Organic chemistry 4th edition brown foote iverson

Part of the explanation for the greater deshielding of vinylic hydrogens compared with alkyl hydrogens lies in the hybridization of carbon.

Vinylic hydrogens are deshielded by this electronegativity effect and their nuclei resonate farther downield relative to alkyl hydrogens. Simi- larly, signals for acetylene and aldehyde hydrogens also appear farther downield compared with alkyl hydrogens. If the chemi- cal shift of vinylic hydrogens d 4.

Yet the chemical shift of acetylenic hydrogens is only d 2. It seems that either the chemical shift of acetylenic hydrogens is abnormally small or the chemical shift of vinylic hydrogens is abnormally large. In either case, another factor must be contributing to the magnitude of the chemical shift. The- oretical and experimental evidence suggest that the chemical shifts of hydrogens bonded to p-bonded carbons are inluenced not only by the relative electronegativi- ties of the sp2- and sp-hybridized carbon atoms but also by magnetic induction from p bonds.

Diamagnetic Effects from p Bonds To understand the inluence of p bonds on the chemical shift of an acetylenic hydrogen, imagine that the carbon-carbon triple bond is oriented as shown in Figure Because of magnetic induction, the applied ield induces a circulation of the p electrons, which in turn produces an induced magnetic i eld.

Given the geometry of an alkyne and the cylindri- cal nature of its p electron cloud, the induced magnetic ield is shielding in the vicinity of the acetylenic hydrogen. Therefore, lower frequency electromagnetic radiation is required to make an acetylenic hydrogen nucleus resonate; the local magnetic ield induced in the p bonds shifts the signal of an acetylenic hydrogen upield to a smaller d value.

The effect of the induced circulation of p electrons on a vinylic hydrogen Figure The direction of the induced magnetic ield in the p bond of a carbon-carbon double bond is parallel to the applied ield in the region of the vinylic hydrogens. The induced magnetic ield deshields vinylic hydrogens and, thus, shifts their signal downield to a larger d value. The presence of the p electrons in the carbonyl group has a similar effect on the chemical shift of the hydrogen of an aldehyde group.

Applied ield, B 0 Figure Applied ield, B 0 The bottom line when comparing acetylinic versus vinylic hydrogens is that the hybridization effect, which would predict a larger chemical shift greater deshield- ing for the acetylinic hydrogen signals, is more than overcome by the strongly shielding diamagnetic effect of the cylindrically oriented alkyne p bonds and the deshielding diamagnetic effect of the alkene p bond. The net result is that acetyl- inic hydrogens have signals that appear less downield from TMS as compared to the signals for vinylic hydrogens.

The effects of the p electrons in benzene are even more dramatic than in alk- enes. All six hydrogens of benzene are equivalent, and its 1H-NMR spectrum is a sharp singlet at d 7. Hydrogens bonded to a substituted benzene ring appear in the region d 6.

Few other hydrogens absorb in this region; thus, aryl hydro- gens are quite easily identiiable by their distinctive chemical shifts, as much as 2 ppm higher than comparably substituted alkenes. That aryl hydrogens absorb even farther downield than vinylic hydrogens is Ring current accounted for by the existence of a ring current, a special property of aromatic An applied magnetic ield causes rings Figure When the plane of an aromatic ring tumbles in an applied the p electrons of an aromatic ring to circulate, giving rise to the magnetic ield, the applied ield causes the p electrons to circulate around the ring, so-called ring current and an giving rise to the so-called ring current.

This induced ring current has associated associated magnetic ield that with it a magnetic ield that opposes the applied ield in the middle of the ring but opposes the applied ield in the reinforces the applied ield on the outside of the ring. Thus, given the position of middle of the ring but reinforces the applied ield on the outside of aromatic hydrogens relative to the induced ring current, they are deshielded and the ring.

From the number of signals, we can determine the number of sets of equivalent hydrogens. From integration of signal areas, we can determine the relative numbers of hydrogens giving rise to each signal. From the chemical shift of each signal, we derive information about the types of hydrogens in each set.

A fourth kind of information can be derived from the splitting pattern of each signal. This molecule contains two sets of equivalent hydrogens. According to what we have learned so far, we predict two signals with relative areas corre- sponding to the three hydrogens of the! CH3 group and the one hydrogen of the!

CHCl2 group, respectively. You see from the spectrum, however, that there are, in fact, six peaks. These peaks are named by how the signal is split: two peaks are a Chapter 13 Nuclear Magnetic Resonance Spectroscopy Copyright Cengage Learning. The grouping of two peaks at d 2. CH3 group, and the grouping of four peaks at d 5. CHCl2 group. We say that the CH3 resonance at d 2.

