The IR and proton (1H NMR) and carbon (13C NMR) spectra of the molecules of IR Tutor are given along with correlation tables.

Transcription

1 C143 NMR omework The IR and proton (1 NMR) and carbon (13C NMR) spectra of the molecules of IR Tutor are given along with correlation tables. This assignment will challenge you to interpret NMR spectra by correlating spectra data with molecular structure. 1

2 The following IR spectra that appear in IR Tutor are shown on the following slides together with the 1 NMR and the 13C NMR spectra of the same compounds. Correlation Tables for the IR, 1 NMR and 13C NMR are given. In this homework exercise you should try to make sense out of the 1 NMR and 13C NMR spectra based on the empirical approach that we used for the interpretation of IR spectra. The different signals in the 1 NMR spectra correspond to signals for chemically different protons. The intensity of the signal is roughly proportional to the number of protons in the structure. The different signals in the 13C NMR spectra correspond to signals for chemically different carbon atoms. The intensity of the signal is roughly proportional to the number of carbons in the structure. 2

3 Some Important and Characteristic Infrared Absorption Frequencies and Wavelengths for Some Common Stretching Motions Atom Group Typical of Frequency (cm -1 ) Wavelength (µ) O- (free) Alcohols (dilute) cm µ O- ( bonded) Alcohols (concentrated) Carboxylic acids cm µ C C Acetylene (C) 33 cm µ C C Benzene (C), Ethylene (C) cm µ C C Ethane (C) cm µ C C Acetylene cm µ C N Nitriles 2-23 cm µ C O Carbonyl cm µ C C Alkene cm µ C C Alkane 6-15 cm µ C O Alcohols, Ethers 1-13 cm µ In general we will only be using the data in an IR spectrum for stretching vibrations which have energies higher than 162 cm -1. Although the bands at lower energy are known and assigned, the region below 162 cm -1 is very congested with single bond stretches of two heavy atoms (see C-C and C-O in table) and C- bends and are beyond the scope of what we want to do. 3

4 NMR units are ppm (parts per million). We discuss where this ppm unit comes from in class. These are the signals of protons (1 NMR). We think of the position (in ppm) of the NMR signals in the same way we viewed the frequency of IR signals. The 1 NMR signals are characteristic of certain types of protons in molecules 4

5 These are the position of the signals of carbon atoms (13C) in ppm. We think of the position of a NMR signal in ppm in the same way we viewed the frequency of IR signals. The 13C NMR signals are characteristic of certain types of carbon atoms in molecules. 5

6 1 NMR 1 range = to ~ 1 ppm Increasing magnetic field, 13C NMR 13C range = to ~ 2 ppm Use the correlation tables and note scale of x-axis 6

7 Example: n-hexane. You ve seen the IR and have considered the C- strecthes C- stretches At ~ 29 cm -1 C 2 C 2 C 2 C 2 n-hexane 7

8 Now let s consider the 1 (proton) and 13C (carbon) NMR spectra of n-hexane (next slide). The 1 NMR shows the signals of ALL of the protons of which a molecular structure is composed. The position of the signal depends on the chemical environments of the electrons so 1 NMR provides information on molecular structure of any molecules containing X- bonds. The 13 C NMR shows the signals of ALL of the carbons of which a molecular structure is composed. The position of the signal depends on the chemical environments of the carbon atoms so 13C NMR provides information on organic molecules. 8

9 Now consider the structure of n-hexane below. ow many different kinds of protons and carbon atoms does the structure suggest? C 2 C 2 C 2 C First consider the protons: there are three different kinds of protons: the protons on C 1, the C 2 protons on C 2 and the C 2 protons on C 3. We note that C 1 is equivalent to C 6, C 2 is equivalent to C 5 and C 3 is equivalent to C 4. We expect to see three signals in the 1 NMR of n-hexane (next slide) for the three different types of protons. We can t predict where they will be but we will empirically try to correlate them with the structure of n-hexane. The intensity of the signals in a 1 NMR are proportional to the relative number of protons in a structure. This will make the assignments easier. 9

