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Modern Organic Chemistry II

by: Marilou Hyatt Jr.

Modern Organic Chemistry II CHEM 343

Marketplace > Colorado State University > Chemistry > CHEM 343 > Modern Organic Chemistry II
Marilou Hyatt Jr.
GPA 3.81

Yian Shi

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Yian Shi
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This 43 page Class Notes was uploaded by Marilou Hyatt Jr. on Tuesday September 22, 2015. The Class Notes belongs to CHEM 343 at Colorado State University taught by Yian Shi in Fall. Since its upload, it has received 13 views. For similar materials see /class/210347/chem-343-colorado-state-university in Chemistry at Colorado State University.

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Date Created: 09/22/15
15 February Reductive amination the reaction ofan aldehyde or a ketone with ammonia or with an amine in the presence of a reducing agent Addition of oxygen nucleophiles Addition of water Hydrate a molecule with two OH groups on the same carbon gemdiols The extent to which an aldehyde or a ketone is hydrated in an aqueous solution depends on the aldehyde or ketone Addition of alcohol Hemiacetal hemiketal the product of the addition of one equivalent of an alcohol to an aldehyde ketone Acetal ketal the product formed when a second equivalent of an alcohol is added Acetal or ketal formation requires an acid catalyst The reaction is reversible Protecting group Protects a functional group from a synthetic operation that it would otherwise not survive Addition of sulfur nucleophiles The CS bond of thioacetal or thioketal can be converted into CH bonds using H2 and Raney Ni The Wittig reaction a reaction in which an aldehyde or ketone reacts with a phosphoniumylide to form an alkene Ylide a compound that has opposite charges on adjacent covalently bonded atoms with complete octets E isomers dominate with stabilized ylides and Z isomers dominate with unstabilized ylides 27 February KetoEnolTautomerism Tautomers two constitutional isomers differing in the location of a double bond and a hydrogen For most ketones the enol tautomer is much less stable than the keto tautomer The fraction of enol tautomer is considerably greater for a Bdiketone because its enol tautomer is stabilized by hydrogen bonding and by conjugation of the carboncarbon double bond with the second carbonyl group The interconvesion of the tautomers can be catalyze by either acids or bases Basecatalyzed ketoenolinterconversion The first step is removal of anot proton and the second step is protonation of the oxygen Acidcatalyzed ketoenolinterconversion The first step is protonation of the oxygen and the second step is removal of anot proton Reactivity Considerations Ambident 39 39 quot a 39 39 quot with two 39 I 39 quot39 sites Which nucleophile site will react with an electrophile depends on the electrophile and on the reaction conditions Protonation occurs preferentially on oxygen For other electrophiles carbon is more likely to be the nucleophile 27 January Ch 16 Reactions of Substituted Benzenes Nomenclature of Disubstituted Benzenes If the two substituents are different they are listed in alphabetical order Nomenclature of Polysubstituted Benzenes Numbered so lowest possible numbers used Listed in alphabetical order with appropriate numbers Substituents can either increase or decrease the reactivity of benzene Inductive electron donation through 0 bond 4 reactivity Inductive electron withdrawal thorough 0 bond J reactivity Resonance electron donation and withdrawal Substituent for example NHZ OH OR Cl has pair of nonbonded e on atom directly attached to benzene ring delocalized into ring through porbital overlap Atom double or triply bonded to more electronegative atom for example CO CEN NOZ 03H n e withdrawn through the ring Strongly activating substituents ex OH NHZ OR Donation into ring by resonance greater than withdrawal by induction Moderately activating esters amides Weakly activating alkyl group benzene Weakly deactivating halogens Moderately deactivating carbonyl groups Withdraw inductively and by resonance Stroneg deactivating nitro groups Substituent orientation All activating substituents are orthopara directors Weakly deactivating halogen also orthopara All orthopara have lone pair on atom directly attached or alky aryl or CHCHR groups n n J L I u t 1 39 39 U only meta directors All meta have positive charge or partial positive charge on atom attached to ring The orthopara ratio More ortho product expected due to two ortho positions available one para Ortho sterically hindered Para not Para formed if substituent on ring or incoming electrophile large Activating groups allow reaction to proceed under less rigorous conditions FriedelCrafts reactions slowest no reaction if meta director 17 February Stereochemistry of nucleophilic addition reactions re and si faces Prochiral carbonyl carbon a carbonyl carbon bonded to two different substituents The two faces may be classified as re or si according to the sequence rule If the substituents viewed from a particular face appear clockwise in order of decreasing priority then that face is re if counterclockwise then si Nucleophilic addition to aBunsaturated aldehydes