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chemistry
organic chemistry

Questions and Answers of Organic Chemistry

Identify the reagents appropriate for each step in the following syntheses:
Give the structure of the product derived by intramolecular aldol condensation of the keto aldehyde shown:
Prepare each of the following compounds from the starting materials given and any necessary organic or inorganic reagents:
Terreic acid is a naturally occurring antibiotic substance. Its actual structure is an enol isomer of the structure shown. Write the two most stable enol forms of Terreic acid, and choose which of
In each of the following, the indicated observations were made before any of the starting material was transformed to aldol addition or condensation products: (a) In aqueous acid, only 17% of
(a) For a long time attempts to prepare compound A were thwarted by its ready isomerization to compound B, The isomerization is efficiently catalyzed by traces of base. Write a reasonable mechanism
Consider the ketones piperitone menthone, and isomenthone.
Many nitrogen-containing compounds engage in a proton-transfer equilibrium that is analogous to keto–enol tautomerism: Each of the following compounds is the less stable partner of such a
Outline reasonable mechanisms for each of the following reactions:
Suggest reasonable explanations for each of the following observations: (a) The C═O stretching frequency of a, ß -unsaturated ketones (about 1675 cm-1) is less than that of typical dialkyl
Bromination of 3-methyl-2-butanone yielded two compounds, each having the molecular formula C5H9BrO, in a 95:5 ratio. The 1H NMR spectrum of the major isomer A was characterized by a doublet at
Treatment of 2-butanone (1 mol) with Br2 (2 mol) in aqueous HBr gave C4H6Br2O. The 1H NMR spectrum of the product was characterized by signals at 1.9 ppm (doublet, 3 protons), 4.6 ppm (singlet, 2
What is the absolute configuration (R or S) at the a carbon atom in each of the following L-amino acids?
Which of the amino acids in Table 27.1 have more than one stereogenic center?
Write the most stable structural formula for tyrosine: (a) In its cationic form (c) As a monoanion (b) In its zwitterionic form (d) As a dianion
Write structural formulas for the principal species present when the pH of a solution containing lysine is raised from 1 to 9 and again to 13.
Outline the steps in a synthesis of valine from 3-methylbutanoic acid.
Outline the steps in the preparation of valine by the Strecker synthesis.
Outline the steps in the synthesis of valine from diethyl acetamidomalonate. The overall yield of valine by this method is reported to be rather low (31%). Can you think of a reason why this
Suggest a reasonable mechanism for the reaction of an a-amino acid with ninhydrin.
One of the amino acids in Table 27.1 is the biological precursor to y-aminobutyric acid (4-aminobutanoic acid), which it forms by a decarboxylation reaction. Which amino acid is this?
Write structural formulas showing the constitution of each of the following dipeptides. Rewrite each sequence using one-letter abbreviations for the amino acids. (a) Gly-Ala (d) Gly-Glu (b)
Expand your answer to Problem 27.10 by showing the structural formula for each dipeptide in a manner that reveals the stereochemistry at the a-carbon atom.
What is the amino acid sequence (using three-letter abbreviations) of methionine enkephalin? Show it using one-letter abbreviations.
Amino acid analysis of a certain tetrapeptide gave alanine, glycine, phenylalanine, and valine in equimolar amounts. What amino acid sequences are possible for this tetrapeptide?
Digestion of the tetrapeptide of Problem 27.13 with chymotrypsin gave a dipeptide that on amino acid analysis gave phenylalanine and valine in equimolar amounts. What amino acid sequences are
Give the structure of the PTH derivative isolated in the second Edman cycle of the tetrapeptide Val-Phe-Gly-Ala.
Lysine reacts with two equivalents of benzyloxycarbonyl chloride to give a derivative containing two benzyloxycarbonyl groups. What is the structure of this compound?
Show the steps involved in the synthesis of Ala-Leu from alanine and leucine using benzyloxycarbonyl and benzyl ester protecting groups and DCCI-promoted peptide bond formation.
p-Nitrophenyl esters are made from Z-protected amino acids by reaction with p-nitrophenol in the presence of N, N’-dicyclohexylcarbodiimide. Suggest a reasonable mechanism for this reaction.
Show how you could convert the ethyl ester of Z-Phe-Gly to Leu-Phe-Gly (as its ethyl ester) by the active ester method.
Starting with phenylalanine and glycine, outline the steps in the preparation of Phe-Gly by the Merrifield method.
5-Fluorouracil is a drug used in cancer chemotherapy. What is its structure?
The names of the principal nucleosides obtained from RNA and DNA are listed. Write a structural formula for each one. (a) Thymidine (thymine-derived nucleoside in DNA) (b) Cytidine
It was pointed out in Section 27.22 that sickle cell hemoglobin has valine in place of glutamic acid at one point in its protein chain. Compare the codons for valine and glutamic acid. How do they
The imidazole ring of the histidine side chain acts as a proton acceptor in certain enzymecatalyzed reactions. Which is the more stable protonated form of the histidine residue, A or B? Why?