In many situations, the degree of signal splitting can be predicted on the basis of the n 1 1 rule. According to this rule, if a hydrogen has n hydrogens nonequiv- n 1 1 rule alent to it but equivalent among themselves on the same or adjacent atom s , its If a hydrogen has n hydrogens 1 nonequivalent to it but equivalent H-NMR signal is split into n 1 1 peaks.

The three hydrogens of the! CH3 group have one nonequivalent neigh- is split into n 1 1 peaks. The single hydrogen of the! CHCl2 group has a set of three nonequivalent neighbor hydro- gens n 5 3 , and its signal is split into a quartet.

For these hydrogens, n 5 1; For this hydrogen, n 5 3; their signal is split into 1 1 1 its signal is split into 3 1 1 or two peaks—a doublet. Molecule a has three sets of equivalent hydrogens; its 1H-NMR spectrum shows a singlet, a quartet, and a triplet in the ratio 3 : 2 : 3.

Molecule b has two sets of equivalent hydrogens; its 1H-NMR spectrum shows a triplet and a quartet in the ratio 3 : 2. Molecule c has three sets of equivalent hydrogens; its 1H-NMR spectrum shows a singlet, a septet, and a doublet in the ratio 3 : 1 : 6. Predict the number of signals in the 1 H-NMR spectrum of each isomer and the splitting pattern of each signal.

A common type of spin-spin coupling involves the H atoms on NMR signals. These H atoms are three bonds apart Vicinal Hydrogens and are referred to as vicinal hydrogens.

Coupling between vicinal hydrogens is H atoms on two C atoms that are called vicinal coupling, illustrated in Figure A coupling constant J is the bonded to each other. The magnitude of a cou- Coupling between nuclei of vicinal pling constant is expressed in hertz Hz ; for protons in 1H-NMR spectroscopy, the H atoms.

The values of J depends only Coupling constant J on interactions with other nuclei within a molecule, and so it is independent of the The separation on an NMR spectrum in hertz between adjacent peaks applied ield strength.

Because of spin-spin coupling, alignment of the Hb nuclear spin with the applied magnetic ield results in a slightly different chemical shift of the signal for Ha compared to the situation in which the Hb nuclear spin is aligned against the applied magnetic ield. Across the population of molecules in a sample, there will be similar numbers of molecules Figure Any single molecule gives rise to a single signal The quartet-triplet 1H-NMR for Ha, but the spectrum of the entire sample shows both.

The result is that the signals of 3-pentanone showing signal for the Ha atom appears in the spectrum as a doublet. In this hypotheti- the original trace and a scale cal example, the signal for Hb will also be split into a similar doublet owing to Ha expansion to show the signal because the effect operates in both directions.

The coupling constant for two vicinal hydrogens on adjacent sp3 hybridized carbon atoms is approximately 7 Hz. For a spectrometer operating at MHz, a coupling constant of 7 Hz corresponds to only 0. Because peaks with this and comparable values of J are so narrowly spaced, splitting patterns from spectra taken at MHz and higher are often very dificult to see by inspection of the spectra. It is, therefore, common practice to retrace certain signals in expanded form so that splitting patterns are easier to observe Figure Given in Table Predicting Peak Intensities As stated previously, in the general case, n equivalent H atoms will cause signal split- ting into n 1 1 peaks.

We can now understand this rule and the relative intensities of these peaks by analyzing all possible spin state combinations. There are n 1 1 different spin state combinations of n spins aligning with or against an applied mag- netic ield. The probability of a molecule having a given set of spins is proportional to the number of possible spin alignments giving rise to that spin state.

The arrows in Figure If there is just one Hb nucleus to consider, there are only two possibilities q or p , both Each arrow represents an Hb nuclear spin orientation.

Two equivalent Hb nuclei can have three different possible com- binations that occur in a ratio middle , while three equivalent Hb nuclei can have four possible combinations that occur in a ratio right. Here is a note of caution in counting the number of peaks in a multiplet: if the signal of a particular hydrogen is of low intensity compared with others in the spectrum, it may not be possible to distinguish some of the smaller side peaks because of elec- Doublet tronic noise in the baseline.

The extent of Triplet coupling is related to a number of factors, including the number of bonds between 1 : 2 : 1 the H atoms in question.