and ketones The reaction pattern direct addition vs conjugated addition depends on the nature of the nucleophile Nucleophiles that are strong bases such as RMgX H tend to form direct addition products Nucleophiles that are relatively weak bases such as cyanide ions amines thiols and halide ions usually form conjugate addition products Direct addition occurs more rapidly than conjugate addition therefore the direct addition product is formed first When nucleophile is a strong base the direct addition is irreversible and the resulting product will be the final product of the reaction When nucleophile is a relatively weak base the direct addition is reversible and the initially formed direct addition product can collapse back to the starting material allowing conjugated addition to occur Since conjugate addition is irreversible the conjugate addition product will accumulate and eventually become the major product of the reaction 19 March Oxidation loss of electrons the molecule that loses electrons is oxidized Reduction gain of electrons the molecule that gains electrons is reduced Oxidation is always coupled with reduction Reduction increases the number of OH bonds or decreases the number of C0 ON or CX halogen bonds Oxidation decreases the number of OH bonds or increases the number of 00 ON or C X halogen bonds Reduction Reactions Reduction by addition of two H atoms catalytic hydrogenation Reduction by addition of an electron a proton an electron a proton Dissolving metal reduction metal donates an electron to the compound and ammonia donates a proton Sodium in liquid ammonia cannot reduce a carboncarbon double bond Reduction by addition of a hydride ion and a proton LiAlH4 is stronger reducing agent than Na BH4 only LiAlH4 is strong enough to reduce carboxylic acids esters and amides Oxidation of Alcohols Common oxidizing reagents Cr03 chromium trioxide HzCrO4 chromic acid NazCr207 sodium dichromate and KMnO4 potassium permanganate Swern oxidation Oxidation of Aldehydes and Ketones Aldehydes are generally easier to oxidize than primary alcohols Tollens reagent a test for an aldehyde functional group BaeyerVilliger oxidation Relative migration tendencies H gttertalkyl gt secalkyl phenyl gt primary alkyl gt methyl Oxidation of alkenes with peroxyacids The addition of oxygen to an alkene is a stereospecific reaction Hydroxylation of Alkenes Oxidative cleavage of 12diols Oxidative cleavage of alkenes ozonolysis 16 April Catalyst substance that increases the rate of a chemical reaction without itself being consumed or changed in the overall reaction A catalyst does not change the equilibrium constant it does not change the amount of product formed during the reaction It only changes the rate at which the product is formed A catalyst can decrease AG in one of three ways The catalyzed and uncatalyzed reactions can follow can similar mechanism with the catalyst providing a way to make the reactant less stable The catalyzed and uncatalyzed reactions can follow a similar mechanism with the catalyst providing a way to make the transition state more stable A catalyst can completely change the mechanism of the reaction providing an alternative pathway with a smaller AG than that of the uncatalyzed reaction Acid catalysis Acid catalyst increases the rate of a reaction by donating a proton to a reactant The acid increases the rates of both slow steps of the reaction A protonated carbonyl group is more electrophilic than an unprotonated carbonyl group Increasing the reactivity of the carbonyl group is an example of making the reactant less stable ie more reactive Base catalysis Base catalyst increases the rate of a reaction by removing a proton from a reactant The base increases the rate of the reaction by providing a pathway with a more stable transition state Nucleophilic catalysis or covalent catalysis Nucleophilic catalyst increases the rate of a reaction by acting as a nucleophile a nucleophilic catalyst increases the reaction rate by changing the mechanism of the reaction Iodide ion increases the rate of formation of ethanol by changing a relatively slow onestep reaction into a reaction with two relatively fast steps Metalion catalysis Metal ions exert their catalytic effect by coordinating complexing with atoms that have nonbonding electrons metal ions are Lewis acids Metalion catalysis can increase the rate of a reaction in several ways A metal ion can make a reaction center more susceptible to receiving electrons because it can stabilize a developing negative charge on a transition state A metal ion can make a leaving group a weaker base and therefore a better leaving group A metal ion can increase the rate ofa hydrolysis reaction by forming a complex with water thereby increasing water s acidity Metalbound hydroxide ion while not as good a nucleophile as hydroxide ion is a better nucleophile than water Thus metalion complexation increases the reactivity of the attacking nucleophile I y Cuzi or My catalyzed u39ecm39 Coordination of the metal ion with two oxygen atoms of the reactant makes the carbonyl group more susceptible to receiving the electrons thus increasing the rate of decarboxylation ZnB catalyzed hydrolysis of methyl trifluoroacetate The Zn2 catalyzed hydrolysis