13-1 Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson William H. Brown...
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Transcript of 13-1 Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson William H. Brown...
13-13-11
Organic Organic ChemistryChemistry
William H. BrownWilliam H. Brown
Christopher S. FooteChristopher S. Foote
Brent L. IversonBrent L. Iverson
William H. BrownWilliam H. Brown
Christopher S. FooteChristopher S. Foote
Brent L. IversonBrent L. Iverson
13-13-33
Molecular SpectroscopyMolecular Spectroscopy Nuclear magnetic resonance (NMR) Nuclear magnetic resonance (NMR)
spectroscopyspectroscopy:: a spectroscopic technique that gives us information about the number and types of atoms in a molecule, for example, about the number and types of • hydrogen atoms using 1H-NMR spectroscopy• carbon atoms using 13C-NMR spectroscopy• phosphorus atoms using 31P-NMR spectroscopy
13-13-44
Nuclear Spin StatesNuclear Spin States An electron has a spin quantum number of 1/2
with allowed values of +1/2 and -1/2 • this spinning charge creates an associated magnetic
field• in effect, an electron behaves as if it is a tiny bar
magnet and has what is called a magnetic moment
The same effect holds for certain atomic nuclei• any atomic nucleus that has an odd mass number, an
odd atomic number, or both also has a spin and a resulting nuclear magnetic moment
• the allowed nuclear spin states are determined by the spin quantum number, I, of the nucleus
13-13-55
Nuclear Spin StatesNuclear Spin States
• a nucleus with spin quantum number II has 22II + 1 + 1 spin states; if I = 1/2, there are two allowed spin states
• Table 13.1 gives the spin quantum numbers and allowed nuclear spin states for selected isotopes of elements common to organic compounds
1H 2H 12C 13C 14N 16O 31P 32SElement
Nuclear spinquantum number (I )
Number ofspin states
1/2 1 0 0 01/2 1
2 3 1 2 3 1
1/2
2 1
13-13-66
Nuclear Spins in BNuclear Spins in B00
• within a collection of 1H and 13C atoms, nuclear spins are completely random in orientation
• when placed in a strong external magnetic field of strength B0, however, interaction between nuclear spins and the applied magnetic field is quantized, with the result that only certain orientations of nuclear magnetic moments are allowed
13-13-88
Nuclear Spins in BNuclear Spins in B00
In an applied field strength of 7.05T, which is readily available with present-day superconducting electromagnets, the difference in energy between nuclear spin states for • 1H is approximately 0.120 J (0.0286 cal)/mol, which
corresponds to electromagnetic radiation of 300 MHz (300,000,000 Hz)
• 13C is approximately 0.030 J (0.00715 cal)/mol, which corresponds to electromagnetic radiation of 75MHz (75,000,000 Hz)
13-13-99
Nuclear Spin in BNuclear Spin in B00
• the energy difference between allowed spin states increases linearly with applied field strength
• values shown here are for 1H nuclei
13-13-1010
Nuclear Magnetic ResonanceNuclear Magnetic Resonance
• when nuclei with a spin quantum number of 1/2 are placed in an applied field, a small majority of nuclear spins are aligned with the applied field in the lower energy state
• the nucleus begins to precess and traces out a cone-shaped surface, in much the same way a spinning top or gyroscope traces out a cone-shaped surface as it precesses in the earth’s gravitational field
• we express the rate of precession as a frequency in hertz
13-13-1111
Nuclear Magnetic ResonanceNuclear Magnetic Resonance If the precessing nucleus is irradiated with
electromagnetic radiation of the same frequency as the rate of precession,• the two frequencies couple, • energy is absorbed, and • the nuclear spin is flipped from spin state +1/2 (with
the applied field) to -1/2 (against the applied field)
13-13-1212
Nuclear Magnetic ResonanceNuclear Magnetic Resonance
• Figure 13.3 the origin of nuclear magnetic “resonance
13-13-1313
