2 Infrared spectroscopy

Infrared spectroscopy Certain groups in a molecule absorb infra-red radiation at characteristic frequencies Complicated spectra can be obtained than provide information about the types of bonds present in a molecule Below 1500 cm-1 – “Fingerprinting” Complicated and contains many signals – picking out functional group signals difficult. This part of the spectrum is unique for every compound, and so can be used as a “fingerprint”. A computer will compare the IR spectra against a database of known pure compounds to identify the compound Above 1500 cm-1 – “Functional group identification” Use an IR absorption table provided in exam to deduce presence or absence of particular bonds or functional groups e.g. C=O 1680 – 1750 cm-1 O-H (acid) 2500- 3000 cm-1 3.6 Analysis Carbon has a value of 12.0000 as it is by definintion the standard reference 1 Mass spectrum for butane 43 29 N Goalby chemrevise.org 2000 1500 Use spectra to identify particular functional groups and to identify impurities, limited to data presented in wavenumber form C=O Spectra for butanal “rogue” absorptions can also occur and are indicators of impurities O-H absorptions tend to be broad Spectra for ethanoic acid C=O The absorption of infra-red radiation by bonds in this type of spectroscopy is the same absorption that bonds in CO2 , methane and water vapour in the atmosphere do that maybe causing global global warming. Absorption or trough in between 1680-1750 cm-1 range indicates presence of C=O bond Absorption or trough in between 2500-3000 cm-1 range indicates presence of O-H bond in an acid Always quote the wave number range from the data sheet Absorption or trough in between 1680- 1750 cm-1 range indicates presence of C=O bond