Benzene

Part 1

Starter: name the functional group.1 2 3

4 5

Learning outcomes

• Define arene and aromatic

• Name aromatic compounds

• Describe the structure of benzene

• Review the evidence for this structure

• Show how electrophilic substitutions occurs with different electrophiles

Arene and aromatic

• Look at page 4

• Write your definition of Arene and Aromatic

Benzene

• Isolated by Faraday in 1825

Physical properties:

• Clear, colourless, hydrocarbon

• 92.3% Carbon, 7.7% hydrogen.

• 0.250g was vaporised at 100oC had a volume of 98cm3

• Boiling point 80oC, melting point 5oC

Calculate the empirical formula

• 92.3% Carbon, 7.7% hydrogen.

Element C H

% 92.37.7

Ar 12.01.0

Moles 7.69 7.7

Ratio 7.69 7.69

1 1 empirical formula CH

Calculate the molecular formula

1 mole of gas is 24dm3 at RTP

• 0.250g was vaporised at 100oC had a volume of 98cm3.

• From this it was calculated that the molecular mass was 78g

• So what is the molecular formula?

The relative molecular mass of the sample is 78

The molecular formula78/13 = 6

Therefore the molecular formula is C6H6

Molymods

Find as many structures as possible for C6H6

Draw them in your notes

Problem

• Doesn’t react like an alkene- no reaction with HBr

• Doesn’t undergo electrophilic addition

• Enthalpy change should be about 800 kJmol-1, where as it is only 207kJmol-1

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.

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.

Kekulé 1865

STRUCTURE OF BENZENESTRUCTURE OF BENZENE

HOWEVER...

• it did not readily undergo electrophilic addition - no true C=C bond

• only one 1,2 disubstituted product existed

• all six C—C bond lengths were similar; C=C bonds are shorter than C-C

• the ring was thermodynamically more stable than expected

To explain the above, it was suggested that the structure oscillatedbetween the two Kekulé forms but was represented by neither ofthem. It was a RESONANCE HYBRID.

Next: X-ray diffraction

Bond type

Structure Bond length

C C Cyclohexane 0.15

C C Benzene 0.14

C C cyclohexene 0.13

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 andresisted attempts to break it down through normalelectrophilic addition. However, substitution of anyhydrogen atoms would not affect the delocalisation.

The theory suggested that instead of three localised (in one position) double bonds, the six p () electrons making up those bonds were delocalised (not in any oneparticular position) around the ring by overlapping the p orbitals. There would be nodouble bonds and all bond lengths would be equal. It also gave a planar structure.

STRUCTURE OF BENZENESTRUCTURE OF BENZENE

Exam question

• In this question, one mark is available for the quality of spelling, punctuation and grammar.

• Describe with the aid of suitable diagrams the bonding and structure of a benzene molecule.

Discussion of the π-bonding

p-orbitals overlap (1)

above and below the ring (1)

(to form) π-bonds / orbitals (1)

any of the first three marks are available from a labelled diagram eg

bonds

(π-bonds / electrons) are delocalised (1)

4 marks

Other valid points – any two of:

• ring is planar /

• C-C bonds are equal length / have intermediate length/strength between C=C and C-C /

• σ-bonds are between C-C and/or C-H

• bond angles are 120° 6

MAX 2 out of 4 marks (1)(1)

Quality of written communication two or more sentences with correct spelling, punctuation and grammar 1

[7]

Homework

• Produce a leaflet or a poster showing where benzene is used and hence why it is important. Due Friday 10th September

Learning outcomes

• Define arene and aromatic

• Name aromatic compounds

• Describe the structure of benzene

• Review the evidence for this structure

• Show how electrophilic substitutions occurs with different electrophiles

Naming aromatic compounds

Chlorobenzene

Phenol

Methylbenzene

Many of these compounds are foul smelling and toxic, they are still called aromatic

• When a long alkyl chain with other substitutions is present, think of the benzene as substituted onto the chain, using phenyl and a number to position the chain.