Nuclear Magnetic ResonanceNuclear Magnetic Resonance ResonanceResonance:: in NMR spectroscopy, resonance is
the absorption of electromagnetic radiation by a precessing nucleus and the resulting “flip” of its nuclear spin from a lower energy state to a higher energy state
The instrument used to detect this coupling of precession frequency and electromagnetic radiation records it as a signal• signal:signal: a recording in an NMR spectrum of a nuclear
magnetic resonance
13-13-1414
Nuclear Magnetic ResonanceNuclear Magnetic Resonance
• if we were dealing with 1H nuclei isolated from all other atoms and electrons, any combination of applied field and radiation that produces a signal for one 1H would produce a signal for all 1H. The same is true of 13C nuclei
• but hydrogens in organic molecules are not isolated from all other atoms; they are surrounded by electrons, which are caused to circulate by the presence of the applied field
• the circulation of electrons around a nucleus in an applied field is called diamagneticdiamagnetic currentcurrent and the nuclear shielding resulting from it is called diamagnetic shieldingdiamagnetic shielding
13-13-1515
Nuclear Magnetic ResonanceNuclear Magnetic Resonance
• the difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small
• the difference in resonance frequencies for hydrogens in CH3Cl compared to CH3F under an applied field of 7.05T is only 360 Hz, which is 1.2 parts per million (ppm) compared with the irradiating frequency
360 Hz300 x 106 Hz
1.2 = 1.2 ppm106=
13-13-1616
Nuclear Magnetic ResonanceNuclear Magnetic Resonance
• signals are measured relative to the signal of the reference compound tetramethylsilane (TMS)
• for a 1H-NMR spectrum, signals are reported by their shift from the 12 H signal in TMS
• for a 13C-NMR spectrum, signals are reported by their shift from the 4 C signal in TMS
• Chemical shift (Chemical shift ():): the shift in ppm of an NMR signal from the signal of TMS
Tetramethylsilane (TMS)
CH3
Si CH3
CH3
CH3
13-13-1818
NMR SpectrometerNMR Spectrometer
Essentials of an NMR spectrometer are a powerful magnet, a radio-frequency generator, and a radio-frequency detector
The sample is dissolved in a solvent, most commonly CDCl3 or D2O, and placed in a sample tube which is then suspended in the magnetic field and set spinning
Using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in about 2 seconds
13-13-1919
NMR SpectrumNMR Spectrum 1H-NMR spectrum of methyl acetate
• Downfield:Downfield: the shift of an NMR signal to the left on the chart paper
• Upfield:Upfield: the shift of an NMR signal to the right on the chart paper
13-13-2020
Equivalent HydrogensEquivalent Hydrogens Equivalent hydrogens:Equivalent hydrogens: have the same chemical
environment• a molecule with 1 set of equivalent hydrogens gives 1
NMR signal
H3C
C C
CH3
H3C CH3
CH3CCH3 ClCH2CH2Cl
Propanone(Acetone)
1,2-Dichloro-ethane
Cyclopentane 2,3-Dimethyl-2-butene
O
13-13-2121
Equivalent HydrogensEquivalent Hydrogens
• a molecule with 2 or more sets of equivalent hydrogens gives a different NMR signal for each set
CH3CHCl
Cl ClC C
CH3
H H
O
Cyclopent-anone
(2 signals)
1,1-Dichloro-ethane
(2 signals)
(Z)-1-Chloro-propene
(3 signals)
Cyclohexene (3 signals)
13-13-2222
Signal AreasSignal Areas Relative areas of signals are proportional to the
number of H giving rise to each signal Modern NMR spectrometers electronically
integrate and record the relative area of each signal
13-13-2323
ChemicalChemicalShiftsShifts11H-NMRH-NMR
RCH2OR
(CH3)4Si
ArCH3
RCH3
RC CH
RCCH3
ROHRCH2OH
ArCH2R
O
O
RCH2RR3CH
R2NH
RCCH2R
R2C=CRCHR2
R2C=CHR
RCH
O
RCOH
O
RCH2ClRCH2BrRCH2I
RCH2F
ArHO
O
R2C=CH2
RCOCH3
RCOCH2R
ArOH
9.5-10.1
3.7-3.9
3.4-3.6
Type of Hydrogen
0 (by definition)
Type of Hydrogen
Chemical Shift ()
1.6-2.62.0-3.0
0.8-1.01.2-1.41.4-1.7
2.1-2.3
0.5-6.0
2.2-2.6
3.4-4.0
Chemical Shift ()
3.3-4.0
2.2-2.52.3-2.8
0.5-5.0
4.6-5.05.0-5.7
10-13
4.1-4.73.1-3.3
3.6-3.84.4-4.5
6.5-8.5
4.5-4.7
13-13-2525
