Exam 6: Organic Chemistry

When the compound shown below undergoes an S_N2 reaction with hydroxide, which of the following compounds will most likely form as the major product?

Passage Malaria is caused by the parasite Plasmodium falciparum and is transmitted through a female mosquito bite.  Although there are antimalarial treatments available, P. falciparum has become resistant to many of these drugs.  P. falciparum cells contain a respiratory organelle called the apicoplast that is necessary for the parasite's survival but is not found in humans.  Therefore, a new drug that targets this organelle could be useful in the treatment of malaria.A portion of the apicoplast protein ferredoxin (PfFd) is shown in Figure 1, with certain amino acid residues labeled.  In the apicoplast, ferredoxin NADP⁺ reductase (PfFNR) interacts electrostatically with PfFd and catalyzes an electron transfer reaction.  A compound that selectively inhibits this interaction could be a beneficial antimalarial agent. Figure 1  Structure of a portion of PfFdChalcone (Compound 3) is a compound that interacts with PfFd and exhibits antimalarial properties.  A series of chalcone derivatives (Compounds 4-8) were synthesized via the aldol condensation shown in Scheme 1, where nucleophilic addition of Compound 1 to Compound 2 is followed by elimination to yield α,β-unsaturated carbonyl molecules, Compounds 3-8. Scheme 1An inhibition assay was performed to compare chalcone (Compound 3) and its derivatives by determining the extent to which these compounds inhibited electron transfer between PfFNR and PfFd (Table 1).Table 1  Inhibition of electron transfer assay results Adapted from H. Suwito et al. "Design and synthesis of chalcone derivatives as inhibitors of the ferredoxin - ferredoxin-NADP+ reductase interaction of Plasmodium falciparum: pursuing new antimalarial agents." Molecules. ©2014 MDPI. -What is the most likely explanation why an amino group was chosen as a substituent on the chalcone derivatives?

Which of the following does NOT describe why a nucleophile will react more quickly with acetic anhydride than with N,N-diisopropylisobutyramide?

Passage The N-terminus of an amino acid must be protected with 9-fluorenylmethoxycarbonyl (Fmoc) during solid state peptide synthesis to avoid side reactions (Scheme I). Scheme IThe Fmoc protecting group is typically removed with piperidine (Scheme II), a heterocyclic secondary amine. Scheme IIBecause the use of piperidine is restricted, structurally similar amines were examined as possible replacements.  To determine whether piperidine analogues were suitable alternatives for Fmoc removal, deprotection of Compound 2 was carried out with several amine bases (Scheme III). Scheme IIICompound 2 was prepared from L-tryptophan and Fmoc-Cl, and the reaction was monitored by thin-layer chromatography (TLC) and visualized by ultraviolet light.  The deprotections were done with piperidine and each analogue using a microwave set to 155 watts at 75 °C for 15 seconds.  The products of the deprotections were analyzed by high-performance liquid chromatography (HPLC) and mass spectrometry; the retention times of the products were compared to tryptophan and Compound 2 standards. Adapted from O.P. Luna et al, "Deprotection Reagents in Fmoc Solid Phase Peptide Synthesis: Moving Away from Piperidine?." Molecules. ©2016 MDPI. -Which of the following most accurately describes the process that occurs when visualizing the formation of the Compound 2 reaction mixture on a thin-layer chromatography plate?