Acrylonitrile (CH2=CHC≡PN) readily undergoes conjugate addition when treated with nucleophilic reagents. Describe a synthesis of 0-alanine (H3NCH2CH2CO2) that takes advantage of this fact.
(a) Isoleucine has been prepared by the following sequence of reactions. Give the structure of compounds A through D isolated as intermediates in this synthesis. (b) An analogous procedure has
Hydrolysis of the following compound in concentrated hydrochloric acid for several hours at 100°C gives one of the amino acids in Table 27.1. Which one? Is it optically active?
If you synthesized the tripeptide Leu-Phe-Ser from amino acids prepared by the Strecker synthesis, how many stereoisomers would you expect to be formed?
How many peaks would you expect to see on the strip chart after amino acid analysis of bradykinin? Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg Bradykinin
Automated amino acid analysis of peptides containing asparagine (Asn) and glutamine (Gln) residues gives a peak corresponding to ammonia. Why?
What are the products of each of the following reactions? Your answer should account for all the amino acid residues in the starting peptides. (a) Reaction of Leu-Gly-Ser with 1-fluoro-2,
Hydrazine cleaves amide bonds to form acylhydrazides according to the general mechanism of nucleophilic acyl substitution discussed in Chapter 20: This reaction forms the basis of one method of
Somatostatin is a tetradecapeptide of the hypothalamus that inhibits the release of pituitary growth hormone. Its amino acid sequence has been determined by a combination of Edman degradations and
What protected amino acid would you anchor to the solid support in the first step of a synthesis of oxytocin (see Figure 27.8) by the Merrifield method?
Nebularine is a toxic nucleoside isolated from a species of mushroom. Its systematic name is 9-ß-D-ribofuranosylpurine. Write a structural formula for nebularine.
The nucleoside vidarabine (Ara-A) shows promise as an antiviral agent. Its structure is identical with that of adenosine (Section 27.24) except the D-arabinose replaces D-ribose as the carbohydrate
When 6-chloropurine is heated with aqueous sodium hydroxide, it is quantitatively converted to hypoxantine. Suggest a reasonable mechanism for this reaction.
(a) The 5`-nucleotide of inosine, inosinic acid (C10H13N4O8P), is added to foods as a flavor enhancer. What is the structure of inosinic acid? (The structure of inosine is given in Problem
Treatment of adenosine with nitrous acid gives a nucleoside known as inosine:
In one of the early experiments designed to elucidate the genetic code, Marshall Nirenberg of the U.S. National Institutes of Health (Nobel Prize in physiology or medicine, 1968) prepared a synthetic
Identify each of the following as either D- or L-glyceraldehyde:
Which aldotetrose is the structure shown? Is it D-erythrose, D-threose, L-erythrose, or L-threose? (Be careful! The conformation given is not the same as that used to generate a Fischer projection.)
L-(+)-Arabinose is a naturally occurring L sugar. It is obtained by acid hydrolysis of the polysaccharide present in mesquite gum. Write a Fischer projection for L-(+)-arabinose.
Name the following sugar
Write Haworth formulas corresponding to the furanose forms of each of the following carbohydrates: (a) D-Xylose (c) L-Arabinose (b) D-Arabinose (d) D-Threose
Clearly represent the most stable conformation of the -pyranose form of each of the following sugars: (a) D-Galactose (c) L-Mannose (b) D-Mannose (d) L-Ribose
The specific optical rotations of pure a- and ß-D-mannopyranose are +29.3° and -17.0°, respectively. When either form is dissolved in water, mutarotation occurs, and the observed rotation of the
How many ketotetroses are possible? Write Fischer projections for each.
Write Fischer projections of (a) Cordycepose (3-deoxy-D-ribose): a deoxy sugar isolated by hydrolysis of the antibiotic substance cordycepin (b) L-Fucose (6-deoxy-L-galactose): obtained from seaweed
Write structural formulas for the a- and ß-methyl pyranosides formed by reaction of D-galactose with methanol in the presence of hydrogen chloride.
Methyl glycosides of 2-deoxy sugars have been prepared by the acid-catalyzed addition of methanol to unsaturated sugars known as glycals.
The two stereoisomeric forms of maltose just mentioned undergo mutarotation when dissolved in water. What is the structure of the key intermediate in this process?
Does sodium borohydride reduction of D-ribose yield an optically active product? Explain.
Which of the following would be expected to give a positive test with Benedict’s reagent? Why? (a) D-Galactitol (see structure in margin) (d) D-Fructose (b) L-Arabinose (e) Lactose (c) 1,
Another hexose gives the same aldaric acid on oxidation as does D-glucose. Which one?
