Homework Assignment #3

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1 Chemistry Spring 2016 Homework Assignment #3 1. Determine whether each of the following statements is true or false. If the statement is false, modify and rewrite it so that it is a true statement. a. When a molecule has two, degenerate, infrared active, vibrational modes, the two vibrational modes will show absorptions at different frequencies in the infrared spectrum. b. For a given substance, strong intermolecular forces between molecules of the substance can cause peak broadening of some of the absorptions in the infrared spectrum of the substance. 2. List two reasons why water is seldom used as a solvent in sample preparation for infrared spectroscopy. 3. For triatomic molecules (i.e., molecules that contain three atoms) such as difluorine monoxide, F 2 O, there are only two possible molecular geometries, linear or bent. a. Draw the Lewis structure for F 2 O (O is the central atom), and predict the molecular geometry using the VSEPR model. b. Could infrared spectroscopy be used to confirm the molecular geometry that is predicted using the VSEPR model? That is, could the infrared spectrum for F 2 O be used to determine whether the molecule is linear or bent? Explain your reasoning. 4. In the infrared spectrum of the alkene, 1-propene, the normal mode for =CH 2 stretching occurs at a frequency of 3080 cm -1. Would you expect the normal mode for /CH stretching in the alkyne, 1-propyne, to occur at a frequency that is higher, lower, or the same as, that for 1-propene? Explain your reasoning. H H H H C C C H H 1-propene (an "alkene") H H H C C C H 1-propyne (an "alkyne")

2 Chemistry Spring What is a functional group? Explain why infrared spectroscopy can often be used to detect the presence of a functional group in a molecule. 6. For infrared spectroscopy, liquid samples of organic compounds are commonly prepared neat by sandwiching the sample between two NaCl plates. List two reasons why NaCl (rather than some other substance) is used. 7. The force constant for the carbon-carbon stretching mode in acetylene is 1.5 x 10 6 dynes/cm. H C C H acetylene Calculate the frequency (in cm -1 ) for the carbon-carbon stretching mode of acetylene. 8. The bending normal mode for water occurs at about 1885 cm -1 in the infrared spectrum. Explain what happens when an H 2 O molecule absorbs infrared light of this frequency; that is, how is the vibrational motion of the molecule affected by the absorption of the infrared light? 9. List three uses for infrared spectroscopy. 10. Which of the following molecules will not absorb infrared light of any frequency? Explain your reasoning. I. CO II. HI III. N 2 2

3 Chemistry Spring Two possible structures for chlorite ion, ClO 2 -, and the expected normal modes for each structure are shown below O.. Cl.. Ȯ.. linear O... O..... Cl bent - linear O-Cl symmetric stretch O-Cl asymmetric stretch O-Cl-O bend (xy plane) bent O-Cl symmetric stretch O-Cl asymmetric stretch O-Cl-O bend O-Cl-O bend (xz plane) a. For each structure, determine which normal modes would be infrared active, and which would be infrared inactive. Explain your reasoning. Note: a picture is worth a thousand words! The Pauling electronegativities for O and Cl are 3.5 and 2.8, respectively. b. Can infrared spectroscopy be used to determine whether the structure of the ClO 2 - ion is linear or bent? Explain your reasoning. 12. Identify the molecular geometry for each of the following triatomic molecules using the VSEPR model. Then, determine which molecules would be expected to have at least one normal mode that is infrared active. Explain your reasoning. a. NO 2 b. + PCl 2 c. - BrF 2 d. - N 3 3

4 Chemistry Spring The infrared spectrum for HCN (a linear molecule) was obtained using an infrared spectrophotometer which operates in the range: cm -1. The spectrum shows three absorption bands at 3311 cm -1, 2097 cm -1 and 712 cm -1. a. Explain how the number of normal vibrational modes can be determined for any molecule. b. For HCN, the types of vibrational motion are: i. C-N stretching ii. H-C-N bending iii. C-H stretching Complete the following Table by identifying the type of vibrational motion associated with each absorption band in the infrared spectrum for HCN. wavenumber, cm type of vibrational motion c. HCN has four normal vibrational modes. Explain why the infrared spectrum shows only three absorptions (instead of four). 4

5 Chemistry Spring Infrared spectra for four different organic compounds are shown on the following four pages. For each compound, identify the functional group (i.e., alkane (contains CH 2 and CH 3 groups only), alkene, alcohol, aldehyde, ketone, carboxylic acid, or ester) that the compound contains. Note: all four compounds contain CH 2 and/or CH 3 groups. 5

6 Compound 1 6

7 Compound 2 7

8 Compound 3 8

9 Compound 4 9

ORGANIC - BROWN 8E CH INFRARED SPECTROSCOPY.

ORGANIC - BROWN 8E CH INFRARED SPECTROSCOPY. !! www.clutchprep.com CONCEPT: PURPOSE OF ANALYTICAL TECHNIQUES Classical Methods (Wet Chemistry): Chemists needed to run dozens of chemical reactions to determine the type of molecules in a compound.