Passage Fluorescent amino acids are useful for labeling proteins because they can be visualized spectroscopically, facilitating the study of protein structure and interactions.  The noncanonical amino acid dansylalanine, a fluorescent amino acid that is sensitive to the hydrophobicity of its environment, is a good candidate to be incorporated into proteins because of its small size and fluorescent capabilities.  Dansyl chloride and Compound 1 were used to synthesize dansylalanine, as shown in Reaction 1.  Dansylalanine absorbs maximally at 340 nm and emits at approximately 540 nm. Reaction 1  Synthesis of dansylalanineA leucyl-tRNA synthetase from Escherichia coli was engineered to aminoacylate bacterial tRNAs containing a CUA anticodon with dansylalanine.  After verifying that the tRNA/synthetase pair does not interact with endogenous amino acids or tRNAs, it was transfected into the yeast Saccharomyces cerevisiae along with a plasmid encoding a mutated human superoxide dismutase (hSOD).  Using this system, dansylalanine was genetically incorporated into hSOD in place of either Gln-16 or Trp-33, and expressed in S. cerevisiae.Position 16 is located on the surface of hSOD at the N-terminus of a β-strand, and position 33 lies in the center of a strand within a β-barrel.  To probe changes in the structure of hSOD, researchers exposed the purified protein to guanidinium chloride (GdmCl) to denature the protein at concentrations ranging 0-4.5 M.  The experiment was conducted in a 35 mM sodium phosphate buffer at pH 7.2.  The fluorescence of hSOD with dansylalanine at either position 16 or 33 was monitored at each GdmCl concentration, and the results of the unfolding experiment are shown in Figure 1. Figure 1  Plot of dansylalanine fluorescence at different concentrations of GdmCl Adapted from Summerer D, Chen S, Wu N, Deiters A, Chin JW, Schultz PG. A genetically encoded fluorescent amino acid. Proc Natl Acad Sci USA. 2006. -What feature of dansylalanine allows for monitoring protein unfolding?

Passage The protozoan Euglena gracilis expresses glycosyltransferases (enzymes that add carbohydrates to molecules) and glycosidases (enzymes that remove carbohydrates) within its membrane.  These enzymes were previously studied using radiolabeled sugars.  To avoid the use of radioactive materials, fluorescence-based assays were investigated.  Derivatives of the fluorescent molecule coumarin were attached to carbohydrates (Figure 1) for detection in fluorescence assays.Compound 1, a carbohydrate derivative with hydroxyl groups protected by benzoate (Bz), reacted with Compound 2 to form Compound 3.  ¹³C NMR confirmed the formation of Compound 3 by observing a distinct C-H coupling at the anomeric carbon.  Hydroxyl deprotection and addition of Compound 4 (a coumarin analogue) gave Compound 5, a fluorescent coumarin derivative.  Compound 6, another coumarin derivative, was made following a similar sequence except an additional sugar was attached before the addition of the coumarin group. Figure 1  Synthesis of fluorescent coumarin derivatives (Note: HCT denotes the fluorescent butyl hydroxycoumarin triazole group.)Small vesicles called microsomes were isolated from Euglena gracilis membranes, and microsomal glycosyltransferase activities were investigated using a hexose donor substrate and either Compound 5 (Figure 2) or Compound 6 (Figure 3) as the acceptor substrate.  Thin-layer chromatography (TLC) analysis indicated the formation of a new product in each assay.  These products were purified and characterized by liquid chromatography-mass spectrometry (LC-MS).  The molecular ion for each product was subjected to another round of MS, in which individual hexose units (MW) = 162) were cleaved from the coumarin derivatives that were formed during the assay.  The MW of hydroxycoumarin triazole (HCT) is 301. Figure 2  Analysis of Compound 5 as the acceptor substrate: (A) TLC analysis of fluorescent products; (B) Mass spectrum of product A fragmented molecular ion peak Figure 3  Analysis of Compound 6 as the acceptor substrate: (A) TLC analysis of fluorescent products; (B) Mass spectrum of product B fragmented molecular ion peak Adapted from: I. M. Ivanova et al., "Fluorescent mannosides serve as acceptor substrates for glycosyltransferase and sugar-1-phosphate transferase activities in Euglena gracilis membranes." Carbohydrate Research. © 2017 Elsevier. -The assay in lane 4 most likely functions to:

