Note for CHEM 251 at OSU
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Date Created: 02/06/15
organtc cherntstry sn2 E2 S 1E1 Suz SulEl electropth nuctcophttc 9N substrate 1 u f 2N Le emung 7d Nu ck o U slow a 6 e s gt 7 C V LV Cg sum Mew crNu substrate leavmg youp wtthA C 7 7 and m E1 etectrophte A QB a PR R3 Va cc HrB B a L R R H78 1 2 sn1 and E1 haye tdentrcat rate deterrntntng steps so they mm mumpm WE 9 genera11y occur stmu1taneous1y and have the same properttes Snz and E2 Stu131 mechanism one stepithxs stn 1e step ts the raterdetermmm step RDS two stepsiRDS ts formatton of carbocatton big sn2 stenc htnoLrance blockmg Nu Nu ts tn RDS stabt1tztng carbocatron nhstacle E2 blockm B tsn39t abt obstac1e B doesn39ttotn substrate NuB tsn39t tn RDS so blockm tttsn39t an obstac1e Kittens sn2 tnyerston backstde attack stnce LG blocks frontstde sn1 racerntzatron p1anar carbocatron tnterrnedtate chemistry E2 cts vs trans determtnedb anurpenplanar txansmonrstate E1 both cts and trans tsomers wtu be produced regjnc E2 posstb1e products from deprotonatton of any Becarbon E1 posstb1e products from deprotonatton of any are chemistry major product w bulky base 1ess substttuted stertc htnoLrance major product more substttuted alkene major product wtth noncbulky base more substrtuted e39cdonanng alkyl substrtuents stabt1tze alkenes rate Rate ksubstrate Nu39 orB39 so Nu1339 t rateT Rate ksubstrate so Nu53917 rate unchanged expressinn substrate and Nu39B39are tn RDS on1 the substrate ts tn RDS Nu quality requtres good Nustrong B NuB ts tn RDS can work wtth a poor Nuweak B bulky NuB favors E2 vs sn2 blockmg B tsn39t a btg obstac1e NuB tsn39t tn RDS LG quality requtres goodleavmg group because 1eaytng group ts tn RDS requtres goodleavmg group because LGts tn RDS preferred po1ar aprottc no OH or NH bonds po1ar prottc at 1east one OH or NH bond snlvent39 for sn2 hydrogencbonds to so1yentwou1o1 b1ock Nu hydrogencbonds to so1yent stabt1tze carbocatton forE2prot1 so1yent would protonate the base substrate sn2 rnethy1gt1 gt2 3 gtyes no sn2 substrtutents b1ock Nu 3 gt 2 rnethy1 and 1 gtye no StuE1 E2 1 2 or 3 blockm B ts not abt obstac1e atk 1 substrtuents stabt1tze the carbocatron 1 W organic chemistry 8N2 E2 SNl El comparing the same element charge resonance nucleophilicity negative charge 9 better Nu resonance 9 worse Nu charge is stabilized basicity negative charge 9 stronger base resonance 9 weaker base charge is stabilized leaving group ability positive charge 9 better LG more willing to accept electrons resonance 9 better LG charge will be stabilized comparing different elements same row same column big difference electronegativity size nucleophilicity less electronegative 9 better Nu bigger 9 better Nu usually willing to donate electrons big Nu s are less hindered by solvent more polarizable basicity less electronegative 9 stronger base bigger 9 weaker base willing to donate electrons large base can spread out and stabilize electron density leaving group ability more electronegative 9 better LG willing to accept electrons bigger 9 better leaving group big LG s can spread out and stabilize electron density nucleophiles leaving groups bases nucleophiles leaving groups N 0 F N39 039 Fquot good Nu 3N2 or 3N1 N o F N 0 good LG u 7 P 6 g P 6 poor Nu SNl only P S sulfonate R OR P S not a LG 1 I39 I O The a carbon is attached to cyanide NC39 charge on the C not the N the oxygen Qt t0 the sum bases Nucleophiles and bases shown with charges before attacking N O F N39 039 strong base E2 Leaving groups shown with charges before leaving P S Gl P39 S39 Gl weak base E1 The tables for individual atoms assume no resonance Resonance Br Brquot not a base makes atoms into worse nucleophiles and bases and into better leaving l 1 groups wwwfreelanceteachercom organic chemistry SN2 E2 SNl El what happens in SN2 SNl E2 and El mechanisms SNZ One step Nucleophile joins or carbon and leaving group leaves or carbon SNl Step one Leaving group leaves or carbon Step two Nucleophile joins or carbon E2 One step Base takes B hydrogen TE bond forms between or and B carbons leaving group leaves or carbon E1 Step one Leaving group leaves or carbon Step two Base takes B hydrogen TE bond forms between or and B carbons how to determine SN2 vs E2 vs SNl vs El for haloalkanes and alkylsulfonates NUCLEOPHILE poor Nu good Nu weak base good Nu strong base 0 BASE weak base N s Se Cl39 Br39 139 NC39 s Se39 N39 039 uCARBON O non bulky bulky LDA tbutylO39 methyl no reaction SN2 SN2 SN2 1 no reaction SN2 SN2 E2 1 with 3 Bcarbon no reaction SN2 no reaction if Bcarbon is 4 E2 E2 2 SNlEl SN2 no reaction if Bcarbon is 4 E2 E2 3 SNlEl SNlEl E2 E2 SNlEl means both SNl and El occur The ratio of SNl to E1 is generally about 95 to 5 so the El reaction is often not mentioned The El reactions become more important at high temperatures The table displays the major reaction for each caseiin some cases there may be signi cant levels of minor competing re actions The table may not give the correct answer in all realworld situations but it will generally be accurate for the questions that are typical of exams wwwfreelanceteachercom organic chemistry 8N2 E2 SNl El how to determine SN2 vs E2 vs SNl vs El for alcohol substrates First the alcohol must be treated with an acid to give it a better leaving group ACID nonnucleophilic acid nucleophilic acid H2804 HCl HBr HI First acid protonates an alcohol to form a good leaving group First acid protonates alcohol to form a good leaving For substitution another alcohol acts as nucleophile group For elimination you can use another alcohol or HSO439 as base Then Cl39 Br39 or I39 act as nucleophiles or bases tilCARBON methyl SN2 needs A SN2 no reaction with Cl39 1 SN2 needs A SN2 no reaction with Cl39 E2 needs more A 2 SNlEl SNJEJ El dominant with A El dominant with A 3 SNlEl SNlEl El dominant with A El dominant with A The information in italics indicates characteristics that differ from what we would expect for haloalkane substrates The most important new reactivity for alcohols is that treatment of secondary and tertiary alcohols with H2804 plus heat gives El this reaction is called dehydration High temperatures favor El over SNl for haloalkanes as well but this is rarely tested Ethers have mostly similar reactivity to alcohols HBr and HI attack ethers to form alcohols which can then be attacked by more HBr or HI to form haloalkanes SNlEl means both SNl and El occur In these cases the ratio of SNl to E1 is generally about 95 to 5 so the El reaction is often not mentioned But as mentioned above at high temperatures E1 is dominant The table displays the major reaction for each caseiin some cases there may be significant levels of minor competing reactions The table may not give the correct answer in all situations but it will generally be accurate for the questions that are typical of exams 4 wwwfreelanceteachercom