Chemical ShiftChemical Shift Depends on (1) electronegativity of nearby atoms, (2) the
hybridization of adjacent atoms, and (3) diamagnetic effects from adjacent pi bonds
Electronegativity
CH3OH
CH3F
CH3Cl
CH3BrCH3I
(CH3)4C(CH3)4Si
CH3-XElectroneg-ativity of X
Chemical Shift ()
4.03.53.12.82.5
2.11.8
4.263.473.052.682.16
0.860.00
13-13-2626
Chemical ShiftChemical Shift Hybridization of adjacent atoms
RCH3, R2CH2, R3CH
R2C=CHR, R2C=CH2
RCHO
R2C=C(R)CHR2
RC CH
Allylic
Type of Hydrogen(R = alkyl)
Name ofHydrogen
Chemical Shift ()
Alkyl
Acetylenic
Vinylic
Aldehydic
0.8 - 1.7
1.6 - 2.6
4.6 - 5.7
9.5-10.1
2.0 - 3.0
13-13-2727
Chemical ShiftChemical Shift Diamagnetic effects of pi bonds• a carbon-carbon triple bond shields an acetylenic
hydrogen and shifts its signal upfield (to the right) to a smaller value
• a carbon-carbon double bond deshields vinylic hydrogens and shifts their signal downfield (to the left) to a larger value
RCH3
R2C=CH2
RC CH
Type of H Name
Alkyl
VinylicAcetylenic
0.8- 1.0
4.6 - 5.72.0 - 3.0
Chemical Shift ()
13-13-2828
Chemical ShiftChemical Shift
• magnetic induction in the pi bonds of a carbon-carbon triple bond (Fig 13.9)
13-13-2929
Chemical ShiftChemical Shift
• magnetic induction in the pi bond of a carbon-carbon double bond (Fig 13.10)
13-13-3030
Chemical ShiftChemical Shift
• magnetic induction of the pi electrons in an aromatic ring (Fig. 13.11)
13-13-3131
Signal Splitting; the (Signal Splitting; the (nn + 1) Rule + 1) Rule Peak:Peak: the units into which an NMR signal is split;
doublet, triplet, quartet, etc. Signal splitting:Signal splitting: splitting of an NMR signal into a
set of peaks by the influence of neighboring nonequivalent hydrogens
((nn + 1) rule: + 1) rule: if a hydrogen has n hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s), its 1H-NMR signal is split into (n + 1) peaks
13-13-3232
Signal Splitting (n + 1)Signal Splitting (n + 1)
• 1H-NMR spectrum of 1,1-dichloroethane
CH3-CH-ClCl
For these hydrogens, n = 1;their signal is split into(1 + 1) = 2 peaks; a doublet
For this hydrogen, n = 3;its signal is split into(3 + 1) = 4 peaks; a quartet
13-13-3333
Signal Splitting (n + 1)Signal Splitting (n + 1)
ProblemProblem: predict the number of 1H-NMR signals and the splitting pattern of each
CH3CCH2CH3
O
CH3CH2CCH2CH3
O
CH3CCH(CH3)2
O
(a)
(b)
(c)
13-13-3434
Origins of Signal SplittingOrigins of Signal Splitting Signal coupling:Signal coupling: an interaction in which the
nuclear spins of adjacent atoms influence each other and lead to the splitting of NMR signals
Coupling constant (J):Coupling constant (J): the separation on an NMR spectrum (in hertz) between adjacent peaks in a multiplet; • a quantitative measure of the influence of the spin-
spin coupling with adjacent nuclei
13-13-3636
Origins of Signal SplittingOrigins of Signal Splitting
• because splitting patterns from spectra taken at 300 MHz and higher are often difficult to see, it is common to retrace certain signals in expanded form
• 1H-NMR spectrum of 3-pentanone; scale expansion shows the triplet quartet pattern more clearly
13-13-3737
Coupling ConstantsCoupling Constants Coupling constant (J):Coupling constant (J): the distance between peaks in a
split signal, expressed in hertz• the value is a quantitative measure of the magnetic
interaction of nuclei whose spins are coupled
8-11 Hz
8-14 Hz 0-5 Hz 0-5 Hz6-8 Hz
11-18 Hz 5-10 Hz 0-5 Hz
CCHa
C C
HbHaC
Hb
C
Ha
Hb
Ha
Hb
Ha
Hb HbHa
Hb
Ha
C CHaHb
13-13-3939
Signal SplittingSignal Splitting Pascal’s Triangle• as illustrated by the
highlighted entries, each entry is the sum of the values immediately above it to the left and the right
13-13-4040
Physical Basis for (Physical Basis for (nn + 1) Rule + 1) Rule Coupling of nuclear spins is mediated through
intervening bonds• H atoms with more than three bonds between them
generally do not exhibit noticeable coupling• for H atoms three bonds apart, the coupling is referred
to as vicinal coupling
