Question

Synthesis of alkyl fluorides is best accomplished by which of the following methods?
A.Free radical fluorination
B.Sandmeyer’s reaction
C.Finkelstein reaction
D.Swarts reaction

Answer 1

Hint: Synthesis of alkyl fluorides by normal methods is difficult. The reaction designed for the synthesis of alkyl fluorides is called Swarts reaction.

Complete answer:
First let us discuss all the types of reactions given.
-Free radical fluorination is a highly explosive reaction. Hence it cannot be preferred in the synthesis of alkyl fluorides.
-Sandmeyer’s reaction is used for the preparation of aryl halides. Hence it cannot be used for the synthesis of alkyl fluorides.
-In the Finkelstein reaction, alkyl halides (usually chlorides and bromides) react with sodium halide to produce another alkyl halide. The sodium halide used is mostly NaI.
$R - X + NaI \to R - I + NaX$ $(X = Cl,Br)$
-Synthesis of alkyl fluorides is not possible by this method. Because, in order to produce alkyl fluorides, we need NaF in the reaction mixture. But NaF is not capable of carrying out this reaction.
-Swarts reaction is the most suitable method for the synthesis of alkyl fluorides. In this reaction, alkyl chloride or bromide is heated in the presence of metallic fluorides such as $AgF$, $H{g_2}{F_2}$, $Co{F_2}$ or $Sb{F_3}$. The reaction can be represented as,
$R - X + AgF \to R - F + AgX$ $(X = Cl,Br)$
Where R is an alkyl group. It is a halogen exchange reaction. In this reaction, metal-fluoride bond is broken to form a new bond with carbon of alkyl group and fluorine. The metal is then combined with chlorine or bromine.
Hence option D is correct.

Note:
Fluoride is a poor leaving group compared to iodide. That is why, NaF cannot react with alkyl halide, but NaI can (in Sandmeyer’s reaction).