Radicals

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Radicals
• There are 7 valence electrons around the C in
this species.
• Would you expect this species to be stable?
a. no
b. yes
c.not enough information
H
methyl radical
H
C
H
Radicals
H
H
C
H
C
ethyl radical
H
H
• What do you think it indicates about the ethyl radical relative to the
methyl radical that the ethyl radicals BDE is lower?
a. Ethane is less stable than methane.
b. Ethyl radical has more steric hindrance.
c. Ethyl radical has a more pyramidal geometry
d. BDE’s don’t indicate anything about radicals.
e. Ethyl radical is more stable than the methyl radical.
• The questions in slides 5 and 6 are based on
the table in slide 4.
Radical Stability
BDE
kcal/mole
Me-H  Me + H
104.8
Et-H  Et + H
100.3
i-Pr-H  i-Pr + H
96
t-Bu-H t-Bu +H
93.3
DH° Radical
Stability
Radical Stability
• What is happening to DH with increasing
substitution at the radical C?
a.
It increases as substitution at the radical C
increases.
b. It decreases as substitution at the radical C
increases.
c. There is no significant effect as substitution
increases.
Radical Stability
• What is the relative order of radical
stability?
a. 3° > 2° > 1° > methyl
b. 3° < 2° < 1° < methyl
c. Radical stability cannot be determined
from this data
Photohalogenation
• Once methyl chloride forms, is there any reason
why it couldn’t react with Cl2?
a. Yes. The reaction doesn’t produce any other
products.
b. No. The methyl chloride can react and produce
more products.
D or h
CH4 + Cl2
CH3Cl + CH2Cl2 + CHCl3 + CCl4 + HCl
Photohalogenation of Ethane
•
Which step (combination or
disproportionation) is more favorable
entropically?
a. Combination
b. Disproportionation
c. Both  same
Site of Free Radical
Halogenation
Depends on
1. reactivity of H type
H on 3C > 2C > 1C > CH3-H
Why would this be true?
a. This order reflects the order of radical stability.
b. This order reflects the ease with which the halogen can approach the
H.
c. This order reflects the BDE’s of the C-H bonds.
d. Both a. and c.
Site of Free Radical
Halogenation
2. Depends on # of 1, 2, and 3 H’s
Why would this be true?
a. This order reflects the ease with which the
halogen can approach the H.
b. Statistical. Chlorine atom is more likely to
collide and react with more abundant H.
c. Increasing the number of a given type of H
increases its inherent reactivity.
Free Radical Addition to
Alkenes Mechanism
R
Br
a
R
H2C
C
a
b
Br
C
H2
C
H
R
H
b
Br
C
CH2
H
• Which radical is more likely to form (from path a or from
path b)?
Free Radical Addition to
Alkenes Mechanism
• If a free radical added to the alkene, but there was no HBr to react with it, what reaction would be left to the new
alkyl free radical?
a.
b.
c.
d.
Combination with another radical.
Disproportionation with another radical.
Atom abstraction from alkene.
Addition onto another alkene molecule.
Polymers
• The mechanical strength of a polymer material is
dependent on its average molecular weight. The higher
it is, the stronger the material will be. Which termination
step (combination or disproportionation) favors strong
materials
a. disproportionation
b. combination
c. neither
Polymers
• Why is head to tail addition favored?
a. It produces the more stable polymer.
b. It produces the strongest bonds.
c. It is favored entropically.
d. Head to tail addition leads to the more
stable radical.
Predict which product(s) will be formed in
significant amounts.
d.
e.
None of the above
All of the above
Which of the following is not a reactive intermediate in
the mechanism of the reaction producing the product
shown?
d. Br·
e. all reactive intermediates in the reaction
• The question in slide 19 refers to the
structures in slide 18.
+ R-H
H
O
R
O
O
O
O
Antioxidants
• What factor(s) lead to this being a very unreactive radical
that will inhibit further reaction?
a. resonance stabilized radical is unreactive
b. steric hindrance to further reactions with radical
c. oxygen radicals are particularly unreactive due to their
electronegativity
d. b and c
e. a and b
Atherosclerosis
• Which of the following sites would not produce a
resonance stabilized radical after a H was
abstracted? (These H’s would be less reactive.)
O
c
a
b
d
e
OR
Predict the product(s) of the following
reaction.
Reactions of Aromatic Substituents
• What benzyl reactive intermediate would be
involved in the previous reaction?
CH2
CH2
CH2
C
H
a.
b.
c.
d.

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