Answer to Question #32049 in Organic Chemistry for prasad

Question

which having higher C=O ir frequency Cis or trans unsaturated esters? Why?

Answer 1

The C-O stretch occurs as two bands, one stronger and broader than the other, at 1300 – 1000 cm^-1.
Only those vibrations are IR active which involve the change in dipole moment of the molecule and the intensity of absorption bands depends upon the change in dipole moment of molecule. Since there is a change in dipole moment in case of cis-isomer whereas trans-isomers are non-polar in nature, therefore both geometrical isomers show different bands in IR spectrum. Generally, the configuration around a di-substituted double bond is easy to determine from the IR spectra. The trans configuration gives a characteristic band at 965 cm^-1, while the cis configuration gives a weak band at 1640 cm^-1 and a medium band at 3020 cm^-1. Thus, your typical IR spectrum runs from 4000 to 400 cm^-1. Wavenumber is directly related to energy; higher the energy of the absorption. The higher-energy near-IR, approximately 14000–4000 cm^−1 (0.8–2.5 μm wavelength) can excite overtone or harmonic vibrations. The mid-infrared, approximately 4000–400 cm^−1 (2.5–25 μm) may be used to study the fundamental vibrations and associated rotational-vibrational structure. The far-infrared, approximately 400–10 cm^−1 (25–1000 μm), lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy. A cis isomer absorbs more strongly than a trans isomer (cis is less symmetrical than trans).