Sn1 and Sn2: leaving group

- [Instructor] The S N 1 and S N 2 reactions involve leaving groups. Let's look at this pKa table to study leaving groups in more detail. On the left, we have the acid. For example, hydroiodic acid, HI, with an approximate pKa of negative 11. Remember, the lower the pKa value, the stronger the acid. So on this table with the pKa value of negative 11, hydroiodic acid is the strongest acid and the stronger the acid, the more stable the conjugate base. So the conjugate base to HI is I minus the iodide anion. And since this is the conjugate based to the strongest acid, this is the most stable base. Let me write that down here. This is the most stable base on the table, which means that the iodide anion is an excellent leaving group because it is very stable. Next, we have hydrobromic acid, approximate pKa of negative nine. So the conjugate base would be the bromide anion, so also a stable conjugate base. So therefore, a good leaving group. For HCl, it's the chloride anion, also a good leaving group. So you see these halide anions as leaving groups all the time in organic mechanisms. Let me write this down here. So these are all examples of good leaving groups. Next, let's look at this acid on the left here. This is p-Toluenesulfonic acid with a pKa value of negative three, so it's still pretty acidic. The conjugate base to this is on the right here and we call this anion a tosylate group. Let me write this down. This is called a tosylate group. And since it's kind of a bulky group, instead of drawing this out all the time, you often see OTs written. So, OTs, like that, and you could put a negative charge on the oxygen here if you wanted to. So you'll see the tosylate group function as a leaving group in many reactions. Let's look at an example of another acid. So if I move down here to H3O+, the hydronium ion, with a pKa value of negative two. The conjugate base to H3O+ is H2O and water is also a good leaving group. So let's go back up here to the topic and we can see that all the acids that we talked about have negative pKa values, so negative 11, negative nine, negative seven, negative three, and negative two. And notice all of the conjugate bases are good leaving groups. So you can say that if an acid has a negative value for the pKa, the conjugate base will be a good leaving group. Let's look at another example of an acid. So, water. Water's pKa value is positive 15.7, so it's not a very strong acid. The conjugate base to water is the hydroxide anion, OH-, and this is a bad leaving group. So hydroxide ion is a bad leaving group and that's because water is not a strong acid. Look at this value for the pKa, positive 15.7. So if we look at ethanol, similar story here. So ethanol has a pKa value of positive 16. So the ethoxide anion is not a good leaving group, so this pKa values are in the positive and these conjugate bases must not be very stable which means they are bad leaving groups. Let me write that down here. So these are examples of bad leaving groups. Both S N 1 and S N 2 reactions need good leaving groups. However, the S N 1 reaction is even more sensitive. Let's look at tert-Butyl chloride. Let's say it's reacting via an S N 1 mechanism. The first step should be loss of leaving group. So these electrons come off onto the chlorine. We would form the chloride anion which has a negative one formal charge. We just saw on our pKa table that the chloride anion is a stable conjugate base. So therefore, this is a good leaving group. We're taking a bond away from the carbon in red, so the carbon in red gets a plus one formal charge and we form a tertiary carbocation as well. Since this is the rate determining step of our S N 1 mechanism, the formation of our stable anion, this formation of a good leaving group helps the S N 1 mechanism occur. Next, let's look at this alcohol here. If we approach it the same way as we did in the previous problem and we said, "Okay, first step is loss of the leaving group "and these electrons come off onto the oxygen." Think about what leaving group that is. That would be the hydroxide ion which we know from our pKa table is not a good leaving group. So the hydroxide ion is not as stable of an anion as the chloride anion. So the chloride anion is a good leaving group. The hydroxide anion is a bad leaving group. So that's not the first step of this mechanism. We need to make a better leaving group and you can do that by having a proton source. Let's say we have a source of protons, an acid in solution, so let's say there's an H+ here. The first step would be to protonate our alcohol, so our alcohol is gonna act as a base and pick up a proton. Let's draw the results of that. We have our ring. Let's put in that methyl group. And now our oxygen is bonded to two hydrogens. There's still a lone pair of electrons on this oxygen which give the oxygen a plus, which gives the oxygen a plus one formal charge. So the electrons here in magenta, let's say, pick up this proton to form this bond. Now we're ready for loss of a leaving group because if these electrons come off onto the oxygen now, we form water as a leaving group. Let me draw that in here. So here is the water molecule. Let me highlight those electrons in blue. These electrons come off onto the oxygen then we form water and we know from our pKa table that water is a good leaving group. We're taking a bond away from this carbon in red, so we're also gonna form a tertiary carbocation. Let me draw that in here. Here's our ring. Here's our methyl group. A plus one formal charge on the carbon in red. So by thinking about your pKa values, you can determine the stability of the conjugate base and therefore, if a leaving group is a good leaving group or a bad leaving group and that helps you out when you're drawing mechanisms.