Basic concepts of Aromatic

What are the three issues with Kekulé’s Model of benzene

1) Benzene does not undergo electrophilic addition and doesn’t readily react with bromine water

2) Benzene is 152 kJmol^-1 more energetically stable than expected

3) All the carbon-carbon bonds are the same length

What are the observations and conclusion of issue 1?

• Benzene does not decolourise bromine water

• Benzene does not easily take part in other electrophilic reactions

• Conclusion : Benzene has a lower chemical reactivity than is predicted

What are the observations and conclusions of issue 2?

• Hydrogenation of cyclohexane (1 double bond) has a Δ_H of -120kJmol^-1

• Hydrogenation of theoretical Kekule’s benzene should be Δ_H of -360kJmol^-1

• Actual hydrogenation of benzene was Δ_H of -208kJmol^-1 which is 152 kJmol^-1 more stable than thought

• Conclusion : So the actual structure is different to Kekule’s model

What are the observations and conclusion of issue 3?

• Double bonds should have a length of 0.134 nm and single bonds a length of 0.153nm

• X-ray crystallography shows that al 6 bonds had the same length of 0140nm

• Conclusion : so benzene must contain 6 bonds that are equal in lenth in between a single and double bond

What are the main features of the delocalised model of benzene?

• Each carbon has 4 outer shell electrons

• 3 electrons used to form sigma bonds

• The 4th electron occupies a p-orbital which is at right angles to the plane of benzene

• 6 p-orbitals overlap sideways, above and below the plane of the ring, to form a delocalised ring of 6 pi electrons

What are the reaction conditions for nitration of benzene?

• Concentrated sulfuric acid catalyst

• Concentrated nitric acid

• Reflux reaction

What is a halogen carrier?

A type of catalyst that help to form halogen electrophiles in the reaction vessel

Why are halogen carriers required for Halogenation of Benzene?

Due to benzene having a lower electron density compared to traditional alkenes due to its delocalised pi electrons. So it cannot induce a dipole on its own.

What are the two types of halogen carriers?

AlX₃ or FeX₃

What is Friedel-crafts acylation and it’s reaction conditions?

It is and ROCl (Acyl chloride) reacting with AlCl₃ to form and AlCl₄^- ion and an RCO⁺ (acylium ion). The acylium ion then reacts with benzene to form a aromatic ketone.

It’s reaction conditions are Acyl chloride + halogen carrier + reflux at 60^oC

What is the starting equation for Alkylation?

RCl + FeCl₃ → R⁺ + FeCl₄^-

What occurs when phenol is brominated?

From orange to white solid

Why are halogen carriers not required for phenols?

• Phenol and benzene both contain delocalised pi electrons

• Lone pair of electrons from OH^- is delocalised into the ring

• This increases electron density

• So is able to induce dipoles on halogens

What are the 4 properties of phenols?

• Partial solubility in water

• Orange colour after UI

• Dissolves completely when in a NaOH solution

• Orange solution to white ppt in bromine water

Why does property 1 occur?

The hydroxy group can form hydrogen bonds with the water but the aromatic ring (non-polar) cannot so it is partially soluble

Why does property 2 occur?

As phenol is a weak acid of 5 ~ 6 pH range

Why does property 3 occur?

Phenols can react with strong bases such as NaOH to form a soluble salt + water.

What are positions 2,3,4 called on a phenol?

Ortho, meta and para

What are all the 2 and 4 directing groups?

• NH₂ - Amino

• OH - Hydroxy

• -NHR - Amine

• -OR - Ether

• -R - Alkyl

• -X - Halogen

What are 2 and 4 directing groups known as?

Activating groups

What are 3 directing groups known as?

Deactivating groups

What are all the 3 directing groups?

• NO₂ - Nitro

• -COOR - Ester

• -CHO - Aldehyde

• -COOH - Carboxylic acid

• -CN - Nitrile

• -SO₃H - Sulfonic acid