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5: Organic Spectrometry
•Spectrometry in Organic Chemistry •Mass Spectrometry •Spectrometry Using Electromagnetic Radiation •Nuclear Magnetic Resonance Spectrometry •13C NMR Spectrometry •1H NMR Spectrometry •Infrared Spectrometry •UV-Visible Spectrometry
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This chapter describes four instrumental methods that organic chemists routinely use to determine the structures of organic compounds. They are Mass Spectrometry (MS) , Nuclear Magnetic Resonance Spectrometry (NMR) , Infrared Spectrometry (IR) , and Ultraviolet-Visible Spectrometry (UV-Vis).
These four methods use electronic instruments called spectrometers to generate spectra that contain the structural information about molecules. We will describe these spectrometers only in the most general terms. This chapter is primarily designed to introduce you to the utility and limitations of these four instrumental methods, and to illustrate how organic chemists use their spectral data to determine structures of organic molecules.
Analytical chemistry is the branch of chemistry that deals with the development and use of instrumental techniques such as these to determine structures of molecules, and it is the subject of other courses in the undergraduate chemistry curriculum. However, these four instrumental methods are of such great importance to organic chemists that we give this early introduction to show the kinds of structural information they provide.
Spectrometry in Organic Chemistry
Organic chemists must determine structures of the organic compounds that they use in chemical reactions, that form in these chemical reactions, and that they isolate from living organisms. They accomplish this using several instrumental techniques collectively described as organic spectrometry. Organic spectrometry makes use of electronic instruments called spectrometers
that provide energy to molecules and then measure how the molecules respond to that applied energy.
In order to fully understand spectrometry , we should learn about the design and construction of spectrometers. However we can develop a practical understanding of how these different types of organic spectrometry provide information about molecular structure without a detailed knowledge of spectrometers. We illustrate this in the following sections using as examples the classes of organic molecules introduced in Chapters 2 and 3.
Types of Spectrometry The four most important types of spectrometry that organic chemists routinely use are: Mass Spectrometry (MS) Nuclear Magnetic Resonance Spectrometry (NMR) Infrared Spectrometry (IR) Ultraviolet-Visible Spectrometry (UV-Vis)
Each of these methods provides unique information about organic molecular structure because each monitors the response of an organic molecule to a different type of energy input. In MS , a molecule is bombarded with a beam of high energy electrons , in NMR it is irradiated with radio waves , in IR it is subjected to heat energy , while in UV-Vis spectrometry the molecule is placed in a beam of ultraviolet or visible light.
We discuss mass spectrometry ( MS ) first since it is fundamentally different from the other three types of spectrometry. Of the other three methods, we consider NMR in much greater detail than either IR or UV-Vis because of its overwhelming importance to organic chemists as an aid in structure determination. Our discussions of IR and UV-Vis in this chapter are brief because these methods are best suited to analyzing types of molecules that we have not yet introduced. We discuss them in more detail in later chapters.
Spectrometry versus Spectroscopy. You may see other books refer to the techniques in this chapter as organic spectroscopy rather than organic spectrometry. This is not technically correct, but it is done so often that it has become accepted practice. Chemical spectroscopy actually involves the study of the interaction of electromagnetic energy , described later in this chapter, with molecules. In contrast, chemical spectrometry is the practical use of instruments, including those based on spectroscopy , to probe molecular structure.
e-^ + R1-R2 → (R1-R2)+^ + 2e- (M) (M)+ (R1-R2)+^ → R1+^ + R2 and/or R1 + R2+ (M)+
Mass spectrometers detect the presence of positively charged ions and measure their masses. As a result, a mass spectrometer provides masses of molecular ions ((R1-R2)+^ ) as well as masses of the positive fragment ions (R 1 +^ and R 2 +) that result from fragmentation of the molecular ion. Fragment ions are like pieces of a jig saw puzzle that chemists can often fit back together to give part or all of the detailed molecular structure of the original organic molecule.
