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

IR Spectroscopy

Group Frequencies and AnalysisIntroductionWhen approaching any IR spectrum be sure to use the larger-to-smaller region approach- do not immediately focus on any one single peak (even OH or C=O)

From the Hookes Law derivation we are using we find that the IR can be conveniently be divided into four major regions:

4000 cm-12700 cm-12000 cm-11600 cm-1400 cm-1

Bonds to HTriple bondsDouble bondsSingle BondsO-HN-HC-H

CCCN

C=OC=NC=CC-CC-NC-OC-X

Fingerprint Region

IR Spectroscopy

Group Frequencies and AnalysisIntroductionIf supporting information is available molecular formula, chemical inferences (i.e. this was the product of an oxidation reaction), assume this information is correct and the analysis of the IR should support it (later in your careers you can doubt information given to you)

If a molecular formula is available, do an HDI!

Many texts list various methods for approaching an IR spectrum; use the method that works best for you and stick to it.

The most common mistakes in spectral analysis are those of jumping the gun to a conclusion (usually based on some small, insignificant peak) or taking a random haphazard approach to the spectrum (gee, here is an IR, oh, lets start looking for phosphorus this time)

Be methodical, develop a scheme and stick to it!

IR Spectroscopy

Group Frequencies and AnalysisBefore we begin Each functional group will be described as follows:Group General What is most recognizable? What makes it different from similar groups?

Group Frequencies (cm-1):Scale on bottom summarizes band positions and strengths Strong -Medium - Weak -

Bondobservedn in cm-1type of vibrationExceptions and things to watch

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkanes General due to the small electronegativity difference between C and H, hydrocarbon bands are of medium intensity at best and give simple spectra

Group Frequencies (cm-1):

C-H3000-2800StretchStrained ring systems may have higher n-CH2-~1465Methylene bend (scissor)-CH3~1375Methyl bend (sym)-(CH2)4-~720Rocking motion 4 or more CH2- (long chain band)C-CNot interpretively useful, small weak peaks

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkanes Dodecane C12H26

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkanes Cyclopentane C5H10

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkanes Additional If the 1400-1350 region is free of interference, the presence of certain alkyl groups can be discerned:

Methylene MethylScissor1465Bendasymm1450Bendsymm1375usually overlap13801370gem-dimethyl13701390t -butyl

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkanes Additional Example: Compare 2,2-dimethylpentane vs. 2-methylhexane:vs.

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkenes General slightly more complex than alkanes; asymmetric C=C is observed as well as the sp2-C-H stretch. Still, bands are weak to medium in intensity

Group Frequencies (cm-1):

=C-H3095-3010Stretch- Diagnostic for unsaturation- may be aromatic as well=C-H1000-650Out-of-plane (oop) bend- Can be used to determine degree of substitutionC=C1660-1600Stretch- Can be reduced by resonance- Symmetrical C=C do not absorb- trans- weaker than cis-

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkenes 1-octene C8H16

Note you still have alkane present!

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkenes trans-4-octene C8H16

Note absence of C=C band, shouldering of C-H band

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkenes cis-2-pentene C5H10

Note shouldering of C-H band

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkenes cyclopentene C5H8

Note increased complexity due to ring vibrations

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkenes Substitution The out of plane =C-H bend produces strong bands but interference can come from aromatic rings (similar oop) and C-Cl bonds (~700)monosubstituted

cis-1,2

trans-1,2

1,1-disubstitued

trisubstituted

tetrasubstituted1000900800700none, with weak C=C

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkenes Substitution The monosubstitued band is very reliable; and the variance induced by electronic effects is observed

monosubstituted (R-)

monosubstitued w/lone pair group(ex. Cl, -F, -OR)

monosubstitued w/conj. group(ex. C=O, CN)

The shifts are similar for 1,1-disubstitued systems1000900800700overtone usually observed

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkenes Rings Incorporation of a double bond endocyclic or exocyclic to a ring may shift the observed band

Endocyclic: Ring strain shifts the C=C band to lower n (ex. cyclopropene)

The adjacent C-C bond couples with the C=C system if the resulting component vector is along the line of the C=C bond an increase in n occurs this reaches a minima at 90o for cyclobutene (no net component along C=C bond) and rises again with cyclopropene

1646161115661656nC=C 1650

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkenes Rings Endocyclic: If C=C at a ring fusion, absorption is reduced as if one further carbon was removed from the ring:

The presence of additional alkyl groups on the ring dramatically raises nC=CnC=C 1611nC=C 1656nC=C 15661788188316411675

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkenes Rings Exocyclic: these C=C bonds give an increase in absorption n with decreasing ring size:

As the angle between the two C-C bonds is reduced more p character is required (sp = 180, sp2 = 120, sp3 = 109.5, sp>3 =

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkynes General can be symmetric, psuedo-symmetric or internal greatly reducing the number of observed bands

Group Frequencies (cm-1):

C-H~3300Stretch- Diagnostic for terminal alkyneCC~2150Stretch- Can be reduced by resonanceSymmetrical and psuedo-sym. CC do not absorbC-H900-700Bend(Text does not list)Possible not to observe any bands for the CC system

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkynes 1-hexyne C6H10

Nice terminal, asymmetric, well behaved alkyne

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkynes 3-hexyne C6H10

A not-so-nice, internal, symmetrical alkyne

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsAlkynes 1-hexyne C6H10

Nice terminal, asymmetric, well behaved alkyne

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsMononuclear aromatic rings General not true alkenes; most of the small bands associated with them are not of diagnostic value; electronic effects of a single group on the ring can change the observed bands drastically