of methyl trifluoroacetate has two slow steps Zn increases the rate of the first slow step by providing metalbound hydroxide ion a better nucleophile than water It increases the rate of the second slow step by decreasing the basicity of the group that is eliminated from the tetrahedral intermediate Mechanism for Carboxypeptidase A Carboxypeptidase A catalyzes the hydrolysis of the C termianl peptide bond in peptides and proteins releasing the C terminal amino acid Carboxypeptidase A is a metalloenzyme an enzyme that contains a tightly bound metal ion First step Zn2 polarizes the CO making the carbonyl carbon more susceptible to nucleophile attack and stabilizing the negative charge that develops on the oxygen atom in the transition state Arg 127 also participates in increasing the carbonyl group s electrophilicity and in stabilizing the developing negative charge on the transition state Zn2 also complexes with water which increases its acidity thereby making the nucleophile more like hydroxide ion Glu 270 functions as a generalbase catalyst further increasing water s nucleophilicity Second step Glu 270 functions as a generalacid catalyst increasing the leaving tendency of the amino group ntramolecular reactions An intramolecular reaction that forms a five or a sixmembered ring occurs more readily than the analogous intermolecular reaction number of collis ions Rate of reaction x fraction with sufficient energy x fraction with proper unit of time orientation In an intramolecular reaction the reacting groups are tied together in the same molecule and have a better chance of finding each other than if they were in two different molecules in a solution of the same concentration increasing the frequency of the collisions The reaction rate can be further increased if the reacting groups are arranged in the proper orientation ntramolecular catalysis The reaction rate increases when a reacting group and a catalyst in the same molecule than when they are in separate molecules 13 February Nomenclature Aldehydes IUPAC removing the quotequot from the alkane name and adding llal The carbonyl carbon is always at the number 1 position Ketones IUPAC removing the quotequot from the alkane name and adding quotonequot The chain is numbered in the direction that gives the carbonyl group the smaller number Derived names the substituents attached to the carbonyl group are cited in alphabetical order followed by llketone Relative reactivities of carbonyl compounds Aldehydes are more reactive toward nucleophilic attack than ketones Because alkyl groups are electrondonating compared to a hydrogen Because alkyl groups are sterically larger than the hydrogen Bigger alky group 9 less reactive Reactivity Considerations Nucleophilic acyl substitution reactions All carboxylic acid derivatives react by the same mechanism Nucleophilic addition reactions Aldehydes and ketones react with nucleophiles to form addition compounds If a nucleophile is a strong base the reaction is irreversible Ifa nucleophile is a weak base the reaction is reversible Nucleophilic additionelimination reaction If the attacking atom of the nucleophile has a pair of nonbonding electrons in the addition product water will be eliminated from the addition product Addition of carbon nucleophiles Grignard reagents Acetylide anions CH3CEC Hydrogen cyanide 9 cyanohydrin n basic solutions a cyanohydrin is converted back to the carbonyl compound Hydride ion reduction reaction Reactions of carbonyl compounds that have leaving groups with Grignard reagents and hydride ion donors Grignard reagents The reaction does not stop at the ketone stage because ketones are more reactive than esters Reaction with a hydride ion donor NaBH4 is safer but less reactive than LiAlH4 Using an equivalent of DIBALH diisobutylaluminum hydride an ester can be converted into aldehyde The reaction of a carboxylic acid with LiAlH4 forms a single primary alcohol Addition of nitrogen nucleophiles Addition of ammonia primary amines and other ammonia derivatives mine a compound with a carbonnitrogen double bond Schiff base the imine obtained from reaction of a carbonyl compound and a primary amine Overall imine formation is a nucleophilic additionelimination reaction The pH at which imine formation is carried out must be carefully controlled There must be sufficient acid present to protonate the tetrahedral intermediate so that H20 rather than the much more basic H0 is the leaving group However if too much acid is present it protonates the reactant amine Protonated amines are not nucleophiles so they cannot react with carbonyl groups Oxime the imine obtained from reaction with hydroxylamine HZNOH Hydrazone the imine obtained from reaction with hydrazine HZNNHZ Semicarbazone the imine obtained from reaction with semicarbazide H 2NNHCONH2 The WolffKishner reduction a ketone or an aldehyde is converted into a methylene group upon heating in a basic solution of hydrazine Addition of Secondary Amines Enamine a Bunsaturated tertiary amine obtained from the reaction of an aldehyde or a ketone with a secondary amine The mechanism for enamine formation is exactly the same as the mechanism for imine formation except for the last step of the reaction 23 March Amines are compounds in