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by: Michelle Gulgowski DVM


Marketplace > University of Kentucky > Chemistry > CHE 538 > PRIN OF ORGANIC CHE
Michelle Gulgowski DVM
GPA 3.91


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Class Notes
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This 5 page Class Notes was uploaded by Michelle Gulgowski DVM on Friday October 23, 2015. The Class Notes belongs to CHE 538 at University of Kentucky taught by Staff in Fall. Since its upload, it has received 31 views. For similar materials see /class/228302/che-538-university-of-kentucky in Chemistry at University of Kentucky.

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Date Created: 10/23/15
Principles of Organic Chemistry lecture 22 page 1 DEMYSTIFYING THE REACTION ENERGY DIAGRAM 1 We are working to capture aspects of the reaction energy diagram 11 We want to know where in terms of numbers the transition state effectively becomes a stationary state intermediate Ts TS Ts Ts Ts Ts TS Int int Int Int Ener R W R R P p P Reaction Coordinate 1 Reaction Coordinate 2 Reaction Coordinate 3 Reaction Coordinate 4 2 Molecular Clocks 21 All clocks have one thing in common they count a particular occurrence at a constant frequency 211 The more constant the frequency the better the clock 3 Rate constants are clocks 31 Rate constants are expressed in terms of hz or lsec depending on molecularity Mins in this case frequency is also concentration dependent rate constants are frequencies 311 How often does it happen Ask the rate constant 32 The lifetime of a species is inversely proportional to the rate constant of the disappearance ofthe species 321 kR l life time 3211 or framed more personally your average life span is inversely related to how fast they die in your home town 32111 IfI do something 10 times in a minute how long does it take me to one thing 32112 The frequency is 10 mini1 and the life time of the only one operation is 110 minute or 6 sec 32113 The relationship between frequency and lifetime is 139 v71 32114 You get the idea by now 33 The phenomenon of IR can be thought of in terms of a molecular clock 331 T1116 plain vanilla organic bond absorbs in the IR between 3000 and 1000 cm 332 v C1x 3 x1010 cmsec 10003000cm71 30 x1013 7 90 x1013 Hz Principles of Organic Chemistry lecture 22 page 2 333 Thus the lifetime of the typical organic vibration is between 3XlO3914 7 1x103914 s 3331 This is a very short period of time in human experiential terms 34 Reactive intermediates operational de nition 341 76 vibrations is the lower estimate for any species that can be de ned as an intermediate 3411 I don t remember where I read this but I like it 3412 From the organic vibration argument IR the minimum lifetime associated with such an intermediate that breaths six times before it dies is on the order of 20XlO3913 7 67103914 sec 3413 This is simple 6X the numbers quoted above 3414 These numbers correspond to an energy barrier between 11 to 065 kcalmol 34141 NOTE I needed to actually use the vibrational partition function eq 19 below to avoid getting negative numbers 341411 Negative energy barriers are not barriers 34142 This is due the error that arises when lieihvkT 1 z kThv at low v The frequency rate is the on the high end at this point 34143 RTlnmj AG eq 19 341431 where v is the frequency divided by 6 34144 instead ofthe equation 3415RTlnkThkR AG eq 18 342 It is a bit more complex than this because the intermediate can go up and over at least two barriers One gives products and the other gives starting material 3421 So the lifetime 11k1k2 3422 or approx 10XlO3913 7 33X103914 sec 34221 Notice these numbers are a little less than those above the 6 vibration criteria for the line between tstate and stationary state is a bit arbitrary in any case 34222 Let s stick with the previous numbers 343 The worst enemy of the reactive intermediate in solution is solvent molecules 3431 You see reactive intermediates are well reactive Principles of Organic Chemistry lecture 22 page 3 3432 The diffusion constant for neutral species is N 6 X 109 Ms71 thus for water 55 M 6 X109Ms 1X 55M 3 x1011s 1 3433 Therefore it takes a neutral solvent molecule about 3 X 1011 s71 1 3 X 103912 sec to diffuse in and collide with a reactive intermediate after it is created in water 34331 Other liquid values of comparable viscosity are in this ball park 3434 On the heavy atom single bond end of the organic IR spectrum C7 O 1000 cm39l a bond can vibrate about 100 times before it is hit by a solvent molecule 34341 1000 cm3913x1010 cm s391 3 x 1013 s 1 Vibessec 34342 3 X 1011 s71 diffusions or solvent collisionssec 34343 X 1013 s71 Vibessec3 X 1011 s71 solvent collisionssec 100 Vibes solvent collisions 35 We can compare 24 kcalmol to what we get out of the theory 3511This value is quoted to be the lowest barrier that one can have between intermediates to be able to isolate one from the other on the laboratory time frame 35111 A reactive intermediate that is bound by a 24 kcalmol barrier according equation eq 18 will have an average lifetime about one day at 25 OC 35112 RTlnkThkR AG eq 18 35113 2 calmol lK 129s KLn13s0x10 23JK 129s K 6626x103934 Js kR 2400 calm0 35114 When I solve for 1kR I get 64300 sec or 18 hours TS 06 kcalmol Ts Ts Reaction Coordinate 2 Reaction coordinate 4 3521 We have now arrived at a temporal energetic separation between stationary and transition state 35211 If the energy barrier protecting the species is lower than 06 kcalmol we have to call it a transition state 35212 If the energy barrier protecting the species is higher than 06 kcalmol we can start to think about its eXistence in terms of a lifetime and thus a stationary state 6 Principles of Organic Chemistry lecture 22 page 4 Two aspects of kinetic theory that are important in Organic Chemistry are the Hammond Postulate and the Curtin Hammett Principle The Hammond Postulate 51 Elemental changes in molecular structure that are resemble in both structure and energy the highest energy stationary state 511 What does this mean for an exergonic reaction 512 What does this mean for an endergonic reaction 52 With all other things equal latin ceteris paribus if two reactions occur by a similar mechanism the more exothermic reaction happens faster 521 Thus the Hammond postulate says that there is a relationship between structure and energy on the potential energy surface 522 The Hammond postulate states that the energy and structure of the transition state is closest to the highest energy stationary state for any elementary chemical transformation 52211 instructor illustrates 3E AEI 5222 52221 p is structural change Rxn Coordinate p 52222 and Hammond says AEig Ap1 oc AEg Ap2 52223 So lAEl is inversely proportional to AE 522231 lAEl the absolute value ofthe difference between Eintermediate and Emolecule 522232 So with all other things equal reactions characterized by more exothermic AH lt 0 more exergonic AG lt 0 should occur faster 52224 And with all other things equal reactions characterized by more endothermic AH gt 0 more endergonic reactions AG gt 0 should occur slower The Cultin Hammett Principle Principles of Organic Chemistry lecture 22 page 5 61 It is not the lowest energy intermediate that determines the product but the most reactive 611 In general you can t look at product mixtures and guess which intermediate is the most stable especially when the two intermediates are close in energy 612 Instructor draws diagram to illustrate int 1 int 2 P2 Rxn coordinate 1 Rxn coordinate 2 6121


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