1. Alkyl Halides & Radical Rx’s
chapter 8
Alkyl Halides & Radical Rx’s
Structure
Nomenclature
Physical Properties
Halogenation of Alkanes
Mechanism of Halogenation
Allylic Halogenation
Examples
Radical addition
Note the Chapter Summary and Key Rx’s
ss18 12

2. free radical substitution mechanism
Chapter 8
synthesis
free radical substitution mechanism
Chapter 7
synthesis
allylic
benzylic
Hammond
radical addition

3. free radical substitution
Chapter 8
synthesis
classification
nomenclature
mechanism
and details Hammond postulate
free radical substitution

4. 8.1 Structure of ‘R-X’ haloalkane R-X H3C-Cl haloalkene sp3 haloarene
(a vinyl halide)
sp3
(alkyl halide)
haloarene
sp2
(aryl halide)

5. Haloalkane (alkyl halide) sp3
8.1 Structure of ‘R-X’
Haloalkane (alkyl halide) sp3
R-X
H3C-Cl
methyl chloride X R C
" '
3o halide R C H X '
2o halide R C H X
1o halide

6. 8.2 Nomenclature #-haloalkane #-halocycloalkane
IUPAC - halides (X) are substituents
Substituent names: halo
fluoro, chloro, bromo, iodo
#-haloalkane
#-halocycloalkane
(R)-4-bromo-1-chloro-4-fluoro-1-cyclopentene
structure ?

7. Common Names: (alkyl halide) or (special names)
(isopropyl bromide) vs 2-bromopropane H 3 C B r
(chloroform) vs trichloromethane
H-CCl3

8. 8.3 Physical Properties “polar covalent bond” - dipole
- mismatch of electronegativity
-size H 3 C 
 
) H C B r H 3 C

9. 8.4 Halogenation of Alkanes
substitution of X for H
hv = ultraviolet light,  = heat
X2 = Cl2, Br2
seldom F2 (too reactive - exothermic)
or I2 (endothermic, unreactive)

10. Substitution, products and by-products
+ other R-X’s hv
Cl2 hv
Cl2

11. A few rx’s are useful, e.g.:
Generally halogenation not useful
- mixtures (separate)
A few rx’s are useful, e.g.:
+ Br2  Br
+ HBr + other Br's
Others - allylic & benzylic

12. Substitution, products and by-products
monobromination
+ HBr + diBr + etc
+ HBr + diBr +...
(92) (8)
+ HBr + diBr +...
bromination favors
3o > 2o > 1o

13. initiation
propagation
and/or
terminations

14. Regioselective for 3o > 2o > 1o C-H
(92) (8)
(57) (43)

15. radical stability
like carbocations -
R. is electron deficient (not charged)

16. Order of stability of R(+) / R.
same order

17. free radical substitution mechanism
Chapter 8
synthesis
free radical substitution mechanism
Chapter 7
synthesis
Hammond
allylic
benzylic
radical addition

18. Selectivity 3o > 2o > 1o, but Cl and Br are different
major mono-X product Br Cl
3o o o
Cl. more reactive less selective than Br.

19. Hammond’s Postulate Cl vs Br
Hammond’s Postulate: the structure of the transition state:
for an exothermic reaction looks more like the reactants of that step
- for an endothermic reaction looks more like the products of that step

20. Hammond’s Postulate
In halogenation of an alkane, the rate-limiting step is hydrogen abstraction
this step is endothermic for bromination
and exothermic for chlorination.  H°
(kcalmol)

21. Hammond’s Postulate
For chlorination (hydrogen abstraction is exothermic):
transition state resembles the alkane and chlorine atom
little radical character on carbon in t.s.
regioselectivity only slightly influenced by radical stability

22. Hammond site of collision important PE stability of R. important
early t.s. - like SM
site of collision
important PE
SM prog rx Prod
SM prog rx Prod PE
late t.s.
stability of R.
important

23. (free radical substitution)
Halogenation
(free radical substitution)

24. Allyl Radical - resonance

25. initially
H-Br
with progress rx

26. NBS for Br2

27. Free Radical Stability

28. Free Radical Stability
allylic
benzylic

34. Radical ADDITION - note rx conditions!!!
Add H-X in the presence of peroxides
reverses normal addition
“anti-Markovnikov”
Markovnikov
“normal”

35. Radical ADDITION - note rx conditions!!!
Add H-X in the presence of peroxides
reverses normal addition
“anti-Markovnikov”
X = Cl, Br, I
end