Norrish reaction
Encyclopedia
The Norrish reaction in organic chemistry
describes the photochemical reactions taking place with ketone
s and aldehydes. This type of reaction is subdivided in Norrish type I reactions and Norrish type II reactions . The reaction is named after Ronald George Wreyford Norrish
.
of aldehydes and ketones into two free radical intermediates. The carbonyl group accepts a photon and is excited to a photochemical singlet state
. Through intersystem crossing
the triplet state
can be obtained. On cleavage of the α-carbon carbon bond from either state, two radical fragments are obtained.
Several secondary reaction modes are open to these fragments depending on the exact molecular structure.
The synthetic utility of this reaction type is limited. It often is a side reaction for instance in the Paternò–Büchi reaction. One organic synthesis
based on this reaction is that of bicyclohexylidene .
abstraction of a γ
-hydrogen (which is a hydrogen atom three carbon positions removed from the carbonyl group) by the excited carbonyl compound to produce a 1,4-biradical as a primary photoproduct (IUPAC definition).
Secondary reaction are either intramolecular recombination of the two radicals to a cyclobutane
compound (path A) or fragmentation to an enol
and an alkene
.
The reaction was first reported in 1937 by Ronald George Wreyford Norrish
.
with respect to the photolysis of the aldehyde
heptanal
, a prominent compound in Earth's atmosphere . Photolysis of heptanal in conditions resembling atmospheric conditions results in the formation of 1-pentene and acetaldehyde
in 62% chemical yield together with cyclic alcohols (cyclobutanols and cyclopentanol
s) both from a Norrish type II channel and around 10% yield of hexanal from a Norrish type I channel (the initially formed n-hexyl radical attacked by oxygen).
In one study the photolysis of an Acyloin
derivative in water in presence of hydrogen tetrachloroaurate
(HAuCl4) generated nanogold particles with 10 nanometer diameter. The species believed to responsible for reducing Au3+ to Au0 is the Norrish generated ketyl
radical.
No less than three Norrish-type reactions feature in the classic 1982 total synthesis of dodecahedrane
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
describes the photochemical reactions taking place with ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
s and aldehydes. This type of reaction is subdivided in Norrish type I reactions and Norrish type II reactions . The reaction is named after Ronald George Wreyford Norrish
Ronald George Wreyford Norrish
Ronald George Wreyford Norrish was a British chemist. He was born in Cambridge and attended The Perse School. He was a former student of Eric Rideal...
.
Norrish I
The Norrish type I reaction is the photochemical cleavage or homolysisHomolysis
In general it means breakdown to equal pieces There are separate meanings for the word in chemistry and biology.-Homolysis in chemistry:...
of aldehydes and ketones into two free radical intermediates. The carbonyl group accepts a photon and is excited to a photochemical singlet state
Diradical
A diradical in organic chemistry is a molecular species with two electrons occupying two degenerate molecular orbitals . They are known by their higher reactivities and shorter lifetimes. In a broader definition diradicals are even-electron molecules that have one bond less than the number...
. Through intersystem crossing
Intersystem crossing
Intersystem crossing is a radiationless process involving a transition between two electronic states with different spin multiplicity.-Singlet and triplet states:...
the triplet state
Triplet state
A spin triplet is a set of three quantum states of a system, each with total spin S = 1 . The system could consist of a single elementary massive spin 1 particle such as a W or Z boson, or be some multiparticle state with total spin angular momentum of one.In physics, spin is the angular momentum...
can be obtained. On cleavage of the α-carbon carbon bond from either state, two radical fragments are obtained.
Several secondary reaction modes are open to these fragments depending on the exact molecular structure.
- The fragments can simply recombine to the original carbonyl compound (path A).
- By extrusion of carbon monoxideCarbon monoxideCarbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
in path B, two organic residues can recombine with formation of a new carbon carbon bond - When the carbon fragment has an α-protonHydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
available it gets abstracted forming a keteneKeteneA ketene is an organic compound of the form R'RC=C=O. The term is also used specifically to mean ethenone, the simplest ketene, where R' and R are hydrogen atoms.Ketenes were first studied as a class by Hermann Staudinger.-Formation:...
and a saturated hydrocarbon in path C - When the alkyl fragment contains a β-proton it gets abstracted with formation of an aldehydeAldehydeAn aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
and an alkeneAlkeneIn organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
.