which one or more of the hydrogens of ammonia NHg have been replaced by an alkyl group Heterocyclic compounds heterocycles are cyclic compounds in which one or more of the atoms of the ring are hetero atoms Heterocycles make up an exceedineg important class of compounds Almost all the compounds we know as drugs most vitamins and many other natural products are heterocycles Hetero atom is an atom other than carbon such as N O S Se P Si B and As More about amine nomenclature Common names for amines are obtained by citing the names of the alkyl substituents alphabetically that have replaced the hydrogens of NH3 Systematic names use quotaminequot as a functional group suffix Saturated Heterocycles Do not contain double bonds A saturated heterocycle is easily named as a cycloalkane using a prefix to denote the hetero atom quotaza for N quotoxaquot for O quotthiaquot for S Reaction of Amines Amine can act both as base and nucleophile Synthesis of Amines Gabriel synthesis Aromatic fivememberedring heterocycles Ex pyrrole furan and thiophene Pyrrole is an extremely weak base because nitrogen s nonbonding pair of electrons is needed for pyrrole s aromaticity Pyrrole furan and thiophene undergo electrophilic substitution at C2 Substitution occurs preferentially at C2 because the intermediate obtained by putting a substituent at this position is more stable than the intermediate obtained by putting a substituent at 03 If both position adjacent to the hetero atom are occupied electrophilic substitution will take place at C3 Pyrrole furan and thiophene are more reactive than benzene toward electrophilic substitution Benzene requires a relatively strong Lewis acid for electrophilic aromatic substation thiophene needs a weak Lewis acid SnCl4 furan requires an even weaker Lewis acid BFg and pyrroles require no catalyst in this case When pyrrole is protonated it is not protonated on nitrogen it is protonated on CZ Pyrrole is unstable in strongly acidic solutions because it polymerizes readily Indole benzofuran and benzothipohene contain a fivemembered aromatic ring fused with a benzene ring Pyridine is a tertiary amine and undergoes reactions characteristic of tertiary amines Like benzene pyridine is aromatic and undergoes electrophilic aromatic substitution reactions Pyridine s electronwithdrawing nitrogen causes the ring carbons to have significantly less electron density than the ring carbons of benzene Pyridine undergoes electrophilic aromatic substation reactions only under vigorous conditions and the yields of these reactions are often quite low Pyridine undergoes electrophilic substitution at C3 Nucleophilic aromatic substitution Pyridine is more reactive than benzene toward nucleophilic aromatic substation reaction Pyridine undergoes nucleophilic substitution at C2 and C 4 If the leaving groups at C2 and 04 are different the incoming nucleophilie will preferentially substitute for the weaker base the better leaving group 6 February Carbonyl group a carbon doubly bonded to an oxygen Nomenclature Carboxylic acids IUPAC replacing the quotequot ending of the alkane name with quotoic acid Acyl halides IUPAC replacing the quotic acid of the acid name with quotyl chloride Acid anhydride Symmetrical anhydride IUPAC using the acid name and replacing quotacidquot with quotanhydridequot Mixed anhydride IUPAC starting the names of both acids in alphabetical order followed by quotanhydridequot Esters IUPAC the name of the group attached to the carboxyl oxygen is stated first This is followed by the name of the acid with quotic acid replaced by quotatequot Lactones cyclic esters Amides IUPAC replacing quotoic acid of the acid name with quotamidequot if there is a substituent bonded to the nitrogen the name of the substituent is stated first Lactams cyclic amides Nitriles RCEN adding quotnitrilequot to the alkane name Nucleophilic acyl substitution reaction The substituent that was attached to the acyl group in the reactant is replaced by a nucleophile Which one is preferentially eliminated The weaker the base the better it is as a leaving group A carboxylic acid derivative will undergo a nucleophilic acyl substitution reaction provided that the incoming nucleophile is not a much weaker base than the substituent attached to the acyl group in the reactant Relative reactivities of carboxylic acids acyl halides acid anhydrides esters and amides The weaker the base attached to the acyl group the easier it is for both steps of the reaction to take place ie the less basic the leaving group the more reactive the carboxylic acid derivative Why For the first step of the reaction The weaker the basicity of Y the less the resonance stabilization of the molecule The weaker the basicity of Y the more it withdraws electrons inductively from the carbonyl carbon For the second step of the reaction The weaker the base the easier it is to eliminate Relative reactivities of carboxylic acid derivatives Acyl chloride gt acid anhydride gt ester gt carboxylic acid gt amide General mechanism for nucleophilic acyl substitution reactions All carboxylic acid derivatives react by the same mechanism If the nucleophile is neutral the mechanism has an additional step loss of a proton Reactions of esters Hydrolysis a reaction with