1. 1. Reactions involving the ring
a) Reduction
Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11)
b) Electrophilic aromatic substitution (Chapter 12)
c) Nucleophilic aromatic substitution (Chapter 23)
2. The ring as a substituent (Sections ) 2

2. 11.12 Free-Radical Halogenation of Alkylbenzenes

3. The Benzene Ring as a Substituent• C • C
allylic radical
benzylic radical
benzylic carbon is analogous to allylic carbon 2

4. Bond-dissociation energy for C—H bond is equal to DH° for:
Recall:
Bond-dissociation energy for C—H bond is equal to DH° for:
R—H R• + •H
and is about 400 kJ/mol for alkanes.
The more stable the free radical R•, the weaker the bond, and the smaller the bond-dissociation energy. 13

5. Bond-dissociation energies of propene and tolueneH
H2C CH C H C •
368 kJ/mol
H2C CH
-H• H C H C •
356 kJ/mol
-H•
Low BDEs indicate allyl and benzyl radical are more stable than simple alkyl radicals. 14

6. Resonance in Benzyl RadicalH H • C H H H H H
unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

7. Resonance in Benzyl RadicalH •
unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

8. Resonance in Benzyl RadicalH •
unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

9. Resonance in Benzyl RadicalH H C H H • H H H
unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

10. Free-radical chlorination of toluene
industrial process
highly regioselective for benzylic position
CH3
Cl2
CH2Cl
light or heat
Toluene
Benzyl chloride 16

11. Free-radical chlorination of toluene
Similarly, dichlorination and trichlorination are selective for the benzylic carbon. Further chlorination gives:
CHCl2
CCl3
(Dichloromethyl)benzene
(Trichloromethyl)benzene 16

12. Benzylic Bromination
is used in the laboratory to introduce a halogen at the benzylic position
CH3
NO2
CH2Br
CCl4, 80°C
light
+ Br2
+ HBr
NO2
p-Nitrotoluene
p-Nitrobenzyl
bromide (71%) 22

13. N-Bromosuccinimide (NBS)
is a convenient reagent for benzylic bromination Br
NBr O
CH2CH3
CHCH3 NH O
CCl4 + +
benzoyl
peroxide,
heat
(87%) 23

14. 11.13 Oxidation of Alkylbenzenes24

15. Site of Oxidation is Benzylic Carbon
CH3 or
Na2Cr2O7
H2SO4
COH O
CH2R
H2O
heat or
CHR2 25

16. p-Nitrobenzoic acid (82-86%)
Example
COH O
CH3
NO2
Na2Cr2O7
H2SO4
H2O
heat
NO2
p-Nitrotoluene
p-Nitrobenzoic acid (82-86%) 26

17. Example
COH O
CH(CH3)2
CH3
Na2Cr2O7
H2SO4
H2O
heat
(45%) 26

18. 11.14 Nucleophilic Substitution in Benzylic Halides24

19. Primary Benzylic Halides
CH2Cl
O2N
NaOCCH3 O
Mechanism is SN2
acetic acid
CH2OCCH3
O2N O
(78-82%) 29

20. Relative solvolysis rates in aqueous acetone
What about SN1?
Relative solvolysis rates in aqueous acetone C
CH3 Cl C
CH3 Cl
600 1
tertiary benzylic carbocation is formed more rapidly than tertiary carbocation; therefore, more stable 30

21. Relative rates of formation:
What about SN1?
Relative rates of formation:
CH3 +
CH3 + C
CH3 C
more stable
less stable 30

22. benzylic carbon is analogous to allylic carbon
Compare. + C + C
allylic carbocation
benzylic carbocation
benzylic carbon is analogous to allylic carbon 2

23. Resonance in Benzyl CationH + H C H H H H H
unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

24. Resonance in Benzyl CationH H C H H + H H H
unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

25. Resonance in Benzyl CationH H C H H + H H H
unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

26. Resonance in Benzyl CationH H C H H + H H H
unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

27. Solvolysis C
CH3 Cl
CH3CH2OH C
CH3
OCH2CH3
(87%) 37