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Date Created: 11/16/15
11162015 Organic Chem I Study Guide Ch 6 9 I Ch 6 An Overview of Organic Rxns 0 Kinds of Organic Rxns 4 General Types I Addition Rxn 2 molecules combine to create one product H H 1quot 139 fccx GlGl H c c H H H 1 1 I Elimination Rxn 1 molecule splits into 2 Opposite of Addition Rxns H H H H I I H04 I HcccH Icc H20 H H H H I Substitution Rxn 2 reactants exchange parts H H l 1111 I Hquot E ctci Huietc H et if H 439quot IEll H H I Rearrangement Rxn Same formula but different connections Constitutional Isomers Hi HJ EH x a HI D 393 5 Hi 0 How Orgo Rxns Occur The Mechanisms I The mechanism describes the steps that occur during an organic rxn I The steps involve either of the 2 processes 1 Bonds Forming o Symmetrical radical Each reactant donates an e39 reactants are radicals o Unsymmetrical polar One reactant donates both e39 reactants are ions 2 Bond Breaking o Symmetrical radical Each product gets an e39 products are radicals o Unsymmetrical polar One product gets both e39 products are ions 3 Note Use fish hook arrows for radical rxns and regular arrows for polar rxns o The 3 Steps of Radical Rxns 1 Initiation One reactant splits into two radicals 2 Propagation A radical combines with a molecule to generate another radical 3 Termination Two radicals combine to end the chain rxn and generate product Initiation 0 Polar Rxns Partial neg amp pos charges Cl CI 3 C 0 Table 61 Polarity Patterns 0 Polar bonds can be made more polar by rxn Propagation with acids amp bases C CH4 gt HCI ocn o Nonpolar bonds can be made polarizable by T interaction w solvent or other polar I molecules 0 Cquot Cl I39I CquotC39 Cquot o Electrophile Lewis Acid e39 poor accepts e39 pair empty orbital positively charged Termination positively polarized central atom CI CH H 2C Cl 0 Nucleophile Lewis Base e39 rich donates e39 1110 TH gt 1C C111 pair lone pair e39 negatively charged negatively polarized central atom 0 Arrow goes from nucleophile to electrophile EX of Polar Rxn Addition of HBr to Ethene C2H4 The Mechanism Step i Pruittttatiuii nut attteue Step 3 aidttiti39un tit t in carbonation i i Former 39i39it E THEE 7 1 y E wk mi 32 4 i BEEF HEr Eatbttttatitm Note When using curved arrows the octet rule must be followed Describing a Rxn Equilibrium Rates and Energy Changes 0 Equilibrium Constant Keq ProductsyReactantsX a Keq lt l reactants eXist at equilibrium Keq gt 1 products eXist at equilibrium b Keq is controlled by Gibbs free energy of the products and reactants 0 Gibbs Free Energy AG energy released on the favored side of the equilibrium rxn Gprod Greac a AG lt O Keq gt1 energy is released to surroundings spontaneous exergonic b AG gt O Keq ltl energy is absorbed from surroundings nonspontaneous endergonic 0 Entropy AS change in molecular randomness in a rxn a AS lt O randomness decreases AS gt O randomness increases E g Br2 gt Br Br39 AS increases 0 Heat of Reaction AH difference in strength between bonds broken and bonds formed BEreac BEprod BE is Bond Dissociation Energy Energy needed to break bond amp form two radicals AH lt 0 energy is released to the surroundings exothermic AH gt 0 energy is absorbed from surroundings endothermic 0 AG O Keq l rxn at equilibrium AGO AHO TASO under STP 25 C 298 K amp 1 atm 0 Activation Energy Ea Difference between energy of reactants amp energy of transition state Higher the Ea slower the rxn and vice versa transition state highest point in a rxn step 0 Intermediates occur in between steps such as carbocation always in the valley 0 Enzymes proteins that catalyze speed up rxns by either providing another pathway or lowering Ea Ch 7 Alkenes Structure and Reactivity O O Calculating Degree of Unsaturation I 1 degree of unsaturation for every double bond ring amp 2 hydrogens lost I Saturated Compound CnH2n2 Unsaturated Compound CnHzn