Passage The drug paracetamol, also known as acetaminophen, is used to relieve pain and fever, and is a metabolite of the antipyretic drug phenacetin (Figure 1).  These two drugs have similar medicinal properties, but phenacetin has been shown to be carcinogenic and cause kidney damage.  Acetaminophen is a safer alternative to phenacetin when taken in therapeutic doses. Figure 1  Structure of phenacetinA student synthesizes acetaminophen in a laboratory from phenol according to the reactions shown.  The synthesis begins with the nitration of phenol by sodium nitrate (NaNO₃) to produce the mixture of the isomers 2-nitrophenol and 4-nitrophenol (Reaction 1).  These isomers can be easily separated by steam distillation to isolate the desired product, 4-nitrophenol.  Reduction of the nitro group with sodium borohydride (NaBH₄) (Reaction 2) is followed by acetylation of the amine with acetic anhydride (Reaction 3) to afford the final product, acetaminophen. Reaction 1 Reaction 2 Reaction 3 -Which of the following statements most accurately describes the component that remains in the reaction flask during the steam distillation?

Passage The protozoan Euglena gracilis expresses glycosyltransferases (enzymes that add carbohydrates to molecules) and glycosidases (enzymes that remove carbohydrates) within its membrane.  These enzymes were previously studied using radiolabeled sugars.  To avoid the use of radioactive materials, fluorescence-based assays were investigated.  Derivatives of the fluorescent molecule coumarin were attached to carbohydrates (Figure 1) for detection in fluorescence assays.Compound 1, a carbohydrate derivative with hydroxyl groups protected by benzoate (Bz), reacted with Compound 2 to form Compound 3.  ¹³C NMR confirmed the formation of Compound 3 by observing a distinct C-H coupling at the anomeric carbon.  Hydroxyl deprotection and addition of Compound 4 (a coumarin analogue) gave Compound 5, a fluorescent coumarin derivative.  Compound 6, another coumarin derivative, was made following a similar sequence except an additional sugar was attached before the addition of the coumarin group. Figure 1  Synthesis of fluorescent coumarin derivatives (Note: HCT denotes the fluorescent butyl hydroxycoumarin triazole group.)Small vesicles called microsomes were isolated from Euglena gracilis membranes, and microsomal glycosyltransferase activities were investigated using a hexose donor substrate and either Compound 5 (Figure 2) or Compound 6 (Figure 3) as the acceptor substrate.  Thin-layer chromatography (TLC) analysis indicated the formation of a new product in each assay.  These products were purified and characterized by liquid chromatography-mass spectrometry (LC-MS).  The molecular ion for each product was subjected to another round of MS, in which individual hexose units (MW) = 162) were cleaved from the coumarin derivatives that were formed during the assay.  The MW of hydroxycoumarin triazole (HCT) is 301. Figure 2  Analysis of Compound 5 as the acceptor substrate: (A) TLC analysis of fluorescent products; (B) Mass spectrum of product A fragmented molecular ion peak Figure 3  Analysis of Compound 6 as the acceptor substrate: (A) TLC analysis of fluorescent products; (B) Mass spectrum of product B fragmented molecular ion peak Adapted from: I. M. Ivanova et al., "Fluorescent mannosides serve as acceptor substrates for glycosyltransferase and sugar-1-phosphate transferase activities in Euglena gracilis membranes." Carbohydrate Research. © 2017 Elsevier. -If researchers needed to synthesize Compound 1 before beginning the synthesis of Compound 5, which of the following sugars would require the fewest steps if used as a starting material?

