KHS ChemistryUnit 3.4 Structural Analysis1 Structural Analysis 2 Adv Higher Unit 3 Topic 4 Gordon...

KHS Chemistry Unit 3.4 Structural Analysis 1

StructuralStructuralAnalysis 2Analysis 2

Adv Higher Unit 3 Topic 4Adv Higher Unit 3 Topic 4

Gordon WatsonGordon Watson

Chemistry Department, Kelso High SchoolChemistry Department, Kelso High School


IntroductionIntroduction

This topic continues to explore methods used in the Structural Structural AnalysisAnalysis of organic molecules including IR & NMR SpectroscopyIR & NMR Spectroscopy and X-Ray CrystallographyX-Ray Crystallography.


Aspects of SpectroscopyAspects of Spectroscopy

Spectroscopy was introduced in Unit 1 as a technique that uses the interaction between Electromagnetic RadiationElectromagnetic Radiation and particles to help determine structure.


Visible SpectrumVisible Spectrum

400 nm 750 nm

Visible Light

Longer Wavelength ()Shorter Wavelength ()

Higher Frequency () Lower Frequency ()

Higher Energy (E) Lower Energy (E)

E = h E = h


Infra-Red RadiationInfra-Red Radiation

Visible Light

Just below red in the visible region.

Wavelengths usually 2500-25000 nm.

IR SpectroscopyIR Spectroscopy uses units called wavenumbers wavenumbers ( ), nunu, the reciprocal of the wavelength, (1/1/),, in centimeters (cmcm-1-1).

UltravioletUltraviolet InfraredInfrared

WavenumbersWavenumbers are proportional to frequencyfrequency and energyenergy.

Wavenumbers usually 4000-400 cm-1


Molecular VibrationsMolecular VibrationsQuickTime™ and aGraphics decompressorare needed to see this picture.

Molecules have a variety of possible vibration statesvibration states.

Some of these states are due to StretchingStretching: which changes the changes the distancedistance between atoms in the molecule.

QuickTime™ and aGraphics decompressorare needed to see this picture.

Some of these states are due to BendingBending: which changes the changes the angleangle between atoms in the molecule.


Stretching & BendingStretching & Bending


Molecular DipolesMolecular DipolesMany of these vibrations can cause a change in the Molecular DipoleMolecular Dipole - especially if the bond is polarpolar.

The fluctuating electrical fieldelectrical field produced can interact with the electric field of electroelectromagneticmagnetic radiation

If the frequencyfrequency of the radiationof the radiation matches the frequency of the vibrationfrequency of the vibration - then energy will be absorbed.

Molecular vibrations are relatively low energy - Infra RedInfra Red


IR ActiveIR ActiveQuickTime™ and aGraphics decompressorare needed to see this picture.


If a vibration has no effect on the dipole of the molecule then it will be unable to absorb radiation - IR inactiveIR inactive

The symmetricsymmetric stretching of the O—H bonds in water will be IR inactiveIR inactive

The assymmetricassymmetric stretching of the O—H bonds in water will be IR activeIR active


Stretching & BendingStretching & BendingQuickTime™ and aGraphics decompressorare needed to see this picture.StretchingStretching vibrations are of higher higher

energyenergy - 4000 - 1600 cm-1

StretchingStretching vibrations tend to absorb absorb stronglystrongly to produce large distinct peaks.


4000 - 1600 cm-1 is where functional functional groupsgroups can be identified.

BendingBending vibrations tend to be of lower lower energyenergy - 1400 - 400 cm-1

BendingBending absorb weaklyabsorb weakly to produce complex indistinct peaks.

1400 - 400 cm-1 is the fingerprint regionfingerprint region


Frequencies 1Frequencies 1

The spectra of AlkanesAlkanes are among the ‘simplest’ and, since most organic molecules contain alkyl groups, this is effectively the background upon which other functional groups will appear.



AlcoholsAlcohols and aminesamines display broadbroad O-H and N-H stretching bands in the region 3400-3100 cm-1. The O-H absorbtion is particularly strongstrong.


Alcohols & AminesAlcohols & Amines



AlkeneAlkene and alkynealkyne C-H bonds display sharpsharp stretching absorptions in the region 3100-3000 cm-1. The bands are of mediummedium intensity


Alkene & AlkyneAlkene & Alkyne



CarbonylCarbonyl stretching bands occur in the region 1800-1700 cm. The bands are generally very strongvery strong and can be broadbroad.


AldehydesAldehydes

CarbonylCarbonyl band tends to be at lower end of 1800-1700 cm-1 region . A characteristic double peak between 2700-2850 cm-1.


KetonesKetones

CarbonylCarbonyl band also tends to be at lower end of 1800-1700 cm-1

region . NoNo characteristic double peak between 2700-2850 cm-1.


EstersEsters

CarbonylCarbonyl band tends to be slightly higher in 1800-1700 cm-1 region. C—O stretches can sometimes be picked out (unreliableunreliable).


AcidsAcids

CarbonylCarbonyl band tends to be typical.

HydroxylHydroxyl band tends to be even broadereven broader than usual. This is due to strong hydrogen hydrogen bondingbonding between molecules - often leads to dimerisationdimerisation


AmidesAmides

CarbonylCarbonyl band tends to be much lowermuch lower than normal .

N—H stretches also present.



AromaticAromatic stretching bands occur in various places. They are often difficult to pick out.


Aromatics 1Aromatics 1

Sometimes the best indication that a molecule is aromaticaromatic is a reasonable number of sharp bandssharp bands in the fingerprint regionfingerprint region


Aromatics 2Aromatics 2


Summary of IR Absorptions 1Summary of IR Absorptions 1


Summary of IR Absorptions 2Summary of IR Absorptions 2


Strengths & LimitationsStrengths & Limitations

IR aloneIR alone cannot determine a structure.

Some signals may be ambiguousambiguous.

The functional groupfunctional group is usually indentifiable.

The absenceabsence of a signal is definite proof that the functional group is absent.

Correspondence with a known sample’s IR spectrum confirms the

identity of the compound - fingerprintfingerprint


End of Topic 4End of Topic 4

StructuralStructuralAnalysis 2Analysis 2

KHS ChemistryUnit 3.4 Structural Analysis1 Structural Analysis 2 Adv Higher Unit 3 Topic 4 Gordon...

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