water Alcoholysis transesterification a reaction with an alcohol Aminolysis a reaction with an amine Hydrolysis of an ester involves two relatively slow steps formation of a tetrahedral intermediate and collapse of a tetrahedral intermediate H increases the rates of both slow steps H increases the rate of formation of a 39 39 quot quot by r 39 g the carbonyl oxygen to increase the electrophilicity H increases the rate of collapse of a tetrahedral intermediate by decreasing the basicity of the leaving group Hydroxideion promoted ester hydrolysis The final product is a carboxylate ion rather than a carboxylic acid the hydroxideion promoted hydrolysis of an ester is not a reversible reaction The hydroxide ion is consumed in the overall reaction it is actually a reagent rather than a catalyst Reactions of Carboxylic Acids Fischer esterification reaction Reactions of Amides Mechanism for acidcatalyzed hydrolysis The addition of acid increases the rate of formation of the tetrahedral intermediate by protonating the amide The acid also changes the relative leaving abilities of the two groups 30 March Stronger oxidizing agent such as HN03 One or more of the alcohol groups can be oxidized in addition to the aldehyde group The primary alcohol is the one most easily oxidized Aldaric acid the product obtained when both the aldehyde and the primary alcohol groups are oxidized Chain elongation The KilianiFischer synthesis Chain shortening The Wohl Degradiation The Ruff degradation Cyclic structure of monosaccharides hemiacetal formation Hemiacetal hemiketal the product of the addition of one equivalent of an alcohol to an aldehyde ketone Acetal ketal the product formed when a second equivalent of alcohol is added Anomers two sugars that differ in configuration only at the carbon that is the carbonyl carbon in the openchain form anomers like epimers are a particular kind of diastereomer Anomeric carbon the carbon that is the carbonyl carbon in the openchain form the only carbon bonded to two oxygens Pyranoses sixmembered ring sugars Furanoses fivemembered ring sugars If an aldose can form a five or a sixmembered ring if will exist predominately as a cyclic hemiacetal in solution Haworth projection For a Dpyranose The sixmembered ring is represented as being flat and is viewed edge on The ring oxygen is always placed on the back righthand corner of the ring with the anomeric carbon 01 on the righthand side and the primary alcohol group drawn up from the back lefthand corner CS Groups on the right in a Fischer projection are down in a Haworth projection while groups on the left in a Fischer projection are up in a Haworth projection The Haworth projection ofa Dfuranose is viewed on edge with the ring oxygen away from the viewer The anomeric carbon is on the righthand side of the molecule and the primary alcohol group is drawn up from the back lefthand corner Ketoses also exist predominantly in cyclic forms Stability of Glucose BDglucose predominates at equilibrium in an aqueous solution Remember all the OH groups in BDglucose are in equatorial position Based on glucose it is easy to draw the chair conformation of any other pyranose Formation of Glycosides Glycoside the acetal or ketal of a sugar Glycosidic bond the bond between the anomeric carbon and the alkxoy oxygen Glycosides are nonreducing sugars because they are acetals or ketals and are not in equilibrium with the openchain aldehyde or ketone in aqueous solution they cannot be oxidized by reagents such as Ag or Brz Nglycoside a nitrogen replaces the oxygen at the glycosidic linkage Disaccharides if the hemiacetal group of a monosaccharide uses an alcohol group of another monosaccharide to form an acetal the formed glycoside is called disaccharide Lactose is a disaccharide found in milk Lactase is an enzyme that specifically breaks the B14 glycosidic linkage of lactose Galactosemia without the enzyme which converts galactose to glucose galactose accumulates in the bloodstream This accumulation can cause mental retardation in infants and even death Galactosemia is treated by excluding galactose from the diet Polysaccharides Polysacchardies contain as few as ten or as many as several thousand monosaccharide units jointed together by glycosidic linkages Starch is the major component of flour potatoes rice beans corn and peas It is a mixture of two different polysaccharides amylose about 20 and amylopectin about 80 The surfaces of many cells contain short polysaccharide chains These polysaccharides are linked to the cell surface by the reaction ofan OH or an NHZ group ofa protein with the anomeric carbon of a cyclic sugar Proteins that are bonded to polysaccharides are called glycoproteins 1 February Nucleophilic aromatic substitution reactions SNAr reaction The benzene ring has one or more substituents that stroneg withdraw electrons from the ring by resonance The benzene ring has a good leaving group such as halogen The electronwithdrawing groups must be positioned ortho or para to the leaving group because the electrons of the attacking nucleophile can be delocalized onto the substituent only if the substituent is in one of those positions The leaving group must be a weaker base than the incoming nucleophile Benzyne An aryl halide