The synthetic utility of this reaction type is limited. It often is a side reaction for instance in the Paternò–Büchi reaction. One organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
based on this reaction is that of bicyclohexylidene .
Norrish II
A Norrish type II reaction is the photochemical intramolecularIntramolecular
Intramolecular in chemistry describes a process or characteristic limited within the structure of a single molecule, a property or phenomenon limited to the extent of a single molecule.- Examples :...
abstraction of a γ
Gamma
Gamma is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. It was derived from the Phoenician letter Gimel . Letters that arose from Gamma include the Roman C and G and the Cyrillic letters Ge Г and Ghe Ґ.-Greek:In Ancient Greek, gamma represented a...
-hydrogen (which is a hydrogen atom three carbon positions removed from the carbonyl group) by the excited carbonyl compound to produce a 1,4-biradical as a primary photoproduct (IUPAC definition).
Cyclobutane
Cyclobutane is an organic compound with the formula 4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes...
compound (path A) or fragmentation to an enol
Enol
Enols are alkenes with a hydroxyl group affixed to one of the carbon atoms composing the double bond. Alkenes with a hydroxyl group on both sides of the double bond are called enediols. Deprotonated anions of enols are called enolates...
and an alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
.
The reaction was first reported in 1937 by Ronald George Wreyford Norrish
Ronald George Wreyford Norrish
Ronald George Wreyford Norrish was a British chemist. He was born in Cambridge and attended The Perse School. He was a former student of Eric Rideal...
.
Scope
The Norrish reaction has been studied in relation to environmental chemistryEnvironmental chemistry
Environmental chemistry is the scientific study of the chemical and biochemical phenomena that occur in natural places. It should not be confused with green chemistry, which seeks to reduce potential pollution at its source...
with respect to the photolysis of the aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
heptanal
Heptanal
Heptanal or heptanaldehyde is an alkyl aldehyde with a strong fruity odor which is used as an ingredient in cosmetics, perfumes, and flavors. It can be obtained from castor oil by distillation under reduced pressure. Industrially, it is used in the manufacture of 1-heptanol and ethyl heptanoate....
, a prominent compound in Earth's atmosphere . Photolysis of heptanal in conditions resembling atmospheric conditions results in the formation of 1-pentene and acetaldehyde
Acetaldehyde
Acetaldehyde is an organic chemical compound with the formula CH3CHO or MeCHO. It is one of the most important aldehydes, occurring widely in nature and being produced on a large scale industrially. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants as part...
in 62% chemical yield together with cyclic alcohols (cyclobutanols and cyclopentanol
Cyclopentanol
Cyclopentanol or cyclopentyl alcohol is a cyclic alcohol.-Reactions:The dehydration of cyclopentanol produces cyclopentene:...
s) both from a Norrish type II channel and around 10% yield of hexanal from a Norrish type I channel (the initially formed n-hexyl radical attacked by oxygen).
In one study the photolysis of an Acyloin
Acyloin
Acyloins are a class of organic compounds in organic chemistry sharing a common functional group consisting of a hydroxyl group placed on the α-position of a carbonyl group.- Nomenclature :Common types of ketols include:...
derivative in water in presence of hydrogen tetrachloroaurate
Chloroauric acid
Chloroauric acid is a inorganic compound with the formula HAuCl4. This pale yellow compound is a common precursor to gold in a variety of purposes. The term chloroauric acid is also sometimes used to describe other gold chlorides....
(HAuCl4) generated nanogold particles with 10 nanometer diameter. The species believed to responsible for reducing Au3+ to Au0 is the Norrish generated ketyl
Ketyl
A ketyl group in organic chemistry is an anion radical with the general structure C-O. in which an oxygen radical is bonded directly to carbon. This radical is very unstable and appears in chemical reactions as a reactive intermediate...
radical.
Dodecahedrane
Dodecahedrane is a chemical compound first synthesised by Leo Paquette of Ohio State University in 1982, primarily for the "aesthetically pleasing symmetry of the dodecahedral framework"....
External links
- IUPAC Gold BookGold BookThe Compendium of Chemical Terminology is a book published by the International Union of Pure and Applied Chemistry containing internationally accepted definitions for terms in chemistry...
Norrish type I definition - IUPAC Gold BookGold BookThe Compendium of Chemical Terminology is a book published by the International Union of Pure and Applied Chemistry containing internationally accepted definitions for terms in chemistry...
Norrish type II definition