10 Next let s consider the carbon atoms of n-hexane C 2 C 2 C 2 C There are only three different kinds of carbon atoms in n-hexane: C 1 = C 6, C 2 = C 5, and C 3 = C 4 We therefore expect three signals for the three different carbon atoms The signals in a 13C NMR are proportional to the relative number of carbon atoms in a structure. This will make the assignments easier. 1

11 1NMR n-hexane C 2 and C 3 are very similar C 2,3 C 1 Task: Assign the 1 signals to the protons of n-hexane C 2C 2C 3 C 3 25 C C NMR Use the correlation tables and note scale of x-axis C 1 5 C 2 C 2 C 2 C Task: Assign the 13C signals to the carbon atoms of n- hexane 11

12 1NMR n-hexane Assign the 1 signals to the protons of n-hexane C 2 C 2 C 2 C C NMR Assign the 13C signals to the carbon atoms of n- hexane

13 3 C C C 2,3-dimethyl butane 13

14 1 NMR Assign the signals in the NMR spectra to the and C in the structures C C 13C NMR 2 C 4 C C C Use the correlation tables and note scale of x-axis 14

15 C 2 CC 2 C 2 C 2 1-hexene 15

16 1 NMR 4 3 Assign the signals at ~ 5 ppm and ~ C 2 C 2C C NMR Assign the signals at ~ 115 ppm and ~ 14 ppm. C 2 C 2C Use the correlation tables and note scale of x-axis 16

17 C C C C C C Toluene methyl benzene 17

18 2 2 1 NMR 3 Assign the signal at ~ 2.3 ppm. 1 (overlaps) toluene 2C 2C 13C NMR Assign the signal at ~ 2 ppm. 1C 1C 1C C C C C C C Use the correlation tables and note scale of x-axis 18

19 C 2 C 2 C 2 C 2 C 2 C 2 C N 1-heptyl cyanide 19

20 N C C 2C2 C 2C2 C 2C2 1 NMR Assign the 1 signals N C C 2C2 C 2C2 C 2C2 13C NMR Assign the 13C signals N 1-heptylcyanide Use the correlation tables and note scale of x-axis 2

21 C CC 2 C 2 C 2 C 2 1-heptyne 21

22 1 NMR Can you find the C- signal associated with the triple bond? heptyne 13C NMR Which signals are due to the triple bond carbons? C CC 2 C 2 C 2 C Use the correlation tables and note scale of x-axis 22

23 NMR Can you find the C- signal associated with the triple bond? C CC 2 C 2 C 2 C C CC 2 C 2 C 2 C 2 13C NMR Which signals are due to the triple bond carbons? Use the correlation tables and note scale of x-axis 23

24 O C 2 C C 2 C 2 C 2 3-heptanone 24

25 1 NMR 1: ow many signals can you assign? O heptanone O C 2 C C 2 C 2 C 2 13C: What is the signal at ~ 21 ppm due to? 13C NMR Which signal is associated with this signal in the IR of 3-heptanone? O C 2 C C 2 C 2 C

26 O 3 C C OC 2 EthylAcetate 26

27 1 NMR What is the signal at ~ 4.2 ppm due to? The signal at ~ 2. ppm? O O ethyl acetate 13C NMR What are the peaks at ~ 17 ppm and ~6 ppm due to? O 3 C C OC Use the correlation tables and note scale of x-axis 27

28 O C 2 C 2 C 2 C 2 C 2 C 1-heptanal 28

29 1 NMR What is the signal at ~ 1 ppm due to? Can you find the analogous signal in the IR? O heptanal 13C NMR What is the signal at ~ 2 ppm due to? O C 2 C 2 C 2 C 2 C 2 C Use the correlation tables and note scale of x-axis 29

30 C 2 C 2 C 2 C 2 C 2 O 1-heptanol 3

31 1 NMR What is the signal at ~ 3.5 ppm due to? O hexanol 13C NMR What is the signal ~63 ppm due to? C 2 C 2 C 2 C 2 C 2 O Use the correlation tables and note scale of x-axis 31

32 C 2 C 2 C 2 C 2 C 2 CO 1-eptanolicAcid O 32

33 1 NMR What is the signal at ~ 11 ppm due to? O O heptanoic acid What is the signal at ~18 ppm due to? 13C NMR O C 2 C 2 C 2 C 2 C 2 CO Use the correlation tables and note scale of x-axis 33