Molecular and Fragment Ions from Methane. We use methane (CH 4 ) to illustrate molecular ion formation and fragmentation because all of its chemical bonds are identical.
(a) Electron bombardment (formation of the molecular ion) e-^ + CH4 → CH4+^ + 2e- 10p 10p 1e 10e 9e 2e (b) Fragmentation (formation of radical and cation) CH4+^ → CH3 + H+ 10p 9p 1p 9e 9e or CH4+⋅^ → CH3+^ + H⋅ 10p 9p 1p 9e 8e 1e
Each of these equations is chemically and electrically balanced. Both the total number of protons ( p ) as well as the total number of electrons ( e ) are the same on both sides of each equation, and the same is true for the net electrical charge on both sides of each equation. The relative numbers of protons ( p ) and numbers of electrons ( e ) for each species show you why a species has a negative (-) charge, a positive (+) charge, and/or an unpaired electron ( ). The species with single (+) charges have one more p than e , while those labelled with a ( ) have an odd number of e's. (By convention, we do not show a ( ) on e-^ even though it is simply a single electron.)
This detailed analysis is a useful exercise, but you will not need to do it routinely in order to interpret results of MS structure determinations of organic compounds. The two important points
are that a mass spectrometer (a) generates and detects positively charged ions ( molecular and fragment ions ) from the original compound, and (b) determines their masses. We describe this in more detail in the following sections.
The Mass Spectrometer and Mass Spectrum There are several different designs for mass spectrometers, but all of them form, detect, and measure the mass of positively charged species formed by electron bombardment.
Mass Spectrometer. We show the typical component parts of these mass spectrometers using the simple "block" diagram in Figure 5.4. Figure 5. The mass spectrometer bombards the organic sample in the sample chamber (Figure 5.4) with high energy electrons from the source , and detects the resulting positive ions in the analyzer/detector region of the spectrometer. The analyzer and detector are usually separate components, but some mass spectrometers, used for routine mass spectral analysis in organic laboratories, analyze and detect positive ions in the sample chamber where they form.
Mass Spectrum. The mass spectrometer determines the amount and mass of each positively charged species, stores these data in a computer, and subsequently prints out these results in a table or displays them as a mass spectrum (Figure 5.5). Figure 5. A mass spectrum consists of a collection of lines at different m/z values (described below) along the horizontal axis or base line of the spectrum. Each line corresponds to a positively charged species detected by the spectrometer.
Mass-to-Charge Ratios (m/z Values). The m/z values ( mass-to-charge ratios ) on the horizontal axis of the spectrum correspond to the mass ( m ) ( amu ) of each positively charged species divided by its electrical charge ( z ). Most positive species formed in a mass spectrometer have a charge of +1 (z = +1), so their m/z values usually are the same as their masses ( m/z = m/(+1) = m ). The m/z values for the taller lines in the mass spectrum often appear as labels at the top of those lines.
The height of each line (or signal or peak ) corresponds to the relative amount formed of the positive species with a particular m/z value. We call the tallest peak in any mass spectrum the base peak and usually assign it a value of 100% on the vertical axis. In the spectrum in Figure 5.5, the base peak is the line at m/z = 42. We describe the heights of the other peaks in the spectrum as a
Table 5.1. Exact and Unit Resolution m/z Values for Cations formed from Hexane in a Mass Spectrometer. m/z Value ( amu) Ion Structure Exact Unit Resolution (C6H14)+^ (CH3CH2CH2CH2CH2CH3)+^ 86.1096 86 C5H11+^ (CH3CH2CH2CH2CH2+) 71.0861 71 C4H9+^ (CH3CH2CH2CH2+) 57.0705 57 C3H7+^ (CH3CH2CH2+) 43.0548 43 C2H5+^ (CH3CH2+) 29.0391 29 CH3+^ (CH3+) 15.0235 15
You can see peaks at all of these m/z values in the hexane mass spectrum (Figure 5.6). In addition, there are prominent peaks for fragments that have m/z values other than those in Table 5.1. Some are 1 or 2 amu less than those mentioned in Table 5.1 and they correspond to ions with one or two fewer H atoms than the ions shown in Table 5.1. It is also important to note that there are several "groups" of peaks made up of individual peaks that are each 14 amu (the mass of a CH 2 group) larger or smaller than individual peaks in a neighboring group.
