Organolead compound
Encyclopedia
Organolead compounds are chemical compound
s containing a chemical bond
between carbon
and lead
. Organolead chemistry is the corresponding science. The first organolead was hexaethyldilead synthesised in 1858. Sharing the same group with carbon, lead is tetravalent.
Going down the carbon group
the C–X (X = C, Si. Ge, Sn, Pb) bond becomes weaker and the bond length
larger. The C–Pb bond in tetramethyllead is 222 pm long with a dissociation energy of 49 kcal
/mol
(204 kJ/mol). For comparison the C–Sn bond in tetramethyltin is 214 pm long with dissociation energy 71 kcal/mol (297 kJ/mol). The dominance of Pb(IV) in organolead chemistry is remarkable because inorganic lead compounds tend to have Pb(II) centers. The reason is that with inorganic lead compounds elements such as nitrogen
, oxygen
and the halides have a much higher electronegativity
than lead itself and the partial positive charge on lead then leads to a stronger contraction of the 6s orbital
than the 6p orbital making the 6s orbital inert.
By far the most important organolead compound is tetraethyllead used as an anti-knocking agent. The most important lead reagents for introducing lead are lead tetraacetate and lead chloride
.
The use of organoleads is limited partly due to their toxicity, although the toxicity is only 10% of that of palladium
compounds.
. For example methylmagnesium chloride
reacts with lead chloride to tetramethyllead, a water-clear liquid with boiling point
110 °C and density 1.995 g/cm³. Reaction of a lead(II) source with sodium cyclopentadienide
gives the lead metallocene
, plumbocene
.
Certain arene compounds react directly with lead tetraacetate to aryl lead compounds in an electrophilic aromatic substitution
. For instance anisole
with lead tetraacetate forms 'p-methoxyphenyllead triacetate in chloroform
and dichloroacetic acid
:
Other compounds of lead are organolead halides of the type RnPbX(4-n), organolead sulfinates (RnPb(OSOR)(4−n)) and organolead hydroxides (RnPb(OH)(4−n)). Typical reactions are :
compounds are amphoteric. At pH
lower than 8 they form R2Pb2+ ions and with pH higher than 10, R2Pb(OH)3- ions.
Derived from the hydroxides are the plumboxanes:
which give access to polymeric alkoxides:
2O + R'OH → 1/n (R3PbOR')n - n H2O
of organolead compounds to free radicals is easy. In its anti-knocking capacity, its purpose is that of a radical initiator
. General reaction types of aryl and vinyl organoleads are transmetalation
for instance with boronic acid
s and acid-catalyzed heterocyclic cleavage. Organoleads find use in coupling reaction
s between arene compounds. They are more reactive than the likewise organotin
s and can therefore be used to synthesise sterically crowded biaryls.
In oxyplumbation, organolead alkoxides are added to polar alkenes:
The alkoxide is regenerated in the subsequent methanolysis and, therefore, acts as a catalyst.
s such as the phenol
2,4,6-trimethylphenol (mesitol) exclusively at the aromatic ortho position:
The reaction requires the presence of a large excess of a coordinating amine
such as pyridine
which presumably binds to lead in the course of the reaction. The reaction is insensitive to radical scavengers and therefore a free radical mechanism can be ruled out. The reaction mechanism
is likely to involve nucleophilic displacement of an acetate
group by the phenolic group to a diorganolead intermediate which in some related reactions can be isolated. The second step is then akin to a Claisen rearrangement
except that the reaction depends on the electrophilicity (hence the ortho preference) of the phenol.
The nucleophile
can also be the carbanion
of a β-dicarbonyl compound:
The carbanion forms by proton abstraction of the acidic α-proton by pyridine
(now serving a double role) akin to the Knoevenagel condensation
. This intermediate displaces an acetate
ligand to a diorganolead compound and again these intermediates can be isolated with suitable reactants as unstable intermediates. The second step is reductive elimination with formation of a new C–C bond and lead(II) acetate
.
and plumbylenes, the lead carbene
counterparts:
These intermediates break up by disproportionation
.
Plumbylidines of the type RPb (formally Pb(I)) are ligands to other metals in LnMPbR compounds (compare to carbon metal carbynes).
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s containing a chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
between carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
. Organolead chemistry is the corresponding science. The first organolead was hexaethyldilead synthesised in 1858. Sharing the same group with carbon, lead is tetravalent.
Going down the carbon group
Carbon group
The carbon group is a periodic table group consisting of carbon , silicon , germanium , tin , lead , and ununquadium ....
the C–X (X = C, Si. Ge, Sn, Pb) bond becomes weaker and the bond length
Bond length
- Explanation :Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond will be shorter...
larger. The C–Pb bond in tetramethyllead is 222 pm long with a dissociation energy of 49 kcal
Calorie
The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule...