Passage Fatty acids (FAs) are lipids that play important roles in cellular function, including energy storage and membrane structure.  Most polyunsaturated fatty acids (PUFAs) are made in the body; however, omega-3 alpha-linolenic acid and omega-6 linoleic acid must be obtained from the diet.  These two PUFAs are precursors to other omega-3 and omega-6 long-chain FAs that are necessary for physiological processes such as neurological development, normal growth, and immune response.  Insufficient omega-3 and omega-6 long-chain FAs can lead to an increase in omega-9 fatty acids such as mead acid, and to essential fatty acid deficiency (EFAD), which can cause infertility, decreased growth, and increased susceptibility to infection.Total FA levels in plasma and serum can be used to diagnose EFAD.  Plasma and serum samples from several individuals were hydrolyzed to release FAs from triacylglycerols and phospholipids.  Once isolated, the free FAs were converted to pentafluorobenzyl (PFB) derivatives and separated by gas chromatography (GC).  Total levels of each type of fatty acid were quantified by negative chemical ionization-mass spectrometry (NCI-MS) immediately after GC.  NCI is an ionization technique that creates negatively charged ions with little to no fragmentation.  The NCI-MS revealed that two of the peaks from GC had the same m/z ratio (Figure 1).  They were both confirmed to be forms of an unsaturated, unbranched 22-carbon FA called omega-9 docosenoic acid. Figure 1  Selected ion chromatogram of docosenoic acidResearchers have proposed that an elevated ratio of the triene omega-9 mead acid to the tetraene omega-6 arachidonic acid, known as the triene/tetraene (TT) ratio, can signify EFAD.  Using GC and NCI-MS, they measured the TT ratio, concentration of mead acid, and individual concentrations of omega-3 and omega-6 FAs in plasma and serum samples from three patients and compared their results to normal ranges.  An increased TT ratio and mead acid concentration and decreased omega-3 and omega-6 FA concentrations were observed in the patient samples compared to normal ranges. Adapted from E. Kish-Trier et al., "Quantitation of total fatty acids in plasma and serum by GC-NCI-MS." Clinical Mass Spectrometry. ©2016 Elsevier. -If three equivalents of NaOH are used for triacylglycerol saponification, the triacylglycerol will be:

Passage Solid-phase peptide synthesis is normally carried out on a resin bathed multiple times in dimethylformamide (DMF)-a toxic, polar, aprotic solvent-which poses an environmental problem because a large amount of toxic solvent waste is created.  Cyclic carbonates such as propylene carbonate (PC) and ethylene carbonate (EC) are nontoxic, polar, aprotic solvents that could be used as an alternative to DMF.Compounds 1 and 2 are modified L-amino acids.  They were coupled to form Compound 3, a dipeptide, which was then deprotected to yield Compound 4 (Figure 1). Figure 1  Synthesis of Compound 4To determine the effect that solvent type and temperature have on the synthesis of this peptide, the nontoxic solvents PC and EC as well as the toxic solvent DMF were each used in separate syntheses of Compound 4 at different temperatures.  The reaction solvent was also used to rinse the resin.  Table 1 shows the results of these syntheses.Table 1  Results from the Synthesis of Compound 4 The successful synthesis of Compound 4 using nontoxic solvents encouraged researchers to pursue the synthesis of a larger, biologically active peptide.  Bradykinin, a vasodilator containing nine amino acids (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg), was synthesized using the same solid-state peptide synthesis protocol used to synthesize Compound 4.  Separate syntheses were done using either DMF or PC, and the purity of bradykinin from each synthesis was analyzed by high-performance liquid chromatography (HPLC) (Figure 2).In each chromatogram, peak 1 has the same retention time as commercially obtained bradykinin, and the other peaks correspond to impurities.  In chromatogram A, the impurity has a similar retention time to the product peak whereas the impurity peaks in chromatogram B have distinct retention times compared to the product peak. Figure 2  HPLC of synthetic bradykinin: (A) synthesis in DMF; (B) synthesis in PC Adapted from S. B. Lawrenson et al., "The greening of peptide synthesis."  Green Chem. ©2017 The Royal Society of Chemistry. -The most likely reason the resin was rinsed with the same solvent used for the reaction was to:

