Radicals

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   

methyl radical

CH

H

H

Radicals

ethyl radical 

• 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 geometryd. BDE’s don’t indicate anything about radicals.e. Ethyl radical is more stable than the methyl radical.

CH

H

H

C

H

H

• The questions in slides 5 and 6 are based on the table in slide 4.

Radical Stability

BDEkcal/mole

DH° RadicalStability

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

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° < methylc. 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. 

CH4 + Cl2D or h

CH3Cl + CH2Cl2 + CHCl3 + CCl4 + HCl

Photohalogenation of Ethane

• Which step (combination or disproportionation) is more favorable entropically?

a. Combinationb. Disproportionationc. 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

• Which radical is more likely to form (from path a or from path b)?

Br

H2C C

R

H

CH2

C

R

H

Br

Br C

R

H

CH2a

a

bb

Free Radical Addition to Alkenes Mechanism

• If a free radical added to the alkene, but there was no H-Br to react with it, what reaction would be left to the new alkyl free radical?

a. Combination with another radical.b. Disproportionation with another radical.c. Atom abstraction from alkene.d. 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. disproportionationb. combinationc. 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. None of the abovee. All of the above

CH

H3C

CH3

H2C C

H

CH3

CH3+ Cl2

excess

h

CH

H3C

CH3

H2C C

H

CH3

CH2 Cl CH

H3C

CH3

HC C

H

CH3

CH3

Cl

CH3C

CH3

H2C C

H

CH3

CH3

Cla. b. c.

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

H2C C

CH3

CH3

+ H-Br

O O

Br CH2 C

CH3

CH3

H

CH2 C

CH3

CH3

Br Br CH2 C

CH3

CH3

Oa. b. c.

• The question in slide 19 refers to the structures in slide 18.

O

H R

O

+ R-H

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 unreactiveb. steric hindrance to further reactions with radicalc. oxygen radicals are particularly unreactive due to their

electronegativityd. b and ce. 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.)

OR

O

abc

d

e

Predict the product(s) of the following reaction.

CH3

Br2, h, D

CH3

Br

CH3

CH3

CH2

Br

Br

Br

CH3

Br

Br

a. b.

c. d.

e.

Reactions of Aromatic Substituents

• What benzyl reactive intermediate would be involved in the previous reaction?

CH2 CH2 CH2 CH

a. b. c. d.