⚗️ Preparation of Wittig Ylide (Route 2)

A Wittig ylide is an essential intermediate used in the Wittig reaction to convert aldehydes or ketones into alkenes. It is prepared from a triphenylphosphine and an appropriate alkyl halide (in this case, benzyl bromide).

🔬 Step-by-Step Mechanism

The phosphorus atom in triphenylphosphine (Ph₃P) contains a lone pair of electrons, making it a strong nucleophile. It attacks the electrophilic carbon in the benzyl halide via an SN2 reaction:

The product is a phosphonium salt (a positively charged phosphonium ion paired with bromide as the counterion).

A strong base (such as butyllithium, NaNH₂, or a strong alkoxide) abstracts a proton from the methylene group (–CH₂–) attached to phosphorus:

This leads to the formation of the ylide (Ph₃P=CHPh), which is a resonance-stabilized compound having both positive and negative charges on adjacent atoms.

🧪 Final Note

This ylide is now ready to be used in the Wittig reaction with a carbonyl compound (like an aldehyde or ketone) to form an alkene product.

The choice of base and solvent is crucial in ylide formation to ensure the reaction proceeds efficiently without decomposing sensitive intermediates.