CH3– CH – CH – CH3

Cl

2-Chloro-3-phenylbutane

Naming dominos

Identify the following molecules as alkene, arene or cycloaklane

1. CH3CH2CH2CH2CH3

2. C6H5CH3

3. CH3CH=CHCH2CH3

4. CH3CH(CH3)CH2CH3

5.

Identify the following molecules as alkene, arene, alkane or cycloalkane

1. CH3CH2CH2CH2CH3 alkane

2. C6H5CH3 arene

3. CH3CH=CHCH2CH3 alkene

4. CH3CH(CH3)CH2CH3 alkane

5. cycloalkane

Learning outcomes

• Define arene and aromatic

• Name aromatic compounds

• Describe the structure of benzene

• Review the evidence for this structure

• Show how electrophilic substitutions occurs with different electrophiles

ELECTROPHILIC SUBSTITUTIONELECTROPHILIC SUBSTITUTION

Theory The high electron density of the ring makes it open to attack by electrophiles

Addition to the ring would upset the delocalised electron system

Substitution of hydrogen atoms on the ring does not affect the delocalisation

Because the mechanism involves an initial disruption to the ring,

electrophiles must be more powerful than those which react with alkenes

Overall there is ELECTROPHILIC SUBSTITUTION

ELECTROPHILIC SUBSTITUTIONELECTROPHILIC SUBSTITUTION

Theory The high electron density of the ring makes it open to attack by electrophiles

Addition to the ring would upset the delocalised electron system

Substitution of hydrogen atoms on the ring does not affect the delocalisation

Because the mechanism involves an initial disruption to the ring,

electrophiles must be more powerful than those which react with alkenes

Overall there is ELECTROPHILIC SUBSTITUTION

Mechanism

• a pair of electrons leaves the delocalised system to form a bond to the electrophile

• this disrupts the stable delocalised system and forms an unstable intermediate

• to restore stability, the pair of electrons in the C-H bond moves back into the ring

• overall there is substitution of hydrogen ... ELECTROPHILIC SUBSTITUTION

ELECTROPHILIC SUBSTITUTION REACTIONS - ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATIONNITRATION

Reagents conc. nitric acid and conc. sulphuric acid (catalyst)

Conditions reflux at 55°C

Equation C6H6 + HNO3 ———> C6H5NO2 + H2O

nitrobenzene

ELECTROPHILIC SUBSTITUTION REACTIONS - ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATIONNITRATION

Reagents conc. nitric acid and conc. sulphuric acid (catalyst)

Conditions reflux at 55°C

Equation C6H6 + HNO3 ———> C6H5NO2 + H2O

nitrobenzene

Mechanism

ELECTROPHILIC SUBSTITUTION REACTIONS - ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATIONNITRATION

Reagents conc. nitric acid and conc. sulphuric acid (catalyst)

Conditions reflux at 55°C

Equation C6H6 + HNO3 ———> C6H5NO2 + H2O

nitrobenzene

Mechanism

Electrophile NO2+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction...

2H2SO4 + HNO3 2HSO4¯ + H3O+ + NO2+

acid base

ELECTROPHILIC SUBSTITUTION REACTIONS - ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATIONNITRATION

Reagents conc. nitric acid and conc. sulphuric acid (catalyst)

Conditions reflux at 55°C

Equation C6H6 + HNO3 ———> C6H5NO2 + H2O

nitrobenzene

Mechanism

Electrophile NO2+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction...

2H2SO4 + HNO3 2HSO4¯ + H3O+ + NO2+

acid base

Use The nitration of benzene is the first step in an historically important chain of reactions. These lead to the formation of dyes, and explosives.

