Download - Lecture 7 - Chapter 2-9-16-05
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CHMBD 449 Organic Spectral AnalysisFall 2005Chapter 2: IR SpectroscopyAnalysisGroup Frequencies
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IR Spectroscopy
Group Frequencies and Analysissp3 Nitrogen AminesAmines Once presence is determined, the substitution at nitrogen is easy to determine; only the 3 amine may present a problem
Group Frequencies (cm-1):
N-H (-NH2)3650-3600(2 bands)1640-1560Stretch (sym. and asym.)
BendN-H(-NHR)3400-3300(1 band)1500Stretch
BendFor alkyl amines, very weak for aromatic 2 amines, strongerN-H ~800Oop bendN-N1350-1000StretchRemember 3 amines have no N-H bands
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IR Spectroscopy
Group Frequencies and Analysissp3 Nitrogen Amines1 Amine tert-butylamine
Two band NH2 peak appears as small w
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IR Spectroscopy
Group Frequencies and Analysissp3 Nitrogen Amines2 Amine dibutylamine
Note weakness of NH- band (can be mistaken as C=O overtone, if carbonyl is present)
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IR Spectroscopy
Group Frequencies and Analysissp3 Nitrogen Amines3 Amine tributylamine
Difficult to discern from alkane molecular formula for confirmation almost requisite
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IR Spectroscopy
Group Frequencies and Analysissp3 Nitrogen AminesAmmonium Salts
Almost certainly never encountered in neat samples, but an important component of amino acids and many pharmaceuticals
Group Frequencies
N-H 3300-2600Stretch1 salts are at the higher n end of this band, 3 salts at the lower endAdditional band sometimes obs. at 2100 N-H1600-1500Bend1 as two bands (sym. And asymm.), 2 at the upper end of this range, 3 absorbs weakly
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IR Spectroscopy
Group Frequencies and Analysissp3 Nitrogen AminesAmmonium Salts anilinium hydrochloride
Spectrum is of a KBr disc sample:
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsGeneral Along with alcohols, the most ubiquitous group on the IR spectrum. Although it is easy to determine if the C=O is present, deducing the exact functionality and factors that influence the position of the band provide the challenge
Base C=O Frequencies (cm-1):
C=O1810Stretch (sym.)Anhydride band 11800Acid Chloride1760Anhydride band 21735Ester1725Aldehyde1715Ketone1710Carboxylic Acid1690Amide
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsGeneral The carbonyl C=O frequency is very sensitive to the effects we went over previously a quick recap
Electronic Effects: Inductive vs. Resonance:
On first inspection, the ester, amide and acid halide/anhydride all possess lone pairs of electrons that can resonate with the C=O (which should lower n)
CNOFClS
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsGeneral Electronic Effects: Inductive vs. Resonance:
In the case of an oxygen or chlorine being adjacent to the carbonyl, each of these atoms resist the positive charge in the contributing resonance structure, and the inductive effect becomes a stronger factor
CNOFClSThis inductive effect draws in s electrons from the C=O, which strengthens the p bond these carbonyls appear at higher n
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsGeneral Electronic Effects: Inductive vs. Resonance:
In the case of nitrogen, it is less electronegative than oxygen and has a greater acceptance of the positive charge in the contributing resonance structure, so the carbonyl is lowered in n CNOFClSThe inductive effect of nitrogen compared to an sp2 carbon is negligible by comparison
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsGeneral Electronic Effects: Inductive vs. Resonance:
Likewise in aldehydes and ketones there is the inductive donation of electrons to the s bond of the carbonyl which slightly weakens and reduces the n of the p bond (and explains the small difference between aldehydes and ketones) CNOFClSThe inductive effect of nitrogen compared to an sp2 carbon is negligible by comparison
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsGeneral Electronic Effects: Inductive vs. Resonance:
In addition, we discussed this effect in regards to a-halogenated carbonyls as one of the effects that can change group n CNOFClSThe inductive effect of chlorine will draw s electrons through a-carbon, weakening the C=O s and strengthening the p
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsGeneral Electronic Effects - Resonance:
Not only is the C=O n lowered by the effects of conjugation, the peak may also be broadened or split by the contribution of the two electronic conformers The s-cis absorbs at higher n than the s-trans. Why?