Exact Mass Values. The mass values of these peaks are shown at unit resolution in Figure 5.6, but high resolution mass spectrometers give their exact mass values. The exact mass of the hexane molecular ion (C 6 H 14 )+^ is virtually identical to the exact mass of a hexane molecule (C 6 H 14 ) since (C 6 H 14 )+^ differs from (C 6 H 14 ) by just one electron that has negligible mass. However, if you use atomic masses from a periodic table or the Handbook of Chemistry and Physics to calculate the molecular mass of hexane, you obtain a value of 86.18 amu rather than the exact mass value of 86.11 (86.1096 rounded off to 4 significant figures). These two values of 86.18 and 86.11 may seem very close to each other, but their difference of 0.07 amu is greater than any experimental or calculational error.
A clue that we may have overlooked something in this analysis of hexane masses is the observation that the mass 86 peak is not the highest mass peak in this mass spectrum. If you look closely at Figure 5.6 you will see a very small peak at mass 87 that is not due to an impurity in our sample. We explain below both the origin of this M+1 peak , and why we cannot calculate exact mass values using atomic mass data from a periodic table.
M+1 Peaks and Isotopes. The exact mass values of all of the cations in Table 5.1 are slightly less than we would calculate using atomic mass values from a periodic table. This is because atomic masses of C and H from periodic tables are weighted averages of exact mass values
of their naturally occurring isotopes. In contrast, mass spectrometers detect individual ions that do not have "average" isotopic distributions as we describe below.
The 12.01 amu atomic mass of C from a periodic table is a weighted average based on the 99% natural abundance of 12 C (6 protons, 6 neutrons, atomic mass 12.00000 amu) and 1% 13 C ( protons, 7 neutrons, atomic mass 13.00335 amu). Similarly, the 1.008 amu atomic mass of H from a periodic table is a weighted average based on the 99.985% natural abundance of 1 H ( proton, 0 neutrons, atomic mass 1.007825 amu) and 0.015 % 2 H (1 proton, 1 neutron, atomic mass 2.0140 amu). However, the detector of the mass spectrometer determines the masses of individual molecular fragments that cannot contain a statistical distribution of isotopes.
While most hexane molecular ions contain only 12 C and 1 H and are (^12 C 61 H 14 )+., there are also molecular ions in which one 12 C is replaced by a 13 C to give (^13 C 112 C 51 H 14 )+.^ that we call the M+1 peak. Their masses are both different from that calculated for (C 61 H 14 )+.^ using atomic masses from a periodic table. In any sample we also expect a few molecular ions of hexane to contain two or more 13 C atoms, but their number is so small that they are not visible in the spectrum. While an M+1 peak in the hexane spectrum could also reflect the presence of a 2 H atom in the molecular ion (^12 C 62 H 11 H 13 )+., the natural abundance of 2 H (0.015%) is so small that such ions constitute a trivial part of the M+1 peak.
Most fragment ions also contain just 12 C and 1 H, so their exact masses in Table 5.1 are also less than we would calculate using weighted average masses from a periodic table. However like the molecular ion, fragment ions with relatively intense peaks also have neighboring isotopic peaks one mass unit higher due to replacement of a 12 C by 13 C.
Mass Spectra of Hexane Structural Isomers In order to see how mass spectra can provide information to help distinguish between isomers with the same molecular formula, we compare the mass spectrum of hexane with those of its isomers 2- methylpentane , and 2,2-dimethylbutane that are all C 6 H 14 ( Figure 5.8). Figure 5.
