FREE RADICAL REACTIONS A STUDENT WO AS MASTERED TE MATERIAL IN TIS SECTION SOULD BE ABLE TO: 1. Define, recognize, and give examples of: homolytic cleavage (homolysis), heterolytic cleavage, free radical, exothermic and endothermic reactions. 2. Given homolytic bond dissociation energies (enthalpies), calculate s (enthalpies) of reaction. Bond forming is an exothermic process ( is negative); bond breaking is endothermic ( is positive). The of reaction equals the sum of the bond dissociation enthalpies of the bonds broken minus the sum of the enthalpies of the bonds formed. 3. Predict the relative stabilities of free radicals and the relative rates of reactions that give rise to free radicals. The order of free radical stablilties is: 3 > 2 > 1 > C 3. Relatively stable species are relatively easy to form (and difficult to destroy). 4. Predict the product(s) of free radical reactions. Important reactions include: alogenation of alkanes. I 2 does not react, the order of reactivity is F 2 > Cl 2 > 2, and the order of selectivity is 2 > Cl 2 > F 2. Stereochemistry of reactions (these occur via a planar intermediate) is important. Anti-Markovnikov addition of hydrogen bromide to alkenes and alkynes. Free radical polymerization of alkenes. 5. Propose mechanisms for free radical reactions, and predict and explain experimental results using a knowledge of these mechanisms. Important reactions include: alogenation of alkanes Anti-Markovnikov addition of Free radical polymerization of alkenes Important concepts include: Energy of activation (effect of temperature changes) For reactions in which bonds are both broken and formed, E act must be greater than both zero and. Collision frequency (effects of temperature and concentration) Initiation, propagation, and termination steps 120
A STUDENT WO AS MASTERED TE OBJECTIVES ON TE PREVIOUS PAGE SOULD BE ABLE TO SOLVE TE FOLLOWING PROBLEMS AND RELATED ONES: 1.1 Which of the following is a free radical? A. C2 B. C2 C. C2N2 D. C2O. 1.2 Which of the following is a homolytic cleavage? O O A. C-O-O-C -----> O 2 C-O. B. ()3C- -----> ()3C C. C2Cl CN -----> C2CN Cl D. C=C2 2O -----> C O 2. Calculate the value of for each of the following reactions using the table of bond dissociation energies in your textbook. A. (C 3 ) 2 C F 2 -----> (C 3 ) 2 CF F B. C 6 5 C 3 Cl -----> C 6 5 C 2 Cl C. C 3 C 2 C 2 C 3 O -----> C 3 C 2 C 2 O C 3 D. C 3 C 2 C 3 OO -----> C 3 C 2 C 2 O 2 O 3. Which of the following free radicals is the most stable? Which is the least stable?. A. (C 3 C 2 ) 3 C B. C 3 CC(C 2 C 3 ) 2 C. (C 3 C 2 ) 2 CC 2 C 2 4. Predict the product(s) of each of the following reactions. Indicate stereochemistry where appropriate. a) 2 -------> 121
b) C2Cl 3C C Cl2 -------> c) ()3C C2 C Cl Cl2 -------> d) (C 3 ) 2 CC=C 2 --------------> e) n C 2 =CCl --------------> 5.1 Propose a mechanism for each of the following reactions. a) (C 3 ) 3 C 2 -------> (C 3 ) 3 C b) C=C2 ------------> C2C2 c) n CF 2 =CF 2 --------------> (CF 2 CF 2 ) n 122
5.2 For which of the following reactions is E act (A) equal to zero, (B) greater than zero, and (C) equal to?. a) Cl 2 -----> 2 Cl b) C 3 C 3 Cl -----> C 3 C 2 Cl c) C 3 C 2 Cl 2 -----> C 3 C 2 Cl Cl d) C 3 C 2 Cl -----> C 3 C 2 Cl SOLUTIONS TO SAMPLE PROBLEMS: 1.1 D 1.2 A 2. a) -280 kj mol-1 b) -57 kj mol-1 c) -7 kj mol-1 d) -257 kj mol-1 3. most stable: A; least stable: C 4. Predict the product(s): a) 2 -------> C2Cl b) 3C C Cl2 -------> CCl2 3C C (major product; bromine is selective) C2Cl 3C C Cl 3C C2Cl C Cl c) ()3C (ClC2)2C Cl C2 C2C2Cl C2 C Cl2 -------> C C Cl ()3C Cl ()2C Cl C2Cl Cl Cl Cl Cl C2CCl2C()3 Cl C()3 C()3 123
d) (C 3 ) 2 CC=C 2 --------------> (C 3 ) 2 CC 2 C 2 e) n C 2 =CCl --------------> (C 2 CCl) n 5.1 Propose a mechanism for each of the following reactions. a) (C 3 ) 3 C 2 -------> (C 3 ) 3 C Initiation: 2 -------> 2 Propagation: (C 3 ) 3 C -----> (C 3 ) 3 C (C 3 ) 3 C 2 -----> (C 3 ) 3 C Termination: 2 -----> 2 (C 3 ) 3 C -----> (C 3 ) 3 C 2 (C 3 ) 3 C -----> (C 3 ) 3 CC(C 3 ) 3 b) C=C2 ------------> C2C2 Initiation: ROOR -----> 2 RO RO ----> RO Propagation: C=C2 -----> CC2 CC2 -----> C2C2 Termination: 2 RO -----> ROOR 2 -----> 2 RO -----> RO CC2 CC2 -----> CC2 RO -----> CORC2 2 CC2 -----> C C2 C C2 124
c) n CF 2 =CF 2 --------------> (CF 2 CF 2 ) n 5.2 E act = 0: d E act > 0: a, b, c E act = : a Initiation: ROOR -----> 2 RO Propagation: RO CF 2 =CF 2 -----> ROCF 2 CF 2 ROCF 2 CF 2 CF 2 =CF 2 -----> ROCF 2 CF 2 CF 2 CF 2 etc. 125
Name Sample Quiz Organic Chemistry 2210 Answer All Questions 1. Which of the following compounds is a product of the reaction shown? Cl2 -------->? F C2Cl F Cl F F Cl F C2Cl 2. What is in kj mol-1 for the process shown? Some bond dissociation energies in kj mol-1 are: C 3 C 3, 368; C 3 C 2, 410; C 3 C 2, 289. C 3 C 3 -----> C 3 C 2 A. -121 B. -79 C. 79 D. 121 3. What is in kj mol-1 for the reaction shown? Some bond dissociation energies in kj mol-1 are: C 3 C 2 C 2, 410; (C 3 ) 2 C, 395; F F, 159; C 3 C 2 C 2 F, 444; (C 3 ) 2 C F, 439; F, 569. F 2 C 3 C 2 C 3 ----> F C 3 C 2 C 2 F A. -444 B. -439 C. -458 D. -454 4. Which of the indicated hydrogens reacts most RAPIDLY with Cl 2 in the presence of? CC2 5. Which of the following reactions proceeds by way of a free radical mechanism? CCl 4 A. C 3 C=C 2 2 ---------> C 3 CC 2 B. C 3 C=C 2 ----------------> C 3 C 2 C 2 C. (C 3 ) 3 C KO -------> (C 3 ) 2 C=C 2 K 2 O D. C 3 C 2 KO -------> C 3 C 2 O K 126