ELECTROPHILIC SUBSTITUTION REACTIONS - ELECTROPHILIC SUBSTITUTION REACTIONS - HALOGENATIONHALOGENATION

Reagents chlorine and a halogen carrier (catalyst)

Conditions reflux in the presence of a halogen carrier (Fe, FeCl3, AlCl3)chlorine is non polar so is not a good electrophilethe halogen carrier is required to polarise the halogen

Equation C6H6 + Cl2 ———> C6H5Cl + HCl

Mechanism

Electrophile Cl+ it is generated as follows... Cl2 + FeCl3 FeCl4¯ + Cl+

a Lewis Acid

Now your turn

Write the mechanisims for the following electrophiles:

1. CH3+

2. CH3CO+

Where does the electrophile end up?

Groups with +I Groups with a - I

Mostly 2, 4 and 6 positions Mostly with 3 and 5 positions

OH Cl

CH3 COOH

NH2 NO2

+ I effect

- I effect

Learning outcomes

• Define arene and aromatic

• Name aromatic compounds

• Describe the structure of benzene

• Review the evidence for this structure

• Show how electrophilic substitutions occurs with different electrophiles

Diploma students only

FRIEDEL-CRAFTS REACTIONS OF BENZENE - FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATIONALKYLATION

Overview Alkylation involves substituting an alkyl (methyl, ethyl) group

Reagents a halogenoalkane (RX) and anhydrous aluminium chloride AlCl3

Conditions room temperature; dry inert solvent (ether)

Electrophile a carbocation ion R+ (e.g. CH3+)

Equation C6H6 + C2H5Cl ———> C6H5C2H5 + HCl

Mechanism

General A catalyst is used to increase the positive nature of the electrophile

and make it better at attacking benzene rings.AlCl3 acts as a Lewis Acid and helps break the C—Cl bond.

FRIEDEL-CRAFTS REACTIONS OF BENZENE - FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATIONALKYLATION

Catalyst anhydrous aluminium chloride acts as the catalystthe Al in AlCl3 has only 6 electrons in its outer shell; a LEWIS

ACIDit increases the polarisation of the C-Cl bond in the haloalkanethis makes the charge on C more positive and the following

occurs

RCl + AlCl3 AlCl4¯ + R+

FRIEDEL-CRAFTS REACTIONS - FRIEDEL-CRAFTS REACTIONS - INDUSTRIALINDUSTRIAL ALKYLATIONALKYLATION

Industrial Alkenes are used instead of haloalkanes but an acid must be presentPhenylethane, C6H5C2H5 is made by this method

Reagents ethene, anhydrous AlCl3 , conc. HCl

Electrophile C2H5+ (an ethyl carbonium ion)

Equation C6H6 + C2H4 ———> C6H5C2H5 (ethyl benzene)

Mechanism the HCl reacts with the alkene to generate a carbonium ionelectrophilic substitution then takes place as the C2H5

+ attacks the ring

Use ethyl benzene is dehydrogenated to produce phenylethene (styrene);

this is used to make poly(phenylethene) - also known as polystyrene

FRIEDEL-CRAFTS REACTIONS OF BENZENE - FRIEDEL-CRAFTS REACTIONS OF BENZENE - ACYLATIONACYLATION

Overview Acylation involves substituting an acyl (methanoyl, ethanoyl) group

Reagents an acyl chloride (RCOX) and anhydrous aluminium chloride AlCl3

Conditions reflux 50°C; dry inert solvent (ether)

Electrophile RC+= O ( e.g. CH3C+O )

Equation C6H6 + CH3COCl ———> C6H5COCH3 + HCl

Mechanism

Product A carbonyl compound (aldehyde or ketone)

FURTHER SUBSTITUTION OF ARENESFURTHER SUBSTITUTION OF ARENES

Theory It is possible to substitute more than one functional group.

But, the functional group already on the ring affects...

• how easy it can be done • where the next substituent goes

Group ELECTRON DONATING ELECTRON WITHDRAWING

Example(s) OH, CH3 NO2

Electron density of ring Increases Decreases

Ease of substitution Easier Harder

Position of substitution 2,4,and 6 3 and 5