I Steps Find unsaturated compound CnH2n2 Subtract the compounds to find of hydrogens lost CnH2n2 CnH2n Determine the degree of unsaturation H s lost2 I Examples 76H 4 saturated alkane Saturated 06 should have 14 lydrogens from the formula H 2n2 H 262 l4 Benzene has 6 C6 l6 Benzene So it quot1033quot 8 Ilydrogens DBEampQ4 I For every halogen add a hydrogen to the unsaturated formula same steps as above I Oxygen doesn t affect the of hydrogens in the unsaturated formula I For every nitrogen subtract a hydrogen from the unsaturated formula same steps above Naming Alkenes ene I Number the longest chainring with the most double bonds giving the substituents amp double bonds the lowest possible I Start numbering from the end closest to a substituent rings have cyclo CisTrans Isomerism Only for Disubstituted Alkenes EZ Stereochemical Nomenclature Disubstituted Trisubstitutted and Tetrasubstituted Alkenes I Assign atomic priorities Higher on opposite sides E Higher on same side Z Stability of Alkenes I Transalkenes are more stable than cisalkenes I Tetrasubstituted gt Tri gt Digt Mono caused by hyperconjugation sp2sp3 interactions I Tertiary Carbocation gt Secondary gt Primary Vinyl gt Methyl inductive effect Energy Path of Electrophilic Addition of HBr to 2methylpropene Exothermic Rxn Electrophilic addition for preparing alkyl halides The reaction is successful with HCl HI and HBr gun I Orientation of the Electrophilic Addition 39 Lg 1 The rxn shown is regioselective because the carbon M m 401 e on the left in 2methylpropene is more substituted then the other 3 I 2 And so according to Markovnikov s Rule Br39 out 5H quotav 1 39 attaches to the more substituted carbon on the left A X 3 If both carbons have the same of substituents 1 739 t there is no regioselectivity WC 539 4 If one carbon has two identical substituents in regioselectivity doesn t apply 5 Regioselectivity depends on the of substituents on each carbon not the size 6 Inductive Effect The shifting of e39 in a sigma bond in response to electronegativity of nearby atoms 0 Hammond Postulate The structure of the transition state is closest to the nearest stable species 0 The transition state resembles the products in an endothermic rxn and resembles the reactants in an exothermic rxn ili1m m starting products starting pruduntszg Transitiun states Endnthermin quotreactinn Prnducts Exnthermin fw reactiun products EE i l i Enondin rr 0 Rearrangements of carbocations 12 H shift 12 alkyl shift and ring expansion 0 These carbocation shifts generate greater stability 0 Ring Expansion Only works for 345 cyclic rings Cyclohexane is the most stable ring 0 You can also condense cyclic shapes with more than 6 membranes into the cyclohexane structure H H3C2K3CH3 6 H3 Filing Expansi n Example HQ HQ H A 39 H33 EH3 I gp dy ih H rearrangemeal ring E39Kpa f j 12H shift H3C 6 CH gt H3 3 12meth lshift H 3 H3 39 Ch 8 Alkenes Reactions amp Synthesis 0 Preparation of Alkenes Elimination 1 Dehydrohalogenation occurs With a strong base e g KOH in an organic solvent like ethanol 2 Dehydration occurs With an aqueous strong acid e g H2S04 in an organic solvent like THF solvent 0 The Diverse Rxns of Alkenes Addition 1 Halide 2 1 2 Dihalide 3 Halohydrin 4 Alcohol 5 Alkane 6 Epoxide 7 12 Diol 8 Carbonyl Compounds 9 Cyclopropane 10 Polymers 1 Halide Electrophilic Addition of HX Simple rxn with HX in which 1 Nucleophilic double bond bonds to electrophilic hydrogen creating a carbocation and halogen anion 2 Most stable carbocation forms through ring expansion amp 12methylhydride shifts 3 X adds to carbocation 2 12 Dihalide Halogenation Addition of X2 1 X2 adds to alkene thru antiaddition via the halonium ion to create the trans product Found in Sec 82 3 Halohydrin Electrophilic Addition of H0X 2 Possible Conditions Forms When there is an excess of H20 1 Simple addition of X2 in the presence of