the topic of this lecture will be predicting whether sn1 or sn2 will predominate it's useful to be able to determine whether sn1 or sn2 will predominate with a given set of substrate nucleophile because this allows us to better predict what the products will be in terms of the product stereochemistry and whether or not rearrangements will occur remember that when we're saying that the sn1 or the sn2 mechanism predominates or really saying is whichever mechanism predominates is happening more quickly the rate of that mechanism is more quick and therefore the products that result from that mechanism are formed as the major products so what we're really talking about when compared the s want sn1 and sn2 mechanisms are which one of these mechanisms has the fastest rate first we'll look at the effect that the nucleophile has on the rate of an sn1 and the sn2 reaction remember that it's only the rate of an sn2 reaction that is effected by the structure of the nucleophile the rate determining step of an sn2 reaction is by molecular meaning that both the substrate and the and the nucleophile are involved in the rate of the reaction in an sn1 reaction the rate determining step is you know molecular only the substrate is involved therefore changing the structure of the nucleophile does not affect the rate of an sn1 reaction a stronger nucleophile will speed up an sn2 reaction so a strong nucleophile is said to promote an sn2 mechanism a weak nucleophile slows the rate of the sn2 reaction this means that the reaction is more likely to go by the sn1 mechanism especially if the substrate favors the sn1 mechanism next let's look at how the substrate affects the rate of an sn1 versus not two reaction because the substrate is involved in the rate determining step of both the sn1 and the - mechanism the structure of the substrate affects both the rate of the sn1 and the sn2 reaction methyl and primary substrates are poor sn1 substrates they slow the rate of the sn1 mechanism because the carbo cation that is produced is very unstable therefore it forms very slowly however methyl and primary substrates are good as in two substrates because the lack of steric hindrance around the leaving group allows a close approach of the nucleophile to the electrophile of carbon on the other side of this spectrum tertiary substrates are too hindered for sn2 reactions the steric hindrance around the electrophilic carbon attached to the leaving group slows the approach of the nucleophile it slowing the rate of the sn2 reaction tertiary strips substrates make good sn1 substrates however this is because the tertiary carbo cation that is formed is very stable and therefore forms very quickly speeding up the rate of the sn1 reaction with tertiary substrates secondary substrates are in between these substrates can go sn1 or sn2 depending on the other factors in the reaction the solvent and the nucleophile next let's look at the solvent has on the rates of the sn1 and sn2 mechanisms because the sn1 mechanism involves charged intermediates it benefits the most from a polar protic solvent a polar protic solvent is able to stabilize the transition state that looks like those charged intermediates which lowers the activation energy speeding up the mechanism an sn2 mechanism on the other hand benefits most from a polar aprotic solvent a polar aprotic solvent is very good at dissolving the reactants additionally it does not weaken the nucleophile like in polar protic solvent does because an sn2 reaction requires a strong nucleophile this helps speed up the sn2 reaction allowing it to predominate in this type of solvent considering the structures of the nucleophile this straight and the solvent can help us predict weather yes and one or the sn2 mechanism will predominate allowing us to make predictions about the structures the stereochemistry the rearrangements of the products