Question

Interpret the major absorption bands in the infrared spectrum of benzil.

Answer 1

Benzil is Ph-C=O The long C-C bond of 1.54 A (Armstrong ) indicates the absence of pi bonding between | the two carbonyl carbon centres. The Ph CO centres are planar , but the pair of benzoyl

Ph C=O group are twisted with respect to other, with a dihedral angle of 117 degrees in less . hindered analogues. The R(CO)₂  group is a planar anticonformation.

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MAJOR ABSORPTION BANDS IN IR SPECTRA OF BENZIL

Absorption peaks due to aromatic C-H stretching ------Reported as 3062 & 2853 cm^-1  

Intense sharp peaks due to C=O groups-------------------------Reported as 1660 & 1590 cm-1

Absorption peaks due to C -H bonding----------------------------Reported as 871 , 788 & 716 cm^-1

Intense peaks due to OH stretching ---------------------------Reported as 3431 cm^-1