CHEM 203 HOMEWORK 5 Alkenes, Radicals 1. Write an Accurate

CHEM 203 HOMEWORK 5 Alkenes, Radicals

1. Write an accurate mechanism for each of the following known radical chain reactions:

Cl approx. ΔHdiss (kcal/mole) CCl4 CCl3 a. Cl3C–Cl 70 radical initiator C–Cl 80 C–C σ 85 C=C 60 (CH3)Sn–H π b. Cl C–H 95 radical initiator C–S 75 S–H 85 CH3SH Sn–H 45 c. SCH3 Sn–Cl 130 radical initiator a.

R R 2 R • Cl CCl3 R–Cl + • CCl3 (initiation steps)

Cl CCl3 Cl • CCl • (propagation 3 CCl3 CCl3 + • CCl3 steps)

R R 2 R • H Sn(CH3)3 R–H + • Sn(CH ) 3 3 (initiation steps)

Cl • Sn(CH3)3 + • Sn(CH3)3 (propagation • H Sn(CH3)3 steps)

R R 2 R • H SCH3 R–H + • SCH3 (initiation steps)

H SCH3 • SCH • (propagation 3 SCH3 SCH3 + • SCH3 steps)

2. Upon standing in air, ethers possessing a C–H bond adjacent to the O atom are converted into hazardous peroxides. Thus, diethyl ether, A, forms peroxide B. Draw a mechanism for the formation of B.

approx. ΔHdiss (kcal/mole) O–OH air C–C–H 95 CH3–CH2–O–CH2–CH3 CH3–CH–O–CH2–CH3 O–C–H 85 C–O 95 A B O–H 100

CHEM 203 HOMEWORK 5 p. 2

elemental oxygen is a diradical, so:

O O HO–O • H then • O–OH O OH • (termination • step) O O either O O

O O O • • O H O OH OR: + • (propagation O O O O O steps)

3. We have learned that carbon radicals cannot rearrange by the 1,2-shift mechanism seen for carbocations. Accordingly, the radical polymerization of ethylene, would be anticipated to give a linear polymer, C ("high density polyethylene", HDPE). However, the reaction actually yields a branched polymer, D ("low density polyethylene" LDPE), with inferior material properties. Propose a mechanism for the formation of D.

[ etc. ] R • R C CH2=CH2 R

D [ etc. ]

evidently, the polymerization starts normally …..

etc. • • • R R R R (CH2–CH2)n •

but at some point:

a primary radical: R (CH2–CH2)n = R less stabilized: intramolecular • H • H-transfer leads to . . .

a new carbon R • chain may now start growing R a better stabilized at the secondary secondary radical H position . . . •

branched repeat, polymer etc.

CHEM 203 HOMEWORK 5 p. 3

4. Propene (a.k.a propylene), vinyl chloride, styrene, and tetrafluoroethylene (see below) undergo radical polymerization to yield polypropylene, polyvinyl chloride (PVC), polystyrene, and Teflon,® respectively. Show how a generic initiating radical R• will promote formation of such important plastic materials by writing a mechanism for the polymerization reaction of the first 3 molecules of each alkene.

F F

Cl F F propene (propylene) vinyl chloride styrene tetraﬂuoroethylene

R R 2 R • R •

etc. R • R • polypropylene

Cl R R 2 R • R • Cl Cl Cl Cl Cl Cl Cl etc. R • R • polyvinyl chloride Cl

Ph R R 2 R • R • Ph Ph Ph Ph Ph Ph Ph etc. R • R • polystyrene Ph

F F F F F F F R R F R • 2 R • F F F F F F F F F F F F F F F F etc. R R • teﬂon • F F F F F F F F F F F F

CHEM 203 HOMEWORK 5 p. 4

5. Write an accurate mechanism for the following known reactions:

(CH3)3Sn–H a. I radical initiator

• R R 2 R H Sn(CH3)3 R–H + • Sn(CH3)3 (initiation steps)

= I • Sn(CH3)3 •

• H Sn(CH ) 3 3 • Sn(CH3)3 • • (propagation steps)

CH O OCH OCH 3 3 (CH3)3Sn–H 3 b. CH3O Br radical initiator

• R R 2 R H Sn(CH3)3 R–H + • Sn(CH3)3 (initiation steps)

CH3O OCH3 CH3O OCH3 • Sn(CH ) = 3 3 • Br

OCH CH O 3 OCH3 OCH 3 • CH3O 3

CH3O • • H Sn(CH3)3 OCH3 + • Sn(CH ) (propagation CH3O 3 3 steps)