A B A B. 2. Homolytic Bond Dissociation Energies (DH ) 1. Introduction: How Radicals Form and How They React Heterolysis

Transcription

1 hapter 10 Radical Reactios reated by Professor William Tam & Dr. Phillis hag h About The Authors These Powerpoit Lecture Slides were created ad prepared by Professor William Tam ad his wife Dr. Phillis hag. Professor William Tam received his B.Sc. at the Uiversity of og Kog i 1990 ad his Ph.D. at the Uiversity of Toroto (aada) i e was a NSER postdoctoral fellow at the Imperial ollege (UK) ad at arvard Uiversity (USA). e joied the Departmet of hemistry at the Uiversity of Guelph (Otario, aada) i 1998 ad is curretly a Full Professor ad Associate hair i the departmet. Professor Tam has received several awards i research ad teachig, ad accordig to Essetial Sciece Idicators, he is curretly raked as the Top 1% most cited hemists worldwide. e has published four books ad over 80 scietific papers i top iteratioal jourals such as J. Am. hem. Soc., Agew. hem., Org. Lett., ad J. Org. hem. Dr. Phillis hag received her B.Sc. at New York Uiversity (USA) i 1994, her M.Sc. ad Ph.D. i 1997 ad 2001 at the Uiversity of Guelph (aada). She lives i Guelph with her husbad, William, ad their so, Matthew. h Itroductio: ow Radicals Form ad ow They React eterolysis A B omolysis A B heterolytic bod cleavage homolytic bod cleavage A A ios radicals B B h A. Productio of Radicals omolysis of covalet bods Need or light () R O O R 2 R O (alkoxyl radical) (chlorie radical) 2 h B. Reactios of Radicals Almost all small radicals are short-lived, highly reactive species omolytic Bod Dissociatio Eergies (D ) o = 436 kj/mol o = 243 kj/mol Bod formatio is a exothermic process. R R h o = 436 kj/mol o = 243 kj/mol Reactios i which oly bod breakig occurs are always edothermic. h. 10-6

2 The eergies required to break covalet bods homolytically are called homolytic bod dissociatio eergies, ad they are usually abbreviated by the symbol D Sigle-Bod omolytic Dissociatio Eergies (D ) at 25 Bod oke kj/mol 436 F F I I 151 h h Sigle-Bod omolytic Dissociatio Eergies (D ) at 25 Sigle-Bod omolytic Dissociatio Eergies (D ) at 25 Bod oke kj/mol Bod oke kj/mol Bod oke kj/mol F I F O I O h h Sigle-Bod omolytic Dissociatio Eergies (D ) at 25 Bod oke kj/mol Bod oke kj/mol Sigle-Bod omolytic Dissociatio Eergies (D ) at 25 Bod oke kj/mol Bod oke kj/mol Ph Ph h h

3 2A. Use omolytic Bod Dissociatio Eergies to alculate eats of Reactio (D o = 436 kj/mol) (D o = 432 kj/mol) 2 (D o = 243 kj/mol) o = 2 (432 kj/mol) (436 kj/mol 243 kj/mol) = 864 kj/mol 679 kj/mol = 185 kj/mol kj is required to cleave 1 mol of 2 bods ad 1 mol of 2 bods -864 kj is evolved i formatio of bods i 2 mol of Overall, the reactio of 1 mol of 2 ad 1 mol of 2 to form 2 mol of is exothermic h h B. Use omolytic Bod Dissociatio Eergies to Determie the Relative Stabilities of Radicals o = 423 kj/mol Propyl radical (a 1 o radical) tert-butyl radical o = 400 kj/mol (a 3 o radical) o = 413 kj/mol Isopropyl radical (a 2 o radical) h Isobutyl radical o = 422 kj/mol (a 1 o radical) h Relative Stability arbo radicals are cosidered to be electro deficiet (similar to carbocatios), thus electro doatig groups stabilize radicals 3 o > 2 o > 1 o > 3 3 > 3 > (positive iductive effect of alkyl groups stabilize radical) 3. The Reactios of Alkaes with aloges Alkaes have o fuctioal group ad are iert to may reagets ad do ot udergo may reactios alogeatio of alkaes is oe of the most typical free radical reactios h h

