Conjugation lowers carbonyl stretching frequency by reducing effective C=O bond order, and intramolecular hydrogen bonding can lower it further. The qualifying structures are the aryl-conjugated ketone, the ortho-hydroxy aromatic ester with intramolecular hydrogen bonding, and the conjugated cyclohexenone. The unconjugated cyclopentanone, ordinary ester, acid chloride, phenyl acetate and the non-carbonyl-conjugated lactone do not meet the threshold. The count is 3.
More Experimental Techniques in Organic Chemistry: Optical rotation (polarimetry). Application of various chromatographic techniques such as thin-layer, column, HPLC and GC. Applications of UV-visible, IR, NMR spectroscopy and Mass spectrometry in the structural determination of organic molecules. questions
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