Finkelstein displacement

Finkelstein displacement. Primary alkyl chlorides are converted to the pure bromides on repeated treatment with LiBr in 2-butanone at 120°C. Typically, the > ields range from 86% to 96%. 

This reaction was initially reported by Finkelstein in 1910. It is a preparation of alkyl iodide from alkyl bromide or chloride with potassium or sodium iodide in acetone. Therefore, this reaction is generally known as the Finkelstein reaction. Occasionally, it is also referred to as the Finkelstein halide exchange, Finkelstein displacement, or Conant-Finkelstein reaction. Mechanistically, this reaction is a simple nucleophilic substitution (often via Sn2), as iodide is a stronger nucleophile than bromide or chloride. The yield of this reaction is very high and can be quantitative if occurs in DMF. It was found that the trifluoromethyl group retards the displacement of bromide when it presents as an a- or /3-substituent but accelerates the reaction as a substituent in an allylic chloride. Under normal conditions, this type of halide displacement does not occur for aryl halides. For dihalides, unsaturated or cyclic compounds may form via carbocation intermediates, which form transient covalent iodides or are reduced directly by iodide to free radicals. However, the aromatic halide exchange reacts smoothly when 10% Cul is present in the reaction... 

C-Alkylation. (Chloromethyl)dimethylphenylsilane (1) can be utilized to install a silylmethyl group on carbon via base promoted C-alkylation of terminal alkynes, dihydropyrazines, mal-onic esters, phenylacetonitriles, sulfoxides, and imines. Although 1 has been used directly in the alkylation, conversion to the corresponding iodide 2 via Finkelstein displacement (eq 1) prior to alkylation is sometimes warranted. Except for the malonic esters (eq 2), strongly basic conditions and low temperatures (with slow warming) are generally employed in the transformation (eqs 3 and 4). 

A typical example of such reactions is the exothermic Sn2 nucleophilic displacement reaction Cl -I- CH3—Br Cl—CH3 - - Br . Table 5-2 provides a comparison of Arrhenius activation energies and specific rate constants for this Finkelstein reaction in both the gas phase and solution. The new techniques described above cf. Sections 4.2.2 and 5.1) have made it possible to determine the rate constant of this ion-molecule reaction in the absence of any solvent molecules in the gas phase. The result is surprising on going from a protic solvent to a non-HBD solvent and then further to the gas phase, the ratio of the rate constants is approximately 1 10 10 The activation energy of this Sn2 reaction in water is about ten times larger than in the gas phase. The suppression of the Sn2 rate constant in aqueous solution by up to 15 orders of magnitude demonstrates the vital role of the solvent. 

FINKELSTEIN GRYSZKIEWICZ TROCHIMOWSKI - M C COM0IE Halide Displacement... 

When the reagent is Nal in the solvent acetone (2-propanone see Chapter 16, Section 16.2), this transformation is known as the Finkelstein reaction, named after Hans Finkelstein (Germany 1885-1938). Because iodide is a better nucleophile than the bromide ion or the chloride ion, it is rmlikely that bromide or chloride will displace iodide ion from an alkyl iodide to give the alkyl bromide or the alkyl chloride. (This point was made in Chapter 7, Section 7.6.) In other words, iodide can displace bromide or chloride, but bromide or chloride will not displace iodide. In general, fluoride is a poor leaving group in the Sn2 reaction and it will not be used. 

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