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) b. 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) c. 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. radical initiator CH3O Br • 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) .