Subject: Using infrared (IR) radiation to deduce molecular structures for IAS at Excel Chemistry.
Basics of IR Radiation
Infrared (IR) is a region in the spectrum just above the red end of the visible light.
Certain substances can absorb IR radiation, causing molecules to change their polarity during vibration.
Key Examples: Carbon dioxide (CO2), water (H2O), methane (CH4) can absorb IR. Oxygen (O2) and Nitrogen (N2) cannot.
Identifying Compounds with IR Radiation
Molecules must be able to change their polarity to absorb IR.
Functional groups in a molecule give characteristic peaks at specific wavelengths.
Analyzing a spectrum involves looking for these peaks to deduce molecular structures.
Example Analysis
Ethanol: Presence of OH group indicated by a broad peak around 3300 wave numbers.
Acids: OH peak around 3300 or between 2500 and 3000. C=O group appears as a strong peak around 1700-1720 wave numbers.
Ketones: Identified by a C=O peak around 1710 or 1720, with no OH peak.
Aldehydes: C=O peak coupled with CH peaks around 2800-2900.
Practical Application
IR spectra can be used to identify if a sample is an alcohol, ketone, aldehyde, etc., based on the presence or absence of specific peaks. Examples provided include butanol and carboxylic acids based on specific IR peaks.
IR Spectroscopy in Analysis
Effect on Covalent Bonds: IR radiation makes bonds vibrate more, affecting bond polarity.
Different functional groups absorb IR at different wavelengths, which helps in distinguishing between molecules like alcohols and aldehydes.
Example Question: IR can indicate if butane-2-ol has been oxidized by looking for the appearance or disappearance of specific peaks (e.g., the OH peak disappearing in a ketone).
Molecular Absorption of IR
Molecules with polar bonds like CO2 and H2O can absorb IR, causing changes in polarity.
Molecules that cannot change their polarity, like O2 and N2, do not absorb IR.
Educational Conclusion
IR spectroscopy is a vital tool for identifying molecular structures.
It can’t be used to distinguish between primary, secondary, or tertiary alcohols.
Next Steps
The course will move on to covering past paper questions for further practice.