Group Frequencies (cm-1):

-C-H3050-3010StretchAlso for alkenesC-H900-690Out of plane (oop) bendCan be used to determine substitution pattern

2000-1667Overtone and combination bandsIf observed, similar too oop=C-H1600-1400Ring stretch observed as two doublets (1600, 1580, 1500 & 1450)Greatly dependent on substituents

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsMononuclear aromatic rings toluene C7H8

Typical mono-substituted (EDG) ring

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsMononuclear aromatic rings o-xylene C8H10

Typical ortho-substituted (EDG) ring

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsMononuclear aromatic rings m-xylene C8H10

Typical meta-substituted (EDG) ring

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsMononuclear aromatic rings p-xylene C8H10

Typical para-substituted (EDG) ring

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsMononuclear aromatic rings a-methylstyrene C9H10

Conjugated mono-substituted ring

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsMononuclear aromatic ringsSubstitution The aromatic out of plane =C-H bend produces strong bands but interference can come from alkenes (similar oop) and C-Cl (~700)Consider this region to only be reliable for alkyl-, alkoxy-, halo-, amino-, and acetyl substituted rings

Interpretation is often unreliable for nitro-, carboxylic- and sulfonic groups

The overtone of these bands is the dominant source of the combination and overtone bands observed at 2000-1667

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsMononuclear aromatic ringsSubstitution 900800700600monoorthometapara1,2,41,2,31,3,5

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsMononuclear aromatic ringsSubstitution The aromatic combination and overtone bands are a set of weak absorptions that occur from 2000-1667. This is often obscured by C=O monoorthometapara1,2,41,2,31,3,5The general shape of the pattern is used for determining substitution pattern; typically only a neat liquid sample gives an intense enough set of bands for analysis

IR Spectroscopy

Group Frequencies and AnalysisThe HydrocarbonsPolynuclear and Hetero- aromatic ringsGeneral All bands for these aromatic systems are similar to the mononuclear systems; shifts should be assumed, and analysis would be case-by-case

IR Spectroscopy

Group Frequencies and Analysissp3 Oxygen Alcohols, phenols and ethersAlcohols General the best recognized group on carefully selected spectra, but H-bonding effects can drastically change the position, intensity and shape of the O-H band

Group Frequencies (cm-1):

O-H (free)3650-3600StretchSeen in dilute solution or gas phase spectraO-H (H-bond)3400-3300StretchThe classic H-bonded band, seen in addition to the free band in solutionC-O-H1440-1220BendOften obscured by -CH3 bendC-O1260-1000StretchCan be used to determine 1o, 2o, 3o or phenolic structure

IR Spectroscopy

Group Frequencies and Analysissp3 Oxygen Alcohols, phenols and ethersAlcohols 1-octanol

Neat liquid sample gives classic spectrum

IR Spectroscopy

Group Frequencies and Analysissp3 Oxygen Alcohols, phenols and ethersAlcohols 1-octanol

Same sample in dilute CCl4 solution (solvent bands deleted for clarity)

IR Spectroscopy

Group Frequencies and Analysissp3 Oxygen Alcohols, phenols and ethersPhenols p-cresol

Presence of aromatic bands, sharper -OH

IR Spectroscopy

Group Frequencies and Analysissp3 Oxygen Alcohols, phenols and ethersAlcohols Substitution Using the position of the C-O stretching band, it is possible to suggest a 1o, 2o, 3o or phenolic structure to the alcohol; but these should be considered as base values, that may be changed by the effects of conjugation or an adjacent ring system base valuephenol1220

tertiary1150

secondary1100

primary1050nC-O 1070nC-O 1070nC-O 1017nC-O 1060nC-O 1030

IR Spectroscopy

Group Frequencies and Analysissp3 Oxygen Alcohols, phenols and ethersEthersGeneral like alkynes, the simplicity of the spectra may allow them to pass unnoticed deduce from molecular formula if one should be present

Group Frequencies (cm-1):

C-O1300-1000Stretch (asymm.)Absence of C=O and O-H will confirm it is not ester or alcoholSimple alkyl ethers usually one band at 1120, aryl alkyl ethers give two bands 1250 & 1040

IR Spectroscopy

Group Frequencies and Analysissp3 Oxygen Alcohols, phenols and ethersEthers diispropyl ether

Spectrum dominated by all other functionality

IR Spectroscopy

Group Frequencies and Analysissp3 Oxygen Alcohols, phenols and ethersEthers Additional Types Aryl and vinyl ethers The effect of conjugation gives the C-O bond a small amount of double bond character, raising the observed n

Furthermore, strongly asymmetric systems (aryl alkyl and vinyl alkyl ethers) may show an additional weak C-O band for the symmetric stretch at 1040 and 850 respectively

IR Spectroscopy

Group Frequencies and Analysissp3 Oxygen Alcohols, phenols and ethersEthers Additional Types Epoxides Most important bands are the ring deformation bands at nasym 950-815 and nsym 880-750

Weaker breathing mode band is present at 1280-1230

Acetals and Ketals Give four or five unresolved bands in the 1200-1020 region

IR Spectroscopy

Group Frequencies and Analysissp3 Nitrogen AminesAmines General 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