Okay, so this is Dr. Sola, Lecture 1 for Organic Chemistry 2, Part B. The overview of this is we're going to be looking at alkyl halides and the general nucleophilic substitution reaction. We're going to discuss why they undergo nucleophilic substitution reactions, briefly review what nucleophiles are, and the effect of the type of alkyl halides, how many different groups are bound to that. carbon of that alkyl halide on the reaction rates.
And we're going to be briefly looking at the rate equations and kinetics of the differences between SN1 and SN2 reactions. So let's start with an overview of a nucleophilic substitution reaction. Well, as we said before, we have your alkyl halide and you have your nucleophile here.
And we're going to be replacing that halogen with that nucleophile. We're going to be forming a bond between that nucleophile to the carbon of the alkyl halide. We're going to be breaking this bond and forming a leaving group which is your halide. Okay so this is a general nucleophilic reaction and this is what we've done as I've just described.
So your halogen here becomes a very good leaving group and leaving group we often indicate with LG we're using X. So this is not an addition reaction, we're substituting one thing for another. So we have to ask ourselves why these alkyl halides undergo nucleophilic substitution reactions. There must be something going on with an alkyl halide to allow this to happen.
Well, it comes down to, as I briefly mentioned before, that relationship between that carbon and that halogen. bond is polarized. Polarized means the electrons in the bond, that pair of electrons which normally might be equally shared, is actually being pulled towards that halogen because halogens are electronegative, which means they pull electrons towards themselves. And that means that this end of that bond has a negative charge, sort of a slightly negative charge, partial negative charge. And if we show the movement of electrons by drawing an arrow in that direction, so the electron density is being pushed towards that halogen, that little cross there on that arrow that indicates at this end is then slightly positive.
So there's your dipole there that you've created. So you've got a polar covalent bond. So the virtue and the nature of that carbon halogen bond is what opens up the doorway for nucleophilic.
aromatic substitution. There's some more factors we're going to explore here. So we've polarized this carbon halogen bond. In this case, remember we're talking about chlorine, bromine, or iodine.
We're ignoring fluorine because fluorine has a very strong covalent bond. Okay, so that means it's a very hard bond to break. And remember here, we're pushing the nucleophile in and we're breaking the halogen then off there. So we are going to ignore fluoroalkanes. here.
We also have to remember that the reactivity of this is going to also depend on this bond strength. So as we go from chlorine to bromine to iodine, the bond strength between these two gets weaker because chlorine is smaller, bromine is bigger, iodine is even bigger. And based on the electronegativities, the bond strength becomes weaker, so these reactions can become faster. So let's talk about electrophiles because, as you can see, we're pulling the electron density away from the carbon to this halogen.
That means that it's becoming a partial positive charge there, which makes this into an atom that loves electrons. It's craving electrons because it's had its own electrons pulled away. It wants electrons. So an electrophile is the electron-seeking because it is now electron-poor.
So in this case, this is now electron poor. Whereas a nucleophile up here is nucleus seeking. It's wanting to move on to a nucleus because it has a pair of electrons, whether it's a negative charge or it's just on the nucleophile, it doesn't necessarily have to be negatively charged, is able to push that pair of electrons and share it to form a bond.
So that means nucleophiles and electrophiles interact with each other as well. It's a match made in heaven. So here is a generic nucleophilic substitution depiction here.
Here's your alkyl halide, your nucleophile, some general nucleophilic substitution reaction. And we form that bond between the nucleophile and the alkyl group. And there's your leaving group, that halide we've driven off. So let's just talk about nucleophiles. Again, nucleophiles, nucleo, they love phyle, means love, nucleus seeking.
They're electron rich. They're willing to give away a pair of electrons. They tend to be negative, but they can be partially negative or not negative at all. So we've got a hydroxy anion. That's negative.
We have ammonia here. That's not negative, but it's got a pair of electrons that can push. We can even look at here. We can even use alkene, a double bond there. That's a pair of electrons that can be pushed in as a nucleophile in a reaction.
