Lecture 7 - Chapter 2-9-16-05

CHMBD 449 Organic Spectral AnalysisFall 2005Chapter 2: IR SpectroscopyAnalysisGroup Frequencies

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

IR Spectroscopy

Group Frequencies and Analysissp3 Nitrogen Amines1 Amine tert-butylamine

Two band NH2 peak appears as small w

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)

IR Spectroscopy

Group Frequencies and Analysissp3 Nitrogen Amines3 Amine tributylamine

Difficult to discern from alkane molecular formula for confirmation almost requisite

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

IR Spectroscopy

Group Frequencies and Analysissp3 Nitrogen AminesAmmonium Salts anilinium hydrochloride

Spectrum is of a KBr disc sample:

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

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

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

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

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

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

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?

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 =

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:

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

IR Spectroscopy

Group Frequencies and AnalysisCarbonylsKetones 2-hexanone

Typical aliphatic ketone

IR Spectroscopy

Group Frequencies and AnalysisCarbonylsKetones 4-methylacetophenone

Typical aromatic ketone,

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

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

IR Spectroscopy

Group Frequencies and AnalysisCarbonylsAldehydes isovaleraldehyde

Typical aliphatic aldehyde note appearance of Fermi doublet and C=O overtone

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

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

IR Spectroscopy

Group Frequencies and AnalysisCarbonylsCarboxylic Acids propionic acid

Aliphatic carboxylic acid neat sample vs. CCl4 solution (right)

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

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.)

IR Spectroscopy

Group Frequencies and AnalysisCarbonylsCarboxylic Acids - Salts ammonium benzoate

Here is an example of ammonium and carboxylate moieties:

IR Spectroscopy

Group Frequencies and AnalysisCarbonylsCarboxylic Acids Amino Acids L-alanine

Amino Acids combine the features of carboxylate and ammonium salts:

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

IR Spectroscopy

Group Frequencies and AnalysisCarbonylsEsters methyl butyrate

Simple aliphatic ester

IR Spectroscopy

Group Frequencies and AnalysisCarbonylsEsters methyl m-bromobenzoate

Conjugation on the carbonyl end:

IR Spectroscopy

Group Frequencies and AnalysisCarbonylsEsters phenyl acetate

Conjugation on the sp3 oxygen end:

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

IR Spectroscopy

Group Frequencies and AnalysisCarbonylsAmides pivalamide

Primary aliphatic amide

IR Spectroscopy

Group Frequencies and AnalysisCarbonylsAmides 2-pyrrolidone

Cyclic secondary amide - lactam

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

IR Spectroscopy

Group Frequencies and AnalysisCarbonylsAnhydrides iso-butyric anhydride

Typical anhydride

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.

IR Spectroscopy

Group Frequencies and AnalysisCarbonylsAcid Halides propionyl chloride

Overtones of low n peaks can confuse some spectra