4 Alkaes react with molecular haloges to produce alkyl halides by a substitutio reactio called radical halogeatio R X 2 or R X X light () 3A. Multiple aloge Substitutio 2 or light h h B. Lack of hlorie Selectivity hloriatio of most higher alkaes gives a mixture of isomeric moochloro products as well as more highly halogeated compouds hlorie is relatively uselective; it does ot discrimiate greatly amog the differet types of hydroge atoms (primary, secodary, ad tertiary) i a alkae h Polichloriated products light (23%) Isobutae Isobutyl chloride (48%) tert-butyl chloride (29%) Because alkae chloriatios usually yield a complex mixture of products, they are ot useful as sythetic methods whe the goal is preparatio of a specific alkyl chloride h A exceptio is the halogeatio of a alkae (or cycloalkae) whose hydroge atoms are all equivalet. [Equivalet hydroge atoms are defied as those which o replacemet by some other group (e.g., chlorie) yield the same compoud.] Neopetae (excess) 2 or light Neopetyl chloride omie is geerally less reactive toward alkaes tha chlorie, ad bromie is more selective i the site of attack whe it does react h h

5 4. hloriatio of Methae: Mechaism of Reactio Most radical reactios iclude 3 stages (steps) (1) chai iitiatio (2) chai propagatio (3) chai termiatio Mechaism of Free Radical hloriatio of 4 (1) hai iitiatio (homolytic cleavage) Radicals are created i this step 2 h h (2) hai propagatio (i) 3 (2) hai propagatio Other tha 3, other chloriatio products ca be formed i the chai propagatio step (ii) 3 3 Repeatig (i) ad (ii) i a chai reactio provides the product 3 I chai propagatio, oe radical geerates aother ad the process goes o h (ia) (iia) h (2) hai propagatio (3) hai termiatio (ib) (iib) (ic) (iic) h h

6 (3) hai termiatio Free radical reactios caot be completed without chai termiatio All radicals are queched i this step Radical reactios usually provide mixture of may differet products Sythesis of 3 or 4 is possible usig differet amouts of reactats ( 4 ad 2 ) h e.g.: 4 (large excess) 2 3 (maily) 4 2 (large excess) 4 (maily) h hloriatio of Methae: Eergy hages hai iitiatio Step 1 2 (D o = 243) o = 243 kj/mol hai propagatio Step (D o = 440) (D o = 432) Step 3 3 o = 8 kj/mol 3 (D o = 243) (D o = 352) o = 109 kj/mol h h hai termiatio 3 3 (D o = 352) o = 352 kj/mol The additio of the chai-propagatio steps yields the overall equatio for the chloriatio of methae o = 378 kj/mol (D o = 378) (D o = 243) o = 243 kj/mol h o = 8 kj/mol o = 109 kj/mol 3 3 o = 101 kj/mol h

7 5A. The Overall Free-Eergy hage G o = o T S o For may reactios the etropy chage is so small that the term T S o i the above expressio is almost zero, ad G o is approximately equal to o h mol of the products are formed from the same umber of moles of the reactats Thus the umber of traslatioal degrees of freedom available to products ad reactats is the same 3 is a tetrahedral molecule like 4, ad is a diatomic molecule like 2 This meas that vibratioal ad rotatioal degrees of freedom available to products ad reactats should also be approximately the same h S o = 2.8 J K -1 mol -1 At room temperature (298 K) the T S o term is 0.8 kj mol -1 o = 101 kj mol -1 G o = 102 kj mol -1 h B. Activatio Eergies A low eergy of activatio meas a reactio will take place rapidly; a high eergy of activatio meas that a reactio will take place slowly hai iitiatio Step E act = 243 kj/mol hai propagatio Step Step 3 E act = 16 kj/mol 2 3 E act = ~8 kj/mol h Estimates of eergies of activatio (1) Ay reactio i which bods are broke will have a eergy of activatio greater tha zero. This will be true eve if a stroger bod is formed ad the reactio is exothermic. The reaso: Bod formatio ad bod breakig do ot occur simultaeously i the trasitio state. Bod formatio lags behid, ad its eergy is ot all available for bod breakig h Estimates of eergies of activatio (2) Activatio eergies of edothermic reactios that ivolve both bod formatio ad bod rupture will be greater tha the of reactio, o 3 3 (D o = 440) o = 8 kj/mol (D o = 432) E act = 16 kj/mol 3 3 (D o = 440) o = 74 kj/mol (D o = 366) E act = 78 kj/mol h

