CHE 275: Exam 3 Study Guide
CHE 275: Exam 3 Study Guide CHE 275
Popular in Organic Chemistry
Popular in Chemistry
This 10 page Study Guide was uploaded by Emily.nicole on Sunday October 18, 2015. The Study Guide belongs to CHE 275 at Syracuse University taught by David Clarck in Fall2014. Since its upload, it has received 129 views. For similar materials see Organic Chemistry in Chemistry at Syracuse University.
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Date Created: 10/18/15
Exam 3 Orgo Review Sheet Chapter 7 Stereochemistry o Stereoisomers isomers with same constitution different spatial arrangement 0 Chiral has mirror image not superimposable on mirror image 0 Usually occurs when C attached to 4 different groups 0 Molecule with single chirality center must be chiral molecule with multiple chirality centers may or may not be chiral O Enantiomer an object and its nonsuperimposable mirror image 2 chiral objects In a chiral atom exchange of 2 atoms groups converts structure to an enantiomer exchange of 3 returns to original structure w diff orientation Equal and opposite rotations Properties physical properties for both enantiomers are same differences when it comes to properties based on arrangement of atoms in space 0 May fit different receptors in body 0 Chiral Recognition chiral receptor or reagent interacts selectively with one enantiomer of chiral molecule 0 Chirality Center tetrahedral carbon 4 diff atoms or groups Aka asymmetric center carbon stereogenic center stereocenter Cs in doubletriple bonds cannot be chirality centers C in a ring can be one if 1 it bears two diff groups and 2 Path traced around the ring from that C in one direction is diff from that traced in other direction sequence of groups is diff Isotopes count as diff substituents o Achiral has mirror image superimposable O A molecule is achiral if it has a plane of symmetry center of symmetry Plane of Symmetry bisects molecule divides molecule into 2 mirror image halves Center of Symmetrypoint in center of molecule any line extended from it an equal distance in opposite direction will encounter an identical element Molecule lacking both center plane of symmetry likely to be chiral 0 Optical Activity ability of a CHIRAL substance to rotate plane of light 0 Light used to measure this consists of single wavelength is plane polarized o Unpolarized light many beams vibrating in diff planes can be transformed into planepolarized light beams vibrate in same plane by passing it through polarizing filter 0 Light shone trough sample of interest it is optically active if it rotates light 0 Direction and magnitude of rotation measured by second polarizing filter Observed rotationquot designated by alpha signal 9 To be optically active the sample must contain a chiral substance and one enantiomer must be present in excess of the other 1 Enantiomers rotate light same magnitude but opposite direction therefore solution with equal amount of enantiomers exhibits no net rotation Racemic mixtures mixtures containing equal quantities of enantiomers g are optically inactive When one enantiomer present in excess rotation is observed 0 Optically Pure Enatiomeric Excess substance containing exclusively one single enatiomer Optical Purity of major enantiomer of minor enatiomer O Optically inactive substances don t rotate light either achiral or racemic mixture 0 Rotation of plane polarized light in clockwise direction positive counterclockwise neg Exam 3 Orgo Review Sheet Sign used to distinguish between 2 enantiomers in nomenclature 0 Observed Rotation a depends on of molecules encountered by light Affected by length of tube and concentration of solution These discrepancies accounted for in Specific Rotation a 100a CL C concentration of sample L length of tube Specific rotation is a physical property of a substance like bp density Optical purity can be calculated from specific rotation OP specific rotation of sample specific rotation of pure enantiomer X100 0 Absolute Configuration exact 3D arrangement of substituents at chirality center 0 Cahn Ingold RS Notational System sequence rules used to determine absolute configuration at a chirality center 0 Rules 1 Rank substituents at chirality center according to rules from before atomic 2 Orient molecule so lowest ranked atom group points away from you 3 Draw 3 highest ranked substituents as they appear to you when molecule is oriented with lowest group pointing away 4 If order of decreasing precedence is clockwise R if counterclockwise S Rings treated in analogous way point lowest priority