[기출문제] 홍익대 서울캠 유기화학 2024-1 중간 기출문제 (정답 포함)

1. [18 points] Uracil is one of the bases for RNA.

(a) Identify the most acidic proton in uracil, and explain your choice with proper resonance structures.

(b) Dray a resonance hybrid Of the conjugate base of the uracil.

2. [12 points] Which isomer in each pair of trisubstituted cyclohexanes is more stable? Explain with chair conformation of each.

3. [10 points] C>H16 can have many constitutional isomers. Among them:
(a) Draw the skeletal structure of the isomer that would have the highest boiling point.

(b) Draw the skeletal structure of the isomer that would have the smallest gap between its boiling and melting point?

4. [12 points] 

(a) Name the given alkane, one component of jet fuels derived from plan and waste sources.
(a) 2, 6- dimethyl- b- propyloctane.

(b) Write a balanced equation for the combustion of this alkane with O2

(c) Explain whysing plant based fuels helps to reduce the carbon footprint of jet fuels.

5. [16 points] Draw all possible stereoisomers of each compound. Label pairs of enantiomers, diastereomers, and meso, including R, S configuration.

6.[14 points] Captopril is a drug used to treat high blood
(a) Designate each stereogenic center as R or S.
(b)Draw the enantiomer of Captopril.
(c) Which H is the most acidic in Captopril? Explain why.
(d) If the specific rotation of Captopril Captopril and 25% of its enantiomer?
(e) What is the ee of a solution with [a] = +10.5

7. [20 points] For 2,2,3-trimethylpentane draw the 6 Newman projections looking down the C3-C4 bond that describe the 3 staggered and 3 eclipsed rotamers. Label them A-F to match them with the energy levels A-F shown in the plot.

1. (a) The most acidic proton in uracil is the proton attached to the nitrogen at position N1. This is because the conjugate base formed by deprotonation at N1 is highly stabilized by resonance involving the ring nitrogens and adjacent carbonyl groups. Experimental and computational studies confirm that the N1 proton is significantly more acidic than others such as N3 hydrogens.

(b) The resonance hybrid of the conjugate base of uracil after losing the N1 proton shows delocalization of the negative charge onto the oxygen atoms of the carbonyl groups and the ring nitrogens, stabilizing the anion. The resonance structures alternate the negative charge over ring oxygens and nitrogens, creating a stabilized conjugate base.

2. The more stable isomer in trisubstituted cyclohexane pairs is usually the one with bulky substituents in the equatorial positions rather than axial, due to minimized 1,3-diaxial steric interactions. Chair conformations placing larger groups equatorially reduce strain and steric hindrance, making them more stable.

3. (a) The isomer of C8H16 with the highest boiling point will be the least branched, typically the straight or longest-chain alkene, since less branching increases surface area and intermolecular forces. For example, 1-octene or linear octene derivatives have higher boiling points.
(b) The isomer with the smallest gap between boiling and melting points is often a highly branched or symmetrical cycloalkane, where melting and boiling points are closer due to crystal packing and intermolecular interactions. Specific examples can be cycloalkanes or heavily branched structures.

4. (a) The correct IUPAC name: 2,6-dimethyl-b-propyloctane (as given) refers to a branched alkane component of jet fuels derived from plant/waste sources.
(b) Balanced combustion reaction of C (2,6-dimethyl-b-propyloctane) with oxygen 
(c) Plant-based fuels reduce carbon footprint because the CO2 emitted during combustion is partially offset by the CO2 absorbed during the plants’ growth, creating a near carbon-neutral cycle, unlike fossil fuels which add ancient carbon to the atmosphere.

5. For each compound, draw all stereoisomers considering chiral centers. Label enantiomers (non-superimposable mirror images), diastereomers (stereoisomers not mirror images), and meso compounds (achiral compounds with chiral centers). Assign R/S configurations to each stereogenic center via Cahn-Ingold-Prelog rules.

6. (a) Assign R or S to each stereogenic center in captopril by prioritization and orientation rules.
(b) Draw the enantiomer by inverting configurations at all chiral centers.
(c) The most acidic hydrogen in captopril is typically the one on the thiol (-SH) group due to sulfur’s ability to stabilize the conjugate base.
(d) If a solution contains 75% captopril and 25% of its enantiomer, the specific rotation would be 75% of the pure enantiomer’s value.
(e) Enantiomeric excess (ee) is calculated byassuming +14 is the specific rotation of pure captopril.

7. Draw six Newman projections looking down the C3-C4 bond:

•        3 staggered conformers correspond to energy minima (labeled A, C, E).
•        3 eclipsed conformers with energy maxima (B, D, F).
Label the conformers from lowest to highest energy based on torsional strain and steric hindrance of methyl and other substituents.