CHEM30B_w10_e02_practice_key

CHEM30B_w10_e02_practice_key - Chemistry 30B 1. Exam 2...

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Chemistry 30B Exam 2 Practice Problems Solutions Page 1 1. (a) In x-ray crystallography x-ray photons are scattered or diffracted by regularly spaced atoms in a crystal . (b) In an early but very important application of x-ray crystallography, the Braggs measured (for the first time) carbon-carbon bond lengths and carbon-carbon-carbon bond angles in a substance called diamond . (c) Rosalind Franklin verified that DNA is a helical molecule when she saw an X pattern in photo 51, one of many DNA diffraction patterns produced by x-ray crystallography. 2. (a) No X-ray diffraction locates an atom is space, and can give some idea as to whether or not it has a higher atomic number than other atoms due to the intensity of the scattering, but it cannot reveal what elements are present. (b) Yes X-ray diffraction reveals the relative position of atoms in space, so stereochemistry can be assessed. (c) Yes X-ray diffraction reveals the relative position of atoms in space, so distance between atoms anywhere in the molecule can be measured. (d) Yes X-ray diffraction reveals the relative position of atoms in space, so the relative position of three (or more) atoms can be evaluated. 3. (a) Benzene ring planarity: Yes (b) Nitro group (–NO 2 ) resonance: No X-ray diffraction may reveal that the NO 2 atoms lie in the same plane and that the N–O bond lengths are equal, but these facts alone are not proof of resonance. X- ray diffraction revels the position of atoms, but not of electrons . (c) Oxygen-nitrogen bond length: Yes (d) Presence of lone pairs on oxygen in nitro group: No (e) Electronegativity of oxygen > nitrogen: No (f) Positive formal charge on nitrogen in nitro group: No 4. (a) OH OH Na 2 Cr 2 O 7 aq. H 2 SO 4 acetone O O OH (b) CH 3 CO 3 H (c) OH NaH O Na (d) OH 1. BH 3 2. NaOH, HOOH OH OH H
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Chemistry 30B Exam 2 Practice Problems Solutions Page 2 (e) O aq. HI I + I (f) OH HO HO PCC O O HO (g) OH Ph H HO O CrO 3 aq. H 2 SO 4 acetone O Ph OH HO O (h) OCH 3 aq. HI HO OH + CH 3 I I (i) OH OH TsCl pyridine NaSCH 3 acetone OTs OH SCH 3 OH The formula of the intermediate indicates it has just one OTs group. TsCl reacts with the less
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CHEM30B_w10_e02_practice_key - Chemistry 30B 1. Exam 2...

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