THE CHEMISTRYTHE CHEMISTRYOF ARENESOF ARENESFoundation Chemistry

Semester 2

Mr A J Crooks

13.6 Aromatic Chemistry

• 13.6.1 Bonding

• 13.6.2 Delocalisation Stability

• 13.6.3 Electrophilic Substitution

• 13.6.4 Nitration

• 13.6.5 Friedel-Crafts reactions (Alkylation and Acylation).

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13.6.1 Bonding

Aim

• Understand the nature of the bonding in a benzene ring,

• Limited to planar structure and bond length intermediate between single and double.

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Aromatic Hydrocarbons

• Families of hydrocarbons studied so far have a linear backbone of carbon atoms.

• Classed as aliphatic. (Arises from the fact that the first compounds of this class to be studied were the ‘fatty’ (carboxylic) acids – Greek: ‘aliphos’ = fat).

• There is another class of hydrocarbon aromatic hydrocarbons. • Aromatic compounds contain a benzene ring, or behave

chemically like benzene.• Named because earliest examples studied were obtained from

natural sources (resins, aromatic oils, etc) and tended to have pleasant smells (Greek: ‘aroma’ = fragrant)

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What are Arenes?Compounds that include benzene and compounds that resemble benzene in certain of their chemical

properties. Common aromatic compounds

other than benzene include toluene, naphthalene, and

anthracene (all of which are present in coal tar).

So What is the structure of benzene and why does it make it special?

AKA: Aromatic Compounds

TolueneNaphthalene

Anthracene

Discovery of Benzene

• Discovered in 1825 by Michael Faraday during experiments in distillation of crude oil.

• He extracted a gas which, out of mere curiosity, he set alight and the gas burned.

• For many years, the benefits of what Faraday called "bicarburet of hydrogen" were unknown.

• In 1846, A. W. Hoffman isolated the same product, later called benzene, in large quantities by distilling coal.

STRUCTURE OF BENZENESTRUCTURE OF BENZENE

Primary analysis revealed benzene had...

an empirical formula of CH and

a molecular mass of 78 and

a formula of C6H6

• Molecular formula, C6H6.

• Structural formula (circular), first suggested by Friedrich August von Kekule in 1865:

• This was the first structure to show alternating double and single bonds.

STRUCTURE OF BENZENESTRUCTURE OF BENZENE

Kekulé suggested that benzene was...

PLANARCYCLIC and

HAD ALTERNATING DOUBLE AND SINGLE BONDS

Kekule’s Dream (1)

The Discovery of Benzene Ring

• One of the most famous instances of dream-discovery was that of the ring structure of Benzene by Friedrich August von Kekule.

• Until then, molecular structures were conceived as linear. • This, however, did not explain many properties of Benzene

(see later). • Kekule was trying to figure out a structure - not with much

success - when the solution came to him in a dream… X

• He described this experience to a assembly of scientists who had met to commemorate his discovery...

Kekule’s Dream

• "...I turned my chair toward the fire place and sank into a doze. Again the atoms were flitting before my eyes. Smaller groups now kept modestly in the background. My mind's eye sharpened by repeated visions of a similar sort, now distinguished larger structures of varying forms. Long rows frequently rose together, all in movement, winding and turning like serpents; and see! what was that? One of the serpents seized its own tail and the form whirled mockingly before my eyes. I came awake like a flash of lightning. This time also I spent the remainder of the night working out the consequences of the hypothesis."

Kekule’s dream

• Arthur Koestler (in "The Act of Creation") called this incident "probably the most important dream in history since Joseph's seven fat and seven lean cows.

Flaws with this Structure

• Structure suggests a highly unsaturated molecule.

• Should show addition reactions.• Although it does show addition reactions…• Its most important reactions are substitution

reactions.• Hence, the above representation has its

limitations.

Resonance Structures

• In 1858, Kekule proposed a new structure, postulating benzene as a mesomer.

• Two (canonical) forms.

• Rapidly oscillating.

• Known as Kekule structures.

Dewar Forms

• Benzene probably contains other forms beside the Kekule mesomers.• i.e. there are 3 more ‘Dewar’ forms:

90% 10% Kekule Dewar

• Note that each benzene molecule contains the character of each of the above forms.

Pauling’s Structure

• There were still problems with this structure, and so…• In 1931 Pauling used quantum physics to propose his ‘resonance

structure:

• Here, benzene is viewed as a hybrid of III & IV and represented by V.

• Because the 6 C-C bond lengths are all equivalent, Kekule’s rapidly oscillating forms must be ruled out.

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• Note

The Principle Protagonists

Principle Objections to the Kekule Structure

• Kekule predicted 4 isomers of dibromobenzene (1,2; 1,3; 1,4 and 1,6.

• However, 1,6 did not exist. • Benzene does not behave like cyclohexatriene i.e a

reactive molecule which would decolorise aqueous bromine.

THERMODYNAMIC EVIDENCE FOR STABILITYTHERMODYNAMIC EVIDENCE FOR STABILITY

When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.

ENTHALPY OF HYDROGENATION (1)

• When the C=C bond is hydrogenated in cyclohexene, the enthalpy change is –119kJ mol-1.

