write Finkelstein reaction
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The Finkelstein reaction (often referred to as a halex reaction or halogen exchange) named after the German chemist Hans Finkelstein,[1]is an SN2 reaction (Substitution Nucleophilic Bimolecular reaction) that involves the exchange of one halogen atom for another. It is an equilibrium reaction, but the reaction can be driven to completion by exploiting the differential solubility of halide salts, or by using a large excess of the halide salt.[2]
R–X + X′− ⇌ R–X′ + X−
The classic Finkelstein reaction entails the conversion of an alkyl chloride or an alkyl bromide to an alkyl iodide by treatment with a solution of sodium iodide in acetone. Sodium iodide is soluble in acetone and sodium chloride and sodium bromide are not.[3] The reaction is driven toward products by mass action due to the precipitation of the insoluble salt. For example, bromoethane can be converted to iodoethane:
CH3CH2Br (acetone) + NaI (acetone) → CH3CH2I (acetone) + NaBr (s)
Alkyl halides differ greatly in the ease with which they undergo the Finkelstein reaction. The reaction works well for primary (except for neopentyl) halides, and exceptionally well for allyl, benzyl, and α-carbonyl halides. Secondary halides are far less reactive. Vinyl,aryl and tertiary alkyl halides are unreactive. Below some relative rates of reaction (NaI in acetone at 60 °C):[4][5]
Me–ClBu–Cli-Pr–Clt-BuCH2–ClCH2=CH–CH2–ClPhCH2–ClEtOC(O)CH2–ClMeC(O)CH2–Cl17910.01460.0000364179160033000
In modern usage the definition of the reaction has been expanded to include the conversion of alcohols to alkyl halides by first converting the alcohol to a sulfonate ester (tosylates ormesylates are usually used), and then performing the substitution. The example below is from a synthesis of chrysochlamic acid.[6]
The chlorine atom in aryl chlorides (withelectron-withdrawing substituents) can often be replaced by fluorine using a solution of potassium fluoride in polar solvents such as DMF and DMSO and high temperatures.
R–X + X′− ⇌ R–X′ + X−
The classic Finkelstein reaction entails the conversion of an alkyl chloride or an alkyl bromide to an alkyl iodide by treatment with a solution of sodium iodide in acetone. Sodium iodide is soluble in acetone and sodium chloride and sodium bromide are not.[3] The reaction is driven toward products by mass action due to the precipitation of the insoluble salt. For example, bromoethane can be converted to iodoethane:
CH3CH2Br (acetone) + NaI (acetone) → CH3CH2I (acetone) + NaBr (s)
Alkyl halides differ greatly in the ease with which they undergo the Finkelstein reaction. The reaction works well for primary (except for neopentyl) halides, and exceptionally well for allyl, benzyl, and α-carbonyl halides. Secondary halides are far less reactive. Vinyl,aryl and tertiary alkyl halides are unreactive. Below some relative rates of reaction (NaI in acetone at 60 °C):[4][5]
Me–ClBu–Cli-Pr–Clt-BuCH2–ClCH2=CH–CH2–ClPhCH2–ClEtOC(O)CH2–ClMeC(O)CH2–Cl17910.01460.0000364179160033000
In modern usage the definition of the reaction has been expanded to include the conversion of alcohols to alkyl halides by first converting the alcohol to a sulfonate ester (tosylates ormesylates are usually used), and then performing the substitution. The example below is from a synthesis of chrysochlamic acid.[6]
The chlorine atom in aryl chlorides (withelectron-withdrawing substituents) can often be replaced by fluorine using a solution of potassium fluoride in polar solvents such as DMF and DMSO and high temperatures.
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Treatment of a primary alkyl halide or pseudohalide with an alkali metal halide (e.g. KF, KI) leads to replacement of the halogen via an SN2 Reaction.
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