1. Chlorination of Higher Alkanes
Chlorination of an alkane involves the substitution of a chlorine atom for a hydrogen atom on the alkane.
All H’s are equivalent in CH4 and CH3CH3 so only one product forms.
In many other alkanes, more
than one type of hydrogen atom
is present and more than one
product can form. 1o 3o 2o

2. Chlorination of Higher Alkanes
The presence of more than one type of hydrogen atom in an alkane often leads to a mixture of products.

3. Chlorination of Higher Alkanes
The predicted (statistical) distribution and experimental results are different because 1-chloropropane forms from a 1o free radical

4. Chlorination of Higher Alkanes
while 2-chloropropane forms from a 2o free radical

5. Chlorination of Higher Alkanes
Example: Draw the mechanism for the chlorination of propane:

6. Chlorination of Higher Alkanes
As a rule of thumb, the major product is often the one that is formed from the most stable free radical.
There are EXCEPTIONS!
Very large ratio of 1o H to 2o or 3o H may lead to 1o alkyl halide as the major product hu
62%
38%

7. Reactivity of Halogens
All halogens react with methane via the same mechanism, but they have different relative reactivities.
Relative reactivity refers to relative rates of reaction
high reactivity: very fast rate
low reactivity or unreactive: slow or zero rate
F2 > Cl2 > Br2 > I2

8. Reactivity of Halogens
The differences in reactivity of the halogens toward alkanes can be explained by differences in:
Activation energy for the propagation steps
DHrxn
As Ea decreases, more collisions will have enough energy for the reaction to occur
higher reactivity

9. Reactivity of Halogens
Since DG ~ DHrxn for most organic reactions, as the reaction becomes more exothermic, the reaction becomes more spontaneous
higher reactivity
Ea Overall Relative
Halogen (kcal/mol) DHrxn Reactivity
F high
Cl medium
Br low
I unreactive

10. Reactivity of Halogens
F2 is the most reactive halogen
low activation energy
strongly exothermic reaction
I2 is very unreactive.
High activation energy
endothermic reaction

11. Reactivity of Halogens
The chlorination of propane leads to a broad mixture of products:
40% 1-chloropropane
substitution of a 1o H
60% 2-chloropropane
substitution of a 2o H
Bromination of propane is more selective:
3% 1-bromopropane
97% 2-bromopropane

12. Reactivity of Halogens
The RDS for chlorination of propane is exothermic.
According to the Hammond Postulate, the transition state will more closely resemble the reactants.
Little bond breakage has occurred.
Small difference in Ea for formation of 1o vs. 2o radical.
Stability of the free radical is less important in determining the structure of the final product.

13. Reactivity of Halogens

14. Reactivity of Halogens
The RDS for the bromination of propane is endothermic.
According to the Hammond Postulate, the transition state will resemble the products.
More bond cleavage and new bond formation has occurred.
Large difference in Ea for formation of 1o vs. 2o radical
Stability of the free radical is much more important in determining structure of product.

15. Reactivity of Halogens

16. Bromination of Alkanes
Example: Predict the major product of the following reactions.

17. Reactive Intermediates
Reactive intermediates are highly reactive, short-lived species that are never present in high concentration.
Four important reactive intermediates:
carbonium ion (carbocation)
free radical
carbanion
carbene

18. Reactive Intermediates
Carbonium ion:
a chemical species in which:
carbon atom is bonded to 3 other atoms
carbon atom has a positive charge
planar, sp2 hybridized
electrophile:
“electron lover”
an electron pair acceptor

19. Reactive Intermediates
The stability of a carbonium ion increases
as the number of alkyl groups attached to the positively charged carbon increases
inductive effect
donation of electron density through the sigma bonds
partial overlap of filled orbitals with empty ones
through resonance stabilization d+

20. Reactive Intermediates
Stability of carbonium ion: 3o
substituted allylic 2o
allyl
methyl
vinyl
> 1o
>
>
Least stable
Most stable

21. Reactive Intermediates
Free radical:
carbon atom with an unpaired electron
The stability of a free radical increases
as the number of alkyl groups attached to the positively charged carbon increases
through resonance stabilization
Stability:
allyl > 3o > 2o > 1o > methyl > vinyl .

22. Reactive Intermediates
Carbanion:
a trivalent carbon atom that bears a negative charge
a nucleophile
electron pair donor
Carbene:
an uncharged, divalent carbon
atom that has a nonbonding pair of electrons