HOME PAGE * KS3 SCIENCES * GCSE BIOLOGY CHEMISTRY PHYSICS * ADVANCED LEVEL CHEMISTRY

Advanced Level Organic Chemistry: Free radical reactions of benzene & methylbenzene

Part 7. The chemistry of AROMATIC COMPOUNDS

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 7.4 Free radical addition reactions of benzene and methylbenzene with hydrogen and chlorination substitution of the methyl group of methylbenzene

Sub-index for this page

7.4.1 Hydrogenation of benzene and methylbenzene

7.4.2 Free radical addition of chlorine to benzene

7.4.3 Free radical substitution by chlorine of the methyl group in methylbenzene


INDEX of AROMATIC CHEMISTRY NOTES

All Advanced A Level Organic Chemistry Notes

[SEARCH BOX] ignore ads at top



TOP OF PAGE and sub-index


7.4.1  Hydrogenation of benzene and methylbenzene

If benzene or methylbenzene is mixed with hydrogen and passed over a heated nickel catalyst addition of three moles of hydrogen per arene molecule takes place.

The higher temperature and Ni catalyst are needed because of the stability of the benzene ring in arenes.

The hydrogen molecules are adsorbed on the Ni surface and split into atoms.

These atoms are effectively very reactive hydrogen radicals (H•) which can break open the pi bond system of the arene and form new C-H bonds until the totally saturated cycloalkane is formed.

 

(1)   +  3H2 ===>  alkanes structure and naming (c) doc b    or    skeletal formula of benzene C6H6 + 3H2 ===> alkanes structure and naming (c) doc b

hydrogenation of the benzene to cyclohexane.

ΔHhydrogenation(benzene) = -208 kJ mol-1

 

(2) (c) doc b  +  3H2  ===> alkanes structure and naming (c) doc b   or    skeletal formula of methylbenzene C7H8 +  3H2  ===> 

hydrogenation of methylbenzene to methylcyclohexane

ΔHhydrogenation(methylbenzene) = -205 kJ mol-1

Note the similarity of the enthalpies of hydrogenation.

diagram free radical catalyttic hydrogenation of benzene or methylbenzene nickel surface catalyst

The mechanism is very complicated, but the very simplified diagram above gives you some about it.

Hydrogen free radicals (hydrogen atoms with their electron) are formed on the nickel surface, and then attack the pi electron cloud of the 'benzene' ring forming new C-H bonds.

A series of free radical chain reactions ensue until all the pi electrons have been used in forming C-H bonds and the fully saturated cycloalkane is formed.


TOP OF PAGE and sub-index


7.4.2 Free radical addition of chlorine to benzene

If chlorine is bubbled into boiling liquid benzene as ultraviolet light is shone on the mixture you get the addition of three chlorine molecules per benzene molecule.

You do NOT get addition under these reaction conditions.

The reaction yield yields 1,2,3,4,5,6-hexachlorocyclohexane.

C6H6  +  3Cl2  ===>  C6H6Cl6

diagram uv catalysed free radical addition of chlorine to benzene giving 1,2,3,4,5,6-hexachlorocyclohexane

Initiation

Cl2  == uv  ==> 2Cl•

Propagation steps

C6H6  +  Cl•  ===> C6H6Cl•

until the final step is

C6H6Cl5•  +  Cl•  ===> C6H6Cl6 

 


TOP OF PAGE and sub-index


7.4.3 Free radical substitution by chlorine of the methyl group in methylbenzene

When methylbenzene is reacted with chlorine and the mixture irradiated with uv light, you do NOT get addition like you do with benzene!

Instead, you get a free radical substitution reaction involving the methyl group - a side-chain substitution.

The initial product is (chloromethyl)benzene (benzyl chloride).

(i) methylbenzene  +  chlorine  ===> (chloromethyl)benzene  +  hydrogen chloride

C6H5CH3  +  Cl2  == uv ==> C6H5CH2Cl  +  HCl

(c) doc b   +  Cl2  ===>  (c) doc b  +  HCl

(chloromethyl)benzene is not an aryl halide, but a primary aliphatic halogenoalkane.

 

With excess chlorine, the free radical chain reaction can continue to form two other substitution products.

 

(ii) (chloromethyl)benzene  +  chlorine  ===> (dichloromethyl)benzene  +  hydrogen chloride

C6H5CH2Cl  +  Cl2  ===>  C6H5CHCl2  +  HCl

 

(iii) (dichloromethyl)benzene  +  chlorine  ===> (trichloromethyl)benzene  +  hydrogen chloride

C6H5CHCl2  +  Cl2  ===>  C6H5CCl3  +  HCl

 

Again, these are aliphatic halogenoalkanes.

 

A similar reaction will happen with bromine to give three possible bromo-substituted products and via  similar free radical chain mechanism outlined below..

The free radical mechanism of the side-chain chlorination of methylbenzene

(• = unpaired electron on a free radical)

Initiation step

Cl2  == uv  ==> 2Cl•

Chain propagation steps

C6H5CH3  +  Cl•  ===>  C6H5CH2•  +  HCl

C6H5CH2•  +  Cl2  ===>  C6H5CH2Cl  +  Cl•   (1st product, monochloro...)

Termination steps

2Cl•  ===> Cl2

C6H4CH2•  +  Cl•  ===>  C6H5CH2Cl

2C6H4CH2•  ===>  C6H5CH2C6H5

Further propagation steps to form the 2nd and 3rd substitution products.

C6H5CH2Cl  +  Cl•  ===>  C6H5CHCl•  +  HCl

C6H5CHCl•  +  Cl2  ===>  C6H5CHCl2  +  Cl•   (2nd product, dichloro...)

C6H5CHCl2  +  Cl•  ===>  C6H5CCl2•  +  HCl

C6H5CCl2•  +  Cl2  ===>  C6H5CCl3  +  Cl•   (3rd product, trichloro...)

 

 


TOP OF PAGE and sub-index


Doc Brown's Advanced Level Chemistry Revision Notes

[SEARCH BOX] ignore ads at top

INDEX of AROMATIC CHEMISTRY NOTES

 All Advanced Organic Chemistry Notes

TOP OF PAGE and sub-index

 

KS3 BIOLOGY QUIZZES ~US grades 6-8 KS3 CHEMISTRY QUIZZES ~US grades 6-8 KS3 PHYSICS QUIZZES ~US grades 6-8 HOMEPAGE of Doc Brown's Science Website EMAIL Doc Brown's Science Website
GCSE 9-1 BIOLOGY NOTES GCSE 9-1 CHEMISTRY NOTES and QUIZZES GCSE 9-1 PHYSICS NOTES GCSE 9-1 SCIENCES syllabus-specification help links for biology chemistry physics courses IGCSE & O Level SCIENCES syllabus-specification help links for biology chemistry physics courses
Advanced A/AS Level ORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level INORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level PHYSICAL-THEORETICAL Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level CHEMISTRY syllabus-specificatio HELP LINKS of my site Doc Brown's Travel Pictures
Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial.

 Doc Brown's Chemistry 

*

TOP OF PAGE and sub-index