The Molecular Ion Peaks. One of the most obvious differences between these spectra in Figure 5.8 is the molecular ion peak at 86. It is much weaker in the spectrum of 2-methylpentane than in that of hexane , and we cannot see it at all in the spectrum of 2,2-dimethylbutane. This is an example of a general phenomenon in mass spectrometry that increasing branching in a molecule increases the probability of fragmentation of its molecular ion. An increase in ease of
characteristic features using mass spectra of 1-pentanol , 1-pentanamine , 1-chloropentane , 1- bromopentane , and 1-iodopentane (Figure 5.11). Figure 5.
General Features. All of these compounds in Figure 5.11 have the general structure CH 3 CH 2 CH 2 CH 2 CH 2 -Y where Y is OH, NH 2 , Cl, Br, or I. Electron bombardment in the mass spectrometer first gives molecular ions (CH 3 CH 2 CH 2 CH 2 CH 2 -Y)+⋅^ and these fragment into smaller cations and radicals. These fragments form by cleavage at C-C bonds as we saw for isomeric hexanes, but the functional group Y influences this fragmentation. We will focus on the molecular ion peaks , on the fragment peaks corresponding to +CH 2 -Y , and on fragment peaks at mass values 55 (C 4 H 7 +) and 70 (C 5 H 10 +) that form as we show in Figure 5.12. Figure 5. Each Y group causes an unusually large amount of fragmentation at its adjacent C-C bond giving the characteristic +CH 2 -Y fragment. The peak at m/z = 70 is due to the cation arising from loss of the molecular species H-Y (that is H-OH, H-NH 2 , or H-X), while that at m/z = 55 arises from loss of both H-Y and CH 3. We briefly highlight each functional group below.
1-Pentanol (Y = OH). The molecular ion peak (m/z = 88) in the mass spectrum of 1- pentanol (CH 3 CH 2 CH 2 CH 2 CH 2 -OH) is very small and this is characteristic of alcohols (ROH). In contrast, the +CH 2 -OH peak at m/z = 31 ( +CH 2 -Y where Y = OH) is intense and so are the peaks at m/z = 55 (loss of H-OH and CH 3 ) and m/z = 70 (loss of H-OH).
1-Pentanamine (Y = NH 2 ). The molecular ion peak (m/z = 87) for 1-pentamine (CH 3 CH 2 CH 2 CH 2 CH 2 -NH 2 ) is relatively more intense than the molecular ion peak from 1- pentanol and this is generally true for amines ( RNH 2 ) compared to alcohols ( ROH ). The M+.^ line is sufficiently intense that its 13 C isotopic M+1 peak is also visible in the spectrum. Although the peaks at m/z = 55 and 70 due to loss of H-NH 2 (ammonia) are barely visible, the +CH 2 -NH 2 peak ( +CH 2 -Y where Y = NH 2 ) is so intense that it is the base peak in the spectrum. All of these observations are characteristic of the mass spectra of amines.
1-Chloropentane (Y = Cl). Molecular ions of chloroalkanes undergo extensive fragmentation, so the M+^ peak at m/z = 106 for 1-chloropentane (CH 3 CH 2 CH 2 CH 2 CH 2 -Cl) is just barely visible. Consistent with this, the fragment peaks at m/z = 55 due to loss of both H-Cl and CH 3 ⋅, and at m/z = 70 due to loss of H-Cl, are very intense.
The +CH 2 -Cl fragment ( +CH 2 -Y where Y = Cl) is also visible in this spectrum, but you may be surprised to learn that it corresponds to the two separate peaks at m/z = 49 and 51. Natually occurring Cl is a mixture of the isotopes 35 Cl (76%) and 37 Cl (24%) so +CH 2 -Cl is an equivalent % mixture of +CH 2 -^35 Cl (m/z = 49) and +CH 2 -^37 Cl m/z = 51). The isotopic mixture for Cl also causes every cation containing Cl to give two peaks separated by 2 amu. The molecular ion with the isotope 37 Cl (m/z =108) is not visible because it would be only one-fourth the size of the already tiny peak for the 35 Cl molecular ion at mass 106, but pairs of fragment ions with 35 Cl and (^37) Cl appear at m/z = 63 and 65 (C 2 H 4 Cl+), and at m/z = 91 and 93 (C 4 H 8 Cl+).