Passage Fluorescent amino acids are useful for labeling proteins because they can be visualized spectroscopically, facilitating the study of protein structure and interactions.  The noncanonical amino acid dansylalanine, a fluorescent amino acid that is sensitive to the hydrophobicity of its environment, is a good candidate to be incorporated into proteins because of its small size and fluorescent capabilities.  Dansyl chloride and Compound 1 were used to synthesize dansylalanine, as shown in Reaction 1.  Dansylalanine absorbs maximally at 340 nm and emits at approximately 540 nm. Reaction 1  Synthesis of dansylalanineA leucyl-tRNA synthetase from Escherichia coli was engineered to aminoacylate bacterial tRNAs containing a CUA anticodon with dansylalanine.  After verifying that the tRNA/synthetase pair does not interact with endogenous amino acids or tRNAs, it was transfected into the yeast Saccharomyces cerevisiae along with a plasmid encoding a mutated human superoxide dismutase (hSOD).  Using this system, dansylalanine was genetically incorporated into hSOD in place of either Gln-16 or Trp-33, and expressed in S. cerevisiae.Position 16 is located on the surface of hSOD at the N-terminus of a β-strand, and position 33 lies in the center of a strand within a β-barrel.  To probe changes in the structure of hSOD, researchers exposed the purified protein to guanidinium chloride (GdmCl) to denature the protein at concentrations ranging 0-4.5 M.  The experiment was conducted in a 35 mM sodium phosphate buffer at pH 7.2.  The fluorescence of hSOD with dansylalanine at either position 16 or 33 was monitored at each GdmCl concentration, and the results of the unfolding experiment are shown in Figure 1. Figure 1  Plot of dansylalanine fluorescence at different concentrations of GdmCl Adapted from Summerer D, Chen S, Wu N, Deiters A, Chin JW, Schultz PG. A genetically encoded fluorescent amino acid. Proc Natl Acad Sci USA. 2006. -In the synthesis of dansylalanine, the Boc group on Compound 1 most likely functions to:

Passage Hemophilia B is a blood clotting disorder caused by a factor IX (FIX) deficiency.  FIX is a 57-kDa, vitamin K-dependent protease that activates factor X, leading to the conversion of prothrombin to thrombin for propagation of the clotting cascade.  Activated FIX has two major domains:  a γ-carboxyglutamic acid domain and a serine protease domain.  The γ-carboxyglutamic acid domain participates in the oxidation of vitamin K using metallic cofactors, as shown in Figure 1. Figure 1  Oxidation of vitamin KTo further analyze the γ-carboxyglutamic acid-rich domain of FIX, an analogous synthetic peptide composed of matching residues 1 through 49 on FIX was evaluated by proton nuclear magnetic resonance (NMR) spectroscopy.  Analysis of the proton chemical shift before the addition of metal ions suggested that the synthetic peptide contained normal structural elements.  Large chemical shifts were observed after the addition of calcium and beryllium, as shown in Figure 2. Figure 2  Results of NMR spectroscopy (tetramethylsilane [TMS] peak has been removed)The synthetic analog was then placed in solution with vitamin K hydroquinone and cofactors required for vitamin K oxidation.  The oxidation products of vitamin K in Reactions 1 and 2 were collected and evaluated under high-performance liquid chromatography (HPLC) using hexane as the mobile phase.  Analysis demonstrated cis and trans isomers of vitamin K₁ and a trans-epoxy vitamin K₁, as shown in Figure 3. Figure 3  Results of HPLC separation Adapted from Freedman SJ, Furie BC, Furie B, Baleja JD. Structure of the metal-free gamma-carboxyglutamic acid-rich membrane binding region of factor IX by two-dimensional NMR spectroscopy. J Biol Chem. 1995. -What are the bond angles of the water molecule formed during Reaction 1, the methylated carbon atom of the product from Reaction 1, and the alkene formed in Reaction 2, respectively?