group away determine clockwise or counterclockwise 0 Relative Configuration configurations of components relative to one another 0 When no bonds are made or broken at the chirality center in a rxn their relative positions in space are the same 0 Two different compounds with same sign of rotation need not have same configuration 0 Fisher Projections Represent 3D arrangement of molecules 0 Vertical bonds at chirality center are directed away from you 0 Horizontal bonds towards you O Chirality center lies at center of cross but is not explicitly shown 0 Customary to orient molecule so that C chain is vertical w lowest ed C at top 0 Switching the positions of 2 groups in a F projection reverses configuration of chirality center 0 Chirality Axis an axis about which a set of atoms groups is arranged so that the spacial arrangement is not superimposable O Molecules don t need a chirality center to be chiral can have an axis instead 0 Biaryls 2 aromatic rings joined by single bond Rotation about bond reduces steric strain Rotation slowed when large groups are added to ring instead of Hs Can slow rotation so much that two rings are isolated as two forms Makes them enantiomers single bond between them is chirality axis Antropisomers Structures which are related by rotation around single bond yet are capable of independent existence 0 Reactions that create a chirality center 0 In general Optically inactive starting materials cannot give optically active products unless at least one optically inactive reactant or catalyst is present If reactants are achiral formation of one enantiomer is just as likely as the other racemic mixture results optically inactive 0 If all components of starting state are achiral any chiral product will be formed as a racemic mixture 0 If reactant is racemic products will also be racemic O Prochiral addition to either face coverts an achiral reactant to chiral product Exam 3 Orgo Review Sheet Prochirality center created when H replaced by another molecule 0 Enantiotopic product from rxn at one face is the enantiomer of product from rxn at other face Achiral molecule reacts at same rate at each enantiotopic faces gives equal amounts of enantiomers racemic 0 Addition of double bonds substitution rxns typically convert achiral molecules to chiral 0 When reactant is chiral but optically inactive b c it is racemic any products derived from rxn w optically inactive reagents will be optically inactive o CHIRAL molecules with 2 chirality centers 0 First C may be R or S Second C may be R or S O Yields 4 stereoisomers 2R 3R 2S3S 2R 3S 2S 3R 0 1 and 2 are enantiomers 3 and 4 are enantiomers o 1 is not mirror image of 3 and 4 therefore it is a diastereomer Diastereomer stereoisomers that ARENT MIRROR IMAGES CIS AND TRANS ISOMERS OF A PARTICULAR COMPOUND ARE DIASTEREOMERS OF EACH OTHER 0 To convert molecule with 2 chirality centers to an enantiomer configuration at both centers must be changed diastereomer results from change at only one center 0 In a ring situation is the same 4 stereoisomers formed 0 In Fischer projections molecules with 2 chirality centers arranged in an eclipsed formation Staggered conformations do not have correct orientation of bonds for F Projections Stereochemical prefixes used to specify relative configuration in molecules with 2 chirality centers these apply when C chain is vertical 0 Erythro Disastereomer like substituents on same side of F projection O Threo Diastereomer like substituents on opposite sides of F projection 0 Relative conformation used in same way in molecules w multiple chirality centers 0 2 erythrostereoisomers possess same relative conformation differ from that of a threostereoisomer o ACHIRAL molecules with 2 chirality centers It is possible for a molecule to have chirality centers yet be achiral Yields 3 stereoisomers 2 are enantiomers 3rd gives achiral structure structure and its mirror image are superimposable recognize achirality in that the eclipsed formation has plane of symmetry perpendicular to C2 C3 bond Meso Form the 3rd stereoisomer achiral molecule that has chirality centers has plane center of symmetry line drawn down center of F projection bisects it into 2 mirror image halves 0 Same in rings 3 stereoisomeric compounds 2 are enantiomers 1 is diastereomer meso form w plane of symmetry 0 Molecules with multiple chirality centers 0 Max of stereoisomers 2quotn where n of chirality centers and cistrans double bonds 0 When two or more of a molecules chirality centers are equivalently substituted meso forms are possible of stereoisomers