Alcohol Reactions: Appel Reaction using CX4 and PPh3

The Appel Reaction converts an alcohol to an alkyl halide using a tetrabromide (CBr4) or tetrachloride (CCl4) combined with triphenylphosphine (PPh3) used as in catalytic quantities. This reaction is similar to the PBr3/SOCl2 reactions and differs from the HX (HBr/HCl/HI etc...) reactions as it will not lead to rearrangement for secondary alcohols. Similar to PBr3/SOCl2, this reaction will invert the stereochemistry of the final product.

Reaction Examples

Stereochemistry Inversion

The reaction mechanism is depicted below:

Mechansim

In the first step, the tetrahalide is activated by the triphenylphosphine by having the lone pair electrons on the P atom pull away a halide atom.

In the second step, the negatively charged trihalide removes the hydrogen atom from the alcohol-oxygen atom, leaving it negatively charged.

In the third step, the negatively charged alcohol-oxygen attacks the positively charged P atom, causing it to release the halide.

In the fourth step, the oxygen atom pi electrons form a double bond with the P atom, leading to a positively charged O atom.

In the fifth step, the halide attacks the carbon attached to the alcohol-oxygen atom, causing the release of the O atom resulting in the final product. This backside attack is where the stereochemistry inversion occurs.

Practice this reaction using our Reaction Solver!