Solution Kinetics of an Organic Reaction Amanda Nienow, adapted from Halpern 1

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1 Soluion Kineics of an rganic Reacion Amanda Nienow, adaped from Halpern Absrac In his lab, he S N 2 reacion beween 2,4-dinirochlorobenzene and piperidine is examined o deermine he rae consan, es he reacion mechanism, and deermine he Arrhenius parameers. The progress of he reacion (as a funcion of ime) will be racked using specrophoomery. Relaed Readings. Halpern, A. M. Experimenal Physical Chemisry: A Laboraory Texbook. 2 nd Ed. Prenice Hall: Upper Saddle River, NJ. [Available on Moodle, Exp 2] 2. McQuarrie, D.A., Simon, J.D., Physical Chemisry: A Molecular Approach. Universiy Science Books: Sausalio, CA. Chaper 28. Background We won be discussing kineics in class unil December. To help you wih his lab, ake advanage of he resources lised in Relaed Readings. This background secion will provide some basic ideas of kineics and some pariculars of he reacion o be invesigaed. Kineics: Thermodynamics is he sudy of he likelihood of a reacion (i.e., wheher i will progress as wrien or no) whereas kineics is he sudy of he raes of a reacion. We are specifically ineresed in how he rae of a reacion depends on he concenraions of all relevan chemical species and on he emperaure. Wih his informaion, one can predic how a reacion will proceed, adjus condiions o make he reacion relevan for an applicaion, and gain some undersanding of he mechanism of he reacion. Consider a general chemical reacion: aa bb cc dd. The rae of he reacion is given by he general rae law: v = k[a] α [B] β [C] γ [D] δ [Y] ε where k is he rae consan, [A], [B], are he molar concenraions, and [Y] is he concenraion of a species no represened in he balanced reacion bu ha sill affecs he rae (e.g., a specaor ion or a caalys). The exponen o which a paricular species concenraions are raised is called he order of reacion wih respec o ha species. The overall order of reacion is he sum of exponens. I is imporan o noe ha he exponens are no relaed o he soichiomeric coefficiens of he balanced reacion and are no known in advance. We also do no know which of he species will or will no be presen in he rae law. This informaion can only be gained by experimen. Reacion raes are generally emperaure dependen. fen, his emperaure dependence follows he Arrhenius equaion: Ea k( T ) = Aexp RT In his equaion, A is he preexponenial erm (a measure of rae a which collisions occur) and E a is he acivaion energy (he poenial energy barrier ha mus be surmouned in convering producs o reacans).

2 DNCB Pip The reacion of ineres: The reacion ha will be examined in his experimen involves he reacion of wo organic molecules, 2,4-dinirochlorobenzene (DNCB) and piperidine (Pip) o produce 2,4-dinirophenylpiperidine (DNPP) and piperidine hydrochloride (Pip:HCl). This reacion is shown below: Cl - N N - 2 N H - N N N - DNCB Piperidine DNPP Pip:HCl N H 2 Cl - This reacion occurs by a wo-sep mechanism summarized here: slow, k DNCB Pip inermediae fas, k - k 2 inermediae DNPP Pip:HCl verall: DNCB Pip DNPP Pip:HCl The elemenary seps shown above can be used o derive a kineic expression for he reacion. We propose ha he rae of he reacion is approximaely equal o he rae of he slow elemenary sep (i.e., apply he rae deermining-sep approximaion). Then, d[ DNPP] v = k[ DNCB][ Pip] d If [DNPP] is zero a =, he following equaions hold rue: [ DNCB ] = [ DNCB] [ DNPP] [ Pip ] = [ Pip] [ Pip] where [X] is he species concenraion a some ime and [X] is he species concenraion a ime =. By subsiuing hese expressions ino he rae law, we ge he differenial rae law: d[ DNPP] = k{ [ DNCB] [ DNPP] }{[ Pip] 2[ DNPP] } d This equaion holds rue for all cases. We will examine wo limiing ses of condiions in lab. These condiions are given below along wih he inegraed rae laws for each case. (Noe for pre-lab, you will be showing how o use he pseudo-firs order inegraed rae law o find k ).