Passage Electrospray ionization mass spectrometry (ESI-MS) employs a soft ionization technique that, unlike traditional ionization via electron bombardment, results in little to no fragmentation of target molecules.  During electron bombardment, electrons collide with the analyte and cause the molecule to lose an electron whereas during ESI-MS, ions are produced by passing the analyte solution through an electrospray needle that has a potential difference applied with respect to a counterelectrode.  This leads to the formation of charged droplets, which are repelled from the needle toward the counterelectrode and mass spectrometer, as shown in Figure 1. Figure 1  ESI-MS ionization mechanismDuring the droplet's flight, the solvent is evaporated off analyte molecules.  Ionization occurs through the protonation of basic sites on target molecules.  Reaction 1 shows the number of protons n added to molecule M.M → (M + nH)ⁿ⁺Reaction 1Researchers used ESI-MS to quantify the formation of four pyridyloxobutyl-DNA (POB-DNA) adducts, shown in Figure 2.  POB-DNA adduct formation involves highly mutagenic pyridyloxobutylating agents, which are formed from cytochrome P450-mediated hydroxylation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), an abundant N-nitrosamine carcinogen found in cigarette smoke.  Adducts were produced in vitro by treating samples of calf thymus DNA with NNK.  The DNA adducts were then separated by reversed-phase high-performance liquid chromatography (RP-HPLC) and analyzed via ESI-MS. Figure 2  Molecular structure of POB-DNA adducts Adapted from Lao Y, Villalta PW, Sturla SJ, Wang M, Hecht SS. Quantitation of pyridyloxobutyl DNA adducts of tobacco-specific nitrosamines in rat tissue DNA by high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry. Chem Res Toxicol. 2006. -Which of the following would most likely be used as the mobile and stationary phases, respectively, for RP-HPLC separation of the POB-DNA adducts?

Passage Two key ingredients found in many soaps are PEG 150 distearate and ethylhexyl glycerin.  Both can be synthesized from glyceryl tristearate (Figure 1). Figure 1  Formation of PEG 150 distearate and ethylhexyl glycerin from glyceryl tristearateThe first step in this process requires the saponification of glyceryl tristearate.  The resulting stearate molecules can then react with PEG 150 in an acid-catalyzed Fischer esterification to form PEG 150 distearate.  The glycerol produced during saponification is used as a nucleophile to form ethylhexyl glycerin through a base-catalyzed S_N2 reaction with an ethylhexyl halide.Studies have shown that hand washing with soap can substantially reduce the number of germs present on hands and that washing hands before preparing food is particularly effective in reducing illness.  Researchers examined the effect of washing for 20 seconds with different amounts of soap containing PEG 150 distearate and ethylhexyl glycerin and found an inverse correlation between the volume of soap used and the amount of bacteria recovered from the hands.  The data are shown in Figure 2. Figure 2  Bacteria recovered from hands after washing with varying amounts of soapThe researchers then investigated the effect of hand washing with different kinds of soap on the transfer of bacteria from hands to objects.  Volunteers washed with a fixed volume of soap for 30 seconds, then handled sterilized plastic spheres for 30 seconds.  Bacteria were then recovered from the spheres.  The results are shown in Table 1.Table 1 Bacteria Recovered from Objects Adapted from  Fuls JL, Rodgers ND, Fischler GE, et al. Alternative hand contamination technique to compare the activities of antimicrobial and nonantimicrobial soaps under different test conditions. Appl Environ Microbiol. 2008. -Ethylhexyl glycerin could be most efficiently synthesized from glycerol and which of the following?

