How is an Appel reaction used to convert alcohols?

An Appel reaction is used to convert alcohols into alkyl halides by using triphenylphosphine and carbon tetrachloride.

The Appel reaction is a popular method in organic chemistry for the conversion of alcohols to alkyl halides. The reaction is named after its developer, Rolf Appel, and is particularly useful because it can be used to convert primary, secondary, and tertiary alcohols. The reaction involves the use of triphenylphosphine (PPh3) and carbon tetrachloride (CCl4) as reagents.

The mechanism of the Appel reaction begins with the alcohol reacting with triphenylphosphine to form a phosphonium ion. This is an intermediate stage where the oxygen of the alcohol is bonded to the phosphorus atom of the triphenylphosphine. The phosphonium ion then reacts with carbon tetrachloride. During this step, one of the chlorine atoms in the carbon tetrachloride is transferred to the phosphorus, creating a phosphorus trichloride ion and a chloroalkane.

The reaction is typically carried out in an inert solvent, such as dichloromethane, at room temperature. The reaction is highly efficient and can be used to produce a wide range of alkyl halides. It is also noteworthy that the reaction proceeds with inversion of configuration at the carbon atom bearing the hydroxyl group, which is important in the synthesis of stereospecific compounds.

The Appel reaction is a versatile and powerful tool in organic synthesis. It allows for the straightforward conversion of alcohols to alkyl halides, a transformation that is often necessary in the synthesis of complex organic molecules. The reaction conditions are mild, and the reagents are readily available, making it a practical choice for many synthetic applications.