1-Bromopentane (Y = Br). Since naturally occurring Br is almost an equimolar mixture of (^79) Br (51%) and 81 Br (49%), cations containing Br also give two mass spectral peaks with almost
equal intensities such as the two weak molecular ion peaks from 1-bromopentane (CH 3 CH 2 CH 2 CH 2 CH 2 -Br) at m/z =150 and 152. You can see other such isotopic pairs of peaks separated by 2 amu including those for +CH 2 -Br ( +CH 2 -Y where Y = Br) at m/z = 93 and 95. The characteristic fragment peaks at m/z = 55 and 70 for C 4 H 7 +^ and C 5 H 10 +^ are present, but significantly less intense than those from 1-chloropentane.
1-Iodopentane (Y = I). In contrast to Cl or Br, naturally occuring iodine (I) is almost entirely the single isotope 127 I. As a result, 1-iodopentane (CH 3 CH 2 CH 2 CH 2 CH 2 -I) gives just a single molecular ion peak at m/z = 198 along with its small M+1 peak at m/z = 199 due to 13 C.
The mass spectrum of 1-iodopentane also illustrates that fragmentation is much less important for iodoalkanes than for bromoalkanes or chloroalkanes. The characteristic fragment peaks at m/z = 55 and 70, and at m/z = 141 for +CH 2 -I ( +CH 2 -Y where Y = I) are all relatively small. However, you can see an intense peak at m/z = 71 in the mass spectrum of 1-iodopropane due to C 5 H 11 +^. This m/z = 71 peak is also present in the mass spectrum of 1-bromopentane and is due to molecular ion fragmentation at C-I or C-Br bonds forming C 5 H 11 +^ and I.^ or Br.^ atoms. We will see in a later chapter that the relative stability of halogen atoms is I.^ > Br.^ > Cl.^ and this explains the very small m/z = 71 peak in the mass spectrum of 1-chloropentane.
Mass Spectrometry Summary If you look back at the mass spectra that we have shown here, you may wonder how a chemist can possibly identify the compound giving that spectrum without knowing the answer in advance. Each spectrum has many peaks and it is not always clear how some of them formed. These are valid feelings on your part, but chemists who use mass spectrometry as an analytical tool have had extensive training in which they have seen and studied thousands of mass spectra of a variety of
from Figure 5.13 that the relative energy of photons used in the three types of spectrometry that we discuss here decreases in the order EUV-Vis > EIR > ENMR.
Mass Spectrometry Does Not Use Electromagnetic Radiation. It is important to state again that mass spectrometry (MS) does not use energy from the electromagnetic spectrum! It employs a beam of high energy electrons, not photons, to interact with molecules as we have described earlier. While the MS electron beam destroys the molecular sample in the mass spectrometer, NMR, IR, and UV-Vis spectrometry are non-destructive analytical methods. The energy provided by their photons leads to changes in the molecules, but these changes are almost always rapidly reversible as we will describe in the sections below.
Frequency and Wavelength of Electromagnetic Radiation. We can assign energies in kJ to the photons from different regions of the electromagnetic spectrum (Figure 5.13), but this is not done in practice. Organic chemists typically characterize electromagnetic radiation used in NMR, IR, and UV-Vis spectrometry in terms of its frequency or wavelength. As a result, you need to understand the general relationships between energy (E) , frequency ( ) , and wavelength ( ) of electromagnetic radiation (Figure 5.14). Figure 5. Our first lesson is that energy (E) and frequency ( ) are directly proportional to each other as we show in equation (1) where h is a proportionality constant called Planck's constant. E = hν (1) Photons with high energy (like X-rays and γ-rays) have high frequencies , while photons with low energy (like microwaves and radiowaves) have low frequencies. As a result, the order of relative energies of the photons used in UV-Vis, IR, and NMR spectrometry (EUV-Vis > EIR > ENMR) is the
same as the order of their relative frequencies (νUV-Vis > νIR > νNMR).
