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Organic Reactions Summary for Use As a Study Guide Beauchamp Typical First Year Organic Reactions Beauchamp 1 Organic Reactions Summary For Use as a Study Guide Beauchamp y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, SN-E, C=O, epoxides chem, with mechs.doc Typical First Year Organic Reactions Beauchamp 2 Important acid/base reactions used in the examples below. Write out every one of these easy mechanisms. sodium hydroxide Na OH thiolates are good nucleophiles, H H2 Na 2 K (RSH) S 2>E2atMe,1o and 2o RX, C H K = a C N eq K (H O) R S a 2 R S and strong bases, E2 > SN2at -8 10 +8 3oRX. Keq = =10 thiolates thiols 10-16 H n-butyl lithium Li n-Bu Li N N LDA is a very strong base that Ka(HNR2) is also very sterically hindered, Keq = it always acts as a base in our Ka(H-C4H9) -37 course. 10 +13 diisopropylamine Keq = =10 10-50 LDA = lithium diisopropylamide y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, SN-E, C=O, epoxides chem, with mechs.doc Typical First Year Organic Reactions Beauchamp 3 LDA = lithium diisopropylamide O Na NR O 2 enolates are good nucleophiles, K (ROCOCH ) S 2 > E2 at Me, 1o and 2o RX, R C H K = a 3 R C N eq K (HNR ) O C a 2 O CR2 Na and strong bases, E2 > SN2 R2 -25 o 10 +12 at 3 RX. esters K = =10 eq 10-37 ester enolates LDA = lithium diisopropylamide 2 eqs. O Na NR2 O Na enolates are good nucleophiles, K ( O CCH ) S 2 > E2 at Me, 1o and 2o RX, H C H K = a 2 3 C Na N eq K (HNR ) O C a 2 O CHR2 and strong bases, E2 > SN2 R2 -25 o 10 +12 at 3 RX. Keq = =10 acid dianion carboxylic acids 10-37 LDA = lithium diisopropylamide 2 eqs. Na NR2 N N Na enolates are good nucleophiles, K ( O CCH ) S 2 > E2 at Me, 1o and 2o RX, C H K = a 2 3 C N eq K (HNR ) and strong bases, E2 > S 2 C a 2 CR2 N R2 -30 at 3oRX. K = 10 =10+7 nitriles eq 10-37 nitrile enolate n-butyl lithium H Li n-Bu R R n-butyl lithium removes proton X Ka(HCR2SPh2) from sulfur salt and makes a C Keq = C Ka(H-C4H9) good nucleophile at ketones and R SPh2 R SPh2 10-33 aldehydes, forming epoxides. K = =10+17 sulfur salt = ylid eq 10-50 betaine n-butyl lithium removes proton from dithiane and makes a n-butyl lithium good nucleophile at all of our Li n-Bu electrophiles. It can react once or twice in SN2 reactions. K (dithiane) Sulfur acetal forms carbonyl S S K = a S S eq K (H-C H ) C a 4 9 C group after hydrolysis using -33 Li Hg+2. Makes aldehydes and H H K = 10 =10+17 H dithiane eq 10-50 dithiane carbanion ketones. y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, SN-E, C=O, epoxides chem, with mechs.doc Typical First Year Organic Reactions Beauchamp 4 Arrow-Pushing schemes for the above reactions O Na O Na Na H H Na O 2 2 C H O H C C H H C H H H R O R O R O R O H carboxylic acids sodium hydoxide carboxylates alcohols sodium hydride alkoxides H H Na C Na C 2 H2 Na C H O C Na O H C C NR2 R S R S H H R R sodium hydoxide thiols thiolates terminal alkynes sodium amide terminal acetylides H O O Li Li Na H2 N H C N 2 C C H NR C C CH3 2 H2 R C R CH Na H 2 LDA 2 LDA diisopropylamine n-butyl lithium lithium diisopropylamide ketones / aldehydes ketone enolates 2 eqs. O O O O Na Na Na Na R C H R C H C H C NR2 NR Na O C 2 O CHR O C O CR2 2 R R2 2 LDA esters ester enolates carboxylic acids LDA acid dianion N Li Na N Na H C H 2 C H2C C C NR2 S S C CH3 S S CR2 H R2 C 2 CH Li nitriles LDA nitrile enolate H H n-butyl lithium H dithiane dithiane carbanion H H Li Li H R H2 R R 2 R X H C C X H2C C C 2 C C C CH C C CH3 3 H R SPh H2 R SPh R PPh3 2 R PPh3 2 2 Wittig salt = ylid n-butyl lithium betaine sulfur salt = ylid n-butyl lithium betaine Organometallics used in our course (Mg, Li and Cu) Br Br Li Br R Li R R R Li Li bromohydrocarbons Li organolithium discard reagents Br +2 Mg Br Br R Mg Br R R R bromohydrocarbons Mg Mg R (MgBr) Grignard reagents (organomagnesium reagents) Cu Br reacts with RBr (SN2) Cu R R Cu reacts with acid chlorides R R Li R Li Li