lecture 5: Nucleophilic Substitution Reactions
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1 lecture 5: Nuclephilic Substitutin Reactins Substitutin unimlecular (SN1): substitutin nuclephilic, unimlecular. It is first rder. The rate is dependent upn ne mlecule, that is the substrate, t frm the carbncatin intermediate. The rate determining step is the frmatin f this carbncatin because it is an unstable species s it will frm slwly frm a stable neutral rganic mlecule. Hwever, after it is frmed, it is very reactive and the prceeding reactins will be very fast regardless f the strength f the nuclephile. The nuclephile will thus attack the carbncatin by dnating electrns t the empty p- rbital and frm a bnd. Frm either side f the carbncatin empty p-rbital. Because we knw that the first step, the frmatin f the carbncatin is slw, that in an energy diragram, that step must be the step with the higher energy transitin barrier. The energy f that transitin state which determines the verall rate fr the reactin, is clsely linked t the stability f the carbncatin intermediate (Hammnd s pstulate) and it is fr this reasn that the mst imprtant factr in determining the efficiency f an SN1 reactin is the stability f the carbncatin intermediate. The 4 factrs will affect its reactivity: L.G.: it needs t be a gd leaving grup. The ability fr a mlecule t be a leaving grup increases dwn a grup in the peridc table in case f halgens. Substrate: a secndary and tertiary carbnatin can frm depending whether the leaving grup is bnded t a secndary r tertiary carbn. Hwever, if it is a secndary, then ther reactins will cmpete and depeding n the slvent it can prmte carbncatin frmatin. Nuclephile: the strength f the nuclephile nr the cncentratin f it matters. Hwever, we use pr nuclephiles fr SN1. These needs t be realtively small in size s they experience lw steric effects as they apprach the susbtrate. Slvent: plar prtic slvents can slvate bth negatively and psiticely charged species, it can thus prmte SN1 by stabilising the intermediate and spreading the lcalised psitive charge arund the mlecule. This will als stabilize the transitin state (Hammnd s pstulate) lwering the activatin energy and increasing the rate f the reactin. Hwever, using plar aprtic slvents, althugh it can slvate psitively charged species, it des it prly, and s this is nt enugh t prmte carbncatin frmatin (this nly applies fr secndary because a tertiary carbncatin is stable enugh t exist even thugh the slvent is nt aiding in its frmatin) and thus cannt increase the rate f an SN1 reactin, it rather slws it dwn because the energy f the transitin state is much greater than cmpared t a stabilized transitin state (Hummnd s pstulate.
2 Nte: Using acidic slvents can als prmte SN1 by increasing the rate f carbncatin frmatin. Substitutin bimlecular (SN2): substitutin nuclephilic, bimlecular and secnd rder. The rate f the reactin is dependent upn the strength f nuclephile and the substrate. Thus it is dependent upn 2 mlecules (bimlecular). There is n carbncatin frmatin s the nuclephile will directly attack the carbn bnded t the L.G. it will dnate a pair f electrns int the antibnding mlecular rbital f f the C-Br bnd, this is at 180. In the transitin state, the nuclephile and the 3 hydrgens and the Br atm are all bnded. Hwever, the bnd between the nuclephile and the carbn is nt fully frmed, and the bnd between the carbn and Br atm is nt yet cmpletely brken. These are partial bnds. The nuclephile will then cmpletely dnate the pair f electrns int the antibnding mlecular rbital f the C-Br bnd and the sigma bnding mlecular rbital between C-Br is brken and thus the L.G. leaves. Thus the highest energy barrier that this reactin encunters in the reactin is the frmatin f the transitin state. Thus if we can stabilise the intermediate r we can increase the nucelphilicity f the nuclephile we can increase the rate f the reactin. The 4 factrs will affect its reactivity: L.G.: it needs t be a gd leaving grup. The ability fr a mlecule t be a leaving grup increases dwn a grup in the peridc table in case f halgens. Nuclephile: the strength and cncentratin f the nuclephile is very imprtant. We generally use strng and gd nuclephiles. They need t be small in size s that the steric effects experience when it appraches the mlecule are minimized and thus it can directly attack the electrphile attached t the leaving grup. Substrate: it needs t be unhindered, the leaving grup can be attached t a secndary r primary carbn. Hwever, If it is tertiary, it is t hindered SN2 will nt prceed as the grups blck the antibnding mlecular rbital f the C-X bnd and thus the nuclephile cannt dnate its electrns int it. Slvent: if we use plar and aprtic slvent, this will be able t slvate psitive charged species, hwever prly as cmpared a prtic slvent, but will nt be able t slvate the nuclephile effectively and thus it makes it mre reactive creating naked ins. This increases the energy f the starting materials, decreases the activatin energy and thus increases the reactin rate. The transitin state is als less plar than a lcalised negative charge (r psitive charge like in SN1) since the charge is spread between the nuclephile and substrate, thus the transitin state is less in need fr slvatin as in SN1, s the plar aprtic slvent can be used.
