Topic 6: Aromatic Compounds

What type of bonds do carbon atoms create in benzene?

Each carbon atom creates 3 covalent sigma bonds with 3 of its electrons.

Where are the remaining electrons of carbon found in benzene?

The remaining electrons are found in a p-orbital at a right angle above and below the carbon atom.

What is the significance of p-orbitals in benzene?

P-orbitals overlap evenly, creating a ring of delocalised electrons above and below the benzene structure.

What does delocalised electrons mean in the context of benzene?

Delocalised electrons are bonding electrons that are not fixed between two atoms but are shared between three or more atoms.

How does the ring structure of benzene contribute to its stability?

The ring allows charge to evenly spread across the molecule, making it stable and allowing equal bond lengths.

What is the electron density like in the delocalised model of benzene?

There is low electron density because electrons are delocalised across the ring.

How does delocalisation affect benzene's reactivity towards electrophiles?

Benzene is less able to attract electrophiles and will be destabilised if groups are added to two neighbouring carbons.

What is the expected enthalpy change of hydrogenation for benzene according to Kekulé's structure?

It is expected to be 3 times the enthalpy change of hydrogenation of cyclohexane.

What is the actual enthalpy change of hydrogenation of benzene?

The actual enthalpy change of hydrogenation of benzene is -208kJmol-1

What does a less exothermic enthalpy change indicate about benzene's stability?

It indicates that the actual structure of benzene is more stable than expected.

Define aromatic compounds.

Aromatic compounds are those that contain a benzene ring.

What is hydrogenation enthalpy?

It is the enthalpy change when one mole of an unsaturated compound reacts with an excess of hydrogen to become a fully saturated compound.

What are some uses of benzene?

Benzene is used as a major feedstock for pharmaceuticals, dyes, and explosives.

Why is benzene considered highly carcinogenic?

Benzene is considered highly carcinogenic due to its chemical properties and effects on human health.

What is the empirical formula of benzene?

The empirical formula of benzene is CH.

What is the molecular formula of benzene?

The molecular formula of benzene is C6H6.

What did Kekulé suggest about the structure of benzene?

Kekulé suggested a cyclic structure for benzene with alternating double bonds.

What observation suggests that benzene does not undergo electrophilic addition reactions?

Benzene does not decolorize bromine water under normal conditions without a catalyst.

What does the lack of reactivity in benzene imply about its structure?

It implies that benzene does not have double bonds as previously thought.

What did X-ray diffraction reveal about the structure of benzene?

X-ray diffraction shows that benzene has a regular hexagonal shape with equal bond lengths of 0.139 nm.

What is the significance of bond lengths in determining benzene's structure?

Different bond lengths would suggest an irregular hexagon if Kekulé's model was true.

Skeletal Formula Benzene

Shape and Bond Angle of Benzene

planar regular hexagonal shape

• 120 degrees