8 Estimates of eergies of activatio (3) The eergy of activatio of a gasphase reactio where bods are broke homolytically but o bods are formed is equal to o 2 o = 243 kj/mol (D o = 243) E act = 243 kj/mol h h Estimates of eergies of activatio (4) The eergy of activatio for a gasphase reactio i which small radicals combie to form molecules is usually zero o = 378 kj/mol (D o = 378) E act = 0 5. Reactio of Methae with Other aloges hai iitiatio FLUORINATION o E act F 2 2 F hai propagatio F 4 F F 2 3 F F 302 small h Overall o = 432 h hai iitiatio LORINATION o E act hai propagatio small hai iitiatio BROMINATION o E act hai propagatio small Overall o = 101 Overall o = 26 h h

9 hai iitiatio IODINATION o E act I 2 2 I hai propagatio I 4 I I 2 3 I I 89 small Overall o = 53 h alogeatio of igher Alkaes Mechaism for radical halogeatio of ethae hai iitiatio light Step or hai propagatio Step Step h hai termiatio 2 light 25 o light 25 o o h h A. Selectivity of omie omiatio is slower tha chloriatio because the 1st propagatio step is more edothermic (overall still exothermic). As a result, bromiatio is more selective tha chloriatio 2 (99%) (< 1%) h Mechaism 2 (major; 3 o radical more stable) (mior; 1 o radical less stable) h

10 Mechaism (major) o (trace) (> 99%) (mior) 2 25 o (63%) (37%) h h The Geometry of Alkyl Radicals p-orbital R R R 8. Reactios That Geerate Tetrahedral hirality eters Petae (achiral) 2 achiral 1-hloropetae (achiral) ( )-2-hloropetae (a racemic form) * sp 2 hybridized Plaar, similar to carbocatio 3-hloropetae (achiral) h h The Stereochemistry of chloriatio at 2 of petae 3 2 (a) (S)-2-hloropetae (50%) (b) trigoal plaar radical (achiral) eatiomers (R)-2-hloropetae (50%) h A. Geeratio of a Secod hirality eter i a Radical alogeatio 2 3 (S)-2- hloropetae (chiral) 2 trigoal plaar 2 3 (2S,3S)- Dichloropetae (chiral) from bottom face diastereomers (2S,3R)- Dichloropetae (chiral) from top face 2 h

11 Note that other products are formed, of course, by chloriatio at other carbo atoms 9. Radical Additio to Alkees: The Ati-Markovikov Additio of ydroge omide Ati-Markovikov additio of to alkees peroxide effect Additio of to alkees usually follows Markovikov s rule ot h h I the presece of peroxides (RO OR), ati-markovikov additio is observed RO OR ot Mechaism Via a radical mechaism RO OR 2 RO (homolytic cleavage) RO (chai iitiatio) RO h h (3 o radical, more stable) Sythetic applicatio ot (via more stable 2 o carbocatio) (1 o radical, less stable) h RO-OR (via more stable 2 o radical) h

12 ydroge bromide is the oly hydroge halide that gives ati-markovikov additio whe peroxides are preset 10. Radical Polymerizatio of Alkees: hai-growth Polymers ydroge fluoride, hydroge chloride, ad hydroge iodide do ot give ati- Markovikov additio eve whe peroxides are preset peroxide (moomer) (polymer) h h Via radical mechaism (i) RO OR 2 RO (v) RO OR 2 (ii) RO 2 2 RO 2 2 RO OR (iii) RO RO (vi) RO (iv) RO RO h RO RO 2 2 OR x h Other commo polymers Polypropylee ROOR ROOR PV (plumbig polymer) ROOR F 2 F 2 F 2 F 2 Polytetrafluroethee (Teflo) h Other commo polymers 2 3 OOMe ROOR Polymethyl methacrylate (widshield, cotact leses) 2 3 OOMe ROOR 2 Ph 2 Ph Polysteree (styrofoam, coffee cup, etc.) h

13 END OF APTER 10 h