H atoms with more than three bonds between them gen- erally do not exhibit noticeable coupling, although longer range coupling can be seen in some cases. As described in detail above, a common type of coupling is Quartet between vicinal H atoms. H s bonds and whether or not it is ixed. H bonds, so an average angle and an average coupling are Figure As illustrated for alkanes and other lexible molecules. It entry is the sum of the values is only when coupling is between nonequivalent hydrogens that signal splitting immediately above it to the left results; coupling between equivalent hydrogens, whether they are on the same or and the right.

More Complex Splitting Patterns So far, we have concentrated on spin-spin coupling with only one other nonequivalent set of H atoms. However, more complex situations often arise in which the nuclei of a set of H atoms are coupled to the nuclei of more than one set of nonequivalent H atoms in molecules that do not have rapid bond rotation. In these situations, the coupling from adjacent nonequivalent sets of H atom nuclei combines to give more com- plex signal splitting patterns.

Use of a tree diagram is helpful in understanding split- ting in these cases. In a tree diagram, the different couplings are applied sequentially. For example, the atom labeled Hb in Figure Here, the signal for Hb is split into a doublet with coupling constant Jab by Ha, and this doublet is split into a dou- blet of doublets with coupling constant Jbc by Hc.

If there were no other H atoms in the molecule to be considered, then the signal for Ha would be a doublet with coupling constant Jab and the signal for Hc would be a doublet with coupling constant Jbc. If Jab and Jbc are equal, the peaks overlap, a situation discussed in Section This analysis assumes no Spin-spin other coupling in the molecule coupling Jbc Jbc and that Jab?

If Hc is a set of two equivalent H atoms and Ha is still a single H atom, then the observed coupling would be a doublet of triplets, in other words, a signal with six peaks.

Again we are assuming that Jab? The tree diagram in Figure The peaks for Ha and Hb would each integrate to a relative value of one H atom, while the peaks for Hc would integrate to a relative value of 2 H atoms. In the general case, a signal will be split into n 1 1 3 m 1 1 peaks for an H atom that is coupled to a set of n H atoms with one coupling constant, and to a set of m H atoms with another coupling constant.

Note that, in a tree diagram, you get the same splitting patterns no matter which order the two coupling constants are analyzed. Bond Rotation Because the angle between C! H bonds determines the extent of coupling in a molecule, bond rotation is a key parameter. In alkanes and other molecules with relatively free rotation about C! C s bonds, H atoms bonded to the same C atom R1 Ha in! CH3 groups are generally equivalent because of the rapid bond C C Geminal coupling rotation. An exception is when a!

However, when there is restricted bond rotation, Figure Nonequivalent 1H nuclei when two H atoms on the same on the same C atom will couple to each other and cause splitting. This is referred carbon atom are not equivalent. Geminal coupling constants are generally This is most common in small, on the order of 0 to 5 Hz. For example, ethyl propenoate ethyl acrylate is an unsymmetrical terminal alkene; therefore, the three alkenyl H atoms are nonequiv- alent Figure As a result, their nuclei couple with each other.

In alkenes, trans coupling generally results in larger coupling constants J trans 5 11—18 Hz compared to cis coupling J cis 5 5—10 Hz , with geminal coupling being by far the smallest J gem 5 0—5 Hz. Unless a high-resolution spectrum is taken, the geminal coupling constant is so small that it is often dificult to see in terminal alkenes. In the spectrum of ethyl propenoate, the geminal coupling is only visible upon close inspection of the signals labeled a and c.

You should be able to recognize the char- acteristic ethyl group pattern of a quartet integrating to two H atoms! CH3, He. Tree diagrams are provided in Figure C s bonds and 2-methylvinyl oxirane. The two can have constrained conformations. The result is that the two H atoms on! H atoms on the oxirane ring are groups in cyclic molecules can be nonequivalent, leading to complex spin-spin cou- nonequivalent, so they exhibit pling.

Substituted epoxides such as 2-methylvinyloxirane provide a good example geminal coupling. The two H atoms on the three-membered epoxide ring are nonequiv- alent.

Hd is cis to the vinyl group and trans to the methyl group, while He is the re- verse. Because they are in different chemical environments, they are nonequivalent and exhibit geminal coupling Figure The geminal coupling constant is small but discernable in the spectrum because the signals for both Hd and He are doublets. Vinyl H atom Ha is split by both Hb trans coupling and Hc cis coupling , giving rise to a doublet of doublets, or four peaks.

Hb is split by Ha trans coupling along with Hc, the latter geminal coupling constant is so small that it is barely discernable at this resolution. Hc is split by Ha cis coupling as well as Hb geminal coupling. The sin- glet near 1. Coincidental Overlap Here is a word of caution: quite often, because peaks can overlap by coincidence, there are fewer distinguishable peaks in a signal than predicted.

