CHE 331.800--11.24

CHE 331.800--11.24 - ’ Laiwoa C306 351/830 INK 114...

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Unformatted text preview: ’ Laiwoa. C306 351/830 INK/+114 11/24/14 Chapter 10 Radical Reactions m we. Mug 0 Introduction 0919 YXGCW-e I? Homolytic bond cleavage leads to the formation of radicals (also called free radicals) A ' « 0 -~ I. Radicals are highly reactive, short-lived species 1+1 (4 ,, homolysis AN; B —-——-> A- + -B Radicals 0 Production of Radicals It Homolysis of relatively weak bonds such as 0-0 or X-X bonds can occur with addition of energy in the form of heat or light : :X : homolynis 2 :X ‘ K) -- heal orlight " (‘1' . hc‘dl " R—OIO—R ——-* 2 R—O' Dialkyl peroxide Alkoxyl radicals Chapter 10 2 e Dom use ywoggqns (x I a v .5 \QNL Qowfi 101115318 6 ° One {170141, 2—? mo Cl (71W 11/24/14 0 Reactions of Radicals IQ Radicals tend to react in ways that lead to pairing of their unpaired electron. Alkane Alkyl radical o dxfiwen’r 9mm dethevwmg Chapter 10 O Homolytic Bond Dissociation Energies I} Individual atoms are less stable than the molecules they can form. It Breaking covalent bonds requires energy (i.e. is endothermic) +C) WI ‘ H—H—-—->H-+ H- AH° = +436 kJmol" 4"”ch ) Cl—Cl —>C1- + Cl- AH° = +243 kJ morl 101(9ch *1th also! o The homolytic bond dissociation energy is abbreviated DH° H_H Cl—Cl (DH° = 436 kJ mol") (DIE!° = 243 U ml") Chapter 10 o \“ 06mg! m mm CL Wm Wm ‘10 I“ m ~ 6W: m Draw/7b Weak mid JO fima/xfic‘a fir 2 11/24/14 0 Homolytic Bond Dissociation Energies and Heats of Reaction -> Homolytic Bond Dissociation energies can be used to calculate the enthalpy change (AH°) for a reaction. I} DH° is positive for bond breakage and negative for bond formation. H—H + CI—Cl -—-> 2H—Cl (DH° = 436 kJ mol") (DH° = 243 kJ mol") (DH° = 432 kJ mol“) x 2 +679 U is required to cleave —864 kJ is evolved in 1 mol of Hz bonds and formation of the bonds in 1 mol of Cl; bonds. 2 mol of HCl. AH° = (—864 k] + 679 k1) = - 185 kJ for 2 mo] HCI produced See Table 10.1 Chapter [0 o Homolytic Bond Dissociation Energies and the Relative Stabilities of Radicals ID The formation of different radicals from the same starting compound offers a way to estimate relative radical stabllities. I) Examples: Cll5Cll1Cll: n —-CH,C1LCH_¢ l- Il~ All“ -— >411“an l’ropyl radical (u 1" radical) CH.C|IHCH. —* (THENCE + H- Al!" +413 k.) mnl ‘ H lmprnpyl radical (a 2" radical) (in3 ("(1, CH,—C—C‘H1—H~—-v CH.QCH‘ ~ H~ AH" «(mum ‘ n ten-[hug] radical in .l" radical) r“ ' ._' (I' CH: —||-—- (THELIKIHI l [I‘ -_\h’0 = ‘42:”. H moi" H H lsahutyl mdiml (u I" radial) Chapter 10 6 ' :50 7?: > 10> mm»? Km‘mwd‘tom gap/LL OAS) ° a5 F07 ram/a" .- 3 ml): smut :1) mow: 5mm Moi r V €5 Uzss U’Wuj 1‘0 farm 11/24/14 l radical > (I‘ll; 3‘ radical CHRCHCHL; +H' (Inn | 22 kJ mol 1 CII1QCIIH+ tr 1 CH30H20H2' +H' (THQCllCI—In fl +H' 10 kJ mol‘1 AH” = +423 kJ mol1 AH“ = +413 kJ mol‘1 AH“ = +400 kJ mol’1 \ AH° = +422 kJ mol‘1 $“3 (31147112011:l Ciificncu:j 6 O 1 I. The relative stabilities of radicals follows the same trend as for carbocations. * The most substltuted radlcal Is most stable i: Radicals are electron deficlenl, as are carbocatlons, and are therefore also stabilized by hyperconjugation Tertiary > Secondary > Primary > Methyl 330— jg: 7f C C H H / / / / C—C- > C—C~ > C—C' > H—C- HOSJV $*ab\i \c \H \H \H Chapter 10 3 __J AV Subfii fode mom cw 2: may 3mm Aéme) 4 - Monet 06C £400 recur“ o WS‘HLWOW d? x 1N0qu QWVWUL subsfi'hrfim,’ "IWS 73 Free BOLCUCM . 