can undergo a nucleophilic substitution reaction in the presence of a very strong base such as NH The aryl halide does not have to contain an electronwithdrawing group and the reaction proceeds via a benzene intermediate 20 January Organic chemistry science of organic compounds which are defined as compound containing carbon Organic compounds are vital to life Synthetic sweeteners materials super glue are all organic compounds Superglue is a polymer of methyl ot cyanoacrylate DNA double helix Bases are aromatic Antiinflammatory drugs inhibit synthesis of prostaglandin Bonding in benzene Each n e not localized on a single C nor between C s Each ne shared by all 6 C Resonance energy 36 kcalmol More stable than cyclohexatriene Criteria for aromaticity Cyclic and planar One p orbital on each atom in ring Contain 4n 2 He odd of pairs 28 March Bioorganic compounds are organic compounds that are found in biological systems Carbohydrates have molecular formulas CnHZOn now refer to either polyhydroxy aldehydes such as Dglucose and polyhydroxy ketones such as Dfructose or to compounds such as sucrose that can be hydrolyzed to polyhydroxy aldehydes or polyhydroxy ketones Carbohydrates are abundant in the biological world The most abundant carbohydrate is Dglucose Cells of organisms oxidize Dglucose in the first of a series of processes that provide energy to the cells Animals obtain glucose by eating plants or by eating food containing glucose Plants obtain glucose by a process known as photosynthesis Classification of carbohydrates 1 u The terms quotcarbohydrate saccharidequot and quotsugarquot are often used interchangeably Simple carbohydrates are monosaccharides single sugars Disaccharides have two linked sugar subunits Oligosaccharides have three to ten sugar subunits Polysacchardies have more than ten sugar subunits linked together Aldoses polyhydroxy aldehyde Ketoses polyhydroxy ketones Trioses monosacchardies with three carbons Tetroses monosaccharides with four carbons Pentoses monosaccharides with five carbons Hexoses monosaccharides with six carbons Heptoses monosaccharides with seven carbons The D and L notation Fisher projections of monosaccharides the carbonyl group is always placed on top in the case of aldoses or as close to the top as possible in the case of ketoses If the OH group attached to the bottommost chirality center the second from the bottom carbon is on the right the sugar is a Dsugar The mirror image of the Dsugar is an Lsugar Almost all sugars found in nature are Dsugars Diastereomers are configurational isomers that are not enantiomers Epimers two diastereomers that differ in configuration at only one chirality center epimers are a particular kind of diastereomer Configuration of the ketoses Naturally occurring ketoses have the ketone group in the 2position A 2ketose has one fewer chirality center than does an aldose with the same number of carbon atoms Therefore a 2 ketohexose has only half as many stereoisomers as an aldohexose Redox reactions of monosaccharides Reduction The carbonyl group of aldoses and ketoses can be reduced by the usual carbonyl group reducing agents Alditol a polyalcohol resulting from the reduction Oxidation Brz oxidation Brz is a mild oxidizing agent and easily oxidizes the aldehyde group it is not a sufficiently strong oxidizing agent to oxidize ketones or alcohols Consequently ifa small amount ofan aqueous solutikon of El is added to an unknown monosaccharide the reddish brown color of Brz will disappear if the monosaccharide is an aldose but will persist if the monosaccharide is a ketose Aldonic acid the product of the oxidation reaction Tollen s reagent AgZO NH3 Both aldose and ketoses are oxidized to aldonic acids by Tollens reagent 29 February Halogenation of the otcarbon of aldehydes and ketones If Brz Clz or Iz is added to an acidic solution of an aldehyde or ketone a halogen will replace one of the othydrogens of the carbonyl compound If excess Brz CIZ or I2 is added to a basic solution rather than to an acidic solution of an aldehyde or a ketone the halogen will replace all the othydrogens The haloform reaction in the presence of excess base and excess halogen a metyl ketone will be converted into a carboxylic acid CH3 strong base poor leaving group Halogenation of the otcarbon of carboxylic acids the HellVolhardZelinski reaction HVZ reaction Using Lithium Diisopropylamdie LDA to form an enolate LDA is a strong base but a poor nucleophile Removes H at otcarbon Kinetic vs thermodynamic product Kinetic product is the major product if the reaction is carried out at 78 C Thermodynamic product is the major product is the reaction is carried out under conditions that make enolate formation reversible high temperature and a protic solvent so that the kinetic enolate can be reprotonated Reaction of enamines An advantage of using an enamine intermediate to alkylate an aldehyde or ketone is that only the monoalkylated product is formed When a carbonyl compound is alkylated directly dialkylated and Oalkylated products can also be formed Aldehydes and ketones can also be acylated via an enamine intermediate Alkylation of the Bcarbon The Michael reaction Addition of an enoate the enoates that work best in Michael reactions are enoates