Our second lesson is that the wavelength ( ) of photons is inversely proportional to the frequency ( ) of those photons as we show in equation 2 where the proportionality constant c is the velocity of light. ν = c/λ (2) Electromagnetic radiation of higher frequency (higher energy ) has shorter wavelengths than electromagnetic radiation of lower frequency (lower energy ). This means that the relative order of energies of photons used in NMR, IR, and UV-Vis (EUV-Vis > EIR > ENMR) is opposite to that of their photon wavelengths (λUV-Vis < λIR < λNMR). The wavelengths of electromagnetic radiation used in UV-Vis spectrometry are shorter than those used in IR , while those of NMR are the longest.
Units of Frequency or Wavelength. We could use a single set of units to describe the full range of frequencies (or wavelengths) for the whole electromagnetic spectrum, but again this is not done in practice. Each type of spectrometry traditionally has its own set of units and conventions that describe its portion of the electromagnetic spectrum. This may seem confusing, but these methods evolved during different periods of time and in the laboratories of different types of scientists. This is a fact of organic spectrometry that may never change, so we describe their units and conventions as we present and discuss each of these types of spectrometry. Nevertheless, it is very important for you to keep in mind the relative energy, frequency, and wavelength relationships between UV-Vis, IR, and NMR that we show in Figure 5.13 even if you have difficulty seeing the relationship between the different units of frequency or wavelength that are used for each type of spectrometry.
Basic Spectrometer Design Each spectrometric technique uses a spectometer to (a) provide the electromagnetic energy to the organic compound, and (b) monitor the results of that application of electromagnetic energy.
Spectrometer Components. The detailed design, construction, and overall appearance of NMR, IR, and UV-Vis spectrometers differ greatly, but they share common features that we show using the "block" diagram in Figure 5.16. Figure 5. The source provides photons of electromagnetic energy to the sample that is enclosed in a sample chamber. The detector senses energy absorbed (or transmitted) by the sample , and this absorption (or transmission) of energy is displayed on a spectrum generated by a computer that processes data from the detector.
Spectral Peaks. The names of the components of these spectrometers sound similar to those of mass spectrometers, but there is a major difference in the nature of the data displayed in spectra from UV-Vis, NMR, and IR spectrometers compared to mass spectra. Individual peaks in a mass spectrum correspond to different molecular species (cations) that have different mass values. In contrast, all of the individual peaks (or signals) in an NMR, IR, or UV-Vis spectrum are due to a single molecular species when a single compound is placed in the sample chamber.
The multiple peaks that we will see in most NMR, IR, or UV-Vis spectra show the response of the molecule to irradiation with energy of a value that corresponds to the position of each peak in the spectrum. As a result, the horizontal axes of NMR, IR, and UV-Vis spectra correspond to the
by NMR, we call the spectrum that we obtain a Hydrogen NMR spectrum, or a Proton NMR spectrum. When we examine C atoms by NMR, we refer to the resulting spectrum as a Carbon- (or^13 C ) NMR spectrum.
Since 1 H isotopes of hydrogen have a natural abundance of 99.985%, almost every hydrogen atom in an organic compound is 1 H. In contrast, the 13 C isotope of carbon has a natural abundance of only about 1% so most carbon atoms are 12 C that do not absorb rf energy. In spite of this, any sample of an organic compound contains many molecules with 13 C atoms randomly distributed at all possible carbon locations. We will show and discuss 13 C and 1 H NMR spectra for a variety of different types of organic compounds in the following sections and focus on how organic chemists use these spectra both to determine and to confirm molecular structure.
