Lead tetraacetate

Physical Properties

Colorless monoclinic crystals; turns pink; unstable in air; density 2.228 g/cm3 at 17°C; melts at 175°C; decomposes in cold water and ethanol; soluble in chloroform, benzene, nitrobenzene, and hot glacial acetic acid; also soluble in concentrated hydrochloric acid.

Uses

Lead tetraacetate is used as a highly selective oxidizing agent in organic synthesis. This includes oxidation of glycols into aldehydes, preparation of cyclohexyl acetate, production of oxalic acid, and in structural analysis of sugars.

Preparation

Lead tetraacetate is prepared by dissolving lead tetroxide in warm glacial acetic acid followed by cooling. On cooling, the tetraacetate crystallizes out while lead acetate remains in the solution:
Pb3O4 + 8CH3COOH→Pb(CH3COO)4 + 2Pb(CH3COO)2 + 4H2O
Yield is improved by adding chlorine gas to the mixture. Alternatively, the compound may be prepared by adding a mixture of glacial acetic acid and acetic anhydride to lead tetroxide and heating the solution gently. Acetic anhydride converts the water formed in the reaction into acetic acid, thus preventing hydrolysis of the product lead tetraacetate.

Reactions

Lead tetraacetate, on treatment with water, hydrolyzes to lead dioxide and acetic acid:
Pb(CH3COO)4 + 2H2O → PbO2 + 4CH3COOH
Lead tetraacetate is a selective oxidizing agent causing oxidative cleavage of polyhydroxy compounds. It cleaves compounds that have hydroxyl groups on adjacent carbon atoms, breaking the carbon-carbon bonds to form carbonyl compounds, such as aldehydes, ketones or acids. The reaction is carried out in organic solvents. A typical example is as follows:
OH—CH2—CH2—OH + Pb(CH3COO)4 → 2CH2O + Pb(CH3COO)2 + 2CH3COOH

Description

Lead (IV) acetate or lead tetraacetate is a chemical compound with chemical formula Pb(C₂H₃O₂)₄ and is a lead salt of acetic acid. It is commercially available often stabilized with acetic acid.

Chemical Properties

White solid

Chemical Properties

Lead tetraacetate (plumbic acetate), Pb(C2H3O2)4, is a colorless, monoclinic crystalline solid that is soluble in chloroform and in hot acetic acid, but decomposes in cold water and in ethyl alcohol. Lead tetraacetate can be prepared by adding warm, water-free, glacial acetate acid to red lead, Pb3O4, and subsequent cooling. The salt decomposes with the addition of water to give PbO2, but the yield can be improved by passing in chlorine gas. Lead tetraacetate is available in laboratory quantities as colorless to faintly pink crystals stored in glacial acetic acid.

Uses

Oxidation with lead tetraacetate is often used in organic syntheses, because the lead salt is highly selective in the splitting of vicinal glycols. The rate of oxidation of cis glycols is more rapid than of the trans isomers, a property widely used in the structural determination of sugars and other polyols. Lead tetraacetate readily cleaves α-hydroxy acids as oxalic acid at room temperature. Another use is the introduction of acetoxy groups in organic molecules, as in the preparation of cyclohexyl acetate and the acetoxylation of cyclohexanol. At high temperature, methylation takes place. In these reactions, the organic molecule must contain double bonds or activating substituents.

Uses

Lead(IV) acetate is an important oxidizing agent and a source of acetyloxy group used in organic synthesis. For example, 1,4-dioxene is prepared from dioxane involving 2-acetoxy-1,4-dioxane as an intermediate. Similarly, it is used for the preparation of bis(trifluoromethyl)diazomethane from hexafluoroacetone hydrazone. It also reacts with alkenes, alcohols having a delta-proton and di-n-butyl d-tartrate to get gamma-lactones, cyclic ethers and n-butyl glyoxylate respectively. It induces the cleavage of 1,2-diols to the corresponding aldehydes or ketones. It is actively involved in the Kochi reaction for the decarboxylation of carboxylic acids to alkyl halides and used as an alternative reagent to bromine in the Hofmann rearrangement.

Uses

Selective oxidizing agent in organic syntheses: Criegee, Angew. Chem. 53, 321 (1940); Newer Methods of Preparative Organic Chemistry (Interscience, N. Y., 1948) pp 1-17.

Application

Lead tetraacetate is a strong oxidizing agent, a source of acetyloxy groups and a general reagent for the introduction of lead into organolead compounds. Some of its many uses in organic chemistry :
Acetoxylation of benzylic, allylic and α-oxygen ether C-H bonds, for example the photochemical conversion of dioxane to 1,4- dioxene through the 2-acetoxy-1,4-dioxane intermediate and the conversion of α-pinene to verbenone
* Oxidation of hydrazones to diazo compounds for example that of hexafluoroacetone hydrazone to bis(trifluoromethyl)diazomethane * Aziridine formation, for example the reaction of Naminophthalimide and stilbene
* Cleavage of 1,2-diols to the corresponding aldehydes or ketones often replacing ozonolysis, for instance the oxidation of di-nbutyl d-tartrate to n-butyl glyoxylate
* Reaction with alkenes to γ-lactones
* Oxidation of alcohols carrying a δ-proton to cyclic ethers.
* Oxidative cleavage of certain allyl alcohols in conjunction with ozone.

Preparation

Lead tetraacetate can be prepared by reaction of red lead with acetic acid The other main lead acetate is lead (II) acetate.

Definition

ChEBI: An acetate salt with formula Pb(OAc)4. It is used as a selective oxidising agent in organic synthesis.

General Description

Faintly pink wet crystals with an odor of vinegar.

Air & Water Reactions

Unstable in air. Reacts with water to form brown lead dioxide and acetic acid [Merck 11th ed. 1989].

Reactivity Profile

Organometallics are strongly reactive with many other groups. Incompatible with acids and bases. Organometallics are good reducing agents and therefore incompatible with oxidizing agents. Often reactive with water to generate toxic or flammable gases. Generally highly toxic. Often react on contact with tissues to give toxic products.

Health Hazard

Early symptoms of lead intoxication by ingestion are most commonly gastrointestinal disorders, colic, constipation, etc.; weakness, which may go on to paralysis chiefly of the extensor muscles of the wrists and less often of the ankles, is noticeable in the most serious cases. Ingestion of a large amount causes local irritation of the alimentary tract; pain, leg cramps, muscle weakness, paresthesias, depression, coma, and death may follow in 1 or 2 days. Contact causes severe irritation of eyes and can burn skin.

Fire Hazard

Behavior in Fire: Can increase the intensity of a fire when in contact with combustible material. Cool containers with plenty of water.

Safety

Lead (IV) acetate may be fatal if ingested, inhaled, or absorbed through skin. It causes irritation to skin, eyes, and respiratory tract. It is a neurotoxin. It affects the gum tissue, central nervous system, kidneys, blood, and reproductive system.