Suggest a reasonable structure for the intermediate in the conversion of dihydroxyacetone phosphate to D-glyceraldehyde 3-phosphate
Give the products of periodic acid oxidation of each of the following. How many moles of reagent will be consumed per mole of substrate in each case? (a) D-Arabinose (b) D-Ribose (c) Methyl
Refer to the Fischer projection of D-(+)-xylose in Figure 25.2 (Section 25.4) and give structural formulas for (a) (-)-Xylose (Fischer projection) (b) D-Xylitol (c) ß-D-Xylopyranose (d)
From among the carbohydrates shown in Figure 25.2, choose the D-aldohexoses that yield (a) An optically inactive product on reduction with sodium borohydride (b) An optically inactive product on
Write the Fischer projection of the open-chain form of each of the following:
What are the R, S configurations of the three stereogenic centers in D-ribose? (A molecular model will be helpful here.)
From among the carbohydrates shown in Problem 25.20 choose the one(s) that (a) Belong to the L series (b) Are deoxy sugars (c) Are branched-chain sugars (d) Are ketoses (e) Are furanose
How many pentuloses are possible? Write their Fischer projections.
The Fischer projection of the branched-chain carbohydrate D-apiose has been presented in Section 25.12 (a) How many stereogenic centers are in the open-chain form of D-apiose? (b) Does D-apiose
Maltose and cellobiose (Section 25.14) are examples of disaccharides derived from Dglucopyranosyl units. (a) How many other disaccharides are possible that meet this structural requirement? (b) How
Gentiobiose has the molecular formula C12H22O11 and has been isolated from gentian root and by hydrolysis of amygdalin. Gentiobiose exists in two different forms, one melting at 86°C and the other
Cyanogenic glycosides are potentially toxic because they liberate hydrogen cyanide on enzyme-catalyzed or acidic hydrolysis. Give a mechanistic explanation for this behavior for the specific cases of
The following are the more stable anomers of the pyranose forms of D-glucose, D-mannose, and D-galactose:
Basing your answers on the general mechanism for the first stage of acid-catalyzed acetal hydrolysis
D-Altrosan is converted to D-altrose by dilute aqueous acid. Suggest a reasonable mechanism for this reaction.
When D-galactose was heated at 165°C, a small amount of compound A was isolated:
Phlorizin is obtained from the root bark of apple, pear, cherry, and plum trees. It has the molecular formula C21H24O10 and yields a compound A and D-glucose on hydrolysis in the presence of emulsin.
Emil Fischer’s determination of the structure of glucose was carried out as the nineteenth century ended and the twentieth began. The structure of no other sugar was known at that time, and none of
Treatment of D-mannose with methanol in the presence of an acid catalyst yields four isomeric products having the molecular formula C7H14O6. What are these four products?
The list of carboxylic acids in Table 19.1 is by no means exhaustive insofar as common names are concerned. Many others are known by their common names, a few of which follow. Give a systematic IUPAC
Peroxyacetic acid CH3COOH is a weaker acid than acetic acid; its Ka is 6.3 x 10-9 (pKa 8.2) versus 1.8 x 10-5 for acetic acid (pKa 4.7). Why are peroxy acids weaker than carboxylic acids?
Write an ionic equation for the reaction of acetic acid with each of the following, and specify whether the equilibrium favors starting materials or products: (a) Sodium ethoxide (d) Sodium
Which is the stronger acid in each of the following pairs?
What is the most acidic neutral molecule characterized by the formula C3HxO2?
The value cited for the “apparent K1” of carbonic acid, 4.3 x 10-7, is the one normally given in reference books. It is determined by measuring the pH of water to which a known amount of carbon
Of the two procedures just described, preparation and carboxylation of a Grignard reagent or formation and hydrolysis of a nitrile, only one is appropriate to each of the following RX → RCO2H
When benzoic acid is allowed to stand in water enriched in 18O, the isotopic label becomes incorporated into the benzoic acid. The reaction is catalyzed by acids. Suggest an explanation for this
Write the structure of the hydroxy acid corresponding to each of the following lactones. The structure of each lactone is given in Figure 19.7. (a) Mevalonolactone (b) Pentadecanolide (c)
A-lodo acids are not normally prepared by direct iodination of carboxylic acids under conditions of the Hell–Volhard–Zelinsky reaction. Show how you could convert octadecanoic acid to its 2-iodo
What will be the product isolated after thermal decarboxylation of each of the following? Using curved arrows, represent the bond changes that take place at the transition state.
Show the bonding changes that occur, and write the structure of the intermediate formed in the thermal decarboxylation of (a) Benzoylacetic acid (b) 2, 2-Dimethylacetoacetic acid
Many carboxylic acids are much better known by their common names than by their systematic names. Some of these follow. Provide a structural formula for each one on the basis of its systematic
Give an acceptable IUPAC name for each of the following: (a) CH3 (CH2)6CO2H (e) HO2C (CH2)6CO2H (b) CH3 (CH2)6CO2K (f) CH3 (CH2)4CH (CO2H)2 (c) CH2=CH (CH2)5CO2H

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