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Alkylhalides: Substitution reactions 2 (Sn2)

Alkylhalides: Substitution

Inversion

When a Sn2 reaction occurs, the stereo configuration of the molecule which is attacked will changed, it will be inverted relative to the original reactant. This happens because of the attack on the back-side, explained in the previous post. The fenomenon is called the Walden inversion. In the example illustrated below the bromobutane is R configurated, after the reaction attacked by the nucleophile OH- the butanol is S configurated: 

Leaving groups

As seen before, the reaction rate of the Sn2 reaction is dependent on the kind of leaving group. Good leaving groups are leaving groups which conjugated base is derived from strong acids, which means: weak bases (which are also the most stable bases). What does this mean? This means that Iodide is a much better leaving group than Fluoride. Basically what this means is:

I-  <<  Br-   <<   Cl-   <<   F-   BASICITY
RI  >>  RBr  >>  RCl   >>  RF   REACTIVITY

RF won't even undergo a reaction, it is too unreactive.
What are good leaving groups then? As seen above Cl-, Br- and I- are good for substitution reactions. But also sulfonates and sulfates can be used. As neutral leaving groups water, amines and sulfides can be used and are the most common. 

Nucleophiles from the second row of the table have approximately the same size as each other. If the base is more stronger, the nucleophile will react better. Thus it is determined by their conjugated acid, if the conjugated acid is more acid, the base will not be as strong and the nucleophile will not be very reactive:

NH3   <<    H2O   <<    HF   ACIDITY 
NH2-   >>   OH-   >>   F-   STRONGEST BASE
Strongest base and thus the better nucleophile is NH2-

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