Bringing light to dark reactions Keto-enol tautomerization derivation of the rate law of tautomerization pH-rate profiles general acid and general base catalysis examples rate-equilibrium relationships (Bronsted, Marcus) mechanism of the "uncatalyzed" reaction 9931 o OH Br2 O HoO/H © Br + HBr Cßr2 í amount of Br2 added amount of Br2 calculated by extrapolation to t = 0 j___i__i__i__i___i___i__i__I * t KE = cE/cK = tČ/kK CH OH CH2 Ph ^ 1 *•< O Ph © OH K. K® O K © K, k\-\®C\-\® + ^0 rate determining ko + koHGc0HQ OH K 'K. , 'K k0 + k0Hoc0HQ ^O + ^H CH® o 0 K, Q IArG° = 23RTY^Kr = 0 vK = :k K o r 3 H & AE + c <*«(') = *%*(') H" J a H' vE = £nK + Ä:'KeAE) + /íKec e+£ O A^1 H' A' K E „k* K* +c a H Ktot M=k\,< (0 ± "L-f*^**)' E,tot^ s |_ e,totv ' E,rotv 'J E,tot v ' Can we measure kE and kK separately? 'E.tot CEitot(oo)/M t A a) 250 300 i/nm £ 4 -OJD 2 ^ ■ i i i i i i i i 1 ■ o ^ 0 - x_x ^: -2 --4 - p*= /*- ■O'i -3 - . i^t—i-^T I.I.I. 1 3 5 7 9 11 13 c) pH b) 'o t-----'----r Ai(270 nm) j________i_________i________L 9 10 11 d) pH vK = ■K K a (1) o -I—» © o c (D H * r+c E.tot \ / E,tot \ H 8 pH >ŕW ^/d B>/d (t_s/v)ßo| Buffer catalysis O CH- O N-O CH2R Ph O hv OH «CH2 OH CH2 ^ " hv OH ------^ 2 O OH *A hv ■*- Ar OH/NH2 + CO O hv hv O II C + CH2CO HO. .OH HoO - N- \ / h v HO —^ p=c=o HO OH HRC=0 Ph Ph OH HblvbticaChiuica Acta - Vol. 84 (2001) 3803 Fig. 2* Titration curve for the ionization of E determined by flash photolysis. The data points are initial absorbances A0ai 270 nm immediately after the flash. The solid line is the best hi oi Eqn. 5 to the data points. ZV Ol 8 1 I Hd 1 ' 1 ' * s • t - • 3>/d/' ....m^f "(j-s/^ßoi i /É \ / (v' \ _s/sqo^)ßo| ' 1 i . i . 9 f S t--------'---------r -9- (l_s/3j/)6o| (,_S/J/)60| J_______i_______L CH NO. pKa « 20.6 H/D-exchange 17.0 pKaaci = 3.6 + H+ pH-rate profile: o-nitrotoluene 4.0 3.5 3.0 2.5 2.0 1.5 1.0 \-0.5 l 0.0 l Llog(/c/s-1) /cH//cD = 5.1 ±0.1 H0 pH J___i___L kH/kD = 7.4 ± 0.2 as-ö- iwrq j___I___i___I___i___I___i___I___i___I___i___L -5 -3 -1 11 13 15 Helv. Chim. Acta 2001, 84, 14 0-Nitrobenzyl methyl ether: pH-profile ~>----------------1----------------r i----------1----------r- ~>----------------1----------------r- acz-decay rate-, determining ->► pH 12 14 NO, h v N© HO" "O© OH H0-N^ ho o© NHOH + NH N-C-O H—I------' H H N-C-O N-C-O water diffusion diffusion membrane N-C-O -i—h H H hv n------ N-C-O + H H N-C-O H----------- H water N-C-O H—I------' H H N-C-O] H--------W H H CHNH+ N02H NHOH" N-C-O H NH 240 260 280 300 320 2.0 \- 1.5 f 1.0 k 0.5 h 0.0 240 260 280 300 320 fast CHNH+ CH + H+ slow h (JNU , h (JNU~ , h (JNU~ /Vn ~r iKrwj C nu "r A. a C CNO, ^0 OH uOH "•" "A UA k0 + ku Cu+ &HA Cjja CNO~ + 2H+ slow j, CHNH , j, CHNH., , 7, CHNH t NHr , jt NHr ^OH cOH+ KA CA NH + H+ fast 4 - 2 - O - -2 - -4 - ■6 - p/C = 5.94 ± n ' r log(/cobs/s-i) 420 nm pH 7 ^T^^^^T* T"T^ ^"^^ III I i Ml I ,^rW 0.5 1.0 I ■ ■ t 1.5 t/s Jl kr ln -e-----§■ k u out • ^obs ^ v/ -8 - 0 j__________L pH J________i________I________i_________I________i________L 8 10 12 Intercepts of buffer dilution plots 8 IV