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Kyoto University 2013 Entrance Examination—Chemistry (Partial)

Problem III

Read the following paragraphs (a) and (b), and answer the following questions 15. Follow the example when drawing a structural formula. Note that it is not necessary to distinguish geometric or optical isomers, if relevant. Use the following values if necessary: Atomic mass: H = 1.00, C = 12.0, O = 16.0; Volume of 1.00 mol of gas is 22.4 L.

Example of a structural formula:



(a) Hydrogen gas was added to a flask containing 10.0 g compound A (molecular formula: C16H18O4) and a platinum catalyst. The subsequent reaction yielded compound B while consuming 1.64 L hydrogen gas at atmospheric pressure. The contents of the reaction flask were maintained in the standard state.
Hydrolysis of compound B yielded compounds C and D. Compound C underwent a dehydration reaction upon heating in the presence of concentrated sulfuric acid, yielding 1-butene. Oxidation of compound C using a sulfuric acid solution of potassium dichromate yielded compound E, and further oxidation yielded carboxylic acid. On the other hand, compound D underwent dehydration upon heating to yield an aromatic acid anhydride. This aromatic acid anhydride reacted with one molecule of methanol to yield a compound having a molecular weight of 180. Hydrolysis of compound A yielded compound D as well as compounds E and F, which were structural isomers. Note that compound F did not have a geometric isomer.

Question 1
: Referring to the underlined text, how many moles of hydrogen reacted with 1 mole of compound A? Provide your answer in the form of an integer.

Question 2: Compound D may be synthesized by oxidizing an aromatic compound. Provide the chemical name of one such aromatic compound.

Question 3: Draw the structural formulae of compounds A, C, E, and F.

Question 4: The molecular weights of compounds E and F are identical, but their boiling points are 85°C and 114°C, respectively. Explain within the frame of the answer column why their boiling points differ.



(b) Consider a compound expressed by the molecular formula C9H10O and having a cyclic structure in which adjacent carbon atoms in the ring are connected as in benzene. Compounds G, H, I, and J include a 5-membered ring (a ring structure made of five atoms) that includes two adjacent asymmetric carbon atoms that form a benzene-like bond. Compound G reacts with sodium metal to produce hydrogen gas. The hydrogen atoms bonded to asymmetric carbon atoms in compounds H, I, and J are replaced with hydroxyl groups in compounds K, L, and M. Compounds L and M include a hemiketal structure (Fig. 1). A ketonic compound N is in equilibrium with the hemikeral L and contains an alcoholic hydroxyl group that is not a phenol.


                        
                       Figure 1. Hemiketal structure

Question 5: Draw the structural formulae of compounds G, H, L, and N.

Problems IV

Certain bacteria synthesize peptidoglycans, which are a structural component of cell walls and are composed of a polysaccharide linked to a polypeptide. Figure 1 illustrates one such structure, where S1–S2 indicate monosaccharides that make up the polysaccharide and A1–A5 indicate amino acids that make up the polypeptide. Read the following paragraphs (a) and (b), and answer the questions 1–4.

                               Figure 1. A peptidoglycan containing a pentapeptide moiety.


(a) The peptidoglycan illustrated schematically above in Fig. 1 contains a polysaccharide composed of a linear chain of the alternating N-acetylglucosamine (S1) and N-acetylmuramic acid (S2) monosaccharides connected by a β-1,4-glycosidic bond. N-acetylglucosamine is formed from a glucose molecule, in which the hydroxyl group bonded to the carbon atom at the 2-position is replaced with an acetylated amino group. N-acetylmuramic acid is formed from an N-acetylglucosamine molecule, in which the hydroxyl group bonded to the carbon atom at the 3-position is replaced with an ether linkage to a lactate moiety.

Question 1: Select the chemical structure, from the following choices (i)–(iv), that correctly shows the repeating disaccharide unit composed of the N-acetylglucosamine (S1) and N-acetylmuramic acid (S2) monosaccharides, and identify your selected answer by writing the corresponding numeral (i)–(iv) in the space provided. Note that the numbers (1)–(6) in each chemical structure indicate the carbon atom position numbers. Note also that R1–R4 represent functional groups bonded to the N-acetylmuramic acid structure, the chemical structures of which are shown to the right of the corresponding chemical structures.

(i)

(ii)

 (iii)

(iv)

(b) As shown in Fig. 1, two types of polypeptides are present in a peptidoglycan: tetrapeptides composed of four amino acids, A1–A4, and linked through amide linkages to the N-acetylmuramic acid (S2) in the polysaccharide, and pentapeptides composed of five A5 amino acid groups linked to the tetrapeptides. The bonds between A2 and A3 and between A3 and A5 are unusual in that an amide group of one amino acid is bonded to the side chain (R) functional group of the other amino acid. Hydrolysis of the peptidoglycan shown in Fig. 1 produced amino acids having the chemical structures shown in Fig. 2. The isoelectric points of these amino acids are listed in Table 1. A1 is an optical isomer of A4, and A5 does not have an optical isomer.

 

Figure 2. Chemical structures of an amino acid, the side chains (R) corresponding to the amino acids produced by hydrolysis of the peptidoglycan shown in Fig. 1, and the names of these amino acids.


Table 1

Amino acids Isoelectric points
A1 6.00
A2 3.22
A3 9.74
A4 6.00
A5 5.97


Question 2: List the names corresponding to each of the amino acids A1–A5 using the information provided. Note that it is not necessary to distinguish the optical isomers, if relevant.

Question 3: How many different types of dipeptides may be produced by linking one molecule of A2 to one molecule of A3 through an amide bond? Note that it is not necessary to distinguish the optical isomers, if relevant.

Question 4: Draw the chemical structure of the A2–A3 dipeptide contained in the peptidoglycan shown in Fig. 1. Follow the example when drawing the chemical structure. Note that it is not necessary to draw the optical isomers. Circle the functional groups involved in the amide linkage to A1 (using a solid line) and to A5 (using a dashed line) in the context of the peptidoglycan, as shown in the example.

Example of an amino acid. Here, the primary amine is circled using a solid line to indicate a linkage to A1 and the carboxylic acid is circled using a dashed line to indicate a linkage to A5 in the context of the peptidoglycan.


(End of the problems)

*The answers are available and please confirm if necessary.