H20 to create antiaddition trans product via the halonium ion 2 Addition of X2 by using NBSN CS and an organic solvent like DMSO in water 0 Antiaddition trans product Halonium intermediate See diagram in Sec 83 4 Alcohol Hydration Addition of H20 3 Possible Conditions 1 AcidCatalyzed Rxn Water is added to alkene in the presence of a strong acid at a high temperature 0 Mechanism 1 Double bond attacks H atom on the acid to create carbocation 2 Newly formed H20 adds to carbocation 3 H20 is deprotonated 0 Used mostly in the industry 2 0xymercuration Hg OAc2 H20T HF ampNaBH4 Forms Markovnikov Product 0 Mechanism 1 Hg 0Ac2 is added to give the mercurinium ion similar to halonium ion 2 Water is then added and deprotonated 3 Mercury is removed by NaBH4 Which replaces mercury With an H creating the alcohol 0 Used mostly in the lab 3 Hydroboration Involves BH3 THF H202 amp 0H39 Forms AntiMarkovnikov Product 0 Mechanism 1 BH3 splits into BH2 amp H 2 They both add to the same side of the double bond syn addition With BH2 adding to less substituted carbon to avoid steric strain 3 H202 removes the BH2 replacing it With 0H39 0 Used mostly in lab 5 Alkane Hydrogenation Reduction Addition of H2 0ccurs With a metal catalyst such as palladium PdC or platinum Pt02 0 Reduction Carbon gains electron density by forming a CH bond or breaking a CXhalogen0N bond 0 Hydrogen absorbs onto the catalyst and adds to the alkene thru synaddition 0 H2 only bonds to CC double bonds 6 Epoxide Epoxidation Oxidation Addition of Oxygen 2 Possible Conditions 0 Oxidation Carbon loses electron density by breaking a CH bond or forming a CXON bond 0 Condition 1 Peroxy acid e g MCPBA gt Oxygen atom farthest from the carbonyl group in peroxy acid is transferred to the alkene to produce epoxide 0 Condition 2 Halohydrin gt When halohydrin reacts With a base HX is eliminated producing an epoxide 0 Coupled With Hydroxylation Epoxide has synstereochemistry 7 12 Diol Hydroxylation Oxidation Addition of Water 2 Possible Conditions 0 Condition 1 AcidCatalyzed 9Acid catalyzed ringopening reaction occurs WWater to produce trans 12diol o Mechanism 1 The oxygen in the epoxide ring is protonated by H3O to produce a carbocation intermediate 2 Addition of H20 opens the epoxide ring deprotonation creates the alcohol 0 Condition 2 Alkene reacts With osmium tetroxide to produce cis12diol o Mechanism 1 OsO4 adds to alkene With syn stereochemistry to produce cyclic osmate intermediate 2 The OsO4 is then cleaved by NaHSO3 to produce alcohol 8 Carbonyl Compounds Cleavage 3 Possible Conditions 0 Condition 1 Ozonoloysis 9 Most useful double bond cleavage reagent 1 Ozone O3 adds to the CC double bond at a low temperature to give molozonide intermediate Which rearranges to ozonide 2 Zn in acetic acid cleaves the ozonide 0 Forms only ketones amp aldehydes Tetrasubstituted alkene gt two ketones Tri gt ketone amp aldehyde and so on 0 Condition 2 Permanganate 9 NeutralAcidic Soln Not often used 0 Creates C02 amp carboxylic acids if H s are on double bond 2 H s on one double bond carbon gt C02 1 H on a double bond carbon gt carboxylic acid 0 Condition 3 Periodic Acid 9 H104 cleaves cis12diol to create carbonyl compounds 0 OH groups are in an open chain 2 carbonyl compounds 0 OH groups are on a ring Single open chain dicarbonyl compound 9 Cyclopropane Addition of Carbenes 2 Possible Conditions 0 Condition 1 Chloroform 9 CHCl3 is deprotonated by water creating the trichloromethanide anion intermediate Which loses a C139 to create the dichlorocarbene C12C the dichlorocarbene then adds on to an alkene Stereochemistry Produces one stereoisomer 0 Condition 2 Diiodomethane Simmons Smith RxnBest Method 9 o Carbenoid intermediate e g iodomethyl zinc iodide metalcomplexed reagent 0 Transfers CH2 to double bond to create cyclopropane 10 Polymers Chain Reaction Addition of Radicals 3 Steps Initiation