13-13-4141
Coupling ConstantsCoupling Constants
• an important factor in vicinal coupling is the angle between the C-H sigma bonds and whether or not it is fixed
• coupling is a maximum when is 0° and 180°; it is a minimum when is 90°
13-13-4242
More Complex Splitting PatternsMore Complex Splitting Patterns
• thus far, we have concentrated on spin-spin coupling with only one other nonequivalent set of H atoms
• more complex splittings arise when a set of H atoms couples to more than one set H atoms
• a tree diagram shows that when Hb is adjacent to nonequivalent Ha on one side and Hc on the other, the resulting coupling gives rise to a doublet of doublets
13-13-4343
More Complex Splitting PatternsMore Complex Splitting Patterns
• if Hc is a set of two equivalent H, then the observed splitting is a doublet of triplets
13-13-4444
More Complex Splitting PatternsMore Complex Splitting Patterns
• because the angle between C-H bond determines the extent of coupling, bond rotation is a key parameter
• in molecules with relatively free rotation about C-C sigma bonds, H atoms bonded to the same carbon in CH3 and CH2 groups generally are equivalent
• if there is restricted rotation, as in alkenes and cyclic structures, H atoms bonded to the same carbon may not be equivalent
• nonequivalent H on the same carbon will couple and cause signal splitting
• this type of coupling is called geminal couplinggeminal coupling
13-13-4545
More Complex Splitting PatternsMore Complex Splitting Patterns
• in ethyl propenoate, an unsymmetrical terminal alkene, the three vinylic hydrogens are nonequivalent
13-13-4646
More Complex Splitting PatternsMore Complex Splitting Patterns
• a tree diagram for the complex coupling of the three vinylic hydrogens in ethyl propenoate
13-13-4747
More Complex Splitting PatternsMore Complex Splitting Patterns
• cyclic structures often have restricted rotation about their C-C bonds and have constrained conformations
• as a result, two H atoms on a CH2 group can be nonequivalent, leading to complex splitting
13-13-4848
More Complex Splitting PatternsMore Complex Splitting Patterns
• a tree diagram for the complex coupling in 2-methyl-2-vinyloxirane
13-13-4949
More Complex Splitting PatternsMore Complex Splitting Patterns Complex coupling in flexible molecules• coupling in molecules with unrestricted bond rotation
often gives only m + n + I peaks• that is, the number of peaks for a signal is the number
of adjacent hydrogens + 1, no matter how many different sets of equivalent H atoms that represents
• the explanation is that bond rotation averages the coupling constants throughout molecules with freely rotation bonds and tends to make them similar; for example in the 6- to 8-Hz range for H atoms on freely rotating sp3 hybridized C atoms
13-13-5050
More Complex Splitting PatternsMore Complex Splitting Patterns
• simplification of signal splitting occurs when coupling constants are the same
13-13-5151
More Complex Splitting PatternsMore Complex Splitting Patterns
• an example of peak overlap occurs in the spectrum of 1-chloro-3-iodopropane
• the central CH2 has the possibility for 9 peaks (a triplet of triplets) but because Jab and Jbc are so similar, only 4 + 1 = 5 peaks are distinguishable
13-13-5252
Stereochemistry & TopicityStereochemistry & Topicity Depending on the symmetry of a molecule,
otherwise equivalent hydrogens may be• homotopic• enantiotopic• diastereotopic
The simplest way to visualize topicity is to substitute an atom or group by an isotope; is the resulting compound• the same as its mirror image• different from its mirror image• are diastereomers possible
13-13-5353
Stereochemistry & TopicityStereochemistry & Topicity Homotopic atoms or groups
• homotopic atoms or groups have identical chemical shifts under all conditions
Achiral
H
C
H
Cl
Cl
H
C
D
Cl
Cl
Dichloro-methane(achiral)
Substitution does not produce a stereocenter;therefore hydrogensare homotopic.
