$ , alkyl, or acyl group).
Benzene is the nucleophile; powerful electrophiles are needed.
The mechanism involves an addition step followed by an elimination step.
Powerful electrophiles are produced by stripping halide ions or from nitric acid and sulphuric acid.
All reactions occur via the same mechanism once the electrophile is formed.
The Wheland intermediate cation is delocalised.