Carbonyl Chromophore:

The carbonyl group contains, in addition to a pair of σ-electrons, a pair of π-electrons and 2 pairs of nonbond-ing (n or p) electrons. Saturated ketones and aldehydes display 3 absorption bands, 2 of which are observed in the far ultraviolet region. A Π → Π * transition absorbs strongly near 150 nm; an n → σ* transition absorbs near 190 nm.

The third band (R-band) appears in the near ultraviolet in the nm region. The R-band is weak (ɛ max < 30) and results from the forbidden transition of a loosely held n-electron to the Π * orbital, the lowest unoccupied orbital of the carbonyl group. R -bands undergo a blue shift as the polarity of the solvent is increased. Acetone absorbs at 279 nm in n-hexane; in water the λ max is The blue shift results from hydrogen bonding which lowers the energy of the n orbital. The blue shift can be used as a measure of the strength of the hydrogen bond.

{ α-Di Ketones AND α –Keto Aldehdes } Acyclic α-diketones, such as biacetyl, exist in the s-trans conformtion (with a dihedral angle, ϕ, of 180°). The spectrum of biacetyl shows the normal weak R-band at 275 nm and a weak band near 450 nm resulting from interaction between the carbonyl groups. The position of the long- wavelength band of α-diketones incapable of enolization reflects the effect of coplanarity upon resonance, and hence depends on the dihedral angle ϕ between the carbonyl groups (I, II, III):

β-DIKETONES.

The ultraviolet spectra of β-diketones depend on the degree of enolization. The enolic form is stabilized when steric considerations permit intramolecular hydrogen bonding. Acetylacetone is a classic example. The enolic species exists to the extent of about 15% in aqueous solution and 91-92% in the vapor phase or in solution in nonpolar solvents. The absorption is directly dependent on the concentration of the enol tautomer.

Cyclic β-diketones, such as 1,3-cyclohexanedione, exist almost exclusively in the enolic form even in polar solvents. The enolic structures show strong absorption in the nm region due to the Π → π* transition in the s-trans

enone system. 1,3- Cyclohexanedione, in ethanol, absorbs at 253 nm (ɛ max 22,000). The formation of the enolate ion, in alkaline solution, shifts the strong absorption band into the 270 to 300 nm region.

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