Fragmentation of the carbon-carbon bond adjacent to carbonyl group
Norrish Type-I Cleavage
The excited (n, π*) saturated acyclic carbonyl compounds (aldehydes and ketones) undergo an initial cleavage of carbon-carbonyl bond to give an alkyl and an acyl radical. This process is known by α-cleavage or Norrish type I cleavage. Norrish Type-I cleavage occurs more readily in vapour phase than in solution. It is efficient in liquid phase only if a stable radical is formed. The stable radical may be allylic, benzylic, tert alkyl and acyl radicals.
The above mentioned reaction is initiated by low-lying 3(n, π*) state because triplet state cleaves more rapidly than the corresponding singlet state, instead of fact that cleavage from singlet is more exothermic.
The radicals formed in Norrish type I cleavage may undergo following three reactions-
Disproportionation Reaction: If there is α-Hydrogen present in acyl radical then an α-Hydrogen abstraction is takes place, which yields a ketene and alkane.
Decarboxylation: Decarboxylation of acyl radical yields carbon monoxide and an alkyl radical.
The alkyl radical formed reacts with another alkyl radical to give an alkane or undergo H-abstraction to give an alkene and alkane.
Intermolecular Hydrogen Abstraction by Acyl Radical: The acyl radical can pick-up a Hydrogen from alkyl radical and yields an
alkene and aldehyde.
α-cleavage occurs in such a way that the most stable alkyl radical is formed.
Example: α-cleavage of 2,2-Dimethyl heptane-3-one at the bond between C-2 and C-3 to give tertiary butyl radical, instead of, cleavage of bond between C-3 and C-4 which yields less stable primary butyl radical.
Photolysis of 2,2,4,4- tetramethyl pentane-3-one, result in a high yield (-90%) of CO from both singlet and triplet excited state. The life-time of singlet excited state 4.5 to 5.6 x 10-9 S as compared with 0.11 x 10-9 S for triplet excited state. The Norrish type I cleavage occur 100 times faster from triplet than the singlet excited state.
As the excitation wavelength is reduced (i.e. energy increased), the selectivity of carbon-carbonyl bond cleavage decreases. For example, butan-2-one is little selective of band cleavage at 254 nm, but as the energy of the radiation is reduced, a greater preference for cleavage of the weaker bond is observed at 313 nm.
Norrish type I cleavage or α-Cleavage is also shows by Saturated cyclic ketones and β,γ-Unsaturated ketones.
In case of cyclic ketones, it forms diradicals followerd by decarbonylation and the resulting new diradical may cyclize if a strainless ring formation is possible.
The diradical formed in the first step (primary reaction) may also abstract a δ-H atom to form an unsaturated aldehyde.
If the carbonyl compound contains a γ-H atom, intramolecular H-atom abstraction can occur. This β-cleavage is known as Norrish type-II reaction.
Norrish type-II reaction