is less than 2quotn if meso forms are possible 0 Remember any object can have only one mirror image Therefore ex of all the stereoisomers of cholic acid one is cholic acid one is its enatiomer and the rest are diastereomers 00000 O Exam 3 Orgo Review Sheet 0 A molecule that contains both chirality centeres and double bonds has more opportunities for stereoisomers chirality center can be R or S double bond can be E or Z c Rxns that produce Diastereomers O 2 factors determine which stereoisomers are actually formed in rxn 1 The stereochemistry of alkene cis or trans E or Z 2 The stereochemistry of mechanism syn or anti 0 Stereospecific Rxn rxn in which stereoisomeric starting materials yield products that are stereoisomers of each other 0 More connected to features of mechanism 0 Terms like syn anti additionquot 0 Optically inactive starting materials yield optically inactive products racemic mixture or meso structure 0 A rxn that introduces a second chirality center into a starting material that ALREADY HAS ONE does not need to produce equal quantities of 2 possible diastereomers this is key concept of rxns that produce diastereomers Less hindered face of double bond side opposite to methyl group etc leads to faster rate of formation of cis or trans stereoisomer 2 faces of double bond are prochiral but NOT enantiotopic Diastereotopic name given to prochiral faces of this kind Stereoselective rxn stereoisomeric products formed in unequal amounts from single starting material 0 More connected to structural effects of reactant 0 Resolution of enantiomers 0 Resolution separation of a racemic mixture into its enatiomeric components 0 Usually involves temporarily converting enantiomers of racemic mixture to diastereomers separating them usually by recrystallization then regenerating the enatiomeric starting materials 0 Diastereomers have different physical properties and so can be separated 0 Kinetic resolution depends on different rates of rxn of 2 enatiomers with chiral reagent Enzymatic resolution form of Kinetic res that uses enzymes as chiral catalysts to SELECTIVELY bring about rxn of ONE enatiomer o Stereoregular Polymers 0 Defined by distinct structures relative to carbons bearing functional groups 0 2 types of stereoregular polymers Isotactic all methyl groups oriented in same direction Syndiotactic methyl groups altering front and back along C chain Both prepared by coordination polymerization under ZieglerNatta conditions 0 Atactic random orientation of methyl groups Formed by free radical polymerization o Chirality centers other than C 0 Silicon has tetrahedral arrangement 0 Trigonal pyramidal molecules are chiral if central atoms bears 3 diff groups Nitrogen can be chiral but enantiomers equilibrate too rapidly to be resolved Phosphorus and Sulfur can be resolved Chapter 8 Nucleophilic Substitution Exam 3 Orgo Review Sheet In this chapter Lewis bases react with alkyl halidessubstratewhen L base acts as a nucleophile o lewis base donates electron pair nucleophile electron rich species has open e pairs Conversion of alkyl halides to other classes of organic molecules by n substitution 0 Choosing experimental conditions best suited to carrying out a particular functional group transformation Functional group transformations by N Substitution 0 Halogen acts as leaving group on carbon lost as an anion o Heterolytic bond breakage of polar CHalogen bond 2 e in bond retained by halogen 0 Most frequently encountered nucleophiles are anions Used as lithium sodium or potassium salts Anionic portion of salt substitutes for halogen of alkyl halide Metal cation becomes Li Na or K halide 0 Carbon attached to halide is sp3 hybridized o Solvents chosen to dissolve both alkyl halide and ionic salt Use of mixed solvents like ethanol to accommodate properties of both elements DMSO dimethyl sulfoxide and NN dimethylformamide DMF make good solvents for nucleophilic substitution rxns o Alkyl halides may be prepared from 1 alcohols by nucleophilic substitution 2 alkanes by free radical halogenation 3 alkenes by addition of hydrogen halides Alkyl halides then become available as starting materials by replacement of halide leaving group with nucleophile Relative reactivity rate of halide leaving groups 0 Alykyl iodides undergo n substitution the fastest alkyl uorides the slowest o Iodine has weakest bond to C therefore best leaving group 0 Leaving group ability related to basicity Weakly basic anions best leaving groups Fluoride most basic worst leaving group Rate at which rxn occurs is important but more than