3 . Simple second order: [Pip] = 2 [DNCB] Inegraed rae law: = [ DNCB] [ DNPP] [ DNCB] 2k 2. Pseudo firs order: [Pip] >> [DNCB] [ DNPP] ln [ DNCB] = { [ Pip] k } Each of hese condiions permis deerminaion of k by using an appropriae plo vs. ime and a linear regression. nce hese condiions are examined experimenally, an addiional experimen can be conduced a various emperaures in order o examine he emperaure dependence of rae which should obey an Arrhenius relaionship permiing deerminaion of he acivaion energy. Pre-Lab Exercises See separae documen for pre-lab quesions. These will be urned in a he beginning of lab. In addiion, read he appropriae chaper of Halpern o gain addiional insigh on he reacion of ineres (book available in Nobel 7). Safey Precauions Safey goggles mus be worn a all imes. I is recommended ha a laboraory coa ha covers clohes and arms be worn. All work MUST BE DNE IN THE HD. -chloro-2,4-dinirobenzene (DNCB) is highly oxic irrian wear gloves a all imes and wash immediaely wih soap and waer if any DNCB ges on your skin. Piperidine is a oxic and flammable liquid wih an objecionable odor. Properly dispose of wase maerials. Procedure Summary: Given ha DNPP is a colored compound we can follow he reacion by using visible absorpion. By using Beer s law, given below, we can find he [DNPP]. A [ DNPP] = where A λ is he absorbance a a given wavelengh, ε λ is he molar aborpiviy for DNPP, and l is he pahlengh. For DNPP: ε 472 nm = 36 dm 3 /mole cm and ε 372 nm = 7, dm 3 /mole cm. Noe: You ll do hese calculaions for a series of ime poins. A each ime, record absorbance a a paricular wavelengh, λ and hen conver he absorbance o [DNPP]. Using he pseudo-firs order inegraed rae law given above, you will find k. You will hen vary he emperaure of he sysem and deermine k a he differen emperaures. From his, you can find A and E a. ε λ l

4 General Seps:. Prepare sock soluions of.62 M Piperidine in absolue ehanol and.4 M DNCB in absolue ehanol. (These sock MAY have been lef by prior groups, bu you may wan o make your own o ensure proper concenraions.) 2. Prior o running he experimen, scan he UV-Vis absorpion specrum of your reacans (afer keeping hem under isohermal condiions in he waer bah; use 25 C). Use he cean pics USB 2 specromeer for his. (There are insrucions on he hardware/sofware a he end of his documen and on he course websie.) 3. Mix he reacans and some absolue ehanol in accuraely measured volumes (e.g,. ml each reacan and 2. ml absolue ehanol) and se aside for 5- minues. Then record, sore, and prin he absorpion specrum of produc; deermine he proper deecion wavelengh. Deermine he molar absorpiviy of he produc a his concenraion (assume DNCB is he limiing reagen and ha he experimen wen o compleion; don forge ha you dilued when mixing.) 4. For he kineics experimens, we will focus on he pseudo-firs order case oulined above, i.e., where [Pip] is in excess. Add.6 ml of DNCB and 2. ml of ehanol ino he es ube. Prepare he insrumen (i.e, ge dark and reference, se-up up for kineic sudy). When ready, add.5 ml of Pip, use probe o lighly mix, and sar collecion. Collec for 5 minues. 5. Carry ou he reacions a a second and hird emperaure. Afer finding he rae consans (see #6), use he Arrhenius equaion o find E a and A. 6. Use he equaion oulined above for he pseudo-firs order kineics condiion, deermine he rae consan for each se of daa. Repor/Analysis Follow he general lab repor guidelines wrie in he syle of a communicaion. In he experimenal secion, be sure o discuss on any deviaions from your planned procedure. Your daa (i.e., resuls secion) should include UV-Vis specra (one of each reacan, one of he produc mixure, and any ohers you deem imporan), any plos used o deermine he rae consan, preexponenial, and acivaion energy, and a able(s) of hose parameers. In he discussion, include he following (and any oher houghs you deem imporan ). Discuss he meaning and imporance of k, A, and E a. Consider wha effec changing from piperidine o anoher amine migh have on he reacion. Give wo examples. How would seric bulkiness of he amine and he elecron-donaing abiliy of he amine affec he sabiliy of he inermediae and he raes of reacion? References. Halpern, A. M. Experimenal Physical Chemisry: A Laboraory Texbook, Exp 2. 2 nd Ed. Prenice Hall: Upper Saddle River, NJ. Pg 34. [Available in Nobel 7] 2. McQuarrie, D.A., Simon, J.D., Physical Chemisry: A Molecular Approach. Universiy Science Books: Sausalio, CA. Chaper secions 23-3 and 23-4 (pp ).

