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1315

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Chemistry

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Dec 6, 2023

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HW #5, page 1 of 5 CHEM 1315 – Homework Set #5 (20 points) Due: 3:30 pm, Thursday, Nov. 16 th , 2023 Reading for this homework set: Klein volume 2, chapters 2 and 3, class notes lectures 19, 20 & 21. Key concepts: Light-matter interactions, UV-Vis Spectroscopy, Combustion Analysis, IR Spectroscopy, Mass Spectrometry, NMR Spectroscopy Please read the following instructions before proceeding:  Answers must be written inside the spaces provided or else they may not be found by Gradescope’s software. Anything written far out side the spaces will not be graded.  Upload this entire document in the correct order to Gradescope or else the software will have trouble organizing the pages properly.  You can reach Gradescope through the Canvas page.  Submit your answers as a PDF. 1. (3 pts) How many signals would you expect to see in the 1 H & 13 C NMR spectra of each compound below, (i.e. how many chemically distinct H & C atoms are present in each molecule?) (.5 pt each) # H signals: _ 1 _ __ 4 __ __ 4 __ # C signals: _ 1 _ __ 4 __ __ 5 __ Name:

HW #5, page 2 of 5 2. (2 pts) Combustion analysis on a molecule yields the following data: 54.53 wt% C, 9.15 wt% H, and 36.32 wt% O. Mass spectrometry data shows the M•+ peak has an m/z of 88 . What is the chemical formula of the molecule? Show your work. (note structure not required) C 4 H 8 O 2 Note: Work can be shown in a variety of ways; one method is shown below Carbon: 54.53% = 0.5453; 0.5453 * 88 = 47.98; mass of carbon = 12.01; # of carbons = 4 Hydrogen: 9.15% = 0.0915; 0.0915 * 88 = 8.052; mass of hydrogen = 1.00; # of hydrogens = 8 Oxygen: 36.32% = 0.3632; 0.3632 * 88 = 31.96; mass of oxygen = 16.00; # of oxygens = 2 3. (3 points) Below are pairs of molecules. You are given a sample that contains one of them, but you don’t know which. Using FTIR, describe a spectral feature, or combination of features, that can be used to unambiguously identify which of the two compounds is the sample (note: be specific – e.g. do not say “I’d expect the spectrum to have a C=O peak if it’s the molecule on the right”; also list the approximate cm -1 where you’d expect to find this peak) (1.5 pts each): Several possible answers: a) Molecule on left will show an N-H stretch around ca. 3300 cm -1 while molecule on right will not b) Molecule on right will show an alkene C-H stretch just above 3000 cm -1 while molecule on left has no alkene H’s present, and therefore will not show that alkene C-H stretch

HW #5, page 3 of 5 4. (3 points) Below are pairs of molecules. You are given a sample that contains one of them, but you don’t know which. Using 1H NMR or 13C NMR, describe a spectral feature, or combination of features, that can be used to unambiguously identify which of the two compounds is the sample (note: be specific – e.g. do not say “the splitting pattern is different”; quantify how many peaks or approximate ppm you’d expect) (1.5 pts each) : Many possible answers: a) Molecule on left will have 5 carbon peaks; molecule on right will have 6 carbon peaks Molecule on left will have 1 singlet in the alkyl peak region around 2 ppm; molecule on right will have a characteristic ethyl splitting (triplet with integral of 3 + quartet with integral of 2) between 1-3 ppm b) In the H NMR, molecule on left will have a carboxylic acid hydrogen singlet peak around 10-12 ppm; molecule on right will not (will have instead an alcohol singlet peak between 1-5 ppm) In the H NMR, molecule on right will have a methyl singlet with integral 3 around 2-3 ppm; none of the alkyl peaks on the left should be singlets between 1-5 ppm 5. A dye molecule is dissolved in a solvent, placed in a cuvette, and its UV-vis spectrum is shown at right. a) (1 pt) What energy of light in eV is being absorbed at 610 nm? Show your work. b) (1 pt) Given the molar absorptivity of the molecule is 122,000 M -1 cm -1 and the cuvette is 1 cm wide, what is the concentration of your sample? Show your work. eV = 1240 /  Beer's Law: Abs = molar absorptivity*path length*concentration eV = 1240 / 610 = 2.03 2 = 120,000 M -1 cm -1 * 1 cm * C; C = 1.66 * 10 -5 M a) b)

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HW #5, page 4 of 5 6. (7 points) A molecule has the formula: C 9 H 10 O 2 . Using the spectra below, answer the following: a) Calculate the degrees of unsaturation: __ 5 __ (1 pt) b) How many non-equivalent carbons are there? __ 9 __ (1 pt) c) In the 1 H NMR spectrum, what structural motif gives rise to peak a ? (write the name or draw the motif) ___ carboxylic acid ____ (1 pts) d) In the 13 C NMR spectrum, circle the peak that corresponds to the structural motif identified above in part c) (1 pt) e) In the FTIR spectrum, circle two peaks > 1500 cm -1 that correspond to the structural motif identified above in part c) (1 pt) f) In the 1 H NMR spectrum, how many neighboring H’s does peak d have? ___ 0 ___ (1 pt) g) Draw the molecule (1 pts)

HW #5, page 5 of 5