Propagation Termination 0 Polymer Molecule made up of repeating monomer units 0 Initiated When a few radicals are made by heating a benzoyl peroxide 0 Polymerization occurs When the carbon radical adds to another ethylene molecule to produce a radical 0 Combination of 2 radicals terminates the chain rxn 0 Industrial synthesis of alkene polymers e g Polyethylene used in packaging amp bottles is made from ethylene Ch 9 Alkynes An Introduction to Organic Synthesis 0 Naming Alkynes Similar to Alkanes amp Alkenes I Name the longest continuous chain containing the most triple bonds I Give triple bonds amp substituents the lowest Parent name ends in yne I Multiple substituents Number from side closest to substituent If both sides are the same distance to give alphabetical order of substituents I Double bonds amp Triple bonds They have the same priority but if they re equal distance from the end double bond first Add prefix if there are several doubletriple bonds 0 Preparation of Alkynes Elimination Rxns of Dihalides I Halogenation of Alkenes 9 12Dihalide Dehydrohalogenation 2x yields Alkyne 0 Reactions of Alkynes Addition 1 Addition of HX amp X2 2 Hydration of Alkynes 3 Reduction of Alkynes 4 Oxidative cleavage of Alkynes 5 Alkyne Acidity Formation of Acetylide Anions 6 Alkylation of Acetylide Anions 1 Addition of HX amp X2 Very Similar to the Electrophilic Addition amp Halogenation equivalent for Alkenes o 1 eq HXX2 gt halogenated alkene 2 eq HXX2 gt halogenated alkane 0 Usually transstereochemistry is observed Markovnikov Product 0 Addition of HX to alkynes produces the vinylic carbocation 2 Hydration of Alkynes 2 Possible Conditions 0 Mercury II Catalyzed Rxn HgSO4H2O H2SO4 Markovnikov Product 0 Enol Intermediate gt Ketone Product Tautomerism o Mechanism 1 Alkyne bond attacks Hg2 creating the vinylic mercury carbocation 2 Water adds to the carbocation and is deprotonated 3 Mercury is replaced by an acidic H to create enol Enol is converted to ketone 0 Internal Alkyne produces racemic mixture of ketones if unsymmetrically substituted 0 Terminal Alkyne produces methyl ketone o Hydroboration Oxidation BH3 THF H2O H202 NaOH AntiMarkovnikov Product 0 Vinylic Borane Intermediate 0 Internal alkyne produces ketone Terminal alkyne produces an aldehyde 0 Alkynes are too acidic to undergo acidcatalyzed hydration 3 Reduction of Alkynes 3 Possible Conditions 0 PdC Catalyst o 2 eq H2 yields the saturated alkane o Lindlar Catalyst 0 1 eq H2 yields the cisalkene 0 NaLi amp NH3 0 1 eq H2 yields the transalkene o Alkyne hydrogenation is used in Commercial Synthesis of Vitamin A 4 Oxidative Cleavage of Alkynes 2 Possible Conditions gt KMnO4 amp O3 0 When cleaving alkynes BOTH KMnO4 amp 03 can create carboxylic acids and carbon dioxide 0 Internal alkyne produces only carboxylic acids 0 Terminal Alykne produces a carboxylic acid and a carbon dioxide 0 Don t worry about this because alkyne cleavage is very rare 5 Formation of Acetylide Anions o Acetylide anions are more stable than alkyl or vinylic anions because of the greater s character 0 This is Why several alkynes including acetylene behave as weak acids and lose their H to become an acetylide anion 0 Acetylide anion acts as a base and is strongly nucleophilic 0 Coupled Walkyl halide to produce alkylated alkyne 6 Alkylation of Acetylide Anions 0 Substitution reaction SN2 because of strong acetylide nucleophile 0 Terminal alkyne is converted to internal alkyne through alkylation quotI V I R fEc H INHgNu gt R E Na 39H AIM C SOCilUHl amide ALGiyilCiC OHlOH Anuncx c 39u H H quot 71L391 7 It H H R HI gt R llt lt Nam rquot l H Acefylide onion 7 7 39 7 l Alk I r i Hlt Primary alkyl bulich ya x a ym 0 Increasing Stability of Carbocation 99 Methyl lt Primary Alkyl Secondary Vinylic Benzylic ArylAromatic lt Secondary lt Tertiary lt Allylic
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