Chemistry - 3.3.10: Aromatic Chemistry

Arenes

Hydrocarbons based on benzene

Benzene formula (molecular and skeletal)

C₆H₆

How do alkyl groups affect the reactivity of (an aromatic) compound in electrophilic substitution reactions?

1. Alkyl groups cause a positive inductive effect

2. So the electrophile is attracted more

What is the effect of delocalised electrons on a molecule?

It provides some extra stability to the molecule

What is special about the structure and bonding of benzene? (2)

1. it is unsaturated, but doesn’t readily undergo addition reactions

2. All the carbon atoms were equivalent, so all the carbon-carbon bonds are the same

Describe the structure and bonding of benzene (4)

• It is a planar (flat), regular hexagon of carbon atoms - each of which is bonded to a single hydrogen atom

• Each carbon atom has 3 covalent bonds (one C-H, 2 C-C)

• Each bond is equal and is between a single and double C-C bond

• The spare electrons of each C atom

Why is the enthalpy of hydrogenation of cyclohexa-1,3-diene not double that of cyclohexane?

The double bonds are separated by a single bond, which allows for some delocalisation

Why is benzene drawn as a hexagonal ring with a circle in it?

Some of the electrons are delocalised (they’re spread over the 6 carbon atoms in the ring)

Why is benzene more stable than cyclohexa-1,3,5-triene?

Its enthalpy of hydrogenation is higher because it has a delocalised ring of electrons

How do you use thermochemical evidence to prove that benzene is more stable than cyclohexa-1,3,5-triene? (3)

1. Find the enthalpy of hydrogenation of cyclohexene

2. Multiply that by 3 to get the enthalpy of hydrogenation of cyclohexa-1,3,5-triene

3. This is greater than the enthalpy change for benzene

Compare benzene’s melting point and boiling point to hexane

1. Boiling point similar to hexane

2. Melting point much higher than hexane

Why does benzene have a higher melting point than hexane?

Benzene’s flat molecules pack together well in the solid state, so are harder to separate during melting

Describe the solubility of arenes

Don’t mix with water (as non-polar), but mix with other hydrocarbons and non-polar solvents

Why can arenes undergo electrophilic substitution reactions? (2)

1. Ring is an area of high electron density so can be attacked by electrophiles

2. Aromatic ring is very stable and needs energy to be put into it to break it before the system can be destroyed

Delocalisation energy

The energy needed to break the ring before the system can be destroyed

What happens when arenes undergo combustion and why?

They burn in air with flames that are noticeably smoky - They have a high C:H ratio, so there is usually unburnt carbon remaining, which produces soot

What is the most typical reaction of benzenes?

Electrophilic substitution

Why is it more likely that substitution reactions take place instead of addition reactions?

Addition reactions would need the delocalisation energy to destroy the aromatic system

Outline the mechanism of electrophilic substitution reactions with benzene/arenes (2)

1. Arrow from delocalised ring of electrons to electrophile (El)

2. C-El bond forms, ring is broken at carbon where C-El bond formed and a + sign in the centre
   C-H bond breaks, arrow from C-H to + sign

Nitration

The substitution of a NO₂ group for one of the hydrogen atoms on an arene ring

How is the electrophile NO₂⁺ generated (overall + 2 steps)?

H₂SO₄ + HNO₃ →HSO₄^- + NO₂⁺ + H₂O

H₂SO₄ + HNO₃ → H₂NO₃⁺ + HSO₄^-

H₂NO₃⁺ → NO₂⁺ + H₂O

What is NO⁺₂ called?

Nitronium ion / nitryl cation

Give the overall reaction when benzene reacts with nitric acid

Benzene + HNO₃ → Nitrobenzene + H₂O

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

What reagents are needed for the nitrification of benzene?

Concentrated nitric acid and a concentrated sulfuric acid catalyst

What are nitrated arenes used for? (2)

They are used to produce explosives e.g. TNT + to make aromatic amines, which are then used to make industrial dyes

What catalyst is used in Friedel-Crafts acylation reactions?

Aluminium chloride

What happens during a Friedel-Crafts acylation reaction between benzene and an acyl chloride? (3)

1. ROCl + AlCl₃ → RCO⁺ + AlCl₄^-

2. RCO⁺ then used in an electrophilic substitution reaction

3. AlCl₄^- + H⁺ → AlCl₃ + HCl

What might be substituted for a hydrogen in a Friedel-Crafts acylation reaction?

RCO, e.g. from an acyl chloride

What is the product of Friedel-Crafts acylation used for?

They’re used in the synthesis of new substituted aromatic compounds

Is benzoic acid soluble in water? Why?

No - the benzene ring is non-polar despite the COOH group’s ability to form H bonds.

Compare the structure of benzene and cyclohexa-1,3,5-triene (2)

1. Benzene has equal C-C bond lengths whereas cyclohexa-1,3,5-triene has bonds of different lengths

2. Both planar, hexagonal rings