5 IChem Sofware for cean pics UV-VIS Specromeers We will use IChem Sofware o collec UV-Vis specra and kineics informaion (i.e., Absorbance vs. Time). This sofware can be found on he lab machines under he Chemisry/cean pics folder. The following noes will help you properly se up he sofware for use in he lab.. The sofware someimes has problems recognizing he hardware. Lab insrucors/tas can help roubleshoo or you can ry he following. a. Go o he Conrol Panel o Add Hardware. Sep hrough each dialog box as hey come up. When promped, choose I ve already insalled. b. nce you ve added he hardware, you need o ell he sofware i is here. pen IChem, go o Configure/Hardware and choose USB under A/D converor ype. Click K. c. Check ha he sofware has recognized he hardware. Go o Configure/Specromeer. Under Serial # here should be a recognizable value such as USB#### (no greek leers). d. nce he sofware and hardware are communicaion, coninue on o #2. 2. Prior o obaining daa on your samples, you need o ake a reference and dark sample o calibrae he specromeer. a. Dark sample: Turn off he lamp (or unplug if here is no off swich). Click on Dark oggle buon (boom righ hand side of screen). Turn lamp back on and le i warm up for 5- minues. b. Reference: The reference is generally your solven for he experimen. Prepare he reference in a cuvee for mos experimens and in a large es ube for kineics experimens (es ube mus be large enough for probe). If using he probe, change your inegraion period o 5 ms and your boxcar smooh o. When lamp is warm, pu cuvee in holder (or pu probe in es ube) and click on Reference oggle buon (boom righ hand side of he screen). 3. Swich o Absorbance mode (using drop down menu on op lef of screen). Change your axes (by clicking on he firs and/or las value on axes) so you have a y-axis range of -.2 o and a x-axis range of 25-7 nm (or oher appropriae values). Your baseline should be presen and be nearly fla. 4. Collecing daa a. UV-Vis specra: Fill cuvee and pu in holder (alernaively, pu probe ino a es ube filled wih sample). Use he scan/sop oggle buon (boom righ) o collec specra. Save specra. b. Kineic samples: Firs configure he sofware properly, and hen collec daa: i. Go o Configure/Kineics. In box, change Prese Duraion (oal collecion ime) o appropriae ime limi (~5-2 min). Change Sampling Inerval (ime beween daa poins) o ~ s. Se wavelengh(s) of ineres in righ hand boxes. Click K. ii. Go o Configure/Specromeer. Choose he Display ab. Choose Specrum and kineics from boom drop box.

6 iii. Choose Char Acive and Coninuous a boom righ. Use Scan/Sop Toggle buon o collec daa. (Noe: Time couns down under he plos.) 5. Saving daa: Save he kineics values o ge daa as wavelengh vs. ime. Save specral values o save specra. Boh ypes will be saved as an ASCII (or ex) file ha can be opened and manipulaed in Excel. See he lab insrucors or TAs if you have quesions no saving daa properly can cause you o repea par of he experimen!