Coincidental peak overlap can occur in any molecule, but it is especially common with lexible alkyl chains. In addition, some coupling constants are so small that peak splitting is hard to see in a spectrum.

Thus, the predicted number of peaks using the n 1 1 3 m 1 1 rule should be considered the maximum that might be observed. Detailed analysis using extremely high resolution spectrometers is often required to distinguish all the peaks in a highly split signal. You should note also that the types of splitting When this is not the case, spectra can become much more complex. Complex Coupling in Flexible Molecules Coupling in molecules having unrestricted bond rotation is often simpliied to give only m 1 n 1 1 peaks, not the expected n 1 1 3 m 1 1.

In other words, the number of peaks actually observed for a signal is the number of adjacent hydro- gens 1 1, no matter how many different sets of equivalent H atoms this represents.

The explanation is that bond rotation averages the coupling constants throughout molecules with freely rotating bonds and tends to make them very similar, in the 6 to 8 Hz range, for H atoms on freely rotating sp3 hybridized C atoms. Very similar or identical coupling constants simplify splitting patterns. For exam- ple, in the hypothetical unsymmetrical molecule depicted in Figure If J ab? J bc, this would lead to a triplet of triplets or nine peaks in the signal for Hb.

However, if the coupling constants are identical so that J ab 5 J bc, the splitting pattern overlaps consid- erably to generate only ive peaks in the signal for Hb. In the general case, simpliica- tion because of very similar or identical J values gives a number of peaks equal to the number of adjacent H atoms 1 1, regardless of patterns of equivalence.

The signal for the H atoms of the central! However, because the values for Jab and Jbc are so similar, only 4 1 1 5 5 peaks are distinguishable in the spectrum for the Hc signal as a result of peak overlap. Another common example is the kind of splitting of the signal for the central!

CH3 group, such as occurs in the molecule 1-chloropro- pane, Cl! CH3 Figure A maximum of 3 3 4 5 12 peaks would be possible for the central!

Fast Exchange Hydrogen atoms bonded to oxygen or nitrogen atoms can exchange with each other faster than the time it takes to acquire a 1H-NMR spectrum. This process is greatly facilitated by even traces of acid or base in a sample. Fast Figure First, signals for exchanging H atoms MHz 1H-NMR spectrum of are generally broad singlets that do not take part in splitting with other signals. Sec- 1-chloropropane. This latter phenomenon can be used to identify signals from exchange- able H atoms by taking spectra with and without added D2O.

Note that these same exchangeable H atoms also can take part in hydrogen bonds, the presence of which can alter chemical shift in a concentration-dependent fashion. Homotopic groups Atoms or groups on an atom that The hydrogens of the CH2 this chapter is slightly oversimpliied because stereochemistry can affect chemical group of propane, for example, shift. Depending on the symmetry of the molecule, otherwise equivalent atoms may are homotopic. Replacing either one of them with deuterium gives be homotopic, enantiotopic, or diastereotopic.

The simplest way to visualize the 2-deuteropropane, which is achiral. Enantiotopic groups whether the resulting compound would be a the same or b different from its Atoms or groups on an atom that mirror image or whether c diastereomers are possible. Depending on the out- give a chiral center when one of the groups is replaced by an come of the test, the atoms or groups are homotopic, enantiotopic, or diastereot- isotope.

A pair of enantiomers opic, respectively. The hydrogens of the CH2 Consider the following molecules. H D Enantiotopic groups have identical chemical shifts in achiral Dichloromethane achiral environments but different chemical achiral shifts in chiral environments.

The hydrogens of the CH2 group of 2-butanol, If one hydrogen in dichloromethane is substituted with one deuterium, an achi- for example, are diastereotopic. This molecule is identical to its mirror image, and the two Diastereotopic groups have different chemical shifts under all conditions, hydrogens in dichloromethane are equivalent and homotopic.

Homotopic groups although the differences are only have identical chemical shifts in all environments. The two hydro- gens in this compound are therefore enantiotopic. Enantiotopic hydrogens have identical chemical shifts except in chiral environments. In a chiral solvent, for exam- ple, the two hydrogens would have different chemical shifts. While the distinction between homotopic and enantiotopic compounds is of little practical consequence in NMR spectroscopy, the two hydrogens in chloroluoromethane can be distin- guished by enzymes, which also provide a chiral environment.

The CH2 hydrogens in this molecule are said to be prochiral. The compound 2-butanol presents a more complex situation. Iverson, Eric Anslyn and Christopher S. Our digital library saves in multipart countries, allowing you to acquire the most less latency time to download any of our books once this one.

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