• Hence cyclohexatriene should evolve 3 x –119kJ (=-357kJ mol-1).

• Actual enthalpy of hydrogenation is –208kJ mol-1

• Hence benzene is 149kJ mol-1more stable than its Kekule structure would suggest.

• This is referred to as resonance energy (preferably delocalisation energy).

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THERMODYNAMIC EVIDENCE FOR STABILITYTHERMODYNAMIC EVIDENCE FOR STABILITY

2 3

- 120 kJ mol-1

When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.

C6H10(l) + H2(g) ——> C6H12(l)

When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.

THERMODYNAMIC EVIDENCE FOR STABILITYTHERMODYNAMIC EVIDENCE FOR STABILITY

2 3

- 120 kJ mol-1

Theoretical- 360 kJ mol-1

(3 x -120)

When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.

C6H10(l) + H2(g) ——> C6H12(l)

Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane

C6H6(l) + 3H2(g) ——> C6H12(l)

When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.

THERMODYNAMIC EVIDENCE FOR STABILITYTHERMODYNAMIC EVIDENCE FOR STABILITY

2 3

Experimental- 208 kJ mol-1

- 120 kJ mol-1

Theoretical- 360 kJ mol-1

(3 x -120)

When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.

C6H10(l) + H2(g) ——> C6H12(l)

Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane

C6H6(l) + 3H2(g) ——> C6H12(l)

Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale

When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.

THERMODYNAMIC EVIDENCE FOR STABILITYTHERMODYNAMIC EVIDENCE FOR STABILITY

2 3

MORE STABLE THAN EXPECTED

by 152 kJ mol-1

Experimental- 208 kJ mol-1- 120 kJ mol-1

Theoretical- 360 kJ mol-1

(3 x -120)

When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.

C6H10(l) + H2(g) ——> C6H12(l)

Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane

C6H6(l) + 3H2(g) ——> C6H12(l)

Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale

It is 152kJ per mole more stable than expected. This value is known as the RESONANCE ENERGY.

When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.

THERMODYNAMIC EVIDENCE FOR STABILITYTHERMODYNAMIC EVIDENCE FOR STABILITY

2 3

MORE STABLE THAN EXPECTED

by 152 kJ mol-1

Experimental- 208 kJ mol-1- 120 kJ mol-1

Theoretical- 360 kJ mol-1

(3 x -120)

When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.

C6H10(l) + H2(g) ——> C6H12(l)

Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane

C6H6(l) + 3H2(g) ——> C6H12(l)

Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale

It is 152kJ per mole more stable than expected.This value is known as the RESONANCE ENERGY.

When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.

ENTHALPY OF HYDROGENATION (2)(Energies in kcal/mol. Multiply by 4.2 for kJ/mol)

X-Ray Diffraction Data

• This showed the C-C bond lengths all to be 0.14nm.

• This is intermediate between the lengths of C-C (0.154nm) and C=C (0.134nm),which would be expected from Kekule’s structure.

• This final piece of evidence provided the clue to the accepted structure for benzene:

STRUCTURE OF BENZENE - STRUCTURE OF BENZENE - DELOCALISATIONDELOCALISATION

The currently accept theory is that instead of benzene having three localised (in one position) double bonds, the six p (p) electrons making up those bonds are delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. This gives a planar structure.

6 single bonds

STRUCTURE OF BENZENE - STRUCTURE OF BENZENE - DELOCALISATIONDELOCALISATION

6 single bonds one way to overlapadjacent p orbitals

The currently accepted theory is that instead of benzene having three localised (in one position) double bonds, the six p () electrons making up those bonds are delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There are no double bonds and all bond lengths are equal. This gives a planar structure.

STRUCTURE OF BENZENE - STRUCTURE OF BENZENE - DELOCALISATIONDELOCALISATION

6 single bonds one way to overlapadjacent p orbitals

anotherpossibility

The currently accepted theory is that instead of benzene having three localised (in one position) double bonds, the six p () electrons making up those bonds are delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There are no double bonds and all bond lengths are equal. This also gives a planar structure.

STRUCTURE OF BENZENE - STRUCTURE OF BENZENE - DELOCALISATIONDELOCALISATION

6 single bonds one way to overlapadjacent p orbitals

delocalised piorbital system

anotherpossibility

The currently accepted theory is that instead of benzene having three localised (in one position) double bonds, the six p () electrons making up those bonds are delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There are no double bonds and all bond lengths are equal. This also gives a planar structure.

STRUCTURE OF BENZENE - STRUCTURE OF BENZENE - DELOCALISATIONDELOCALISATION

6 single bonds one way to overlapadjacent p orbitals

delocalised piorbital system

anotherpossibility

This final structure was particularly stable and

resisted attempts to break it down through normal

electrophilic addition. However, substitution of any

hydrogen atoms would not affect the delocalisation.

The currently accepted theory is that instead of benzene having three localised (in one position) double bonds, the six p () electrons making up those bonds are delocalised (not in any one particular position) around the ring by overlapping the p orbitals. There are no double bonds and all bond lengths are equal. This also gives a planar structure.

Currently Accepted Structure of Benzene

STRUCTURE OF BENZENESTRUCTURE OF BENZENE

ANIMATIONANIMATION

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