Reactionmechanismandreactionorder：级反应机理和反应.ppt

Reaction mechanism and reaction order,Chemical reaction: single or sum of many steps =reaction mechanism A step: (elementary reaction) one, two or three reacting molecules = molecularity unimolecular: isomerization, decomposition bimolecular: energy transfer, combination termolecular: energy transfer + combination Reaction order vs. molecularity Reaction takes place in an elementary reaction unimolecular: first order bimolecular: second order termolecular: third order Is a first order reaction is always an unimolecular reaction?,Consecutive reaction processes,RX + H2O = ROH + H+ + X- Unaffected by the PH of solution, SN1 reaction RX R+ + X- R+ + H2O ROH + H+ RX + OH- = ROH + X- Alkaline solution RX R+ + X- R+ + OH- ROH What happen for 2nd order reaction?,Consecutive reaction processes,A B , B C -dA/dt = k1 A; -d(a-x)/dt = k1 (a-x) dB/dt = k1(a-x) - k2B = k1 a e-k1t - k2B dB/dt + k2B = k1 a e-k1t,Plot of concentrations of A, B, C in consecutive process,Maximum B,Formation of an intermediate complex,A + B X (1) X A + B(2) X C + D(3) Principles of Stationary state 1913 Chapman: the net rate of formation of a reaction intermediate may be put equal to zero. X so reactive, X is low,Formation of an intermediate complex,A + B X k1; X A + B k-1 X + C P + Q k2,Two limiting cases: k-1 k2C, k-1 k2C,Third body effect in atomic recombination,Two atoms molecule + lot of vibrational energy short life time production: needs a collision with other molecular species to shed some excessive energy X + X + M X2 + M before first vibration (10-13 s) effective collision effective molecule=highly negative activation energy higher temperature: lower reaction rate efficiency is different depend on molecule large molecule,Porters interpretation(1961),I + M = I.M (K1); I.M + I I2 + M(k2) I.M: spectroscopically identified for benzene I.M=K1IM dI2/dt = k2II.M = K1k2I2M RT ln K = -G = -(H-TS) k = A exp-Ea/RT,Parallel reaction,AB (1) AC (2) -dA/dt = (k1 + k2)A (a-x)=a exp-(k1+k2)t k1/k2 = Yield of B/Yield of C AC (1), A+BD (2) -dA/dt = (k1 + k2B)A AB (1), A+AC (2),A,B,C,Reactant participating in equilibria,H2 + I2 = 2HI Bodenstein 1899 first order with respect to each reactants Bimolecular process, elementary reaction, four center transition state Sullivan 1967: 0-200C ; non elementary reaction I2 + h I2*, I2* 2 I 2 I + H2 2 HI,A probable mechanism for H2 + I2,I2 = I + I (1) I + I+ H2 HI + HI (2) K1 = I2/I2 I=K11/2I21/2 dHI/dt = 2k2H2I2 = 2k2K1H2I2 Ea(k2) = 21 kJ/mole Ea(k2) + Bond dissociation energy of I2 = Overall activation energy of Bodenstein thermal experiment.,A probable mechanism for H2 + I2,I2 = I + I (1) I + H2 = IH2 (2) I + IH2 HI + HI (3) K1 = I2/I2 I=K11/2I21/2 K2 = IH2/IH2 IH2=K2 IH2 dHI/dt = 2k3IH2I = 2k3K1K2H2I2,Hydrogen-halogen kinetics,H2-F2, H2-Br2, H2-Cl2 complicated experimental rate law interpreted in terms of free radical chain reaction H2-I2 simple kinetic behavior the least understood system,Reactant participating in equilibria,nA=An A=Kn-1/n An1/n = Kn-1/n c/n1/n A: a part of aggregate, aggregate normal concentration of A/n Butyl lithium initiator of anionic polymerization, rate initiator 1/6 butyl lithium associate to hexamer,Reactant participating in equilibria,A = B- + H+ (Ka) B-=KaA/H+; A = H+ B-/ Ka.; A + B- = A(H+ + Ka)/H+ = B(Ka + H+)/Ka = a B- = aKa/(Ka + H+) A = aH+/(Ka + H+) We can formulate rate equation by A & B- PH dependent,Opposing reaction,A B (k1) B A (k-1) dx/dt = k1(a-x) -k-1 x = k1a - k1x - k-1 x final equilibrium. k1(a-xe) = k-1 xe k1a = (k1 + k-1)xe dx/dt = (k1 + k-1)(xe-x) ln (xe)/(xe-x) = (k1+k2)t,Isotope exchange reaction,AX + BX* = AX* + BX Assume K=1 G = -RT ln K = 0 AXt=0 = a; BX*t=0 = b; AX*t=0 = BXt=0 = 0 At equilibrium x2/(a-x)(b-x) = 1 x = ab/(a+b) AX= a-x = a2/a+b, BX*= b-x = b2/a+b AX*= ab/a+b, BX= ab/a+b AXt+ AX*t = a, BXt+ BX*t =