Cyclopropyltriphenylphosphonium bromide is an organophosphorus salt, specifically a phosphonium salt, featuring a cyclopropyl group attached to a triphenylphosphonium cation with a bromide counterion. It is a white to off-white crystalline powder. This compound is primarily utilized as a key reagent in organic synthesis, particularly in Wittig olefination reactions. The cyclopropyl group acts as a stabilized ylide precursor. Upon treatment with a strong base, it generates the corresponding ylide, which reacts with aldehydes or ketones to introduce the cyclopropylidene group, forming methylenecyclopropanes or related strained ring systems. This transformation is valuable for constructing cyclopropane rings, which are important structural motifs in pharmaceuticals, agrochemicals, and materials science due to their unique conformational and electronic properties. It is considered an air-stable and convenient source of the cyclopropylidene synthon. Handling requires standard precautions for laboratory chemicals, including the use of personal protective equipment due to its potential irritant properties.