Free Radical Reactions
Halogenation of Alkanes
RH + X2  RX + HX
Reactive Intermediates
Orbital hybridization in carbocations and carbanions:
Reactive Intermediates:
Free Radicals
Free radicals can be thought of as sp2 hybridized or
quickly interconverting sp3 hybridized.
Energetics
RH + X2  RX + HX
explosive for F2
exothermic for Cl2 and Br2
endothermic for I2
Halogenation Thermodynamics
Chlorination of Methane
Chlorination of Methane
carried out at high temperature (400 °C)
CH4 + Cl2  CH3Cl + HCl
CH3Cl + Cl2  CH2Cl2 + HCl
CH2Cl2 + Cl2  CHCl3 + HCl
CHCl3 + Cl2  CCl4 + HCl
Mechanism of Chlorination of
Methane
Mechanism of Chlorination of Methane
Free-radical chain mechanism.
Initiation step: (Light or Heat is Necessary)
..
.. ..
..
. + . Cl:
: Cl: Cl
:
:
Cl
..
.. ..
..
The initiation step "gets the reaction going"
by producing free radicals—chlorine atoms
from chlorine molecules in this case.
Initiation step is followed by propagation
steps. Each propagation step consumes one
free radical but generates another one.
Question
A)
B)
C)
Which of the above initiates reactions most readily?
Mechanism of Chlorination of Methane
First propagation step:
H3C : H
+
..
. Cl:
..
Second propagation step:
.. ..
:
H3C . + : Cl: Cl
..
..
H3C .
+
..
H : Cl:
..
..
..
H3C: Cl: + . Cl:
..
..
Mechanism of Chlorination of Methane
First propagation step:
H3C : H
+
..
. Cl:
..
Second propagation step:
.. ..
:
H3C . + : Cl: Cl
..
..
.. ..
: Cl
:
H3C : H + : Cl
..
..
H3C .
+
..
H : Cl:
..
..
..
H3C: Cl: + . Cl:
..
..
..
..
H3C: Cl: + H : Cl:
..
..
Almost all of the product is formed by repetitive
cycles of the two propagation steps.
First propagation step:
H3C : H
+
..
. Cl:
..
Second propagation step:
.. ..
:
H3C . + : Cl: Cl
..
..
.. ..
: Cl
:
H3C : H + : Cl
..
..
H3C .
+
..
H : Cl:
..
..
..
H3C: Cl: + . Cl:
..
..
..
..
H3C: Cl: + H : Cl:
..
..
Termination Steps
stop chain reaction by consuming free radicals
..
H3C . + . Cl:
..
..
H3C: Cl:
..
hardly any product is formed by termination step
because concentration of free radicals at any
instant is extremely low
Question
The step shown below is a _____________
step of the free-radical chlorination of
chloromethane.
A) initiation
B) propagation
C) chain-terminating
D) bond cleavage
Question
For the free-radical reaction below, light is
involved in which of the following reaction
steps?
A) Initiation only
B) Propagation only
C) Termination only
D) Initiation and propagation
Halogenation of Higher Alkanes
Chlorination of Alkanes
can be used to prepare alkyl chlorides
from alkanes in which all of the hydrogens
are equivalent to one another
CH3CH3 + Cl2
420°C
CH3CH2Cl + HCl
(78%)
+ Cl2
h
Cl + HCl
(73%)
Chlorination of Alkanes
Major limitation:
Chlorination gives every possible
monochloride derived from original carbon
skeleton.
Not much difference in reactivity of
different hydrogens in molecule.
Example
Chlorination of butane gives a mixture of
1-chlorobutane and 2-chlorobutane.
(28%)
CH3CH2CH2CH2Cl
CH3CH2CH2CH3
Cl2
h
CH3CHCH2CH3
(72%)
Cl
Percentage of Product that Results from
Substitution of Indicated Hydrogen if
Every Collision with Chlorine Atoms is
Productive
10%
10%
10%
10%
10%
10%
10%
10%
10%
10%
Percentage of Product that Actually Results
from Replacement of Indicated Hydrogen
18%
18%
4.6%
4.6%
4.6%
4.6%
4.6%
4.6%
18%
18%
Relative Rates of Hydrogen Atom Abstraction
divide by 4.6
18%
4.6%
4.6
=1
4.6
18
4.6
= 3.9
A secondary hydrogen is abstracted 3.9 times
faster than a primary hydrogen by a chlorine
atom.
Similarly, chlorination of 2-methylbutane
gives a mixture of isobutyl chloride and
tert-butyl chloride
(63%)
CH3
CH3CCH2Cl
CH3
CH3CCH3
H
H
Cl2
h
CH3
CH3CCH3
(37%)
Cl
Question
How many monochlorination products do you
expect to obtain from the chlorination of
2-methylbutane?
A) two
B) three
C) four
D) five
Percentage of Product that Results from
Replacement of Indicated Hydrogen
7.0%
37%
Relative Rates of Hydrogen Atom Abstraction
divide by 7
7.0
7
=1
37
7
= 5.3
A tertiary hydrogen is abstracted 5.3 times faster
than a primary hydrogen by a chlorine atom.
Radicals - Resonance
?
