Lawesson's reagent (LR) is a
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
used in
organic synthesis as a
thiation agent. Lawesson's reagent was first made popular by
Sven-Olov Lawesson, who did not, however, invent it. Lawesson's reagent was first made in 1956 during a systematic study of the reactions of arenes with
P4S10.
Preparation
Lawesson's reagent is commercially available. It can also be conveniently prepared in the laboratory by heating a mixture of
anisole
Anisole, or methoxybenzene, is an organic compound with the formula CH3OC6H5. It is a colorless liquid with a smell reminiscent of anise seed, and in fact many of its derivatives are found in natural and artificial fragrances. The compound i ...
with
phosphorus pentasulfide
Phosphorus pentasulfide is the inorganic compound with the formula (monomer) or (dimer). This yellow solid is the one of two phosphorus sulfides of commercial value. Samples often appear greenish-gray due to impurities. It is soluble in carbon d ...
until the mixture is clear and no more
hydrogen sulfide is formed, then
recrystallized from
toluene
Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) a ...
or
xylene
In organic chemistry, xylene or xylol (; IUPAC name: dimethylbenzene) are any of three organic compounds with the formula . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are s ...
.
Samples give a strong odor of hydrogen sulfide owing to partial hydrolysis. One common and effective method of destroying the foul smelling residues is to use an excess of
sodium hypochlorite
Sodium hypochlorite (commonly known in a dilute solution as bleach) is an inorganic chemical compound with the formula NaOCl (or NaClO), comprising a sodium cation () and a hypochlorite anion (or ). It may also be viewed as the sodium s ...
(
chlorine bleach).
Mechanism of action
Lawesson's reagent has a four membered ring of alternating
sulfur and
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
atoms. The central phosphorus/sulfur four-membered ring dissociates to form two reactive dithiophosphine
ylides (R-PS
2). Much of the chemistry of Lawessons's reagent is in fact the chemistry of this reactive intermediate.
:
In general, the more
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
rich a carbonyl is, the faster the carbonyl group will be converted into the corresponding thiocarbonyl by Lawesson's reagent.
Applications
The chemistry of Lawesson's reagent and related substances has been reviewed by several times.
The main use of Lawesson's reagent is the thionation of carbonyl compounds. For instance, Lawesson's reagent will convert a
carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...
into a
thiocarbonyl
In organic chemistry, thioketones (; also known as thiones or thiocarbonyls) are organosulfur compounds related to conventional ketones in which the oxygen has been replaced by a sulfur. Instead of a structure of , thioketones have the structu ...
. Additionally, Lawesson's reagent has been used to thionate
enones,
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides a ...
s,
lactone
Lactones are cyclic carboxylic esters, containing a 1-oxacycloalkan-2-one structure (), or analogues having unsaturation or heteroatoms replacing one or more carbon atoms of the ring.
Lactones are formed by intramolecular esterification of the co ...
s,
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it i ...
s,
lactams, and
quinones.
:
In one study, reaction of
maltol
Maltol is a naturally occurring organic compound that is used primarily as a flavor enhancer. It is found in the bark of larch tree, in pine needles, and in roasted malt (from which it gets its name). It is a white crystalline powder that is ...
with LR results in a selective oxygen replacement in two positions.
:
A combination of
silver perchlorate
Silver perchlorate is the chemical compound with the formula AgClO4. This white solid forms a monohydrate and is mildly deliquescent. It is a useful source of the Ag+ ion, although the presence of perchlorate presents risks. It is used as a cata ...
and Lawesson's reagent is able to act as an
oxophilic
Oxophilicity is the tendency of certain chemical compounds to form oxides by hydrolysis or abstraction of an oxygen atom from another molecule, often from organic compounds. The term is often used to describe metal centers, commonly the early trans ...
Lewis acid with the ability to catalyze the
Diels–Alder reaction of
diene
In organic chemistry a diene ( ) (diolefin ( ) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alk''ene'' units, with the standard prefix ''di'' of systematic nomenclature. ...
s with α,β-unsaturated
aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
s.
in some cases, alcohols may be converted to thiols by treatment with Lawesson's reagent.
Lawesson's reagent reacts with sulfoxides to form thioethers.
[
]
See also
* Woollins' reagent
References
External links
* {{cite web , url = https://www.organic-chemistry.org/namedreactions/lawessons-reagent.shtm , title = Lawesson's Reagent , publisher = Organic Chemistry Portal , accessdate = 2007-10-16
Reagents for organic chemistry
Sulfur heterocycles
Phosphorus heterocycles
Four-membered rings
Foul-smelling chemicals
Heterocyclic compounds with 1 ring