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the choice of a solvent can have an effect on an sn1 or an sn2 mechanism let's start with polar protic solvents a polar protic solvent is a solvent that has at least one hydrogen connected to an electronegative atom for example if we look at water here we can see we have a hydrogen directly connected to an electronegative atom which is oxygen so water is an example of a polar protic solvent next we have methanol which again has a hydrogen directly connected to an electronegative atom and oxygen and finally acetic acid which has the same thing here here's our hydrogen and here is our oxygen so these polar protic solvents favor an sn1 mechanism so let me write that in here so an sn1 mechanism is favored by a polar protic solvent and let's look at why so down here I have Church butyl bromide and for an sn1 mechanism the first step here would be loss of a leaving group so these electrons come off onto the bromine to form our bromide anion and we're going to form a carbo cation as well let me draw in the carbo cation first so we have a carbon that is bonded to three methyl groups and this is a planar carbo cation so I'm trying to show that our carbon has a plus 1 formal charge and we're also going to have our bromine here which we have three lone pairs of electrons I'll put those in now we're going to get one more lone pair of electrons on the bromine that came from this bond in here so highlighting those electrons in magenta here are those electrons in magenta and bromine that has a negative one formal charge it is the bromide anion so we have this carbo cation and this anion in our sn1 mechanism and we know this is the rate determining step of our sn1 mechanism loss of a leaving group if we're using a polar protic solvent such as water water can stabilize both the cation and the anion for example for our carbo cation we know that carbon has a positive charge on it and if we look at water we know that this oxygen here is a partial negative charge oxygen is more electronegative than hydrogen and the hydrogen would have a partial positive charge so the negative portion of this molecule the oxygen would interact with this positive charge on our carbo cation so let's go ahead and show a water molecule here and the partially negative the partially negative oxygen which is two lone pairs of electrons here on the oxygen will help to stabilize our carbo cation and for our negative anion for our bromide anion here which is negatively charged it'd be the other end of the water molecules if I draw in my water molecule right here so two lone pairs of electrons on the oxygen our partial positive hydrogen's would interact and help to stabilize that anion so polar protic solvent help to stabilize both the carbo cation and the ande anion and that salvation of both cations and anions helps the sn1 mechanism proceed so that's why polar protic solvent will favor an sn1 mechanism now let's look at polar a protic solvent so first let's look at dimethyl sulfoxide so more commonly known as DMSO so here's the DM s and O oxygen is more electronegative than sulfur so the oxygens going to withdraw some electron density and become partially negative and the sulfur would be partially positive a polar a protic solvent does not have a hydrogen directly connected to an electronegative atom so we think about the hydrogen's on DMSO let me just sketch them in here oh that would be three on this carbon and there'd be three on this carbon so here we have hydrogen's directly connected to a carbon and of course carbon is not very electronegative so that's why this is a polar a protic solvent next is look at DMF so DMF is the short way of writing it this one here again no hydrogen connected it directly connected to an electronegative atom this hydrogen is directly connected to this carbon and then this carbon would have three hydrogen's on it and then this carbon would have three hydrogen's on it so this so DMF is a polar a protic solvent and finally let's look at this last one here so the abbreviation would be H M PA so let me write that down here so H M PA again no hydrogen's directly connected to an electronegative atom polar a protic solvent favored an sn2 mechanism so let's look at why down here I have an sn2 reaction on the Left we have this alkyl halide let's say we have sodium hydroxides we could use DMSO as our solvent so let me write that in here so we're going to use DMS M and we know in an sn2 mechanism the nucleophile attacks our alkyl halide at the same time our leaving group leaves so our nucleophile is the hydroxide ion that's going to attack this carbon and these electrons going to come off onto the bromine to form our bromide anion so our Oh H replaces our bromine and we can see that over here in our product and an sn2 mechanism we need a strong nucleophile to attack our alkyl halide and DMSO is going to help us increase the effectiveness of our of our nucleophile which is our hydroxide ion so let's look at some pictures of how it helps us so we have sodium hydroxide here so first let's focus in on the sodium our cation so here's the sodium cation DMSO is a good solve ATAR of cations and that's because the oxygen has a partial negative charge the sulfur has a partial positive charge and these lone pairs of electrons on the oxygen help to stabilize the positive charge on our sodium's the same thing over here partial negative partial positive and again we are able to solve a tar cation so so the fact that polar a protic solvent is a good solve ATAR of the cation means we can separate this ion from our nucleophile and that increases the effectiveness of the hydroxide ion the hydroxide ion itself is not solvated by polar a protic solvent so you might think okay well the oxygen is partially negative and the sulfur is partially positive if the partially positive sulfur could interact with our negatively charged nucleophile but remember we have these bulky methyl groups here and because of steric hindrance that prevents our hydroxide ions from interacting with DMSO the hydroxide ion is all by itself which of course increases its effectiveness as a nucleophile it's better able to attack the alkyl halides if we had used something like water we know that water is a polar protic solvent with the oxygen being partially negative and the hydrogen's being partially positive and a polar protic solvent would interact with our nucleophile solvating it and essentially decreasing the effectiveness of our nucleophile so that's why polar protic solvents don't work as well if you want an sn2 mechanism a polar a protic solvent increases the effectiveness of our nucleophile therefore favoring our sn2 mechanism