Leaving groups iodide

The Sn2 reaction of methyl iodide (iodomethane) with hydroxide ion is a concerted reaction, taking place in a single step with bonds breaking and forming at the same time. The middle structure is a transition state, a point of maximum energy, rather than an intermediate. In this transition state, the bond to the nucleophile (hydroxide) is partially formed, and the bond to the leaving group (iodide) is partially broken. Remember that a transition state is not a discrete molecule that can be isolated it exists for only an instant. 

Of the hahde leaving groups, iodide is most easily displaced, but also most reactive in metal halogen exchange bromide and chloride are successively less reactive. Grignard... 

The Wittig reagent is made by alkylation of the simplest ylid with the same silicon reagent. Notice that the leaving group (iodide) is on the carbon next to silicon, not on the silicon itself. Anion formation occurs next to phosphorus because PhsF is much more anion-stabilizing than MesSi. The ylid reacts with carbonyl compounds such as cyclohexanone in the usual way to produce the allyl silane with no ambiguity over which end of the allyl system is silylated. 

JOSIPHOS (137) gave 138 in 92% yield and 84% ee. As in the preceding case, the leaving group (iodide, bromide, triflate or nonaflate) had almost no effect on stereoinduction, but the yields were lower. 

Next, identify the alkyl halide needed to install this group. Just redraw the alkyl group connected to a leaving group. Iodide and bromide are commonly used as leaving groups. 

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