/mol
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
(204 kJ/mol). For comparison the C–Sn bond in tetramethyltin is 214 pm long with dissociation energy 71 kcal/mol (297 kJ/mol). The dominance of Pb(IV) in organolead chemistry is remarkable because inorganic lead compounds tend to have Pb(II) centers. The reason is that with inorganic lead compounds elements such as nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and the halides have a much higher electronegativity
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
than lead itself and the partial positive charge on lead then leads to a stronger contraction of the 6s orbital
Atomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
than the 6p orbital making the 6s orbital inert.
By far the most important organolead compound is tetraethyllead used as an anti-knocking agent. The most important lead reagents for introducing lead are lead tetraacetate and lead chloride
Lead chloride
Lead chloride may refer to:* Lead chloride , mineral name: cotunnite.* Lead chloride...
.
The use of organoleads is limited partly due to their toxicity, although the toxicity is only 10% of that of palladium
Palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...
compounds.
Synthesis
Organolead compounds can be derived from Grignard reagents and lead chlorideLead chloride
Lead chloride may refer to:* Lead chloride , mineral name: cotunnite.* Lead chloride...
. For example methylmagnesium chloride
Methylmagnesium chloride
Methylmagnesium chloride is a commercially available Grignard reagent. Like methyllithium, it is the synthetic equivalent to the methyl carbanion synthon. It is usually sold as a solution in tetrahydrofuran. The model of the molecule shows methylmagnesium chloride with the magnesium atom in the...
reacts with lead chloride to tetramethyllead, a water-clear liquid with boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
110 °C and density 1.995 g/cm³. Reaction of a lead(II) source with sodium cyclopentadienide
Sodium cyclopentadienide
Sodium cyclopentadienide is an organosodium compound with the formula C5H5Na. The compound is often abbreviated as NaCp or CpNa, where Cp− is the cyclopentadienide anion. Cp is also used as an abbreviation for the cyclopentadienyl ligand in coordination chemistry.-Preparation:Sodium...
gives the lead metallocene
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...
, plumbocene
Plumbocene
Plumbocene is an organometallic compound of lead. It is a member of the class of metallocenes. It is soluble in benzene, acetone, ether, and petroleum ether, and insoluble in water. Plumbocene is stable in cold water.Plumbocene is not commercially available...
.
Certain arene compounds react directly with lead tetraacetate to aryl lead compounds in an electrophilic aromatic substitution
Electrophilic aromatic substitution
Electrophilic aromatic substitution EAS is an organic reaction in which an atom, usually hydrogen, appended to an aromatic system is replaced by an electrophile...
. For instance anisole
Anisole
Anisole, or methoxybenzene, is the 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...
with lead tetraacetate forms 'p-methoxyphenyllead triacetate in chloroform
Chloroform
Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...
and dichloroacetic acid
Dichloroacetic acid
Dichloroacetic acid, often abbreviated DCA, is the chemical compound with formula . It is an acid, an analogue of acetic acid, in which two of the three hydrogen atoms of the methyl group have been replaced by chlorine atoms. The salts and esters of dichloroacetic acid are called dichloroacetates...
:
Other compounds of lead are organolead halides of the type RnPbX(4-n), organolead sulfinates (RnPb(OSOR)(4−n)) and organolead hydroxides (RnPb(OH)(4−n)). Typical reactions are :
- + HCl → R3PbCl + RH
- + SO2 → R3PbO(SO)R
- R3PbCl + 1/2Ag2O (aq) → R3PbOH + AgCl
- R2PbCl2 + 2 OH− → + 2 Cl−
compounds are amphoteric. At pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
lower than 8 they form R2Pb2+ ions and with pH higher than 10, R2Pb(OH)3- ions.
Derived from the hydroxides are the plumboxanes:
- 2 R3PbOH + Na → (R3Pb)2O + NaOH + 1/2 H2
which give access to polymeric alkoxides:
2O + R'OH → 1/n (R3PbOR')n - n H2O
Reactions
The C–Pb bond is weak and for this reason homolytic cleavageHomolysis
In general it means breakdown to equal pieces There are separate meanings for the word in chemistry and biology.-Homolysis in chemistry:...
of organolead compounds to free radicals is easy. In its anti-knocking capacity, its purpose is that of a radical initiator
Radical initiator
In chemistry, radical initiators are substances that can produce radical species under mild conditions and promote radical reactions . These substances generally possess weak bonds—bonds that have small bond dissociation energies. Radical initiators are utilized in industrial processes such...
