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 a halogen substituent of a halogenated hydrocarbon (e.g. haloalkane) by an iodide or a fluoride. Without being restricted to hydrocarbons, transhalogenation is used as a generic term.

Overview reaction

The Finkelstein reaction is an equilibrium reaction in which a halogenated hydrocarbon of the form R – X (R = alkyl, phenyl, X = Cl, Br) is re-halogenated by a second halide (Y = I, F) mostly in Takes place in the form of a sodium salt , the sodium salt is dissolved in acetone:

With primary halogen compounds, the Finkelstein reaction proceeds very quickly. In secondary or tertiary halides, the reaction is difficult or even not take place, so that in such cases, Lewis acid - catalysts such as zinc dichloride (ZnCl ₂ ) or iron (III) chloride are used.

mechanism

The Finkelstein reaction proceeds according to an S _N 2 mechanism . It is shown using the example of chloromethane and sodium iodide with acetone as solvents. The reaction proceeds in concert : the iodide attacks the antibonding orbital of chloromethane, creating a bond between iodide and chloromethane, while at the same time the chlorine-carbon bond is weakened. After passing through the transition state in which both halides are partially bound to the carbon atom, the chloride ion emerges. Since sodium chloride is much less soluble in acetone than sodium iodide, the equilibrium is shifted to the iodomethane side through precipitation of the chloride.

With chiral substrates the reaction proceeds with a Walden inversion , i.e. with inversion of the configuration .

meaning

The reaction is mainly used to obtain iodine alkanes, as these cannot be produced by direct halogenation of alkanes due to the low reactivity of iodine . The reaction is carried out in acetone as the solvent, since sodium iodide dissolves in it , but not sodium chloride or sodium bromide . This allows the direction of the reversible reaction to be shifted towards the products.

Isotope exchange

With the help of the Finkelstein reactions, an isotope exchange of, for example, ¹²⁸ I ^- by ¹³² I ^{- can} take place:

literature

• Author collective: Organikum , 22nd edition, Wiley-VCH, Weinheim, 2004, ISBN 3-527-31148-3 .
• Jie Jack Li: Name Reactions: A Collection of Detailed Reaction Mechanisms. Springer, 2003, ISBN 3-540-40203-9 ( limited preview in Google book search).

Individual evidence

1. ↑ R. Brückner: reaction mechanisms: organic reactions, stereochemistry, modern synthesis methods . 3. Edition. Spectrum, 2004, ISBN 3-8274-1579-9 , pp. 96 ff .
2. ^ BP Mundy, MG Ellert, FG Favaloro, Jr .: Name Reactions in Organic Synthesis . 2nd Edition. Wiley & Sons, 2005, ISBN 0-471-22854-0 , pp. 242-243 .
3. ^ László Kürti , Barbara Czakó: Strategic Applications of Named Reactions in Organic Synthesis - Background and Detailed Mechanisms . Elsevier Inc., 2005, ISBN 0-12-369483-3 , pp. 170-171 .
4. ^ T. Laue, A. Plagens: Name and keyword reactions of organic chemistry . 5th edition. Teubner, Wiesbaden 2006, ISBN 3-8351-0091-2 , p. 124-126 .
5. ^ T. Laue, A. Plagens: Name and keyword reactions of organic chemistry . 5th edition, Teubner, 2006, ISBN 3-8351-0091-2 , p. 125.
6. ^ Zerong Wang: Comprehensive Organic Name Reactions and Reagents . John Wiley & Sons, 2009, ISBN 978-0-471-70450-8 , pp. 1060-1063 .
7. ^ BP Mundy, MG Ellert, FG Favaloro, Jr .: Name Reactions in Organic Synthesis . 2nd Edition. Wiley & Sons, 2005, ISBN 0-471-22854-0 , pp. 242 .
8. ^ CE Mortimer, U. Müller: Chemistry - the basic knowledge of chemistry . 10th edition. Thieme, Stuttgart 2010, ISBN 978-3-13-484310-1 , p. 556 .
9. ^ Siegfried Hauptmann : Organic chemistry . 2nd reviewed edition, VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig 1985, ISBN 3-342-00280-8 , pp. 299-300.