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California State University, Monterey Bay CHEM312

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1 California State University, Monterey Bay CHEM312
Organic Chemistry, 7th Edition L. G. Wade, Jr. Spectroscopy Review California State University, Monterey Bay CHEM312

2 Types of Spectroscopy Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group. Mass spectrometry (MS) fragments the molecule and measures the masses. Nuclear magnetic resonance (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers. Ultraviolet (UV) spectroscopy uses electron transitions to determine bonding patterns. =>

3 The Spectrum and Molecular Effects
=>

4 The IR Region Just below red in the visible region.
Wavelengths usually mm. More common units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters. Wavenumbers are proportional to frequency and energy =>

5 Carbon-Carbon Bond Stretching
Stronger bonds absorb at higher frequencies: C-C cm-1 C=C cm-1 CC <2200 cm-1 (weak or absent if internal) Conjugation lowers the frequency: isolated C=C cm-1 conjugated C=C cm-1 aromatic C=C approx cm =>

6 Carbon-Hydrogen Stretching
Bonds with more s character absorb at a higher frequency. sp3 C-H, just below 3000 cm-1 (to the right) sp2 C-H, just above 3000 cm-1 (to the left) sp C-H, at 3300 cm =>

7 O-H and N-H Stretching Both of these occur around 3300 cm-1, but they look different. Alcohol O-H, broad with rounded tip. Secondary amine (R2NH), broad with one sharp spike. Primary amine (RNH2), broad with two sharp spikes. No signal for a tertiary amine (R3N) =>

8 An Alcohol IR Spectrum =>

9 An Amine IR Spectrum =>

10 Carbonyl Stretching The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around 1710 cm-1. Usually, it’s the strongest IR signal. Carboxylic acids will have O-H also. Aldehydes have two C-H signals around 2700 and 2800 cm =>

11 O-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, cm-1, due to strong hydrogen bonding. =>

12 Variations in C=O Absorption
Conjugation of C=O with C=C lowers the stretching frequency to ~1680 cm-1. The C=O group of an amide absorbs at an even lower frequency, cm-1. The C=O of an ester absorbs at a higher frequency, ~ cm-1. Carbonyl groups in small rings (5 C’s or less) absorb at an even higher frequency. =>

13 An Amide IR Spectrum =>

14 Carbon - Nitrogen Stretching
C - N absorbs around 1200 cm-1. C = N absorbs around 1660 cm-1 and is much stronger than the C = C absorption in the same region. C  N absorbs strongly just above 2200 cm-1. The alkyne C  C signal is much weaker and is just below 2200 cm =>

15 A Nitrile IR Spectrum =>

16 Summary of IR Absorptions
=>

17 Strengths and Limitations
IR alone cannot determine a structure. Some signals may be ambiguous. The functional group is usually indicated. The absence of a signal is definite proof that the functional group is absent. Correspondence with a known sample’s IR spectrum confirms the identity of the compound =>

18 Mass Spectrometry Molecular weight can be obtained from a very small sample. It does not involve the absorption or emission of light. A beam of high-energy electrons breaks the molecule apart. The masses of the fragments and their relative abundance reveal information about the structure of the molecule. =>

19 The Mass Spectrum Masses are graphed or tabulated according to their relative abundance. =>

20 Molecules with Heteroatoms
Isotopes: present in their usual abundance. Hydrocarbons contain 1.1% C-13, so there will be a small M+1 peak. If Br is present, M+2 is equal to M+. If Cl is present, M+2 is one-third of M+. If iodine is present, peak at 127, large gap. If N is present, M+ will be an odd number. If S is present, M+2 will be 4% of M+. =>

21 Mass Spectra of Alkenes
Resonance-stabilized cations favored. Chapter 12 =>

22 Mass Spectra of Alcohols
Alcohols usually lose a water molecule. M+ may not be visible. =>

23 1HNMR Signals The number of signals shows how many different kinds of protons are present. The location of the signals shows how shielded or deshielded the proton is. The intensity of the signal shows the number of protons of that type. Signal splitting shows the number of protons on adjacent atoms =>

24 Typical Values => Chapter 13

25 O-H and N-H Signals Chemical shift depends on concentration.
Hydrogen bonding in concentrated solutions deshield the protons, so signal is around 3.5 for N-H and 4.5 for O-H. Proton exchanges between the molecules broaden the peak =>

26 Identifying the O-H or N-H Peak
Chemical shift will depend on concentration and solvent. To verify that a particular peak is due to O-H or N-H, shake the sample with D2O. Deuterium will exchange with the O-H or N-H protons. On a second NMR spectrum the peak will be absent, or much less intense =>

27 Carboxylic Acid Proton, 10+
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28 Number of Signals Equivalent hydrogens have the same chemical shift.
=>

29 Intensity of Signals The area under each peak is proportional to the number of protons. Shown by integral trace. =>

30 Spin-Spin Splitting Nonequivalent protons on adjacent carbons have magnetic fields that may align with or oppose the external field. This magnetic coupling causes the proton to absorb slightly downfield when the external field is reinforced and slightly upfield when the external field is opposed. All possibilities exist, so signal is split. =>

31 Range of Magnetic Coupling
Equivalent protons do not split each other. Protons bonded to the same carbon will split each other only if they are not equivalent. Protons on adjacent carbons normally will couple. Protons separated by four or more bonds will not couple =>

32 Carbon-13 12C has no magnetic spin.
13C has a magnetic spin, but is only 1% of the carbon in a sample. The gyromagnetic ratio of 13C is one-fourth of that of 1H. Signals are weak, getting lost in noise. Hundreds of spectra are taken, averaged =>

33 Hydrogen and Carbon Chemical Shifts
=> Chapter 13

34 Combined 13C and 1H Spectra
=>

35 Off-Resonance Decoupling
13C nuclei are split only by the protons attached directly to them. The N + 1 rule applies: a carbon with N number of protons gives a signal with N + 1 peaks =>

36 Interpreting 13C NMR The number of different signals indicates the number of different kinds of carbon. The location (chemical shift) indicates the type of functional group. The peak area indicates the numbers of carbons (if integrated). The splitting pattern of off-resonance decoupled spectrum indicates the number of protons attached to the carbon. =>

37 Two 13C NMR Spectra =>

38 Ultraviolet Spectroscopy
Ultraviolet (UV) spectroscopy detects the electronic transitions of conjugated systems. Provides information about the length and structure of the conjugated part of a molecule. UV spectroscopy gives more specialized information than IR or NMR and is less commonly used. UV region is a range of frequencies beyond visible light, usually in the range of 200 to 400nm. UV-Visible spectrometers (like those used in chem110L and chem111L) are often used to see transitions at longer wavelengths.

39 UV light and electron transitions
UV-visible energies correspond to electronic transitions: the energy needed to excite and e- from one molecular orbital to another. Effects of conjugation: in general, a compound that contains a longer chain of conjugated double bonds absorbs at a longer wavelength.

40 End of spectroscopy review


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