049901111 +0 wrml com 606113 and up pom/1 0111 mid . it OUlLOUML 0300660 Correc » '1 LO/ 1 O The Reactions of Alkanes with Halogens ID Alkanes undergo substitution reactions with halogens such as fluorine, bromine and chlorine in the presence of heat or light H H X X I hear n——C——H + x1 —‘rH— ' i .-—-X + X—C—X -+- H~X light l | E—x + H——c——x + H—I H x x H Methane Halogen X = F. (ilarr Hr H Halomethanc Dillalomuthane Trlhanmc-thane Tetrahalomethnne Hydrogen halide l'l'he mm nfrhr number rgfnwlzu‘ nfz'ur'h halogmated met/mm! pmdur'r'd er/ualx flue numbr'r rlfmvlm {lflllt‘l/Itmt’ that rum-ml.) 0 Multiple Substitution Reactions versus Selectivity It Radical halogenation can yield a mixture of halogenated compounds because all hydrogen atoms In an alkane are capable of substitution. ID Monosubstitution can be achieved by using a large excess of the alkane Chapter 10 I) Chlorination of higher alkanes leads to mixtures of monochlorinated products (and more substituted products). * Chlorine ls relatively unselectlve 3+1) ( L , V . ClfiHC _—‘> (.‘H3(.IHCH,C| + CHXCCH; + pnlychlnrinalcd + HCI ~ “514" i ‘ I ‘ products l0 on H 3 CI Isobutane ten-Butyl chloride (29 %) CH} (:113 Isobutyl chloride (23 7:) (48%) '1 n Stead wiHK 39 L ‘- Ib Molecular symmetry is important in determining the number of possible substitution products. C H 3 heat I ‘1“ cuff—C113 + (.‘l, ——> Cll3—C—CHICI + 1101 ' or light CH3 Neopenlnne (excess) CH3 Neopenlyl chloride ID Bromine is less reactive but more selective than chlorine (Sec. 10.6A) Chapter 10 C I ga'oo'éomfs ‘ Q ( 021! W Pmcévct)tc°“‘“”bfl§(tfiw 11/24/14 ‘ mm f3 05 P0110015 Plbotuck 11/24/14 0 Chlorination of Methane: Basic Steps Chain Initiation heat Step] C12——>2Cl- orlight Chain Propagation Step2 CH4 + Cl- —)CH3- + H—Cl Step3 C ,. +C12——>CH3CI +c1- WCfl/Igf “war ‘ V .' Chain Termin tion Nhsm {add Com? +096“ W M115 in Chapter 10 Chlorination of Methane: Mechanism CH4+U A7 CHE—cl + H'Ct Chapter IO 12 11/24/14 6 Halogenation of Higher Alkanes I) Monochlorination of alkanes proceeds to give limited selectivity: * 3° hydrogens are slightly more reactive than 2° hydrogens which are slightly more reactive than 1° hydrogens. ~k But chlorination occurs so rapidly it cannot distinguish well between classes of hydrogen and so Is not very selective. \ x V a CH}CHZC.H3 CH3(,H2CH3LI + CHRCIHCHJ Cl Propane l-Chloropropane 2-Chloropropane (45 ‘7; ) (55 % ) 1”“ CH“ CIH‘ . Cl . , . CH3CHCH3 CHJCHCHZCI + CHJCCH} 25"‘C C] 2-Mcthylpr0pane l-Chloro-Z-methyl- 2-Chlor0‘2-methyl- propane propane (63 %) (37 %) Chapter 10 cliH1 (IZH‘ clm, CH',(|T(‘H,CHV, (‘ICHICHCHICHJ + CH‘CCHICH, 300%? H C] 2-Mclhylbulunc l-ClIIOro-2-rnnhylbutane 2-Chlom—2-mcthyl- (30%) butane (22921 (l‘Ji1 (I‘llx + CH‘CH(|?HCH, ‘ (‘H‘CHCHlCHjCl “~/ cu 2-Chlnm«3~muhyl- l-L‘hlunrJl-methyl- butane butane (33%) [15%) 0 Selectivity of Bromine I) Bromine is much less reactive but Wthan chlorine in radical halogenation. CH3 CH} CH] I Br I I CijL—‘LHE (.,Hf“([.fi“LH3 + CHE—(f—‘CHZBI‘ H Br H 099%) (trace) CI‘H3 (|.‘H3 (IZH3 (‘lh/ CHLHCHZ CHJCISCHR + cugcucnga Cl (37%) (63%) ...
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