Other Nuclei that Absorb Radio Frequency Energy. (F, P, B, N, etc)(magnetically active)
13C NMR Spectrometry
We begin with 13 C NMR spectrometry, but much of what we learn in this section applies directly to the discussion of 1 H NMR that will follow.
General Considerations A typical 13 C NMR spectrum of a compound contains a series of lines (also called peaks or signals ) that correspond to each type of C atom in the compound. The locations of these signals in the spectrum depend on the molecular structure of the organic compound and the types of atoms that are directly and indirectly bonded to each C. We illustrate how these bonded atoms affect the location of 13 C NMR signals using 13 C spectra of several specific compounds as examples.
We present and discuss these spectra before we explain all of the information that they contain. In particular, we will defer our discussion of the horizontal axis labelled chemical shift ( ) until later in this section. For now it is enough to know that the chemical shift values ("delta" values) on the horizontal axis are related to the frequency (or energy) of the rf radiation that each 13 C atom absorbs.
Some 13 C NMR Spectra We show 13 C NMR spectra for six unbranched alcohols in Figure 5.18 along with their chemical structures. Figure 5.
When we ignore the peak at δ 0, and the group of three peaks at about δ 77 that are not due to these alcohols, as we explain later, we see that the spectrum of each alcohol contains the same number of peaks as there are C's in the alcohol. This occurs because each C atom in each alcohol is in a unique environment compared to the other C's in that same alcohol.
Methanol versus Ethanol. Since each methanol (CH 3 -OH) molecule contains just one C atom, its 13 C NMR spectrum has only a single 13 C NMR peak or signal and it is located at δ 50.3. In contrast, each ethanol molecule (CH 3 -CH 2 -OH) has two C's that are in different molecular environments from each other. Both C's of ethanol are bonded to each other by a C-C bond, but one is bonded to an O atom while the other has only C-H bonds. This results in a 13 C NMR spectrum for ethanol that shows two signals that are located at δ 58.0 and δ 18.2, respectively.
The Other Alcohols. Since C atoms in different molecular environments give discrete signals, we should not be surprised that 1-propanol (CH 3 -CH 2 -CH 2 -OH) has a 13 C NMR spectrum with three separate signals ( δ 64.3, δ 25.8, and δ 10.2). Successive additions of CH 2 groups to 1-propanol to give 1-butanol, 1-pentanol, and finally 1-hexanol increases the number of signals by one per CH 2 group so that the 13 C spectrum for each of these alcohols contains the same number of signals as there are C's in the alcohol.
(^13) C NMR Chemical Shifts ( )
The chemical shift value ( ) value) of each peak in these alcohols reflects the chemical environment of the C atom giving the peak.
Generalizations for these Alcohols. Besides the fact that 13 C NMR spectra for these alcohols have the same number of signals as they have C atoms, you can see that 1-propanol, 1- butanol, 1-pentanol, and 1-hexanol each have one signal at about δ 60, and another just below δ
- Organic chemists have determined that the δ 60 signals for these four alcohols correspond to C bonded to O in CH 2 -OH, while the signals below δ 15 are each due to C in CH 3. From these observations, we can conclude that a 13 C NMR signal near δ 60 in a spectrum of a molecule could be due to a CH 2 -OH group in the molecule, while the presence of a peak below δ 15 could indicate that the molecule contains a CH 3.
The signals for the other C's in these molecules have δ values between δ 15 and δ 60. Although each signal corresponds to a C in a CH 2 group between two carbons (C-CH 2 -C), the δ value depends on the number of bonds that separate them from the OH group. Because the chemical
Table 5.2. Additive Effects of Y Groups on 1 3^ C Chemical Shift Values of C's in Compounds with the General Structure C(γ )C(β)C(α)Y Y C(α) C(β) C(γ ) CH 3 +10 +10 - O H +45 +10 - O R +55 +5 - NH 2 +25 +10 - NHR +35 +5 - NR 2 +40 +5 - F +65 +5 - Cl +30 +10 - Br +25 +10 - I 0 +10 0
We can approximate the δ value of C atoms in simple molecules containing these functional groups (Y) by adding the appropriate number from Table 5.2 to the δ value for a comparable C in the corresponding unsubstituted compound where Y = H.
