MOLECULAR STRUCTURE AND PHOTOCHEMICAL REACTIVITY .11. WAVELENGTH AND METHYL SUBSTITUENT EFFECTS IN PHOTOCHEMISTRY OF LACTONES IN LIQUID PHASE

The photochemistry of 7-butyrolactone (I) (2537 A, liquid phase) is extended to α-methylbutyrolactone (II) and γ-methylbutyrolactone (III) to determine (1) the effect of replacing a hydrogen atom of γ-butyrolactone with a methyl group in the α (lactone II) and γ (lactone III) positions on product quantum yields and excited-state lifetimes, and (2) the effect on product quantum yields with a change in wavelength from 2537 to 2380 A. A methyl group in the a position increases the Φformate from 0.23 (allyl formate) to 0.39 (crotyl formate), and the Φcyclopropane increases from 0.013 (cyclopropane) to 0.027 (methylcyclopropane) (2537 A, 25°, neat) while the yield of the aldehyde products does not change (succinaldehyde and methylsuccinaldehyde). A methyl group in the 7 position does not affect the Φformate significantly but Φpentanai-4-one is very much less than ΦSuccinaldehyde and Φmethylsuccinaldehyde- A methyl group in the γ position decreases the lifetime of the excited state from 1.2 X 10-9 to 10-11 sec, but substitution in the γ position has only a small effect. The change in wavelength from 2537 to 2380 Å increases the yield of the dialdehydes (Φ2537succinaidehyde = 0.06; Φ2380succinaidehyde = 0.11; Φ2537methylsuccinaldehyde = 0.05; Φ2380methyl succinaldehyde 0.13), but does not change the quantum yield of formate or cyclopropane significantly. The possibility that this wavelength effect is due to vibrational effects in solution is discussed. © 1969, American Chemical Society. All rights reserved.