reacts with ,-unsaturated C=O dialkyl cuprates y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, SN-E, C=O, epoxides chem, with mechs.doc Typical First Year Organic Reactions Beauchamp 5 o SN2 versus E2 choices at 2 RX. At secondary RX (X= OTs, I, Br, Cl) SN2 and E2 products are in close competition with each other. Anions whose conjugate acids have higher pKa’s (stronger bases have weaker acids) generally produce more E2 relative to SN2. The o examples that we will emphasize at 2 RX centers are carboxlyates (SN2 > E2) vs hydroxide and alkoxides (E2 > SN2), and cyanide (SN2 > E2) vs terminal acetylides (E2 > SN2), azide (SN2 > E2) vs dialkylamides (E2 > SN2) and metal hydrides (SN2 > E2) vs simple hydride (E2 > SN2). Higher basicity and steric hindrance in either RX or the electron pair donor also favors E2 > SN2. The following examples show similar looking base/nucleophiles (used in our course) that react differently with 2oRX o o o structures. (They all react by SN2 at methyl and 1 RX and they all react by E2 at 3 RX.) It is the reactions at 2 RX centers that are ambiguous. 2o RX structures are the most ambiguous. More basic, so E2 > SN2. Less basic, so SN2 > E2. More basic, so E2 > SN2. Less basic, so SN2 > E2. O N C R C C C H O R O R O cyanide terminal acetylides carboxylates hydroxide and alkoxides pKa of conjugate acid = 9 pKa of conjugate acid = 25 pKa of conjugate acid = 5 pKa of conjugate acid = 16-19 More basic, so E2 > SN2. Less basic, so SN2 > E2. More basic, so E2 > SN2. Less basic, so SN2 > E2. H Na H Li H Na N Na H B H N H Al H N H K N R R Na H H hydrides sodium borohydride pK of conjugate acid = 37 azide dialkyl amides lithium aluminum hydride a pKa of conjugate acid = 5 pKa of conjugate acid = 37 pKa of conjugate acid = ? y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, SN-E, C=O, epoxides chem, with mechs.doc Typical First Year Organic Reactions Beauchamp 6 1. Making RBr from alkane and alkene hydrocarbons and alcohols 3 3 a. RBr from alkanes - mechanism using Br2 / h for free radical substitution of alkane sp C-H bonds to form sp C-Br bonds at the weakest C-H bond. overall reaction Br H2 h C Br Br H Br CH H3C CH3 H3C CH3 1. initiation h Br Br Br Br H = 46 kcal/mole weakest bond ruptures first 2a propagation H BE = +95 kcal/mole H H BE = -88 kcal/mole Br C H Br C H = +7 kcal/mole H3C CH3 H3C CH3 (overall) H = -15 2b propagation both steps H Br BE = +46 kcal/mole H BE = -68 kcal/mole Br Br C Br C H = -22 kcal/mole H C CH H3C CH3 3 3 (overall) 3. termination = combination of two free radicals - relatively rare because free radicals are at low concentrations H H Br C Br C H = -68 kcal/mole H3C CH3 H3C CH3 CH3 H H H C CH H = -80 kcal/mole C C 3 CH CH3 H3C CH3 H3C CH3 very minor product CH3 Example reactions Br2 / h CH4 H C Br 3 achiral Br2 / h Br achiral Br Br / h enantiomers 2 (R and S) Br / h Br 2 achiral Br Br 2 Br / h can do E2 twice, 2 to make alkynes y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, SN-E, C=O, epoxides chem, with mechs.doc Typical First Year Organic Reactions Beauchamp 7 Br Br2 / h allylic substitution b. RBr from alkenes (anti-Markovnikov addition of HBr using free radical chemistry): mechanism using HBr / ROOR / h for free radical addition to alkane pi bonds (anti-Markovnikov addition = Br adds to less substituted position to form most stable free radical intermediate, and then H adds to more substituted position) HBr H2 overall reaction H C Br C R2O2 (cat.) H C C H C h 3 3 CH2 H2 1. initiation (two steps) R O R O H = 40 kcal/mole O R h O R (cat.) R BE = +88 kcal/mole BE = -111 kcal/mole O H Br R H Br O reagent H = -23 kcal/mole 2a propagation H H C BE = +63 kcal/mole Br BE = -68 kcal/mole H3C CH2 C Br H3C C H = -5 kcal/mole H2 2b propagation H H2 BE = +88 kcal/mole H = -15 C Br BE = -98 kcal/mole C Br H Br Br H3C C both steps H3C C H2 (2a + 2b) H2 H = -10 kcal/mole 3.
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