Rieke metals are highly reactive metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 powders prepared by the methods developed by Reuben D. Rieke. Rieke metals are highly reactive because they have high surface area and lack surface oxides which retard reaction.

Preparation

Rieke metals are usually prepared by a reduction

Redox

Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 of a THF

ThF

Follicular B helper T cells , are antigen-experienced CD4+ T cells found in the B cell follicles of secondary lymphoid organs such as lymph nodes, spleens and Peyer's patches, and are identified by their constitutive expression of the B cell follicle homing receptor CXCR5...

 suspension an anhydrous

Anhydrous

As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another...

 metal chloride

Chloride

The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...

 with an alkali metal

Alkali metal

The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...

. Typical alkali metals used in this method are potassium

Potassium

Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

, sodium

Sodium

Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

, and lithium

Lithium

Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

. For example, the preparation of Rieke magnesium employs potassium as the reductant:

MgCl₂

Magnesium chloride

Magnesium chloride is the name for the chemical compounds with the formulas MgCl2 and its various hydrates MgCl2x. These salts are typical ionic halides, being highly soluble in water. The hydrated magnesium chloride can be extracted from brine or sea water...

  + 2 K → Mg + 2 KCl

More recent reports emphasize the use of the less hazardous lithium metal in place of potassium. Among the many metals that have been generated by this method are Mg, Ca, Ti, Fe, Co, Ni, Cu, Zn, In.

In some cases the reaction is carried out with a catalytic amount of an electron carrier such as biphenyl

Biphenyl

Biphenyl is an organic compound that forms colorless crystals. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula 2...

 or naphthalene

Naphthalene

Naphthalene is an organic compound with formula . It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings...

. The coprecipitated

Coprecipitation

In chemistry, coprecipitation or co-precipitation is the carrying down by a precipitate of substances normally soluble under the conditions employed...

 alkali metal chloride is usually not separated from the highly reactive metal, which is generally used in situ.

Uses

Rieke magnesium reacts with aryl halides at -78 °C to afford the corresponding Grignard reagents, often with considerable selectivity. Rieke magnesium is famous for enabling the formation of "impossible Grignard reagents" such as those derived from aryl fluorides and from 2-chloronorbornane.

Rieke zinc reacts with bromoesters to give organozinc reagents of value for the Reformatsky reaction.

Safety

The method is straightforward but requires a highly experienced chemist. The starting metal halides must be strictly anhydrous and alkali metals can be very hazardous. The resulting reagents are generally pyrophoric

Pyrophoricity

A pyrophoric substance is a substance that will ignite spontaneously in air. Examples are iron sulfide and many reactive metals including uranium, when powdered or sliced thin. Pyrophoric materials are often water-reactive as well and will ignite when they contact water or humid air...

, the expected trade-off for a highly reactive metal.

External links

• BASF Organozinc Compounds
• Rieke Metals, the owner of the Rieke brand