So I'm just going to give you a brief overview of different types of nucleophiles, so you can get a good gut feeling for things, and then I'll tell you why this is important. So if we're going to compare negative ions to neutral molecules, a negative ion is going to be a better nucleophile. So if we look at the hydroxyanion, that's going to be a better nucleophile than its corresponding neutral molecule, which is water. There's a pair of electrons we can push and form, it can act as a nucleophile, but this is going to be a better nucleophile.
Okay, if we look at this section of the periodic table here, we're going to see a few trends. So if we look at the groups, which are the columns, your electronegativity is going to increase as you go from the bottom to the top in the column, from the bottom to the top. Okay, so electronegativity increases. I'm going to show you all these trends, but what might help you remember if fluorine is. highly, highly, highly electronegative. So if you remember that sort of as the top electronegative element, that's going to help you remember these trends.
So if electronegativity increases, you go up, then elements become better nucleophiles as you go down the column. Oops. So as we go down the column, let's compare HS minus to HO minus here.
these two, one above the other. So as you move down, this is going to be a better nucleophile than that one down that column. Let's just look at the halogens, seeing as we're talking about halogens, because this is what we're looking at here.
As we go from the bottom to the top, most electronegative element, and if you go down, that's going to be, how can I say this? Sorry. You travel down the column.
This is the worst nucleophile. This is going to be the best nucleophile, right? So you can just one, one, one applies in the opposite direction, right?
Hope I've said that okay. So let's look at the trends as we go across a row, okay? So as you go across a row, any of these rows, your electronegativity increases from left to right. You'll remember that because this is your marker stone for. a highly electronegative element.
So if your electronegativity goes up, then you're going to be guessing that your nucleophile is going to be stronger at this end and a worse nucleophile on this end. Like before, worse nucleophile here, better nucleophile here. Okay, so let's do a few comparisons here.
Well, CH3-is going to be a better nucleophile, more able to push that pair of electrons onto an electrophile, then is H2N minus better than hydroxyanion, and that's even better than fluoride, okay? So this is a lot of comparisons here. What I want you to remember is for alkyl halides, okay?
It's good to know all of this stuff, but for alkyl halides, as you go down this... column here because the electronegativity increases in the reverse direction your bond in your alkyl halide rf rcl rbr ri as you go down that bond weakens okay so that means nucleophilic substitution reactions become faster so alkyl iodide reactions are going to be faster than alkyl bromide and alkyl chloride you okay That was a lot of jibber-jabber for that. So now we've talked about nucleophiles, but let's actually think about the effect of the alkyl group that is actually on that alkyl halide. So there's your sp3 hybridized carbon, there's your halogen, in this case we just got bromine on there, and this is a methyl halide because it's a methyl group, right?
Let's substitute one of those hydrogens and put another methyl group on here. So you look at the carbon, we have two hydrogens and one methyl group. That's now called a primary alkyl halide. So what we're doing is we're substituting and putting more alkyl groups around that carbon.
If we have two alkyl groups, in this case two methyl groups, that's a secondary alkyl halide. And if we actually replace that final hydrogen with another methyl group, we have a tertiary alkyl halide. So does this have an effect on the activity?
of an alkyl halide in a nucleophilic substitution reaction. Well, I've drawn this compound out with that sp3 hybridized, so a tetrahedron here. I've circled the bromine there, and bromine's a much bigger atom than hydrogen. Okay, so that's the biggest, bulkiest thing on that atom, our molecule.
Now, we come here, we've added one methyl group. So I put that as R here, and you can see a methyl group is going to be bigger than hydrogen. Okay, so now we have two big groups on that carbon. And now we look here, there's a third methyl group. So I've drawn that here.
