Chemistry 221 Homework set 3

Chem 22 Homework set 3 1. (a) How many lines (peaks) would be observed in the proton-decoupled ("normal") 13C NMR spectrum of each of the isomeric C8H12 hydrocarbons below? (b) The 13C NMR spectrum at right belongs to one of these 6 compounds. Which one? (c) In an off-resonance decoupled 13C NMR spectrum, the peaks at δ 145 and 128 all appear as doublets; the one at δ 112 appears as a triplet. Use this information and some "chemical intuition" to assign these lines to the vinylic Cs. 2 (a) An ester C7H12O2 has the 60-MHz 1H NMR spectrum shown below left. Draw the structure. (b) A compound with the formula C4H8O has the 60-MHz 1H NMR spectrum below right. When a drop of D2O is added to the sample, the 3-H signal at δ 2.3 becomes a 2-H quartet (1:3:3:1), and an HOD peak appears around δ 1.6. Draw the structure. 3. A compound C4H8O has the 180-MHz 1H NMR spectrum shown below. (a) Although the chemical shift is a bit unusual, based on the splitting patterns, what type of Hs are likely responsible for the signals near δ 4 (enlarged in the inset)? (b) Draw the structure. (c) Why have these Hs strayed so far (about 1 ppm) from their normal δ range? (Hint: think resonance structure...) 4. At right are the vinylic 1H NMR signals of one stereoisomer of 1-iodohexene, I—CHa=CHb—CH2–CH2CH2CH3. (a) Which one belongs to Ha and which one to Hb? (b) Draw a tree diagram for each H to account for the appearance of the signals. (c) Based on your analysis, is the compound E or Z? 5. (a) The o, m, and p protons of nitrobenzene appear at δ 8.22, 7.53, and 7.65, respectively. Explain why the o and p appear downfield of the meta. (b) The o, m, and p protons of aniline appear at δ 6.52, 7.02, and 6.62, respectively. Explain why the o and p appear upfield of the meta. Chem 22 Spring 2010 Name _________________________________________ HW set 3 25 points; due Wed, Feb 10 at the beginning of class. 1. (a) For each compound below, how many separate signals would you expect to see in a (room 13 1 temp) proton-decoupled C NMR spectrum, and how many in a H NMR spectrum? (In other words, how many Cl different sets of H N equivalent Cs and Hs are O present?) Write the O numbers in the spaces provided. Cl # 13 C NMR signals: 1 # H NMR signals: _______ _______ _______ _______ _______ _______ _______ _______ 13 2. The proton-decoupled C NMR spectrum at right belongs to one of the compounds shown. (a) Which one? 13 Please circle your choice. (You do not need a table of C δ values.) OH OH OH O O (b) In the structure you chose, draw a box around the C that is responsible for the signal at δ 73. 3. Two NMR signals of o-nitrophenol are upfield (δ7), and the other two are downfield (δ7.5 8) within the aromatic region. Why? ! 8.05 ! 6.94 NO2 ! 7.53 OH ! 7.10 4. A compound with the formula C10H10 displays IR stretches at about 3300 and 2150 cm–1. The compound's 1H NMR spectrum is shown. The number printed below the chemical shift scale are digital integrals. (a) Draw the structure. (b) Draw a tree diagram to account for the splitting pattern of the δ2.0 signal. 5. The 1H NMR spectrum of an ester with the formula C12H14O3 is shown below. (a) Draw the structure. (b) Explain the dramatic downfield shift of the signal at δ7.7.