The lecture discusses the mechanisms of ozonolysis and radical addition reactions to alkenes. Ozonolysis involves a three step mechanism where ozone cleaves the alkene to form an ozonide intermediate which then decomposes to a carbonyl compound. Radical addition reactions involve a three step chain reaction mechanism of initiation, propagation, and termination. The stability of radical intermediates is influenced by resonance stabilization, which explains why styrene reacts with HBr to give a single, benzylic bromide product.

1. ..LECTURE TEN
the last
©gonzales2010@flickr
(mono)alkenes

2. PREVIOUSLY...
diol formation
HO OH
KMnO4
H H

3. ozonolysis
i. O3(g)
ii. Zn/H2O
O O
cleaves
alkenes

4. mechanismwell, the steps involved...
O
O O
O
O O
O O Zn/H2O O O
O

5. radical
chemistry
©Renaissance Pictures

6. free radical
to alkenes
addition of HBr
HBr
ROOR H
Br
anti Markovnikov
addition

7. mechanism
step initiation
O R 2x R O
R O
R O H Br O Br
R H
radical chain reaction

8. ‘curly fishhook’
a
radical is one
electron

9. ‘curly fishhook’
need
two
electrons
to form a bond

10. mechanism
step propagation
Br Br
H
Br H Br Br
Br

11. chain carrier
Br

12. mechanism
step termination
Br Br Br Br
Br
Br Br
Br

13. ©Oberazzi@flickr

14. radical stability
< <
missing
one
electron

15. 11 HBr addition
to styrenes H
HBr Br
Br
we H
known

16. but...
H
Ph HBr Ph
Br
single
product

17. ©Great Beyond@flickr

18. resonance stabilisation
H Br
H H H H
Br
benzylic
cation
H
stable

19. NO resonance stabilisation
H
H Br