anything the nature of the substrate and nucleophile determine what product is formed SN2 mechanism of N Substitution 0 Primary alkyl halides always react this way secondary alkyl halides in presence of a good nucleophile in an aprotic solvent 0 Kinetics measures speed of rxn especially how concentration of reactants and catalysts affects rate 0 Because rate of N substitution depends on leaving group we know that CHalogen bond of alkyl halide breaks in slow step ratedetermining step of rxn Therefore rate depends on concentration of alkyl halide RateKCH3Br OH Rxn rate directly proportional to concentration of both 15t order in each reactant 2nd order overall bimolecular ratedetermining elementary step one step concerted process in which both alkyl halide and nucleophile are involved in transition state C partially bonded to both incoming nucleophile and departing halide in trans state Exam 3 Orgo Review Sheet 0 Stereochemistry Key featurenucleophile attacks C from side opposite bond of leaving group SN2 rxns proceed with Inversion of Configuration at the C that bears the leaving group 3D arrangement of bonds in product opposite to that of reactant Tetrahedral bond arrangement changes to inverted tetrahedral in product In general SN2 is stereospecific stereoisomeric starting materials give stereoisomeric products 0 Steric effects on SN2 rxn rates RATE GOVERNED BY STERIC EFFECTS Rate depends on substitution of alkyl halide primary secondary tertiary Crowding at C w leaving group slows rate Steric hinderance plays role in nucleophilic attack 0 Nucleophile must approach alkyl halide from side opposite bond to leaving group which is hindered by alkyl substituents Tertiary lt secondary lt primary lt methyl Alkyl groups on Cs adjacent to point of nucleophilic attack also decrease rate more chain branching slower the rate When C completely substituted with methyl groups unusual case occurs with primary alkyl halide where it is practically inert to SN2 b c of steric hinderance O Nucleophiles and Nucleophilicity Lewis bases are always nucleophiles Lewis base usually anions sometimes are neutral Many solvents in which N substitution carried out are themselves nucleophiles 0 Used in solvolysis rxns n substitution where nucleophile is solvent 0 Common solvents water methanol ethanol formic acetic acid 0 Solvolysis in water hydrolysis converts alkyl halide to alcohol analogous rxns take place in other solvents containing OH group Because attack by nucleophile is ratedetermining step of SN2 the rate of substitution varies depending on nucleophile some more reactive than others 0 Nucleophilicity nucleophile strength measure of how fast a lewis base displaces a leaving group 0 2 factors in uencing nucleophilicity basicity and solvation O The more basic the nucleophile the more reactive it is For basicity comparison of atoms in same row not column 0 Neutral lewis bases much weaker nucleophiles anionic nucleophile more reactive than neutral one 0 Degree of solvation surrounding of molecule by ion dipole forces Smaller anions high charge to size ratio solvated more strongly are less nucleophilic large neg ions less solvated To act as a nucleophile halide must shed some surrounding solvent molecules When measured in gas phase solvation forces don t exist order of halide nucleophilicity reverses F gt C1 gt Br gt I o SN1 mechanism of N Substitution O Tertiary alkyl halides always react this way secondary alkyl halides in presence of weak nucleophile solvolysis in a protic solvent Exam 3 Orgo Review Sheet Having just learned that tertiary alkyl halides are practically inert to substitution by SNZ this is when SN1 comes in most common examples seen in solvolysis rxns First order rate law Rate k CH33Br Rate depends only on concentration of tertiary alkyl halide NOT nucleophile therefore adding stronger more nucleophile doesn t affect rate Unimolecular ratedetermining step involves only 1 alkyl halide formation of carbocation intermediate SN1 is an Ionization mechanism nucleophile doesn t participate until after rd step Carbocation stability and rxn rate If reactants aren t very nucleophilic SN2 suppressed Reactivity rate opposite to SNZ methyl lt primary lt secondary lt tertiary Clearly steric crowding plays no role in SN1 Carbocation formation is rate determining Parallels carbocation stability more stable to carbocation the faster it is formed the more reactive the alkyl halide rxn 0 methyl primary carbocations too high in energy In general Tertiary alkyl halides always react by SN 1 Primary methyl by SNZ o Carbocation