Appel reaction

The Appel reaction is an SN2 nucleophilic substitution reaction of the type that allows in organic chemistry, the conversion of alcohols into the corresponding alkyl halides. Preference is given to primary and secondary alcohols. Appel the reaction is a special case of the Mitsunobu reaction. The reaction is named after its discoverer Rolf Appel (1921-2012, University of Bonn ) named, who published his research results to 1975.

Overview reaction

As reagents, stoichiometric amounts of triphenylphosphine ( TPP = PPh3 ) and a tetrahalomethane, such as carbon tetrachloride ( CCl4 ) or carbon tetrabromide ( CBr 4 ) is required. With tetrafluoromethane (CF4 ), the reaction can not be performed.

As the product formed during the reaction of an alcohol Appel (R1 and R2 are organic groups), a haloalkane, in the example a chloroalkane.

Reaction mechanism

The mechanism of the reaction is here Appel Umsetztung using the triphenylphosphine and carbon tetrachloride with a secondary alcohol here. The radicals R1 and R2 are organic radicals.

The triphenylphosphine (1) is activated by the carbon tetrachloride (2) and forms the Chlortriphenylphosphines (3) and a deprotonated chloroform (4). The deprotonated chloroform ( 4) engages with the alcohol to give a 5 alkoxide is produced. Then, the reaction between the Triphenylphosphines (3) and the alcoholate is carried out 4, the molecule 7 is formed and cleaved, a chloride ion from Chlortriphenylphosphines (3). In the last step of triphenylphosphine oxide is displaced as a very good leaving group by the nucleophilic attack of the chloride ion 6 and the desired product, in this case, a chloroalkane 8 The driving force of the reaction is the formation of O = P double bonds, so the oxidation of P (III) to P (V).

The Appel reaction is a mild and modern twist to the chlorination or bromination under SN2 conditions. Therefore, it is stereoselectively with inversion of configuration.

Practical significance

Appel, the reaction is purely a laboratory procedures. Due to the formation of stoichiometric amounts of triphenylphosphine oxide and other waste materials the atom economy of the Appel reaction is so bad that no one realized a technical synthesis of a haloalkane based on this reaction.