仪器分析 NMR作业答案

Answers to first assignment for NMR

1.State the number of sets of equivalent hydrogens in each compound and the number of hydrogens in each set.（

(a) 3-Methylpentane (b) 2,2,4-Trimethylpentane

Solution：

(a) 2-Methylpropane has two sets of equivalent hydrogens: 9 equivalent primary hydrogens and 1 tertiary hydrogen.

(b) 2-Methylbutane contains four sets of equivalent hydrogens: 2 sets of 3 primary hydrogens, 1 set of 3 primary hydrogens, 1 set of 2 secondary hydrogens, and 1 tertiary hydrogen.

2.Each compound gives only one signal in its 1H NMR spectrum. Propose a structural formula for each compound.

(a) C2H6O (b) C3H6Cl2(c) C6H12(d) C4H6

3.Each compound gives only one signal in its 1H NMR spectrum. Propose a structural formula for each compound.

C3H6O (b) C5H10(c) C5H12(d) C4H6Cl 4

(a)

4.The line of integration of the two signals in the 1H NMR spectrum of a ketone with the molecular formula C7H14O increases by 62 and 10 chart divisions, respectively. Calculate the number of hydrogens giving rise to each signal, and propose a structural formula for this ketone.

5.The following are structural formulas for two constitutional isomers with the molecular formula C6H12O2

(a)Predict the number of signals in the1H NMR spectrum of each isomer.

(b)Predict the ratio of areas of the signals in each spectrum.

(c) Show how you can distinguish between these isomers on the basis of chemical shift.

6.Predict the number of signals and the splitting pattern for each signal in the 1H NMR spectrum of each of the following molecules:

The sets of equivalent protons in each molecule are labeled a, b, and c. Molecule

(a) has 3 sets of equivalent protons with a singlet, a quartet, and a triplet in the ratio 3 :

2 : 3. (b) has two sets of equivalent protons with a triplet and a quartet in the ratio

3 : 2. (c) has three sets of equivalent protons with a singlet, a septet, and a doublet in the ratio 3 : 1 : 6.

7.The following are pairs of constitutional isomers. Predict the number of signals in the 1H NMR spectrum for each isomer and the splitting pattern of each signal.

Solution：

(a)The 1H NMR spectrum of each consists of two signals

(b)in the ratio 9:3, or 3:1.

(c) We can distinguish these isomers by the chemical shift of the single -CH3 group. The hydrogens of CH3O are more deshielded (appear farther downfield) than the hydrogens of CH3C=O. The approximate values for each chemical shift (taken from the table given earlier) are given between parentheses and the experimentally determined values are shown in pink.

8.Spectral Problem: Molecular formula C5H10O –What is the structural formula? Pls give 6 steps to determine the structure of an unknown:

Step 1: Molecular formula and index of hydrogen deficiency. The reference alkane compound is C5H12 (12H), while the unknown has 10H, no halogen and no nitrogen, so the index of hydrogen deficiency is (12 – 10) / 2 = 1, and the molecule contains either one ring or one bond.

Index of hydrogen deficiency = (Hreference – Hunknown) / 2, where Hreference = 12 for C5H12 (reference alkane) and Hunknown = 10 (no correction necessary for O)

Step 2: Number of signals. There are two signals (a triplet and a quartet) and, therefore, two sets of equivalent hydrogens.

Step 3: Integration. From the integration, the hydrogens in each set are in the ratio 3:2. Because there are 10 hydrogens, 6H must give rise to the signal at 1.07, and 4H must give rise to the signal at 2.42.

Step 4: Pattern of chemical shifts. The signal at 1.07 is in the alkyl region and, based on its chemical shift, probably indicates a methyl group. No signal occurs between 4.6-5.7, so there are no vinylic hydrogens. If a C=C bond is in the molecule, there are no hydrogens on it (it is tetrasubstituted). The chemical shift of the 4H at 2.42 is consistent with two CH2 groups next to a carbonyl group.

Step 5: Signal splitting patterns. The methyl signal at 1.07 is split into a triplet (t), so it must have two neighbors, indicating -CH2CH3 . The signal at 2.42 is split into a quartet (q), so it must have three neighbors. An ethyl group is consistent with these two signals. No other signals occur in the spectrum, so there are no other types of hydrogens in the molecule.

Step 6: Structural formula. Put this information together to arrive at the following

structural formula. The chemical shift of the methylene group (–CH2–) at 2.42 is consistent with an alkyl group adjacent to a carbonyl group.

9.Following are three compounds with the molecular formula C4H8O2, and three 1H NMR spectra. Assign each compound its correct spectrum and assign all signals to their corresponding hydrogens.

Solution：

10.F ollowing are 1H NMR spectra for compounds G, H, and I, all with the molecular formula C5H12O. Each is a liquid at room temperature, is slightly soluble in water, and reacts with sodium metal with the evolution of a gas.

Solution：