Carbonyl Photochemistry: Norrish Type I and II

Introduction

Carbonyl compounds absorb light through electronic transitions associated with the C=O group. Excitation can lead to bond cleavage or hydrogen transfer that is uncommon from the same ground-state structure. The two classic Norrish pathways are easily confused because both begin with an excited carbonyl. Their first chemical bond changes, however, are different: Type I breaks a neighboring carbon–carbon bond; Type II abstracts a hydrogen farther along the chain.

Core explanation

Number the carbonyl carbon as the reference point. A carbon attached directly to it is α , the next is β , and the following one is γ . For a ketone R–CO–R′, Norrish Type I cleavage breaks one of the C–C bonds between the carbonyl carbon and an α carbon. The primary products of an acyclic ketone are an acyl radical and an alkyl radical ; cleavage of a cyclic ketone can create a connected diradical. The IUPAC definition of Type I identifies this α-cleavage and the radical primary products. Subsequent decarbonylation, recombination or hydrogen transfer may alter final products, so a problem asking for the primary photoproduct must be answered before drawing later steps.

Norrish Type II requires an accessible hydrogen on a γ carbon. The excited carbonyl oxygen abstracts that hydrogen intramolecularly through a geometry often represented by a six-membered arrangement involving O, the carbonyl carbon and the α, β and γ chain atoms. Hydrogen transfer gives an OH-bearing radical center near the original carbonyl and a radical at the γ position: a 1,4-diradical . This is the primary product in the IUPAC Type II definition. The diradical can then fragment, commonly giving an alkene from the β–γ segment and an enol-derived carbonyl fragment. Alternatively, the radical ends can combine to give a cyclobutanol, the Norrish–Yang pathway named in the IUPAC definition.

The requirements of Type II are structural and conformational. A carbonyl substrate may contain a formally γ-positioned carbon but no γ-hydrogen, for example if that carbon is fully substituted. Then the named hydrogen-abstraction step cannot occur at that site. Even with a γ-hydrogen, the molecule must adopt a geometry allowing intramolecular transfer, and other excited-state processes may compete. The presence of a suitable hydrogen makes Type II possible , not necessarily exclusive or dominant. Wavelength, solvent, triplet sensitization, substituents and temperature can influence branching.

The radical labels clarify atom bookkeeping. In Type I, the original C=O unit generally remains with an acyl radical before any secondary reaction. In Type II, oxygen gains the γ-hydrogen, and the γ carbon loses it; no carbon atom disappears at this primary step. If fragmentation follows, identify which carbon–carbon bond breaks and where the new alkene appears. Mechanisms that jump from starting ketone straight to a final alkene without a diradical can conceal an incorrect hydrogen location.

Do not equate these pathways with thermal carbonyl chemistry. A ketone can undergo enolisation, nucleophilic addition or condensation in the ground state, but those are different mechanistic families. Photochemical excitation changes the available electronic pathways and can generate radicals even without an external radical initiator. A reaction arrow labeled hν should prompt inspection of the chromophore and any α C–C bonds or γ hydrogens before applying familiar polar mechanisms.

Step-by-step reasoning

Draw the full carbon skeleton and label α, β and γ atoms on each carbonyl side. For Type I, mark the candidate carbonyl–α carbon bond and split it homolytically to an acyl and an alkyl radical. For Type II, first confirm a γ-hydrogen, move that hydrogen to oxygen and place radicals at the affected positions. Only then consider β–γ fragmentation or radical recombination. State which product is primary and which requires a second step.

Visual explanation

Place a ketone at the center of a forked diagram. On the upper branch, highlight the C(carbonyl)–C(α) bond breaking into two radical fragments. On the lower branch, draw a curved six-atom proximity loop from carbonyl oxygen to a γ C–H bond and show the resulting 1,4-diradical. From that diradical draw separate arrows toward cleavage products and a cyclobutanol.

Real-world analogy

Imagine a lit machine with two ways to rearrange a chain. One mode snaps the link immediately next to its central hub; the other reaches down the chain, picks up a handle three positions away and then chooses whether to split or close the chain. The analogy distinguishes the first event of each Norrish pathway, even though later products can be more elaborate.

Real-world example

Organic photochemistry of ketones matters in studies of light-driven degradation of polymers and small molecules. A carbonyl within a material can undergo cleavage or hydrogen transfer after irradiation, potentially shortening chains or creating new products. The detailed outcome depends on local geometry and other reactive groups, so the Norrish labels describe pathways rather than a guaranteed fate for every irradiated carbonyl.

Why?

An excited C=O group has a different distribution of electron density from its ground state. This can weaken an adjacent bond toward homolytic cleavage or make oxygen capable of abstracting a suitably positioned hydrogen. The secondary chemistry of radicals and diradicals then determines observed products. Locating the first radical intermediate gives a more reliable prediction than memorizing a final product sketch.

Common misconception

“Type II” does not mean cleavage two bonds away from the carbonyl. It begins with γ-hydrogen abstraction , and the fragmentation, when it occurs, follows later. “Type I” does not require a γ-hydrogen at all. A molecule may offer competing Type I and Type II opportunities; substrate structure alone rarely proves that only one product will be isolated.

Worked example

Question: Consider CH₃–CO–CH₂–CH₂–CH₃ under irradiation. Does the propyl side support Type II chemistry, and what is the primary Type I event on that side? Reasoning: Moving away from the carbonyl, the right-hand CH₂ is α, the next CH₂ is β, and terminal CH₃ is γ. That γ carbon has hydrogens, so abstraction by excited oxygen can give a 1,4-diradical. Type I instead breaks the bond between the carbonyl carbon and its adjacent right-hand CH₂ to make radical fragments. Answer: Both pathways are structurally possible; Type II begins with a γ-hydrogen transfer, while right-side Type I begins with α C–C cleavage.

Quick check

1. What structural feature is essential for a Norrish Type II first step? Answer: An accessible γ-hydrogen positioned for intramolecular abstraction by the excited carbonyl oxygen is essential.

Exam focus

Label α, β and γ positions before drawing products. Name the primary product: acyl/alkyl radicals for acyclic Type I cleavage, or a 1,4-diradical for Type II abstraction. Draw fragmentation or Norrish–Yang cyclisation only as later possibilities and keep radical electron counts sensible.

Advanced insight

The common six-membered abstraction geometry is a conformational preference, not a permanent ring in the starting molecule. A rigid substrate can favor or suppress Type II by controlling oxygen-to-hydrogen proximity. The 1,4-diradical may also experience spin constraints, so a primary photochemical step and the distribution of later products need not follow a single simplistic pathway.

Summary

Norrish Type I is excited-carbonyl α-cleavage to radical fragments. Norrish Type II begins with intramolecular abstraction of a γ-hydrogen to form a 1,4-diradical, which may fragment or cyclise. Correct predictions start with atom numbering and the first bond change, then distinguish primary intermediates from subsequent products.

Practice questions

1. What bond is broken first in a Norrish Type I reaction of an acyclic ketone? Answer: A carbon–carbon bond between the carbonyl carbon and an adjacent α carbon breaks homolytically.

2. What happens to the γ-hydrogen in Norrish Type II? Answer: It is transferred intramolecularly to the excited carbonyl oxygen during formation of a 1,4-diradical.

3. Can a ketone lacking every γ-hydrogen undergo the defining Type II step? Answer: No. Without an appropriate γ-hydrogen, that abstraction step is unavailable, although other photochemical pathways may occur.

4. How can a Type II diradical lead to a cyclobutanol? Answer: Its radical centres can combine intramolecularly in the Norrish–Yang pathway after the initial hydrogen abstraction.