Substitute one H by D
Achiral
H
C
H
Cl
Cl
H
C
D
Cl
Cl
Dichloro-methane(achiral)
Substitution does not produce a stereocenter;therefore hydrogensare homotopic.
Substitute one H by D
13-13-5454
Stereochemistry & TopicityStereochemistry & Topicity Enantiotopic groups
• enantiotopic atoms or groups have identical chemical shifts in achiral environments
• they have different chemical shifts in chiral environments
Chiral
H
C
H
Cl
F
H
C
D
Cl
F
Chlorofluoro-methane(achiral)
Substitute one H by D
Substitution produces a stereocenter;
therefore, hydrogens are enantiotopic. Both hydrogens are prochiral; one is pro-R-chiral, the other is pro-S-chiral.Chiral
H
C
H
Cl
F
H
C
D
Cl
F
Chlorofluoro-methane(achiral)
Substitute one H by D
Substitution produces a stereocenter;
therefore, hydrogens are enantiotopic. Both hydrogens are prochiral; one is pro-R-chiral, the other is pro-S-chiral.
13-13-5555
Stereochemistry & TopicityStereochemistry & Topicity Diastereotopic groups• H atoms on C-3 of 2-butanol are diastereotopic• substitution by deuterium creates a chiral center• because there is already a chiral center in the
molecule, diastereomers are now possible
• diastereotopic hydrogens have different chemical shifts under all conditions
H OH
HH
H OH
HD
Chiral
Substitute one H on CH2 by D
2-Butanol(chiral)
13-13-5656
Stereochemistry & TopicityStereochemistry & Topicity The methyl groups on carbon 3 of 3-methyl-2-
butanol are diastereotopic• if a methyl hydrogen of carbon 4 is substituted by
deuterium, a new chiral center is created• because there is already one chiral center,
diastereomers are now possible
• protons of the methyl groups on carbon 3 have different chemical shifts
OH
3-Methyl-2-butanol
13-13-5757
Stereochemistry and TopicityStereochemistry and Topicity 1H-NMR spectrum of 3-methyl-2-butanol• the methyl groups on carbon 3 are diastereotopic and
appear as two doublets
13-13-5858
1313C-NMR SpectroscopyC-NMR Spectroscopy Each nonequivalent 13C gives a different signal• a 13C signal is split by the 1H bonded to it according to
the (n + 1) rule • coupling constants of 100-250 Hz are common, which
means that there is often significant overlap between signals, and splitting patterns can be very difficult to determine
The most common mode of operation of a 13C-NMR spectrometer is a hydrogen-decoupled mode
13-13-5959
1313C-NMR SpectroscopyC-NMR Spectroscopy In a hydrogen-decoupled mode, a sample is
irradiated with two different radio frequencies• one to excite all 13C nuclei• a second broad spectrum of frequencies to cause all
hydrogens in the molecule to undergo rapid transitions between their nuclear spin states
On the time scale of a 13C-NMR spectrum, each hydrogen is in an average or effectively constant nuclear spin state, with the result that 1H-13C spin-spin interactions are not observed; they are decoupled
13-13-6060
1313C-NMR SpectroscopyC-NMR Spectroscopy
• hydrogen-decoupled 13C-NMR spectrum of 1-bromobutane
13-13-6161
Chemical Shift - Chemical Shift - 1313C-NMRC-NMR
RCH3RCH2R
R3CH
R2C=CR2
RC CR
R3COR
RCH2Cl
RCH2BrRCH2I