Passage Students used three classification tests to study the reactivity of alcohol, aldehyde, and ketone functional groups in organic compounds.  The organic compounds provided to the students are given in Table 1.Table 1  Organic Compounds Tested First, the students performed the Lucas test by mixing a few drops of each compound with 2 mL of the Lucas reagent (ZnCl₂ and HCl).  This reagent is used to test for secondary and tertiary alcohols.  A positive test result is indicated by a cloudy reaction mixture upon addition of the Lucas reagent due to the formation of an insoluble compound by an S_N1 reaction with a secondary or tertiary alcohol (Reaction 1). Reaction 1Next, a test for alcohols and aldehydes was performed by the addition of a drop of chromic acid reagent (CrO₃ dissolved in aqueous H₂SO₄) into samples of the organic compounds.  The disappearance of the orange color from chromic acid and formation of a blue-green suspension is indicative of a positive result for this test.  Compound A was found to be an example of a positive result, as shown in Reaction 2. Reaction 2Finally, samples of the compounds were also dissolved in water, followed by the addition of NaOH and iodine.  This test is known as the iodoform test, which is used to identify methyl ketones and secondary alcohols.  A positive result is indicated by the formation of iodoform (CHI₃), a yellow precipitate (Reaction 3). 3Table 2  Selected Observations from the Classification Tests -Compound D gives a positive result for the chromic acid test.  What is the structure of the product formed by the reaction of Compound D with the chromic acid reagent?

The infrared spectrum shows the frequencies of infrared light absorbed by a sample of butyraldehyde that was found to be contaminated. Which of the following could be the contaminant?

What types of functional groups react to form the imine intermediate in the Strecker synthesis of amino acids?

Passage Aldehydes are organic compounds encountered in everyday life and used as intermediates in metabolism.  Common aldehydes include formaldehyde, which is used in the fixation of tissues, and cinnamaldehyde, which provides the flavor and smell of cinnamon (Figure 1). Figure 1  Common aldehydesThe aldol condensation shown in Figure 2 is an important carbon-carbon bond-forming reaction that has many applications in the synthesis of bioactive molecules.  It also takes place in some metabolic processes, such as the citric acid cycle and gluconeogenesis.  The aldol condensation begins with the aldol reaction, which can be acid- or base-catalyzed and requires nucleophilic addition to a carbonyl to provide the aldol product, which can be a β-hydroxy ketone or aldehyde.  The final step of the aldol condensation consists of an acid- or base-catalyzed dehydration of the aldol product to yield an α,β-unsaturated ketone or aldehyde. Figure 2  General aldol reaction and condensationThe reverse of an aldol reaction or condensation is known as a retro-aldol, which is important in metabolism, most notably in glycolysis.  This reaction breaks a carbon-carbon bond to form aldehydes and/or ketones.  In glycolysis, fructose 1,6-bisphosphate is broken down into an aldehyde and a ketone, glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP), respectively (Figure 3). Figure 3  Retro-aldol in glycolysis -Which of the following reagents CANNOT be used to make benzoic acid from benzaldehyde?

Passage The drug paracetamol, also known as acetaminophen, is used to relieve pain and fever, and is a metabolite of the antipyretic drug phenacetin (Figure 1).  These two drugs have similar medicinal properties, but phenacetin has been shown to be carcinogenic and cause kidney damage.  Acetaminophen is a safer alternative to phenacetin when taken in therapeutic doses. Figure 1  Structure of phenacetinA student synthesizes acetaminophen in a laboratory from phenol according to the reactions shown.  The synthesis begins with the nitration of phenol by sodium nitrate (NaNO₃) to produce the mixture of the isomers 2-nitrophenol and 4-nitrophenol (Reaction 1).  These isomers can be easily separated by steam distillation to isolate the desired product, 4-nitrophenol.  Reduction of the nitro group with sodium borohydride (NaBH₄) (Reaction 2) is followed by acetylation of the amine with acetic anhydride (Reaction 3) to afford the final product, acetaminophen. Reaction 1 Reaction 2 Reaction 3 - Normal phase thin layer chromatography (TLC) was used to monitor the formation of acetaminophen.  According to the TLC plate shown, which spot has an R_f value of 0.33, and what compound most likely corresponds to this spot?

Researchers want to synthesize valine from isovaleric acid.  The enol shown below, with certain carbon atoms labeled, is formed from the reaction of isovaleric acid with PBr₃. If Br₂ and then water are added to the enol, which of the following carbon atoms will be brominated in the final product?