Question

For IR spectroscopy assigning peaks, could you please explain to me how do we tell which structure belong to each regions of peaks.

or C -H 000 3800 30 3400 3200 3000 2800 2000 2400 2200 C-H stretch of ep Alkene 2 9 10 5 26 2.7 28 293 2000 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 Alkene 3 0r 26 27 28 29 3 NICOLET sx 4000 soo 3500 300 3200 3000 2to0 2000 N 0 2200 olkene region - Ahen no peak in akene region, there's no less dipole moment , so it's internas molecue

Answer 1

Infrared spectroscopy does not directly reveal the structure of a molecule. The bond vibration or vibration a of a particular functional group gives rise to characteristic bands in infrared region of electromagnetic spectrum. So,we can obtain informations about some particular bonds or functional groups of a molecule through IR spectra. The C=O bond produces peaks that differ in intensity and frequency in each case of aldehyde, ketone, ester, carboxylic acid, anhydride etc. Likewise, characteristic bands for C-H differ depending upon the hybridisation of carbon atom in the molecule. Comparing with Infrared Spectroscopy Correlation Table, we can assign the corresponding peaks.

In alkene molecules, there are C-H, C=C, C-C bending and stretching frequencies IR region. Stretching always comes at higher frequencies than bending as it needs higher energy to stretch a bond than to bend. C-H stretch comes at 2800-3200 cm^-1. Among which, C(SP³)-H < C(SP²)-H < C(SP)-H. C=C stretch comes around 1620-1680 cm^-1. Lesser than 1000 cm^-1 is for fingerprint region where C-C stretch and other bending frequencies come.

Here to differentiate the alkene isomers, terminal alkene will show a strong and sharp C-H stretch around 3100-3200 cm^-1 which is weaker in internal alkenes. Moreover, terminal alkene will show sharp C=C stretch at higher frequencies than internal alkenes. These small informations help to differentiate among various structures from IR data. But to perfectly confirm the whole structure along with carbon skeleton; NMR, UV, Mass data along with IR are also needed for practical purposes.