Question
True (A) / False (B)
Hyperconjugation contributes more to thermodynamic
stability than resonance.
Question
True (A) / False (B)
Vinyl free radicals are more thermodynamically
stable than benzylic and allylic free radicals.
Selectivity of Free-radical Halogenation
R3CH
>
R2CH2 >
chlorination:
5
4
1
bromination:
1640
82
1
Chlorination of an alkane gives a mixture of
every possible isomer having the same skeleton
as the starting alkane. Useful for synthesis only
when all hydrogens in a molecule are equivalent.
Bromination is highly regioselective for
substitution of tertiary hydrogens. Major synthetic
application is in synthesis of tertiary alkyl
bromides.
Radicals - Bromination
Question
How many mono-bromination products are expected from the
following reaction?
A. 5
B. 4
C. 3
D. 2
E. 1
Synthetic Application of Chlorination of an Alkane
Cl
Cl2
h
(64%)
Chlorination is useful for synthesis only when
all of the hydrogens in a molecule are equivalent.
Question
An alkane with a molecular formula of C8H18
reacts with Cl2 in the presence of light and
heat to give a single monochloride C8H17Cl.
What is the most reasonable structure for the
starting alkane?
A) CH3CH2CH2CH2CH2CH2CH2CH3
B) (CH3CH2)2CHCH2CH2CH3
C) (CH3)2CHCH2CH2CH(CH3)2
D) (CH3)3CC(CH3)3
Synthetic Application of Bromination of an Alkane
Br
H
CH3CCH2CH2CH3
CH3
Br2
h
CH3CCH2CH2CH3
CH3
(76%)
Bromination is highly selective for substitution
of tertiary hydrogens.
Major synthetic application is in synthesis of
tertiary alkyl bromides.
Question
Which of the following best describes a mechanistic feature of
the free-radical bromination (Br2, light) of 2methylpropane?
A) The initiation step involves cleavage of a C-H bond.
B) The free-radical (CH3)3C· is produced in one
propagation step and reacts with Br2 in another.
C) The reaction is characterized by the homolytic
cleavage of the C-Br bond.
D) The reaction is concerted; i.e., it occurs in a single step.
Question
Which reaction has a faster rate?
Which product is kinetically favored?
I)
A. I) and I)
II)
B. II) and II) C. I) and II)
D. II) and I)
Question
Which reaction has a propagation step that is endothermic?
Which reaction is more regioselective?
I)
A. I) and I)
II)
B. II) and II) C. I) and II)
D. II) and I)
Stereochemistry
Three mono-substituted isomers form in the
halogenation of butane. The products are optically
inactive.
Question
True (A) / False (B)
Bromination of optically active 3-methylhexane
produces only (S)-3-bromo-3-methylhexane.
Question
How many products (including stereoisomers) are
expected in the following halogenation?
A. 5 B. 4 C. 3 D. 2 E. 1
Allylic/Benzylic Bromination
Resonance and regioselectivity:
Br-
A mixture is obtained that includes the di-brominated product.
Di-bromination can be avoided using a specialized reagent:
N-bromosuccinimide (NBS)
Allylic/Benzylic Bromination
N-bromosuccinimide (NBS)
Question
What is the major product of the following reaction?
NBS, h, CCl4
A.
B.
C.
D.
Br
Br
Br
Br
Halogenated Marine Natural Products
(A) Barbamide, a cyanobacterial peptide
containing a trichloromethyl group and (B)
dysidenin, a barbamide-related compound
isolated from a sponge-cyanobacterial
association
Polymers
Radical Polymerization
Free radical conditions are frequently used to form
polymers.
Recall that a polymerization process joins together
many small units called monomers in a long chain.
Radical Polymerization
Radical polymerizations commonly proceed through
a chain reaction mechanism.
Radical Polymerization
Radical Polymerization
Radical Polymerization
Radical polymerizations generally produce chains of
monomers with a wide distribution of lengths.
Because the mechanism proceeds through 1°
carbon free radical intermediates, it is usually difficult.
Radical Polymerization
Branching is common in radical polymerizations.
Branching makes polymer materials
more flexible, such as a polyethylene
used for squeeze bottles.
When catalysts are used to minimize
branching, more rigid materials are produced, such
as the material used for squeeze bottle caps.
Radical Polymerization
Many derivatives of ethylene are polymerized.
Radical Polymerization
Stereoregular Polymers
atactic
isotactic
syndiotactic
Atactic Polypropylene
Random stereochemistry of methyl groups
attached to main chain (stereorandom)
Properties not very useful for fibers etc.
Formed by free-radical polymerization
Isotactic Polypropylene
Stereoregular polymer; all methyl groups on
same side of main chain
Useful properties
Prepared by coordination polymerization
under Ziegler-Natta conditions
Syndiotactic Polypropylene
Stereoregular polymer; methyl groups
alternate side-to-side on main chain
Useful properties
Prepared by coordination polymerization
under Ziegler-Natta conditions
Properties of Polymers
Isotactic and syndiotactic polymers are stronger
and stiffer due to their regular packing
arrangement.
Anionic polymerization usually gives isotactic or
syndiotactic polymers.
Free radical polymerization is nearly random,
giving branched atactic polymers.
Free Radical
Polymerization
Polystyrene & Others