that are flanked by two electronwithdrawing groups The aldol addition a reaction between two molecules of aldehyde or two molecules of ketone Ketones are less susceptible than aldehydes to attack by nucleophiles so aldol additions occur more slowly with ketones Dehydration of aldol addition products formation of a Bunsaturated aldehydes and ketones Aldol condensation the overall reaction aldol addition dehydration 3 39 39 quot 39 39 and 3 39 39 39 can be Iunder acidic or basic conditions Dehydration sometimes occurs under the conditions in which the aldol addition is carried out 23 January Antiaromaticity Less stable than analogous cyclic molecules with localized electrons Cyclic and planar One p orbital on each atom in ring 4n n e even number of pairs Determine relative energies of molecular orbitals by drawing cyclic compound with a vertice pointed down Bonding orbitals below midpoint Nonbonding at midpoint Antibonding above midpoint Relative energy levels correspond to relative levels of vertices Aromatic compounds occupy bonding n molecular orbitals Antiaromatic compounds either cannot fill or have unpaired e39 in nonbonding or antibonding this makes me unstable Nomenclature of monosubstituted benzenes Name of substituent quotbenzenequot Some incorporate substituent name into one word Benzene as substituent quotphenylquot Benzene w methylene as substituent quotbenzylquot Electrophilic aromatic substitution reactions Electrophile put on ring Hi comes off ring Five most common types Halogenation Br CI substitutes Nitration nitro N02 group substitutes Sulfonation sulfonic acid SOgH group substitutes FriedeICrafts acylation acyl RCO group substitutes FriedeICrafts alkylation alkyl R group substitutes 30 January Directing effects of both substituents must be considered in carrying out reactions on disubstitutedbezenes Sandmeyer reactions Reaction of an aryl diazonium salt with a cuprous sat 18 April A polymer is a large molecule made by linking together repeating units of small molecules called monomers The process of linking them up is called polymerization General classes of synthetic polymers Synthetic polymers can be divided into two major classes depending on their method of preparation Chaingrowth polymers or addition polymers are made by the addition of monomers to the end ofa growing chain Stepgrowth polymers or condensation polymers are made by combining two molecules while in most cases removing a small molecule generally water or an alcohol The reacting molecules have reactive functional groups at both ends Chaingrowth polymers Chaingrowth polymerization proceeds by one of three possible mechanisms radical polymerization cationic polymerization or anionic polymerization Each mechanism has three distinct phases initiation step that starts the polymerization propagation steps that allow the chain to grow termination steps that stop the growth of the chain Radical polymerization Chaininitiating steps Chainpropagating steps Termination steps Chain combination Disproportionation Reaction with an impurity Can combine headtotail headtohead tailtotail Headtotail addition is preferred because the propagating site preferentially attacks the less sterically hindered carbon of the alkene Groups that stabilize adjacent radicals also favor headtotail addition Monomers that most readily undergo chaingrowth polymerization by a radical mechanism are those in which the substituent Z is able to stabilize the growing radical species by resonance Cationic polymerization Chaininitiating step Chainpropagating steps Chainterminating steps Monomers that are best able to undergo polymerization by a cationic mechanism are those with electrondonating substituents that can stabilize the positive charge at the propagating site Anionic polymerization Chaininitiating step Chainpropagating steps If the solvent cannot donate a proton to terminate the chain and if all impurities that can react with a carbanion are rigorously excluded chain propagation will continue until all the monomer has been consumed Such nonterminated chains are called living polymers Super Glue is a polymer of methyl otcyanoacrylate Because the monomer has two electron withdrawing groups it requires only a moderate nucleophile to initiate anionic polymerization Ringopening polymerizations Polymerization reactions that involve ringopening reactions such as the polymerization of propylene oxide Stepgrowth polymers condensation polymers are formed by the intermolecular reaction of bifunctional molecules There are two types of stepgrowth polymers Type 1 a single monomer that possesses two different functional groups A and B Type 2 one monomer contains two A functional groups and the other monomer contains two B functional groups 2 April Peptides and proteins are polymers of amino acids linked together by amide bonds The monomeric units are called amino acids residues Dipeptide contains two amino acid residues Tripeptide contains three amino acid residues Oligopeptide contains three to ten amino acid residues Polypeptide contains many amino acid residues Proteins are naturally occurring polypeptides that are made up of 40 to 4000 amino acid residues Proteins and polypeptides serve many functions in biological systems Classification