Slide 1

IRElusidasiCommon Structural Units

Serapan gugus fungsional pada Infra merahRegions of the Infrared Spectrum4000-2500 cm-1 N-H, C-H, O-H (stretching)3300-3600 N-H, O-H3000 C-H2500-2000 cm-1 CC and C N (stretching)2000-1500 cm-1 double bonds (stretching)C=O 1680-1750C=C 1640-1680 cm-1Below 1500 cm-1 fingerprint region

Methyl Isopropyl KetoneSpectra Infrared

Regions of the Infrared Spectrum

IR Spectrum Peak CharacteristicsPrimary Examination Regions of the SpectrumHigh Frequency Region- 4000 to 1300 cm-1Intermediate (Fingerprint Region)- 1300 to 900 cm-1High Frequency Region (Functional Group Region)Characteristic Stretching frequencies of such groups as: =CH, OH, NH, C=O, CO, CN, CC, C=C The Fingerprint RegionAbsorption patterns frequently complexBands originate from interacting vibrational modesValuable when used in reference to other regionsAbsorption unique for every molecular speciesEffective use comes from experienceSpectra InfraredIR Spectrum Peak Characteristics (cont)ShapeSharp (narrow)BroadIntensityWeak(w)Medium(m)Strong(s)Note:Peak intensity is dependent on amount of sample and sensitivity of instrument; therefore, the actual intensity can vary from spectrum to spectrum

Infrared Spectroscopy (IR)

Infrared Spectra of HydrocarbonsC-H, C-C, C=C, C C have characteristic peaks

Hexane

Alkenes

1-Hexene

Alkynes

IR: Aromatic CompoundsWeak CH stretch at 3030 cm1Weak absorptions 1660 - 2000 cm1 rangeMedium-intensity absorptions 1450 to 1600 cm1

Phenylacetylene

Cyclohexane or Cyclohexene?

The infra-red spectrum for a simple carboxylic acid

Ethanoic acid

carbon-oxygen double, C=Ocarbon-oxygen single, C-Ooxygen-hydrogen, O-Hcarbon-hydrogen, C-Hcarbon-carbon single, C-C

IR: Alcohols

Infrared Absorption Spectrum of Ethanol

IR: Carbonyl CompoundsStrong, sharp C=O peak 1670 to 1780 cm1Exact absorption characteristic of type of carbonyl compound1730 cm1 in saturated aldehydes1705 cm1 in aldehydes next to double bond or aromatic ring

C=O in Ketones1715 cm1 in six-membered ring and acyclic ketones1750 cm1 in 5-membered ring ketones1690 cm1 in ketones next to a double bond or an aromatic ring

C=O in Esters1735 cm1 in saturated esters1715 cm1 in esters next to aromatic ring or a double bond

The infra-red spectrum for an esterEthyl ethanoate

Practice problem:

Phenylacetaldehyde

The infra-red spectrum for a ketonePropanone

Amines

The infra-red spectrum for a primary amine1-aminobutane

Analyzing the Spectrum A Suggested ApproachStep 1. Check for the presence of the Carbonyl group (C=O) at 1715 cm-1. If molecule is conjugated, the strong (C=O) absorption will be shifted to the right by ~30 cm-1, i.e. ~1685 cm-1If the carbonyl absorption is present, check for:Carboxylic Acids-Check for OH group (broad absorptionnear 3300-2500 cm-1)Amides-Check for NH group (1 or 2 absorptions near 3500 cm-1) Esters-Check for C-O group (medium absorptions near 1300-1000 cm-1)Anhydrides-Check for 2 C=O absorptions near 1810 and 1760 cm-1Aldehydes-Check for Aldehyde CH group (2 weak absorptions near 2850 and 2750 cm-1)Ketones-Ketones (The above groups have been eliminated)

Isobutyric AcidIR SpectrumCarboxylic AcidsC4H8O2

BenzamideIR SpectrumAmides

C7H7NO

Methyl BenzoateIR SpectrumEsters

C8H8O2

Nonanal

C9H18O

IR SpectrumAldehydesMethyl Isopropyl KetoneIR SpectrumKetones

C5H10O

Step 2. - If the Carbonyl Group is Absent Check for Alcohols, Amines, or Ethers.Alcohols & Phenols-Check for OH group (Broad absorption near 3600-3300 cm-1Confirm present of CO near1300-1000 cm-1Amines-Check for NH stretch (Medium absorptions) near 3500 cm-1Primary Amine- 2 PeaksSecondary Amine- 1 PeakTertiary Amine- No peaksN-H scissoring at 1560 - 1640 cm-1Ethers-Check for C-O group near 1300-1000 cm-1 and absence of OHIR SpectrumAlkynes (CC)Propargyl AlcoholHC CCH2OH

C3H4O

2-NaphtholIR SpectrumAlcohols & PhenolsC10H9O

n-butylamineIR SpectrumAminesC4H11N

IR SpectrumEthersButyl EtherC8H18OCH3(CH2)3 O (CH2)3CH3Step 3. Refine the Structure Possibilities by Looking for Double Bonds, Triple Bonds and Nitro GroupsDouble Bonds -C=C is weak absorption near 1650 cm-1Aliphatics occur on right side of 3000 cm-1.Unsaturated C=C or CC bonds show an absorption on the left side of 3000 cm-1. Triple Bonds-CN (medium, sharp absorption near 2250 cm-1 CC (weak, sharp absorption near 2150 cm-1 Nitro Groups-Two strong absorptions 1600-1500 cm-1and 1390-1300 cm-1 Step 3 (Cont)Aromatic Ring AbsorptionsAromatic unsaturated C=C bonds show an absorption on the left side of 3000 cm-1, but the aromaticity must be verified in the overtone region (1667 - 2000cm-1) and the out-of-plane (OOP) region (900 - 690 cm-1)Medium to strong absorption in region 1650 - 1450 cm-1Many weak combination and overtone absorptions appear between 2000 and 1667 cm-1The relative shapes and numbers (1 - 4) of the overtone absorptions can be used to tell whether the aromatic ring is monosubstituted or di-, tri-, tetra-, penta-, or hexasubstituted.Positional (ortho-, meta-, para-) isomers can also be distinguished.Note: A strong carbonyl absorption can overlap these ovetone bands, making them unusable.Step 3 (Cont)Aromatic Ring Absorptions (Cont)The unsaturated =C-H Out-of-Plane (OOP) bending absorptions in the region 900 690 can also be used to determine the type of ring substitution.The number of absorptions and their relative positions are unique to each type of substitution.Although these absorptions are in the fingerprint region they are particularly reliable for rings with alkyl group substitutions.They are less reliable for polar substituents.BenzonitrileIR SpectrumNitriles

C7H5N

IR SpectrumAlkynes (CC)Propargyl AlcoholHC CCH2OH

C3H4O

Nitro BenzeneIR SpectrumNitro CompoundsC6H5NO2

Step 4. If none of the above apply then the compound is most likely a hydrocarbon.

Generally, very simple spectrum

Hydrocarbons-Check for main absorptions nearright side of 3000 cm-1

DecaneCH3(CH2)8CH3IR SpectrumAlkaneC10H22