rearrangements sometimes occur In rate determining ionization of alkyl halide hydride shift converts secondary to tertiary carbocation When rearrangements occur that is taken as evidence for carbocation intermediates and therefore points to SN1 mechanism 0 Stereochemistry SNZs are stereospecific proceed with inversion of configuration In general N substitution that exhibits first order rate law aren t stereospecific Because steric hinderance doesn t play a role a nucleophile can approach from either face of carbocation so 11 rmixture of enantiomers racemic mixture should result Not always the case because when carbocation is formed it isn t completely free from leaving group Even though ionization is complete leaving group may not have diffused far from carbon to which it was attached which partially blocks attack of nucleophile on Ion Pair Depending on proximity of ion product could proceed with complete inversion of configuration or if leaving group diffuses away could form equal amounts of enantiomers Overall stereochemistry related to attack of nucleophile on ion pair vs separation of the ions 0 Resulting stereochemistry varies according to alkyl halides nucleophile and experimental conditions 0 Effect of solvent rate on N Substitution Solvent affects rate not products made Classified based on 2 criteria 1 proticaprotic 2polarnonpolar Protic have OH groups those most capable of hydrogen bonding interactions Aprotic lack OH groups all of their Hs bonded to carbon Polarity of solvent related to Dielectric Constant E O 0000 Measure of the ability of a material to moderate the force of attraction between oppositely charged particles Exam 3 Orgo Review Sheet Higher the dielectric constant the better the medium can support separated positively and negatively charged species Polar solvents with high dielectric constant Nonpolar solvents with low dielectric constant c Solvent effect on rate of SN2 O O O O SN2 rxn rates increase in Polar Aprotic solvents Polar solvents required needs to be polar enough to dissolve ionic salts to give high concentration of nucleophile which therefore increases rxn rate Polarity not as important as proticaprotic nature Aprotic solvents required lack OH groups so they do not solvate anions like protic solvents and therefore anions are more able to express their nucleophilic character 0 Solvent effects on rate of SN1 O O O SN1 rxn rates increase in Polar Protic solvents Polar solvents required high dielectic constant stabilizes the charge which lowers the activation energy therefore increases the rate Protic solvents required further stabilize transition state with hydrogen bonding o Substitution and Elimination as competing rxns O O O O O Alkyl halide and lewis base can react in both substitution or elimination rxns Substitution either SN1 or SN2 Elimination either E1 or E2 To determine which pathway use rxn of typical alkyl halide secondary with a typical lewis base alkoxide ion as reference point Main rxn is elimination by E2 substitution only occurs in special circumstances 2 most important factors in uencing pathway 1 Structure of alkyl halide 2 Basicity of anion Elimination preferred in presence of strong base when base is crowded Substitution special circumstances When crowding at carbon bearing leaving group decreases Primary alkyl halides react with alkoxide bases by SN2 over E2 Weak basicity of nucleophile less basic than hydroxide 0 Ex cyanide azide alkoxide carboxylate iodide ions anions of type RS Tertiary alkyl halides so sterically hindered that elimination is always favored with anionic nucleophiles Substitution only favored with tertiary ahs in solvolysis rxns Raising temp increases rate of both elimination and substitution Rate of elimination usually increases faster proportion of elim products increases Elimination can always be made to occur Strong bases especially bulky ones react even with primary alkyl halides by an E2 process at elevated temps Substitution more difficult to induce Unhindered substrate good weakly basic nucleophile lowest practical temp o Nucleophilic substitution of Alkyl Sulfonates O 0000 0 Can have leaving groups other than halogens Undergo N Substitution rxns analogous to those of alkyl halides Ability to undergo elimination and nucleophilic substitution Sulfonates used most frequently ptoluenesulfonates TsO aka tosylates or ROTs Tosylates prepared by rxn of alcohols with ptoluenesulfonyl chloride usually in presence of pyridine Very good leaving group even better than alkyl iodides in n substitution Exam 3 Orgo Review Sheet