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsGeneral Ring Strain Effects:C=O groups that can be incorporated into a ring are sensitive to this effect. As ring size decreases more p-character must be used to make the single bonds take on the smaller angle (re: sp>3 =
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsGeneral H-bonding effects:C=O groups are reduced in n if some of the electron density is tapped off to form H-bonds:
This effect can be inter- or intra-molecular:
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsKetones Simplest carbonyl group, for a single carbonyl compound, implied by a lack of any other functionality except hydrocarbon
Group Frequencies (cm-1):
C=O1715Stretch (sym.)n Base, sensitive to changeconj. w/C=C1700-1675nC=C reduced to 1644-1617conj. w/Ph1700-1680nring 1600-1450C=O1815-1705Decreased ring size raises n
1300-1100Bend
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsKetones 2-hexanone
Typical aliphatic ketone
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsKetones 4-methylacetophenone
Typical aromatic ketone,
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsKetones Simplest carbonyl group, for a single carbonyl compound, implied by a lack of any other functionality except hydrocarbon
Group Frequencies (cm-1):
C=O1715Stretch (sym.)n Base, sensitive to changeconj. w/C=C1700-1675nC=C reduced to 1644-1617conj. w/Ph1700-1680nring 1600-1450C=O1815-1705Decreased ring size raises n
1300-1100Bend
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsAldehydes Presence of the unique carbonyl C-H bond differentiates this group from ketones
Group Frequencies (cm-1):
C=O1725Stretch (sym.)n Base, sensitive to changeconj. w/C=C1700-1680nC=C reduced to 1640conj. w/Ph1700-1660nring 1600-1450
2820, 2720StretchFermi doublet; Higher n band often obscured by sp3 C-H
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsAldehydes isovaleraldehyde
Typical aliphatic aldehyde note appearance of Fermi doublet and C=O overtone
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsAldehydes anisaldehyde
Typical aromatic aldehyde, note how C=O obscures the combination and overtone region oop region would be used to determine substitution
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsCarboxylic Acids Various H-bonding effects lead to messy spectra, especially in the upper frequency ranges be aware of the effects of monomeric, dimeric and oligomeric species on the spectrum
Group Frequencies (cm-1):
C=O1710Stretch (sym.)n Base, sensitive to change; conjugation gives reduced nC-O1320-1210Stretch
O-H3400-2400StretchOverlaps C-H region in most cases; multiple sub-peaks can be seen for the dimeric and oligomeric species simplified in non-polar solution or gas phase spectra
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsCarboxylic Acids propionic acid
Aliphatic carboxylic acid neat sample vs. CCl4 solution (right)
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsCarboxylic Acids o-toluic acid
Aromatic carboxylic acid, larger non-polar end of the molecule cuts down on the hydrogen bonding seen with the smaller, previous propionic acid
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsCarboxylic Acids - Salts
Salts are expressed as possesing one single and one double bond the true picture is one that is isoelectronic with the nitro group, with two bonds to oxygen with a bond order of 1.5
Group Frequencies:
16001400Stretch (asymm.)Stretch (sym.)
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsCarboxylic Acids - Salts ammonium benzoate
Here is an example of ammonium and carboxylate moieties:
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsCarboxylic Acids Amino Acids L-alanine
Amino Acids combine the features of carboxylate and ammonium salts:
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsEsters Ester oxygen has an electron withdrawing effect that tends to draw in electrons within the C=O system, strengthening it compared to other carbonyls
Group Frequencies (cm-1):
C=O1735Stretch (sym.)n Base, sensitive to changeconj. C=C1735-1715nC=C reduced to 1640-1625w/Ph1735-1715nring 1600-1450conj. of sp3 O1765-1760
1850-1740nC=O increases with smaller ring
C-O1300-1000Stretch, 2 bands
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsEsters methyl butyrate
Simple aliphatic ester
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsEsters methyl m-bromobenzoate
Conjugation on the carbonyl end:
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsEsters phenyl acetate
Conjugation on the sp3 oxygen end:
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsAmides Amide nitrogen acts as a conjugating group with C=O, reducing double bond character; amide nitrogen appears similar to amime, including the effects of substitution
Group Frequencies (cm-1):
C=O1685Stretch (sym.)n Base, sensitive to changeCan be as low as 1630 w/conj.N-H~3300StretchSimilar to amines, but typically more intense
nC=O increases with smaller ring
N-H1640-1550BendN-H~800oop bend
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsAmides pivalamide
Primary aliphatic amide
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsAmides 2-pyrrolidone
Cyclic secondary amide - lactam
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsAnhydrides With acid halides, typically the highest n C=O; appears as two bands for the symmetric and asymmetric stretching modes
Group Frequencies (cm-1):
C=O1830-1800Stretch (asym.)n Base, sensitive to changeconj. C=C1778-1740Stretch (sym.)Two bands of variable relative intensity
nC=O increases with smaller ring
C-O1300-900Stretch, multiple bands
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsAnhydrides iso-butyric anhydride
Typical anhydride
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsAcid Halides Acid bromides and iodides are not often encountered; acid chlorides are the most prevalent (and useful)
Group Frequencies (cm-1):
C=O1810-1775Stretch (sym.)n Base, sensitive to change
conj. w/Phadd. bandFermi resonance with combination and overtone region of aromatic ringC-Cl730-550StretchIf below 600, not observed using NaCl windowsC-Br650-510StretchTypically too low to obs.C-I600-485StretchTypically too low to obs.
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IR Spectroscopy
Group Frequencies and AnalysisCarbonylsAcid Halides propionyl chloride
Overtones of low n peaks can confuse some spectra