1. 11.10 Reactions of Arenes: A Preview
1. Some reactions involve the ring.
2. In other reactions the ring is a substituent. 1

2. 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

3. Reduction of Benzene Rings
catalytic hydrogenation
(Section 11.4) H
Birch reduction
(Section 11.11) H H 3

4. 11.11 The Birch Reduction 4

5. Birch Reduction of Benzene
Na, NH3
CH3OH
(80%)
Product is non-conjugated diene.
Reaction stops here. There is no further reduction.
Reaction is not hydrogenation. H2 is not involved in any way. 5

6. Mechanism of the Birch Reduction (Figure 11.8)
Step 1: Electron transfer from sodium H H • •• – + +
Na+ Na • 6

7. Mechanism of the Birch Reduction (Figure 11.8)
Step 2: Proton transfer from methanol H H H • •• H H –
OCH3 H •• • H 6

8. Mechanism of the Birch Reduction (Figure 11.8)
Step 2: Proton transfer from methanol H H H H H H • • •• H H H H – H H
OCH3 H •• • H –
OCH3 •• • 6

9. Mechanism of the Birch Reduction (Figure 11.8)
Step 3: Electron transfer from sodium H • + Na 6

10. Mechanism of the Birch Reduction (Figure 11.8)
Step 3: Electron transfer from sodium H • + Na H – H H •• +
Na+ H H H H 6

11. Mechanism of the Birch Reduction (Figure 11.8)
Step 4: Proton transfer from methanol •
OCH3 H •• H – H H •• H H H H 6

12. Mechanism of the Birch Reduction (Figure 11.8)
Step 4: Proton transfer from methanol •
OCH3 H •• – •
OCH3 •• H H – H H •• H H H H 6

13. Birch Reduction of an Alkylbenzene
C(CH3)3 H
C(CH3)3
Na, NH3
CH3OH
(86%)
If an alkyl group is present on the ring, it ends up as a substituent on the double bond. 5