R3COH
RC
RCNR2
O
RCH, RCRO O
RCCOHO
RCORO
0-40
110-160
165 - 180
160 - 180
165 - 185
180 - 215
40-80
40-80
35-80
25-65
65-85
100-150
20-6015-5510-40
Type of Carbon
ChemicalShift ()
ChemicalShift ()
Type of Carbon
13-13-6363
The DEPT MethodThe DEPT Method In the hydrogen-decoupled mode, information on
spin-spin coupling between 13C and hydrogens bonded to it is lost
The DEPT method is an instrumental mode that provides a way to acquire this information• Distortionless Enhancement by Polarization TransferDistortionless Enhancement by Polarization Transfer
(DEPT):DEPT): an NMR technique for distinguishing among 13C signals for CH3, CH2, CH, and quaternary carbons
13-13-6464
The DEPT MethodThe DEPT Method The DEPT methods uses a complex series of
pulses in both the 1H and 13C ranges, with the result that CH3, CH2, and CH signals exhibit different phases;• signals for CH3 and CH carbons are recorded as
positive signals
• signals for CH2 carbons are recorded as negative signals
• quaternary carbons give no signal in the DEPT method
13-13-6666
Interpreting NMR SpectraInterpreting NMR Spectra AlkanesAlkanes • 1H-NMR signals appear in the range of 0.8-1.7 • 13C-NMR signals appear in the considerably wider
range of 10-60
AlkenesAlkenes • 1H-NMR signals appear in the range 4.6-5.7• 1H-NMR coupling constants are generally larger for trans vinylic hydrogens (J= 11-18 Hz) compared with cis vinylic hydrogens (J= 5-10 Hz)
• 13C-NMR signals for sp2 hybridized carbons appear in the range 100-160, which is downfield from the signals of sp3 hybridized carbons
13-13-6767
Interpreting NMR SpectraInterpreting NMR Spectra
• 1H-NMR spectrum of vinyl acetate (Fig 13.33)
13-13-6868
Interpreting NMR SpectraInterpreting NMR Spectra AlcoholsAlcohols 1H-NMR O-H chemical shifts often appears in the
range 3.0-4.0, but may be as low as 0.5. • 1H-NMR chemical shifts of hydrogens on the carbon
bearing the -OH group are deshielded by the electron-withdrawing inductive effect of the oxygen and appear in the range 3.0-4.0
EthersEthers • a distinctive feature in the 1H-MNR spectra of ethers is
the chemical shift, 3.3-4.0, of hydrogens on carbon attached to the ether oxygen
13-13-6969
Interpreting NMR SpectraInterpreting NMR Spectra
• 1H-NMR spectrum of 1-propanol (Fig. 13.34)
13-13-7070
Interpreting NMR SpectraInterpreting NMR Spectra Aldehydes and ketonesAldehydes and ketones• 1H-NMR: aldehyde hydrogens appear at 9.5-10.1• 1H-NMR: -hydrogens of aldehydes and ketones
appear at 2.2-2.6• 13C-NMR: carbonyl carbons appear at 180-215
AminesAmines• 1H-NMR: amine hydrogens appear at 0.5-5.0
depending on conditions
13-13-7171
Interpreting NMR SpectraInterpreting NMR Spectra Carboxylic acidsCarboxylic acids• 1H-NMR: carboxyl hydrogens appear at 10-13, lower
than most any other hydrogens • 13C-NMR: carboxyl carbons in acids and esters appear
at 160-180
13-13-7272
Interpreting NMR SpectraInterpreting NMR Spectra
Spectral Problem 1; molecular formula C5H10O
13-13-7373
Interpreting NMR SpectraInterpreting NMR Spectra
Spectral Problem 2; molecular formula C7H14O