Finkelstein reaction

The Finkelstein reaction is a name reaction in organic chemistry, which was named after the German chemist Hans Finkelstein ( 1885-1938 ). It describes the replacement of the halogen substituent of a halogenated hydrocarbon (e.g., alkyl halide ) of an iodide or a fluoride.

Overview reaction

In the Finkelstein reaction is an equilibrium reaction in which a transhalogenation of a halogenated hydrocarbon of the form RX (R = alkyl, phenyl, X = Cl, Br) by a second halide ( Y = I, F ), usually in the form of a sodium salt takes place, while the sodium salt is dissolved in acetone before:

In primary or aromatic halogen compounds the Finkelstein reaction proceeds very rapidly. In secondary or tertiary halides, the reaction takes place only poorly or not at all, so can be used in such cases, Lewis acid catalysts such as zinc dichloride ( ZnCl2 ) or ferric chloride.

Mechanism

The Finkelstein reaction proceeds by an SN2 mechanism. The example of methyl chloride and sodium iodide, which is dissolved in acetone, the mechanism is to be shown. First, find a nucleophilic backside attack of the iodide place at the carbon atom, resulting in a transition state forms in which both halides are partially bonded to the carbon atom. This is the rate determining step. By the attack of the construction of a covalent bond between the iodine and the carbon atom starts. At the same time, the C- Cl bond is weakened until finally the chloride ion is eliminated and precipitated as sodium chloride.

In chiral educts a Walden inversion, which leads to the configuration change of the product would also be held.

Importance

The reaction is mainly used for the production of alkyl iodides, as they can not be prepared by direct halogenation of alkanes due to the low reactivity of iodine. The reaction is conducted in acetone as solvent, since although sodium, but not sodium chloride or sodium bromide dissolved therein. This can be shifted toward the direction of the reversible reaction to the products.

Isotope exchange

Using the Finkelstein reactions also an isotope exchange can occur, for example 132I - 128I - by: