The Finkelstein reaction involves the reaction of an alkyl chloride or bromide with sodium iodide to produce the corresponding alkyl iodide. The reaction may proceed via either an S_N1 or S_N2 mechanism depending on the nature of the alkyl halide.

Mechanism:

Acetone is employed as solvent in the classical version of the reaction. Acetone is used in order to take advantage of the relatively higher solubility of sodium iodide compared to sodium bromide or chloride. The precipitation of the chloride or bromide salts removes the ions from solution and drives the reaction to completion. In the synthesis of ether compounds, the reaction can be used to prepare a suitable alkyl iodide for use in the Williamson Ether Synthesis.

The reaction can also be performed in other solvents and the reaction can be driven to completion by the addition of a large excess of iodide. The reaction can also be employed using a catalytic amount of iodide. The alkyl iodide product can be generated in situ where it reacts further, regenerating the iodide.

The relative rates of reactivity of various alkyl halides in the Finkelstein reaction resembles the rates of reactivity observed in other nucleophilic substitution reactions. The reaction rate increases as the number of carbons bonded to the carbon atom bearing the halogen decreases. The reaction rate is significantly higher when the carbon atom that undergoes attack is adjacent to a double bond or aromatic ring. Allyl bromide and benzyl bromide have a much higher rate of reaction than simple primary alkyl halides such as ethyl bromide. Electron donors on such double bonds or aromatic rings tend to increase the rate of reaction. Electron withdrawing groups tend to decrease the rate. This is consistent with a transition state that places a developing positive charge on the carbon atom undergoing attack.

One variant of the reaction involves converting an alcohol to an excellent leaving group such as a tosylate, and then converting the tosylate to the iodide. This sequence formally has the effect of converting an alcohol to an alkyl halide.

Experimental Procedure:

For an example of the laboratory procedure for the Finkelstein reaction, see the synthesis of ethyl 5-iodovalerate.

References:

H. Fickelstgein, Ber., 1910, 43, 1528.

C. K. Ingold, Structure and Mechanisms in Organic Chemistry (Cornell Univ. Press, London, 2nd ed., 1969) p. 435.

Streitwieser, A. Chem. Rev., 1956, 56, 571.

J. Hayami et al., Tetrahedron Letters, 1973, 385.

S. Samaan, F. Rolla, Phosphorus and Sulfur, 4, 145 (1978).

W. B. Smith, G. D. Branum, Tetrahedron Letters, 1981, 22, 2055.

F. G. Bordwell, W. T. Brannen, J. Am. Chem. Soc., 1964, 86, 4645.

D. W. Kim, C. E. Song, D. Y. Chi, J. Org. Chem., 2003, 68, 4281-4285.

T. W. Baughman, J. C. Sworen, K. B. Wagener, Tetrahedron, 2004, 60, 10943-10948.

Maloney, D. J. Hecht, S. M. Org. Lett., 2005, 7, 4297.

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