Posses stronger electron attracting ability and leaving group must accept e of bond broken in n substitution therefore good leaving group Make great substrates in SN2 rxns 0 Same correlation with basicity weaker the base better the leaving group O In general any species with pKa greater than 2 for conjugate acid cannot be leaving group in n substitution 0 SN2 rxns most favorable when more basic nucleophile displaces less basic L group 0 Because halides are poorer leaving groups than sulfonates alkyl ptol Can be converted to alkyl halides by SN2 rxns involving Cl Br or I as nucleophile O Advantage sulfonates have to alkyl halides they allow control of stereochemistry Their preparation from alcohols does not involve any bonds to C Configuration of sulfonate is exactly the same as alcohol it was prepared from o Mechanisms for n substitution are the same SN2 inervsion of configuration SN 1 predominant inversion racemization Rearrangements can occur 0 Subject to same limitations Strongly basic elimination weakly basic substitution 0 Chapter 9 Alkynes o Alkynes hydrocarbons containing a triple bond triple bond counts as a functional group 0 Non cyclic alkynes have formula CnH2n2 O Acetylene ethylene is simplest alkyne O Monosubstituted terminal alkynes triple bond at end of chain Most acidic of all hydrocarbons O Disubstituted alkynes internal triple bonds 0 Most distinctive aspect of acetylene terminal alkenes is their acidity 0 Sources of Alkynes 0 At very high temps most hydrocarbons even methane are converted to acetylene O Acetylene serves as stating material from which higher alkynes are prepared 0 Many natural alkynes are Fatty Acids carboxylic acids w unbranched chains of C o Nomenclature O Usual IUPAC names for hydrocarbons followed yne ending 0 If compounds contains both a double and triple bond chain ed so as to give the first multiple bond the lowest number Ties are broken in favor of double bond en suffix precedes yne but1en3yne 0 If there is a triple bond in a substituent designated as ethynyl group 0 Physical properties of alkynes O Resemble alkanes and alkenes 0 Low density water solubility gt NONPOLAR 0 Structure and Bonding Sp hybridization 0 Linear geometry 0 High angle strain smallest cycloalkyne is cyclononyne O Bonds are shorter and stronger 0 Each C has two half filled sp orbitals available to form sigma bonds One sigma bond two pi bonds Characteristics related to s character fraction of hybrid orbital contributed by s orbital O Exam 3 Orgo Review Sheet Sp is one half s one half p sp3 is one quarter s An orbital with more s character will be closer to the nucleus held more strongly ASSOCIATE S CHARACTER WITH ELECTRONEGATIVITY As s character increases so does a carbons electronegativity Is responsible for alkynes high level of acidity compared to other hydrocarbons o Acidity of Acetylene and terminal alkynes O O O O Alkanes alkenes and alkynes are all very weak acids in general Carbanion conjugate base of hydrocarbon anion with ned charge borne by C Because derived from a very weak acid is a VERY strong base Ionization of acetylene gives acetylide ion in which unshared e pair occupied orbital with 50 S character Though acetylene terminal alkynes are far stronger acids than other hydrocarbons they are still very weak acids overall much weaker than say water or alcohols b c acetylene is far weaker acid than H20 alcohols these substances aren t suitable solvent for rxns involving acetylide ions In compounds with OH not strong enough base to deprotonate acetylene Need to use a stronger base like sodium amide instead of sodium hydroxide In acid base reactions equilibrium lies to the side of the weaker acid Anions of acetylene term alkynes are nucleophilic and react with methyl primary alkyl halides to form CC bonds by nucleophilic substitution 0 Separate sheet with all the formulas mechanisms Chapter 9 Supplement 0 Unsaturation structural element that decreases of hydrogens in molecule by 2 Hydrocarbon Degree of Unsaturation Molecular Formula Saturated 0 CnH2n2 Cycloalkane 1 CnH2n Alkene 1 CnH2n Alkyne 2 CnH2n2 0 Multiple bonds rings are elements of unsaturation To calculate find number of hydrogens as if it were saturated subtract the actual number of hydrogens divide by 2 o Heteroatoms halogens replace hydrogen atoms 0 When calculating unsaturations count halides as hydrogen atoms 0 Oxygen doesn t change CH ratio so ignore it in formula 0 Nitrogen acts as half a Carbon add number of nitrogens when calculating unsaturations C2H N1H
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