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1 Classrm In this sectin f Resnance, we invite readers t pse questins likely t be raised in a classrm situatin. We may suggest strategies fr dealing with them, r invite respnses, r bth. "Classrm" is equally a frum fr raising brader issues and sharing persnal experiences and viewpints n matters related t teaching and learning science. G Nagendrappa Department f Chemistry Central Cllege Campus Bangalre University Bangalre , India. nagendrappa@vsnl.net Partl.lntrductin, Resnance, VLlO, N.2, pp.72-78, Part 2. Free Radical Reactins in Industry, Resnance, V1.10, N.3, pp.71-79, Part 3. Free Radicals in Diseases and Health, Resnance, V1.10, N.4, pp.65-74, Part 4. Free Radicals in Atmspheric Chemistry, Resnance, VLlO, N.7, pp.61-72, Part 5. Free Radicals in Organic Synthesis, Resnance, V1.10, N.B, pp.bo-90, Keywrds Allylic brminatin, Sandmeyere reactin, phenlic/cupling benzpinacl, NBS, FeCI 3 and CuCI in radical reactins. An Appreciatin f Free Radical Chemistry 6. Experiments Invlving Free Radicals In the previus five parts, we have seen hw the ubiquitus free radicals tuch ur lives in many ways. In this part, fur experiments invlving free radical intermediates are described. They are very simple, ecnmical and practical, and can be easily carried ut in labratries with meagre resurces. They can, therefre, be adapted fr MSc chemistry practicals as part f free radical chemistry curse. Experiment 1. Free Radical Substitutin Allylic brminatin using NBS - Brminatin f Cyclhexene t 3-Brmcyclhexene Place 7.98 g (0.04 ml) f N-brmsuccinimide, 9.84 g (0.12 ml) f cyclhexene, and 30 ml f carbn tetrachlride in a 100 ml rund-bttmed flask. Add abut 100 mg f benzyl perxide. Cnnect the flask t a water-cled cndenser. Within a few minutes, the reactin cmmences and the flask becmes warm. When the reactin has slightly subsided, reflux the mixture n a water-bath fr abut ne hur. During the reactin, the heavy, ~ RESONANCE I September

2 yellwish NBS is cnverted t lighter, clurless succinimide suspensin. Cl the flask, filter the reactin mixture with suctin, and wash the slid with 2-3 mi f carbn tetrachlride. Distil ff the slvent (b.p. 77 C) and the unreacted cyclhexene (b.p. 83 C) n a water-bath carefully. Distil the residue under water-pump vacuum. Cllect the early fractin cntaining the remaining slvent and cyclhexene separately and then 3- brmcyclhexene biling at C / 20 trr. Experiment 2. Oxidative Free Radical Cupling f2-naphthl Metal Salt fr Radical Generatin by Electrn Acceptance - Preparatin f bis-2-naphthl (binaphthl) Methd A: Place 1.44 g (0.01 ml) f 2-naphthl (m.p. 123 OC) and 60 ml f water in a 250 ml tw-necked rund-bttmed flask cnnected t a reflux cndenser and a drpping funnel. Heat the cntents t biling with fast stirring (magnetic stirrer) and add a slutin f2.8 g (0.01 ml) f crystallised FeC13 in abut 10 ml f water. The ily emulsin f2-naphthl disappears and bis-2- naphthl flakes frm. Bil fr 5-10 minutes mre and filter quickly thrugh a warm Buchner funnel. Wash the slid with 20 ml f biling water and dry in the air. Recrystallise frm tluene, t get abut g f pure prduct, m.p. 218 C. Methd B: Grind a mixture f 2.88 g (0.02 ml) f 2-naphthl and 7.0 g (0.025 ml) with 1-2 drps f water in an agate (r prcelain) mrtar and pestle fr abut 20 minutes. Allw the mixture t stand fr abut 2 hurs with a little grinding nw and then. Wash the mixture with 40 ml f water int a 100 ml beaker, bil fr minutes, filter, wash the slid with 10 mi f biling water, dry, and recrystallise frm tluene. Yield by this methd can be greater than 90%. Experiment 3. Light-induced Free Radical Cupling Preparatin f Benzpinacl frm Benzphenne Prepare a slutin f 3.0 g ( ml) f benzphenne in 35 ml f 2-prpanl in a 100 ml cnical flask (pyrex glass). Add a ~~ RESONANCE I September 2005

