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STRUCTURE OF BENZENE

Benzene = C6H6

Its structure was a mystery for many years - until Kekule’s dream.

STRUCTURE OF BENZENE

Problems with Kekule’s structure:

1) C-C bonds are all the same length (between C-C and C=C length).

2) Benzene does not undergo addition reactions readily (e.g. with bromine water).

3) H hydrogenation about 152 kJ mol-1 less than expected.

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C C

C

CC

C HH

H H

H H

STRUCTURE OF BENZENE

STRUCTURE OF BENZENE

http://www.isis.stfc.ac.uk/

HYDROGENATION OF BENZENE

-120

-240

-360

-208

152

Energy(kJ/mol)

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NAMING AROMATIC COMPOUNDS

methylbenzene(toluene)

CH3 Cl NO2 COOH

chlorobenzene nitrobenzene benzenecarboxylic acid(benzoic acid)

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NAMING AROMATIC COMPOUNDS

1,3-dimethylbenzene(m-xylene)

4-methyl-1-chlorobenzene

2-hydroxybenzenecarboxylic acid

CH3

CH3

CH3

Cl

COOH

OH

COOH

NO2O2N

3,5-dinitrobenzenecarboxylic acid

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NAMING AROMATIC COMPOUNDS

phenylamine(aniline)

2-phenylbutane

phenylethene phenylethanone

NH2

CH CH2

CH2CH3 CH2 CH3

CO CH3

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NAMING AROMATIC COMPOUNDS

phenylethanoate methyl benzenecarboxylate(methyl benzoate)

N-phenylethanamide

OC

CH3

O

CO O

CH3

NH C

O

CH3

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NAMING AROMATIC COMPOUNDS

phenol benzene-1,2-diol

2,4,6-trichlorophenol(TCP)

OH

OH

Cl

Cl

Cl

OH

OH

OH

NO2

2-nitrophenol

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NITRATION

+ HNO3 + H2O

NO2

nitrobenzene

Conditions conc HNO3

conc H2SO4 catalyst

50ºC

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-H+

Formation of electrophile

HNO3

+ 2 H2SO4 + 2 HSO4-

+ H3O+

NO2

+

Reaction of electrophile with aromatic compound

NO2

NO2+ NO2

+H

nitronium ion

NITRATION – ELECTROPHILIC SUBSTITUTION

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NITRATION

Equation (name product)

Conditions

Mechanism

CH3CH3

NO2

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NITRATION

2-methylnitrobenzene

Conditions conc HNO3

conc H2SO4 catalyst

50ºC

CH3

+ HNO3 + H2O

CH3

NO2

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NITRATION

Uses of nitro compounds:

1) as explosives

2) to make aromatic amines (used to make dyes)

CH3

NO2

NO2

O2N

1,3,5-trinitrotolueneTNT

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FRIEDEL-CRAFTS ACYLATION

aromatic ketone

Conditions AlCl3 catalyst

R C

O

Cl+

CR O

+ HCl

+

CR O

+ R C

O

O C

O

R R C

O

OH

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FRIEDEL-CRAFTS ACYLATION

WARNING! - do not mix

Friedel-Crafts acylation & other acylations

(needs AlCl3)

Swap H on benzene ring

for acyl group

H H N

H

R

H O R

Swap H on O/N

for acyl group© www.chemsheets.co.uk A2 019 15-Jul-12

FRIEDEL-CRAFTS ACYLATION

WARNING! - Classic question to test this

NH2 NHCOCH3

CH3COCl

H replaced is on N not benzene ring.

This is NOT Friedel-Crafts acylation!

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Formation of electrophile

AlCl3

+ RCOCl +

AlCl4–+

C

O

R

acylium ion

OR

AlCl3

+ R-C-O-C-R +

–

+

C

O

R

acylium ion

O

O R-C-O-AlCl3

O

FRIEDEL-CRAFTS ACYLATION – ELECTROPHILIC SUBSTITUTION

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-H+

Reaction of electrophile with aromatic compound

CC

+H+

C

O

R

O

RO

R

FRIEDEL-CRAFTS ACYLATION – ELECTROPHILIC SUBSTITUTION

AlCl4– + H+ AlCl3 + HCl

+ H+ + AlCl3– R-C-O-AlCl3

O

R-C-OH

O

or

Regeneration of catalyst

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Equation

Conditions

Mechanism

FRIEDEL-CRAFTS ACYLATION

CH3CH3

CO CH2CH3

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Conditions AlCl3 catalyst

FRIEDEL-CRAFTS ACYLATION

CH3

CO CH2CH3

CH3

+ CH3CH2 C

O

Cl+ HCl

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