Calculations for 1-Hexanol. (aside?)As an illustration, let's use the data in Table 5.2 and Figure 5.20a to estimate δ values for the C's in 1-hexanol and then compare them to those actually observed. We can imagine that 1-hexanol originates from hexane if we replace an H on one of its CH 3 groups by an OH group. We show these two compounds in Figure 5.20b along with experimentally measured δ values for each C atom from 13 C NMR spectra (the numbers not in parentheses), and δ values that we calculate (the numbers in parentheses) as we illustrate below. Figure 5.20b The C of CH 3 in hexane, and the C with the OH group in 1-hexanol, are α carbons and their bonded C's are β, and γ, respectively.
When we add the appropriate numbers from the C(α), C(β), and C(γ) columns for the OH group to the δ values for C(α), C(β), and C(γ) in hexane, we predict a chemical shift for C(α) in hexanol of δ 59 (14 + 45), for C(β) of δ 33 (23 + 10), and for C(γ) of δ 27 (32 + (- 5)). You see that these predicted δ values are very close to those actually observed. You can also see that the observed δ values for C's further from the OH group than C(γ) are the same as those on hexane since correction for a functional group is usually unnecessary at C atoms more than three C's away from it.
Values and Electronegativity. The data in Table 5.2 show that the magnitude of the effect of a functional group on δ values depends on the electronegativity of the atom of that functional group directly bonded to the C skeleton. The effects of OH or OR are greater than those
of NH 2 , NHR, and NR 2 , while they all have greater effects than CH 3 consistent with the order of relative electronegativities O > N > C.
F has the biggest effect since it is the most electronegative atom in the periodic table, and the effect of a halogen on δ values decreases in the electronegativity order F > Cl > Br > I. The magnitude of the effects for Cl, Br, and I are smaller than we might expect based on their actual electronegativity values. The very small effect of I on the δ value on C(α) is particularly striking and there is no simple explanation for this observation.
Chemically Equivalent Carbons. Each C atom in each alcohol in Figure 5.18 gives a separate 13 C NMR signal because each C is in a different environment compared to the others in the same molecule. However, when two or more C's in a molecule are chemically equivalent they do not give separate peaks. We illustrate this by comparing the 13 C NMR spectra of bromoethane, 1-bromopropane and 2-bromopropane in Figure 5.22. Figure 5. Although bromoethane with two C's gives two 13 C NMR signals and 1-bromopropane with three C's gives three signals (remember that the small peaks at δ 0 and δ 77 are not due to these bromoalkanes), the spectrum of 2-bromopropane has only two signals even though it has three C atoms.
This occurs because the two CH 3 groups in 2-bromopropane are chemically equivalent! They each have exactly the same relationship to the Br functional group, and the rest of the molecule, and you can even consider them to be mirror images of each other as we illustrate in Figure 5.23. Figure 5. We can identify the C or C's giving rise to each signal in these spectra from their chemical shift values. In each spectrum the signal with the highest δ value corresponds to the C directly attached to the electronegative Br, while those with the lowest δ values are for CH 3 groups analogous to what we observed with the alcohols. The C-CH 2 -C signal for 1-bromopropane has an intermediate chemical shift for the same reasons as the C-CH 2 -C groups in alcohols (Figure 5.18).
Additional Details about NMR Spectra In order to complete our discussion of 13 C NMR spectrometry we introduce some additional terminology and explain those extra signals at δ 0 and δ 77 that we mentioned earlier and have seen in these spectra.
