Aldehydes and ketones are converted into alkenes by means of a direct nucleophilic addition called the Wittig reaction, In the reaction, a triphenylphosphorine ylide, also called a phosphorane, adds to an aldehyde/ketone to give a four-membered cyclic intermediate called an is not isolated but instead spontaneously decomposes to release triphenylphosphine oxide and an alkene The ylide is formed by reaction of triphenylphosphine, a good nucleophile, with a primary alkyl halide in an Sy2 reaction, followed by deprotonation of the carbon with a strong base, such as butylithium. The carbonyl carbon and the carbon originally bonded to the halogen become the two carbons with the double bond in the product alkene The real value of the Wittig reaction lies in its ability to yield an alkene of predictable structure, as the c-c bond is precisely where the C bond was in the reactant and no isomers (other than E/Z isomers) are formed Draw curved arrows to show the movement of electrons in this step of the mechanism.