CHEM C15: Introduction to Arenes

A huge intro post on the world of aromatic compounds.

  • Structure & Bonding in Arenes
    • Kekule Structure vs. Delocalised electron cloud model
  • Resonance structures
  • Physical Properties of Arenes
  • Chemical Properties of Arenes

Let’s jump right in.


What are arenes?
Aromatic Hydrocarbons.

This means that they only contain C & H AND contain an AROMATIC RING.

What is an aromatic ring?
A ring of 6 Carbons which contain a DELOCALISED RING of electrons.
It is also known as a BENZENE RING, after the simplest arene: Benzene.
They are called “aromatic” because the earliest compounds identified gave nice smells (aromas)!

Benzene
Benzene is a HYDROCARBON just like alkanes & alkenes (it only contains C and H).

  • Ratio of C:H = 1:1
  • Empirical formula: CnHn

There are 2 ways to represent benzene:

Kekule’s Structure (inaccurate) 
Delocalised π electron cloud (updated) 

A bit of history:
All we knew about benzene when it was first discovered was its molecular formula: C6H6

This put it in a different group than alkanes (CnH2n+2) or alkenes (CnH2n).

We didn’t know how it was structured – until Kekule came along. He had a dream about a snake biting its own tail & came up with a theory:
benzene is a ringed compound with alternating double bonds & single bonds.

This is called the Kekule structure:

This satisfies the need for carbon to have 4 bonds. We could name this ‘cyclohexatriene’ – it’s similar to cyclohexene, just having 3 double bonds instead of 1.

HOWEVER, we now know that this structure is INACCURATE, because there are a few issues:

  • The double bonds would be shorter than the single bonds, making the hexagon irregular – but it was later discovered that the bond lengths are all equal
  • Having 3 double bonds would make benzene less stable than it really is
  • The double bonds would undergo all the reactions an alkene would – but they don’t! Benzene does not undergo addition reactions as easily as alkenes do
  • The heat of addition reactions for benzene is LOWER than expected

So, we came up with a better model:
Resonance structure AKA delocalized electron model

  • Each carbon atom undergoes sp2 hybridisation
  • 2 of the sp2 orbitals overlap with other carbons, while another overlaps with a hydrogen, each forming σ-bonds
  • this gives a bond angle of 120°
  • There is 1 p-orbital left for each carbon – all 6 of these p-orbitals overlap to form a π-bond
    • A ring-shaped electron cloud exists above & below the carbon ring – these electrons are DELOCALISED as they are not confined to a single pair of carbons, instead they can exist anywhere within the 2 rings
  • In total: there are 6 C-C σ-bonds, 6 C-H σ-bonds, and a delocalised π-electron cloud
  • The C-C bond length is equal for entire ring

Why do the C-C bonds in the ring all have the same length?
Because the π electrons are DELOCALISED.
Instead of having a fixed double bond position, the double bonds are said to be in RESONANCE.

What is resonance?
In chemistry: resonance is when a molecule alternates between multiple stable configurations. This is NOT isomerism, which is different structures for the same formulae. Instead, we are looking at a single molecule which can switch between different structures periodically.

In the case of benzene, we can say that the 2 resonant configurations are:

  

You can say that the double bonds switch positions at high speeds. Overall, it looks like this:

This is the delocalized ring.

If you replace the hydrogen atoms with alkyl groups:

Methylbenzene 
1,2-Dimethylbenzene 
1,3-Dimethylbenzene 
1,4-Dimethylbenzene 

Physical Properties of Arenes

Melting Points

  • Generally low, since arenes have intermolecular VDW forces & weak permanent dipole-dipole forces (some are slightly polar due to asymmetrical shape)
  • Generally: increasing number of carbon atoms in branches (& thus increasing number of electrons per molecule) increases strength of intermolecular forces, so MP increases
  • BUT the trend largely depends on the shape of the molecule: molecules which are symmetrical & can be packed neatly in a solid lattice have higher MP than those which cannot, as they are more stable & require more energy to melt
  • Benzene & 1,4-Dimethylbenzene have higher MP than the others since they can be packed neatly

Chemical Properties of Arenes
Reactions Overview
Let’s split these into 2 categories: reactions of the arene ring itself, & reactions of side-chains bonded to the ring. Specific reaction mechanisms will be covered in separate posts.

Ring Reactions

Halogenation substitution of H with Cl or Br

Mechanism:
Electrophilic Substitution
Conditions:
Room temperature
MUST be ANHYDROUS

Cannot happen without a catalyst. 
Requires a halogenating agent AKA ‘halogen carriers’ (catalysts):
FeCl3
FeBr3
Fe/heat

Reaction: C6H6 + X2 → C6H5X + HX
Nitration substitution of H with NO2 group

Mechanism:
Electrophilic Substitution
Conditions:
Requires a NO2+ ion, provided by a nitrating mixture:
HNO3 + 2H2SO4 → NO2+ + 2HSO4 + H3O+
This mixture is refluxed with benzene at 55°C  

Reaction:
C6H6 + HNO3 → C6H5NO2 + H+
Friedel-Crafts Reactions
Introduction of side-chains into a benzene ring

Mechanism:
Electrophilic Substitution

2 Reactions:
Friedel-Crafts Alkylation
substitution of H with alkyl groups (CH3)

Friedel-Crafts Acylation
substitution of H with acyl groups: alkyl group & a carbonyl group
Catalyst: ‘halogen carriers’
(AlCl3 / AlBr3 / FeCl3)

Alkylation:
C6H6 + RCl → C6H5-R + HCl

Acylation:
C6H6 + R-COCl → C6H5-COR + HCl  
Hydrogenation of ring*
Changes arene into a cyclic hydrocarbon
C6H6 + 3H2 → C6H12 (cyclohexane)

Side-chain reactions

Halogenation of side chain
(Free-radical substitution)
Same mechanism as alkane Free-radical substitution:
halogen gas is bubbled, requires UV light
Complete oxidation of an alkyl side-chainAlkyl side chains will undergo cleavage & form benzoic acid.  

Reagent:
hot acidified KMnO4
For other reactions of specific functional groups, see my posts on those:
phenols (Ar-OH),
phenylamines (Ar-NH2)
 
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