t-------------1-------------1-------------1-------------1-------------1-------------1-------------1-------------1------------r -1 iog(*obs/s ) o // c HO 6k .A_____A_ H90 // b.OH // -Ä--------A- Aa Ar /cweK /cHelK -B-----& PH 0 ___________i_________________i_________________i_________________i_________________i_________________i_________________i_________________i_________________i_________________i_________________i_____________ 1 2 3 4 5 6 7 8 9 10 11 12 13 6.0 t--------1--------1--------■--------r T--------'--------1--------r iog(Ws"1) -1 log(/ŕ/s-n) 7.0 k 6.5 k 6.0 8 10 12 H pKa(1*) = 0.42 )> z h v -N pKa(1) = a27 N N )VN + 2H MH o-o NH 0.2- 0.1 - 0-0—t----------,----------1----------1----------,---------,----------r 300 350 400 450 500 550 600 Figure 1. Absorption spectra of the transient generated by flash photolysis of 1 using an excitation pulse from a KrF-excimer-laser (A^c = 248 nin, 100 mJ per pulse, pulse width ~25 ns, delay after the pulse maximum ~30 ns). Spectrum a: 1.0 N aqueous NaOH. Spectrum b: 0.001 N aqueous NaOH. NH- N2+ pKa = 12.7 NH N2 + H" k' H rVNN I ,N +H- Bronsted equation { log k. HA _ l P = log G -\-a log ' í*» * A V -' 9- «t je 8 n; »ň X -»- HA / + Ä* 10 12 ä 14 Lcq K = 5 16 17 Figure 1. Rate-equilibrium relation for the keton i za ti on at acetophenone enolate ion in igneous solution at 25 ĎC cat;dy/ed by RC02H (O) and RPO^H (including HOPOjH") (a). The points represent "chcmica]" rate and equilibrium constants with symmetry-induced contributions removed by using the following statistical factors: p m 1, q = 2 for RC02H; p = 3, q = 3 for RP03H"; p = 2, q = 3 for H^POr; p=1,q = 1 for PhCOCHj. Bronsted a variation with AG 100 © OH + K, a b O e + OH0 K K©/K ^a /rvw H © k\-\@C\-\® O H lí Marcus theory o \ \ \ \ \\ \ \ $ i t i / Ě / £ / f * i ) ■~^-" AG° > 0/ V \ \ / / V, \ jg[ -*y^ KCi°- n l \ \ \ / / / y ■"* AG0- A > AetG° « 0 \. ....-■■■"'" Q -------------------------------■> loa— = Ä- -AG t ^ ln(10)i?r ' where A G" = A Gn r i 0 AG' V 1 + -v 4iÄ I log (/(/[M"1 s"1]) -1 e-1 10 0 -10 1----1----1----1----1----1----1----1----1----1----1----1----1----r ArGí/(2.3fí7) --------/ j____I____i____I____i i___I___i___L -25 -20 -15 -10 -5 0 5 10 15 20 ArG°/(2.3fíT) a = ?)Á Gc J T r A G T r A s. - .<>•] AG 1 + —J— o > 4A,G ti G° , sag; log(/c/s-n) -1 O - -5 - -15 t—i—i—i—i—|—i—|—i—|—i—|—r OH O ^obs j___I___i___I___.^i^o J___I___I___I___L 0 2 4 6 8 10 12 14 pH -5 -10 -15 T------1------1------1------1------1------1------1------1------1------1 | I *o'K- V. OH K ■ O J___I___I___I___I___I___I___I___L N ■ ■ ■ .N 0 2 4 6 8 10 12 14 pH o OH hv ^ "n^ pKE (calc) = 25 'OH + ^ 8 ->-------1-------'-------HhÖ1-------r log(A/s"1) rfy^ _, £H+ = (9.5±1.0)x lOSM-is"1 • • • 9 m t--------1--------r V = (5.4 ±1.0) x WM-is"1 piCE=11.4±0. L = 400 ± 60 s-1 : J_____i_____I_____i_____I_____i_____I_____i_____I_____i_____L 0 8 10 12 14 /COH- + 0H- ^ pH>12 fast But: Why does base catalysis saturate? Change in rate-det. step! Conclusions • When will YOU do flash photolysis? • beware of artefacts • buffer catalysis • experts in reading pH-rate profiles • equilibrium constants KE spanning 30 orders magnitude • assignments of elementary reactions •LFER