As you know infrared (IR) spectroscopy can be put to good use in accurately iden

Question

As you know infrared (IR) spectroscopy can be put to good use in accurately identifying carboxylic acids and their derivatives. The C= O stretching absorption band is one of the most prominent in IR spectra since it is always strong in intensity. The table below gives the range of IR stretching absorption bands (expressed in wavenumbers with units of cm^-1) of some non-conjugated C = O containing functional groups. The C = O group of certain functional groups are located in specific IR regions as indicated in the table. Some C = O stretching absorption bands cover a relatively narrow range such as found in ketones whereas others such as found in an amide cover a broader range. Is the italicized statement below TRUE or FALSE? Electron-withdrawing groups bonded to the carbonyl group shift the C=O absorption to longer IR wavelengths.

Explanation / Answer

FALSE,

Electron withdrawing groups strengthen the C=O bond – shifting the IR frequency up, as the frequency goes up, the wavelength goes down. From the table you can see that the acyl chloride and ester are at bigger wavenumbers (lower wavelength), acyl chloride is a good withdrawing group

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