Nanosecond flash studies of reduction of benzophenone by aliphatic amines. Quantum yields and kinetic isotope effects
Nanosecond flash photolysis, steady irradiation, and deuterium substitution studies have been carried out on solutions of benzophenone with added reductants. Quantum yields (phi/sub ketyl/) for reduction in benzene of benzophenone triplet to ketyl radical, based on phi = 2 for benzhydrol (I), were approx. 1 for cyclohexane (II), tert-butylamine (III), 2-aminobutane (IV), cyclohexylamine (V), di-n-propylamine (VI), and triethylamine (VII), approx. 0.7 for 1,4-diazabicyclo(2.2.2)octane (VIII), and approx. 0 for tert-butyl alcohol (IX). Thus, quenching, without radical formation by H abstraction from N and/or ..cap alpha..-C, does not occur with common aliphatic amines but does with Dabco (VIII). The latter quenching is markedly increased by small additions of acetonitrile; the flash spectrum from this compound indicates formation of a triplet amine CT complex or radical ion pair. Triplet-reductant interaction rate constants, k/sur ir/, are high for the amines (approx. 10/sup 8/-10/sup 9/ M/sup -1/ s/sup -1/) but also show significant deuterium kinetic isotope effects: 1.9 with III-N-d/sub 2/; 1.4 with IV-N-d/sub 2/; 1.2-1.3 with IV-..cap alpha..-C-d. It is proposed that k/sub ir/ measures H atom abstraction, favored in the transition state by an initial charge-transfer interaction. Overall steady irradiation quantum yields of reduction by amines, phi/sub Red/, are much lower than phi/sub ketyl/. This is attributed to disproportionationreactions of ketyl and alkylaminyl radicals for primary and secondary amines, and, possibly, aminoalkyl radicals for tertiary amines. In the case of tert-butylamine, the rate constant for disproportionation is obtained from the decay kinetics of ketyl radical and leads to phi/sub Red/ in agreement with that directly measured.
- Research Organization:
- Brandeis Univ., Waltham, MA
- DOE Contract Number:
- EY-76-S-02-3117
- OSTI ID:
- 6520378
- Journal Information:
- J. Am. Chem. Soc.; (United States), Vol. 103:5
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
BENZOPHENONE
PHOTOLYSIS
DEUTERIUM
ISOTOPE EFFECTS
AMINES
BENZENE
CYCLOHEXANE
EXPERIMENTAL DATA
PULSED IRRADIATION
RADICALS
REDUCTION
SCINTILLATION QUENCHING
TRIPLETS
ALKANES
AROMATICS
CHEMICAL REACTIONS
CYCLOALKANES
DATA
DECOMPOSITION
HYDROCARBONS
HYDROGEN ISOTOPES
INFORMATION
IRRADIATION
ISOTOPES
KETONES
LIGHT NUCLEI
MULTIPLETS
NUCLEI
NUMERICAL DATA
ODD-ODD NUCLEI
ORGANIC COMPOUNDS
PHOTOCHEMICAL REACTIONS
STABLE ISOTOPES
400500* - Photochemistry
400302 - Organic Chemistry- Isotope Effects- (-1987)