. General reaction types of aryl and vinyl organoleads are transmetalation
Transmetalation
Transmetalation is a general chemical reaction type in organometallic chemistry describing the exchange of ligands between two metal centers....
for instance with boronic acid
Boronic acid
A boronic acid is an alkyl or aryl substituted boric acid containing a carbon–boron bond belonging to the larger class of organoboranes. Boronic acids act as Lewis acids. Their unique feature is that they are capable of forming reversible covalent complexes with sugars, amino acids, hydroxamic...
s and acid-catalyzed heterocyclic cleavage. Organoleads find use in coupling reaction
Coupling reaction
A coupling reaction in organic chemistry is a catch-all term for a variety of reactions where two hydrocarbon fragments are coupled with the aid of a metal catalyst...
s between arene compounds. They are more reactive than the likewise organotin
Organotin
Organotin compounds or stannanes are chemical compounds based on tin with hydrocarbon substituents. Organotin chemistry is part of the wider field of organometallic chemistry. The first organotin compound was diethyltin diiodide, discovered by Edward Frankland in 1849...
s and can therefore be used to synthesise sterically crowded biaryls.
In oxyplumbation, organolead alkoxides are added to polar alkenes:
- H2C=CH-CN + (Et3PbOMe)n → MeO-CH2-HC(PbEt3)-CN → MeO-CH2-CH2-CN
The alkoxide is regenerated in the subsequent methanolysis and, therefore, acts as a catalyst.
Aryllead triacetates
The lead substituent in p-methoxyphenyllead triacetate is displaced by carbon nucleophileNucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
s such as the phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...
2,4,6-trimethylphenol (mesitol) exclusively at the aromatic ortho position:
The reaction requires the presence of a large excess of a coordinating amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
such as pyridine
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
which presumably binds to lead in the course of the reaction. The reaction is insensitive to radical scavengers and therefore a free radical mechanism can be ruled out. The reaction mechanism
Reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
is likely to involve nucleophilic displacement of an acetate
Acetate
An acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
group by the phenolic group to a diorganolead intermediate which in some related reactions can be isolated. The second step is then akin to a Claisen rearrangement
Claisen rearrangement
The Claisen rearrangement is a powerful carbon-carbon bond-forming chemical reaction discovered by Rainer Ludwig Claisen...
except that the reaction depends on the electrophilicity (hence the ortho preference) of the phenol.
The nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
can also be the carbanion
Carbanion
A carbanion is an anion in which carbon has an unshared pair of electrons and bears a negative charge usually with three substituents for a total of eight valence electrons. The carbanion exists in a trigonal pyramidal geometry. Formally a carbanion is the conjugate base of a carbon acid.where B...
of a β-dicarbonyl compound:
The carbanion forms by proton abstraction of the acidic α-proton by pyridine
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
(now serving a double role) akin to the Knoevenagel condensation
Knoevenagel condensation
The Knoevenagel condensation reaction is an organic reaction named after Emil Knoevenagel. It is a modification of the Aldol condensation.A Knoevenagel condensation is a nucleophilic addition of an active hydrogen compound to a carbonyl group followed by a dehydration reaction in which a molecule...
. This intermediate displaces an acetate
Acetate
An acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
ligand to a diorganolead compound and again these intermediates can be isolated with suitable reactants as unstable intermediates. The second step is reductive elimination with formation of a new C–C bond and lead(II) acetate
Lead(II) acetate
Lead acetate , also known as lead acetate, lead diacetate, plumbous acetate, sugar of lead, lead sugar, salt of Saturn, and Goulard's powder, is a white crystalline chemical compound with a sweetish taste. It is made by treating lead oxide with acetic acid. Like other lead compounds, it is toxic...
.
Reactive intermediates
Organolead compounds form a variety of reactive intermediates such as lead free radicals:- Me3PbCl + Na (77 K) → Me3Pb.
and plumbylenes, the lead carbene
Carbene
In chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is RR'C:, but the carbon can instead be double-bonded to one group. The term "carbene" may also merely refer to the compound H2C:, also called...
counterparts:
- Me3Pb-Pb-Me3 → [Me2Pb]
- [Me2Pb] + (Me3Pb)2 → Me3Pb-Pb(Me)2-PbMe3
- Me3Pb-Pb(Me)2-PbMe3 → Pb(0) + 2 Me4Pb
These intermediates break up by disproportionation
Disproportionation
Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....
.
Plumbylidines of the type RPb (formally Pb(I)) are ligands to other metals in LnMPbR compounds (compare to carbon metal carbynes).