3 drp f acetic acid. Clse the flask with a crk stpper and place the flask in direct sunlight. (The flask may be clamped t an irn stand). Benzpinacl begins t separate within a day. Expse the flask t sunlight till n mre slid appears t separate ut. (That may take 3-4 days). Cl the flask in ice water, filter the mixture under suctin, wash the slid with 3-4 ml f ice-cld 2-prpanl, and dry. The prduct, almst pure benzpinacl, is btained in greater than 90% yield, m.p C. Experiment 4. Reductive Chlrinatin f Diaznium Chlride with Cuprus Chlride (Sandmeyer reactin) Metal Salt fr Radical Generatin by Electrn Dnatin Preparatin f p-chlrtluene Cuprus chlride shuld be prepared freshly as fllws. Disslve 3.5 g (0.014 ml) fcus0 4.5H 2 0 and 0.95 g (0.016 ml) f gd quality NaCI in 13 ml f distilled water in a 100 ml cnical flask with warming n a water bath. Add a slutin f 0.95 g (0.005 ml) f Na 2 S 2 0 S (sdium metabisulphite) in 10 ml f distilled water t the ht slutin with shaking. Clurless CuCI precipitates. Cl the flask in ice water, decant the supernatant slutin, and wash the precipitate by decantatin with a dilute slutin (-0.5% slutin) f Na 2 S 2 0 S ' Disslve the mist precipitate in 6-7 ml f cnc HCI, stpper and keep it aside in an ice-bath, while p-tluene diaznium chlride is prepared. Prepare a slutin f 8.5 ml f cnc HCI in 8.5 ml f distilled water in a 100 ml cnical flask. Disslve 3.5 g (0.032 ml) f freshly distilled p-tluidine and cl the mixture in an ice-salt bath with shaking; p-tluidine hydrchlride precipitates. Add a slutin f 2.4 g (0.035 ml) f NaN0 2 in 5 ml f water slwly (3-5 minutes) with stirring. (The temperature f the reactin mixture shuld be maintained at 0-5 C, by adding a little ice, if necessary). p-tluidine hydrchlride disappears as the highly sluble diaznium chlride is frmed. Pur the cld slutin carefully with stirring int cld CuCI slutin prepared earlier. A thick slid separates. Allw the mixture t warm t rm temperature with ccasinal shaking. The slid starts breaking -RE-S-O-N-A-N-C-E-I--~-p-~-m-re-r ~ m

4 dwn at abut 15 C with the liberatin f nitrgen and the frmatin f p-chlrtluene as an ily prduct. Warm the mixture n a water bath at abut 60 C fr minutes and steam distil until the distillate is freed f ily drps. Cllect p chlrtluene in the distillate by using a separating funnel. Wash it successively with 5 ml f 10% NaOH slutin, 5 ml f water, 5 ml f a slutin f 1: 1 cne HzSO 4: water and finally with 5 ml f water. Dry with a little MgSO 4 and distil the liquid at C t get 3.2 g (76%) fp-chlrtluene. Discussin Fur experiments are described here. Each ne f them is unique in the way the initial free radical reactant is frmed frm the starting cmpund. In the first experiment, a separate radical initiatr, dibenzyl perxide is required. Bradly, the mechanism fllws the fllwing main steps II II Ph-C-O-O-C-Ph 00 II Ph-C-O + heat II ----I.~ 2 Ph-C-O I ----I.. (1 ) ~ 0 + Ph-8-0H (2) The first tw steps may be imprtant nly initially. Subsequently, cyclhexenyl radical is frmed by the abstractin f hydrgen by brmine (eq. 4), Scheme 1. Scheme 1. + Br Br NBr+ HBr Br Br (3) + HBr (4) c?h + Br, (5) ~ R-ES-O-N-A-N-C-E--I-s-e-Pt-em--be-r--Z-O-O-S

5 0Cl ~ 00 ~ ~ 0 ~. 0 H FeCI 2 + HCI (6) 200 ~. 0 (7) H OR 00+ en... 0 ~ ~ OH H (8) Scheme 2. OH + FeCI 2 + Hel (9) Experiments 2 and 4 make use f transi tin metal ins t initiate the reactin thrugh the prcess f electrn transfer. While Fe 3 + in experiment 2 initiates the free radical prcess by remving an electrn frm the phenlic substrate (xidatin, eq 6-9, Scheme 2), Cu + in experiment 4 transfers an electrn t the diaz grup (reductin, eq 10-11, Scheme 3) t cmmence the curse f the free radical reactin. The electrn transfer in bth cases takes place thrugh the frmatin f intimate cmplexes f the metal ins with the substrates. Als nte that Methd B fr binaphthl preparatin is an example f Green Chemistry experiment. + CuCI (10) (11 ) Scheme 3. -RE-S-O-N-A-N-C-E--I-~--p-~-m-b-er ~ ~-

6 hv.. "-max - 330nm (12).. VOA-OC~V OH OH ('t-6 I --r(', I I ~ ~ ~ (14) Scheme 4. Suggested Reading [1] B S Furmiss, A J Hannafrd, P W G Smith and A R Tatchell, Vgefs Textbk f Practical Organic Chemistry, Lngman (ELBS), [2] CCWamserandL TSctt, The NBS reactin, J. Chem. Educ., Vl. 68, pp ,1985. [3] J March, Advanced Organic Chemistry, Wiley, 4th Editin, [4] M B Smith and J March, March's Advanced Organic Chemistry, Wiley, 5th Editin, Binaphthl exhibits atrpismerism as the rtatin arund the bnd cnnecting the tw naphthl rings is restricted. The prduct frmed here is a racemic mixture. It can be reslved int pure enantimers, which find applicatin in enantiselective rganic synthesis. In experiment 3, benzphenne is excited by the UV radiatin in sunlight (the Imax fr nap* transitin in benzphenne is nm). The excited benzphenne (a triplet) starts the radical prcess by abstracting a hydrgen atm frm 2-prpanl (eq 13, Scheme 4). Benzpinacl frms finally as depicted in Scheme 4. Cnclusin The fur experiments described are very simple t carry ut, but they demnstrate varius facets f free radical chemistry. They are als indicative f the prfund and ubiquitus character f free radicals. With this the six-part series, "An Appreciatin f Free Radical Chemistry, ends. It is hped that it has prvided a glimpse f the imprtance f the all-pervading free radicals cntrlling virtually every facet f ur lives. Unfrtunately, ur undergraduate syllabi give a very scanty picture f this tpic. This needs t be changed, and due attentin shuld be devted t its teaching bth UG and PG level ~ R-E-S-O-N-A-N-C-E-I--se-p-te-m-b-e-r--2---S