3 Hwever, if we use plar and prtic slvent, this can slvate bth psitive and negatively charged species and thus will slvate and stabilise the nuclephile via hydrgen bnding, and thus decrease its strng nuclephilicity required fr an SN2 and s the reactin slws dwn. This will lwer the energy f the reactants mre than the transitin state energy thus increasing the activatin energy barrier. The ability f nuclephiles t participate in hydrgen bnding decreases as we g dwn the peridic table. Hence fluride is the strngest hydrgen bnd acceptr, and idide is the weakest. This means that the lne pairs f idide in will be cnsiderably mre free than thse f fluride, resulting in higher rates The prduct btained SN1 and SN2: fr SN1 it will prduce a racemic mixture f prducts because the nuclephile can attack frm abve r belw the mlecules. the sterechemistry fr SN2 f the mlecule will be reversed. If it was S, nw it is R, because the nuclephile needs t attack at 180 degrees t the leaving grup. lecture 6: Hammnd s Pstulate Cmparisn between SN1 and SN2 mechanisms: if cnditins will favur ne particular type f reactins then the prduct frmatin will be dependent upn the mechanism favured. Hwever, there are instances where bth reactins will cmpete
4 Hammnd s pstulate: if 2 states are directly linked n an energy prfile and are clse t each ther in energy, then they are als clse t each ther in structure. That is the structure f the transitin state resembles the structure f the nearest stable species in energy. Fr example: The first transitin state is mre similar t the first intermediate The secnd transitin state is mre similar t the first intermediate. The third transitin state is mre similar t the secnd intermediate As already discussed, fr SN1 reactin, the rate determining step is the frmatin f the carbncatin and thus it is strngly endthermic reactin. Since the first T.S. resembles the first intermediate in energy, stabilizing the first intermediate will als stabilize the first T.S.
5 Kinetic vs Thermdynamic prduct: reactins under thermdynamic cntrl have utcmes that depend n the psitin f the equilibrium and therefre it will prduce the lwest energy prducts. Reactins under kinetic cntrl have utcmes that depend n the rate at which the reactin prceeds, and therefre the relative energies f the transitin states. This leads t the prductin f prducts with the lwest energy pathway. Kinetic prduct: if the system is nt allwed t reach equilibrium, the prduct with the lwest energy pathway will be the majr prduct. Thermdynamic prduct: if the system reaches equilibrium, the lwest energy prduct will be the majr prduct. Case 1: cnsider the fllwing reactin: At lw temperatures, prduct Z fr bth cases is preferred because the reverse reactin is t slw (there is nt enugh energy and time fr the reactin t reach equilibrium, the prducts d knw have time t reverse and vercme the bigger energy barrier t frm the ther prduct). Hwever, if we had a lnger reactin time, the mlecules can reverse and rather frm the mst stable prduct even thugh it is under lw temperature cnditins At high temperatures and lng reactin time, prduct Z and Y (thermdynamic prducts) are preferred because the reverse reactin als prceeds (in this cnditin, all these reactins will ccur until equilibrium is achieved where the mst thermdynamically stable prduct will frm). The mlecules have enugh energy t reverse and vercme the bigger energy barrier. Case 2: cnsider the fllwing reactin: Heat + lng reactin time = C Heat + shrt reactin time = C Cld + lng reactin time = C Cld + shrt reactin time = B Althugh the cnditins are cld, and left fr a lng reactin time, we wuld btain prduct C because it is the mst stable. The mlecules althugh they are in lw temperatures, will reverse back and frm the mst stable prduct r like in this case, the mlecules B can either reverse back r prceed thrugh a different pathway t frm prduct C. Once mlecules f A turn t B, and if given enugh time, mlecules B will turn t C. Case 3: When the reactin is highly exthermic and is irreversible since ther reverse reactin requires massive amunts f energy, the reactin that frms the fastest, wehther it is the thermdymic r kinetic prduct, will be the majr prduct.
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