Sn1 vs Sn2: Solvent effects

- [Lecturer] 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 you look at water here, you can see we have a hydrogen directly connected to an electronegative atom which is oxygen. 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 is our hydrogen and here is our oxygen. So these polar protic solvents favor an SN1 mechanism. 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 tert butyl bromide and for an SN1 mechanism the first step here would be loss of a leaving group so these electrons come off on to the bromine to form our bromide anion and we are gonna form a carbocation as well. So let me draw in the carbocation first. So we have a carbon that is bonded to three methyl groups and this is a plainer carbocation so I'm trying to show that. Our carbon has a plus one formal charge and we are also gonna have our bromine here which we have three lone pairs of electrons. I'll put those in. And then we're gonna 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 has a negative one formal charge as the bromide anion. So we have this carbocation and this anion in our SN1 mechanism and we know this is right determining step of our SN1 mechanism loss of a leaving group. If we are using a polar protic solvent such as water, water can stabilize both the cation and the anion. For example for our carbocation 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 since oxygen is more electronegative than hydrogen. This hydrogen would have a partial positive charge so the negative portion of this molecule, the oxygen would interact with this positive charge on our carbocations. Let's go ahead and show a water molecule here and the partially negative oxygen with its three lone pairs of electrons here on the oxygen will help to stabilize our carbocation. And for our negative anion for our bromide anion here, which is negatively charged, it would be the other end of the water molecule. So if I draw in my water molecule right here so two lone pairs of electrons on the oxygen our partial positive hydrogens would interact and help to stabilize that anion. So polar protic solvents help to stabilize both the carbocation and the anion and that solvation 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 aprotic solvents. So first lets look at dimethyl sulphoxide. So more commonly known as DMSO. So here's the DMS and O. Oxygen is more electronegative than sulfur. So the oxygen is going to withdraw some electron density and become partially negative. And the sulfur would be partially positive. A polar aprotic solvent does not have a hydrogen directly connected to an electronegative atom. So we think about the hydrogens on DMSO. So let me just sketch them in here real fast, there'll be three on this carbon and there'll be three on this carbon. So here we have hydrogens directly connected to a carbon and of course carbon is not very electronegative. So that's why this is a polar aprotic solvent. Next let's look at DMF. DMF is the short way of writing this one here. Again no hydrogen directly connected to an electronegative atom. This hydrogen is directly connected to this carbon and then this carbon would have three hydrogens on it and then this carbon would have three hydrogens on it. So DMF is a polar aprotic solvent. And finally let's look at this last one here. So the abbreviation would be HMPA. So let me write that down here. HMPA. Again no hydrogen is directly connected to an electronegative atom. Polar aprotic solvents favor 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 hydroxide. We could use DMSO as our solvent so let me write that in here. So we are gonna use DMSO. 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. It is going to attack this carbon and these electrons are gonna come off on to the bromide to form our bromide anion. So our OH replaces our bromine and we can see that over here in our product. In an SN2 mechanism we need a strong nucleophile to attack our alkyl halide. And DMSO is gonna help us increase the effectiveness 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 is the sodium cation. DMSO is a good solvator of cations and that's because 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. So same thing over here. Partial negative, partial positive and again we are able to solvate our cation. So the fact that our polar aprotic solvent is a good solvator of a cation means we can separate this ion from our nucleophile. That increases the effectiveness of the hydroxide ion. The hydroxide ion itself is not solvated by a polar aprotic solvent. So you might think, okay well if the oxygen is partially negative and the sulfur is partially positive. 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 ion from interacting with DMSO. So the hydroxide ion is all by itself which of course increases its effectiveness as a nucleophile. It is better able to attack the alkyl halide. If we had used something like water, we know that water is a polar protic solvent with the oxygen being partially negative and the hydrogens 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 aprotic solvent increases the effectiveness of our nucleophile therefore favoring our SN2 mechanism.