and nomenclature of amino acids There are 20 most common naturally occurring amino acids The amino acids differ only in the substituent R attached to the ot carbon The wide variation in these side chains is what gives proteins their great structural diversity and as a consequence their great functional diversity Ten of the amino acids are essential amino acids We humans must obtain these ten amino acids from our diets because we either cannot synthesize them at all or cannot synthesize them in adequate amounts Configuration of amino acids The D and L isomers of monosaccharides and amino acids are defined the same way The Fischer projection with the carboxyl group on the top and the R group on the bottom of the vertical axis is Damino acid if the amino group is on the right of the horizontal axis and is an L amino acid if is on the left Almost all sugars found in nature are Dsugars In contrast most amino acids found in nature have the Lconfiguration AcidBase properties of amino acids The carboxyl groups of the amino acids have pKa values of approximately 2 the protonated amino groups have pKa values near 9 The acidic form predominates if the pH of the solution is less than the pKa of the compound and the basic form predominates if the pH of the solution is greater than the pKa of the compound An amino acid can never exist as an uncharged compound regardless of the pH of the solution A few amino acids have side chains with ionizablehydrogens For example the protonated imidazole side chain of histidine has a mef 604 The isoelectric point soelectric point pl ofan amino acid is the pH at which it has no net charge It is the pH at which the amount of negative charge on an amino acid exactly balances the amount of positive charge p isoelectric point pH at which there is no net charge An amino acid will be positively charged if the pH of the solution is less than its pi and will be negatively charged if the pH of the solution is greater than its p The p ofan amino acid that does not have an ionizable side chain is midway between its two pKa values When pH pKa half the group is in its acidic form and half is in its basic form The pI of an amino acid that has an ionizable side chain is the average of the pKa values of the similarly ionizing groups Separation of amino acids A mixture of amino acids can be separated by several different techniques such as electrophoresis on the basis of their p values paper chromatography on the basis of polarity thinlayer chromatography on the basis of polarity and ionexchange chromatography etc Kinetic resolution the resolution separation of the enantiomers depends on the difference in the rates of reaction of the enzyme with the two Nacetylated compounds Peptide bonds and disulfide bonds Peptide bonds and disulfide bonds are the only covalent bonds that hold amino acid residues together in a peptide or a protein Peptide bonds the amide bonds that link amino acid residues Glu Cys His Val Ala The pentapeptide contains the indicated amino acids but their sequence is not known ValCysAlaGluHis The amino acids in the pentapeptide have the indicated sequence A peptide bond has about 40 doublebond character because of resonance Steric hindrance causes the trans configuration to be more stable than the cis configuration As a consequence of the partial doublebond character of the peptide bond free rotation is not possible about the bond The C and N atoms of the peptide bond and the two atoms to which each is attached are held rigidly in a plane Disulfide bonds When thiols are oxidized under mild conditions they form disulfides A disulfide is a compound with an 55 bond A common oxidizing agent used for this reaction is Brz or Iz in a basic solution Cysteine is an amino acid containing a thiol group Two cysteines therefore can be oxidized to a disulfide The disulfide is called cystine Disulfide bridges are the only covalent bonds that can form between nonadjacent amino acids Some interesting peptides Enkephalins are pentapeptides synthesized by the body to control pain Brandykinin vasopressin and oxytocin are peptide hormones Bradykinin inhibits the inflammation of tissues Vasopressin controls blood pressure by regulating the contraction of smooth muscule Oxytocin induces labor in pregnant women and stimulates milk production in nursing mothers Glutathione is a tripeptide and its function is to destroy harmful oxidizing agents in the body 26 March Substituted pyridines undergo many of the sidechain reactions that substituted benzenes undergo The electronwithdrawing nitrogen causes the ot hydrogens of alkyl groups attached to the 2 and 4 positions of the pyridine ring to have about the same acidity as the othydrogens of ketones Quinoline and soquinoine Have both a benzene ring and a pyridine ring they are known as benzopyridines A porphyrin ring system consists of four pyyrole rings joined by onecarbon bridges Heme found in hemoglobin and myogobin consists an iron atom Fezi igated by the four nitrogens of a porphyrin ring system 4 April Strategy of peptide bond synthesis Nprotection and C activation Automated peptide synthesis Merrifield automated solidphase synthesis 18 January Final is comprehensive


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