Lead tetrachloride

Lead tetrachloride

Names
IUPAC name Lead(IV) chloride
Identifiers
CAS Number	13463-30-4
Properties
Chemical formula	PbCl4
Molar mass	349.012 g/mol[1]
Appearance	yellow oily liquid[2]
Density	3.2 g⋅cm^−3[1]
Melting point	−15 °C (5 °F; 258 K)[1] stable below 0 °C (32 °F; 273 K)[2]
Boiling point	50 °C (122 °F; 323 K)[1]
Solubility in water	Reacts
Solubility	hydrochloric acid
Structure
Coordination geometry	4
Molecular shape	tetrahedral[3]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Lead tetrachloride, also known as lead(IV) chloride, has the molecular formula PbCl₄. It is a yellow, oily liquid which is stable below 0 °C, and decomposes at 50 °C.^[2] It has a tetrahedral configuration, with lead as the central atom. The Pb–Cl covalent bonds have been measured to be 247 pm and the bond energy is 243 kJ⋅mol⁻¹.^[4]

Synthesis

Lead tetrachloride can be made by reacting lead(II) chloride PbCl₂, and hydrochloric acid HCl, in the presence of chlorine gas (Cl₂),^[5] leading to the formation of chloroplumbic acid H₂PbCl₆. It is then converted to the ammonium salt (NH₄)₂PbCl₆ by adding ammonium chloride (NH₄Cl). Finally, the solution is treated with concentrated sulfuric acid H₂SO₄, to separate out lead tetrachloride. This series of reactions is conducted at 0 °C. The following equations illustrate the reaction:

PbCl₂ + 2HCl + Cl₂ → H₂PbCl₆

H₂PbCl₆ + 2 NH₄Cl → (NH₄)₂PbCl₆ + 2HCl

(NH₄)₂PbCl₆ + H₂SO₄ → PbCl₄+ 2HCl + (NH₄)₂SO₄

Reaction with water

Unlike carbon tetrachloride, another group IV (IUPAC: group 14) chloride, lead tetrachloride reacts with water. This is because the central atom is bigger (Pb is bigger than C) so there is less cluttering and water can easily access it.^[3] Also, because of the presence of empty d orbitals on the Pb atom, oxygen can bind to it before a Pb–Cl bond has to break, thus requiring less energy. The overall reaction is thus as follow:

PbCl₄ + 2H₂O → PbO₂(s) + 4HCl(g)

Stability

Lead tetrachloride tends to decompose further into lead dichloride and chlorine gas:^[3]

PbCl₄ → PbCl₂ + Cl₂(g)

This is because the stability of the +4 oxidation state decreases as we travel down this group of the periodic table.^[3] Thus while carbon tetrachloride is a stable compound, with lead the oxidation state +2 is favored and PbCl₄ quickly becomes PbCl₂. Indeed, the inert pair effect causes lead to favor its +2 oxidation state: Pb atom loses all its outermost p electrons and ends up with a stable, filled s subshell.^[6]

Toxicity

Lead is a cumulative poison.^[4] Only limited evidence have been shown of lead's carcinogenic effect, but lead tetrachloride, as well as all other lead compounds, is "reasonably anticipated to be human carcinogens" according to the Report on Carcinogens, Twelfth Edition (2011).^[7] Lead can be absorbed by the body through several routes, primarily inhalation but also ingestion and dermal contact. Lead compounds are also teratogens.^[8]

References

1. "Lead compounds: Lead Tetrachloride". WebElements.com. Retrieved 10 October 2012.
2. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 381. ISBN 0-08-037941-9.
3. "The Chlorides of Carbon, Silicon and Lead". chemguide.co.uk. Retrieved 10 October 2012.
4. Emsley, John (2000). The Elements. Oxford: Oxford University Press. p. 114. ISBN 0-19-855819-8.
5. Neu, John T.; Gwinn, William D. (October 1958). "Raman Spectra of Germanium Tetrachloride and Lead Tetrachloride". ACS 70 (10): 3464–3465. doi:10.1021/ja01190a073. Retrieved 10 October 2012.
6. Miessler, Gary L. (2011). inorganic Chemistry. Boston: Prentice Hall. pp. 275, 289–290. ISBN 978-0-13-612866-3.
7. National Toxicology Program, Department of Health and Human Services (2011). Report on Carcinoens, Twelfth Edition (2011) - Lead and Lead Compounds (PDF). p. 251.
8. "Environmental Health & Safety - 1: General Information About Chemical Safety". Princeton UNiversity. Retrieved 11 October 2012.