So you can see it's becoming crowded around that carbon. And by the time you have three methyl groups here, for example, this is quite a crowded carbon. So as you go down, you see a trend of something called increasing steric. hindrance. Now steric means space, hindrance means to be bumping into each other and getting in the way or hindering something.
So you can see there's more steric hindrance around here than there is up here with these three puny little hydrogens. So in a general nucleophilic substitution reaction, the nucleophile should be substituting the halide here to form that bond and the leaving group. So what they did is they did an experiment here and they were measuring the relative rate of reaction. And in this case, they used an alkyl bromide and then nucleophile hydroxy nucleophile.
And they converted this alkyl bromide into, oh, look at that, into alcohol and bromide. So this is substituting that. And that's your leaving group. And they just measured the rate of reaction. with different substituents on there just to see if there was a difference of adding more and more substituents around that carbon.
So this is the relative reaction rate. And what they just took the secondary one here, these two groups here, and they just called it one. So relative to this, they found out if you had one fewer groups on here. So this is just one group compared to a carbon with two alkyl groups.
This one is 50 times faster than this. So it's telling you that increasing the amount of alkyl groups around here is somehow slowing down a rate of reaction. And then they looked at this one here, which doesn't have any alkyl groups, and look at that.
That's 1500 times faster than this. So there's something happening that by adding these additional groups on there is slowing the reaction. So then what happens with this one?
Well, this reaction under these conditions barely even goes. So something happens with the number of alkyl groups around that carbon. This suggests that there are two different reaction mechanisms for these nucleophilic substitution reactions, and we're going to learn about them, that we have SN1 reactions and SN2 reactions.
And for this module, if you look at your alkyl halide, determine it's a methyl, primary or secondary, think it. SN2 reaction. If it's a tertiary alkyl halide, think SN1 reaction. Okay, so you're going to need to be able to identify this and think this.
Now, yes, there's a blurring of the boundary. Sometimes you will see secondary alkyl halides undertaking SN1. It depends on all these conditions.
But for this module, let's keep it simple and just remember that. Okay, so there's our general... nucleophilic substitution reaction, and we've determined there's two types of mechanisms that were done from experimental kinetic studies.
So they looked at the rates of the reaction, and then they looked at the overall, something called the overall order of the reaction, and they calculated it from their studies. And they used all of this to try to explain how these reactions happen. They try to give a mechanism, what was actually happening between your alkyl halide and your nucleophile to give you your products.
So this was done experimentally. And what this will mean is we're going to learn that there is the SN1 and the SN2 reaction, but we're not going to focus too much on the nitty gritty details. I need to give you some information so you can understand why we're talking about this, and what the differences are between SN1 and SN2 reactions, but we're not going to get too detailed, okay?
So here we go. Here's a general nucleophilic reaction. So this is the general rate equation. So there's something called the rate is equivalent to some constant times the concentration of your alkyl halide in solution and the concentration of your nucleophile. So that's, they're assuming that you need to look at how much of the alkyl halide and how much of the nucleophile you have, and that gives you your rate.
And the rate is moles per liters per second. You've got K as a reaction constant. And these brackets mean concentration in moles per liter.
OK, sometimes you'll see it written like this. Alkyl halide, just make it simpler so you don't have to write out the words. And nucleophile.
OK, so that's a general rate equation. And what we find is if you look at alkyl halides, just as a repetition, if it's a methyl primary secondary, the rate equation you're looking at. the concentration of the alkyl halide and the concentration of nucleophile.
And they determined that the order of the reaction, which is the molecularity of the reaction. Now, these are really big words. What it means is how many molecules are taking place at that rate determining step. What does that mean?
Okay, so you've got a reaction and that very first rate determining step to actually get these two things or to actually to create this. Is it dependent on only the alkyl halide? Something needs to happen at this side and then eventually the nucleophile reacts? Or does it only depend on the nucleophile and later the alkyl halide comes in there?
Or do both of these need to be in the mix, interacting with each other to give you the product? So what they find for these type of alkyl halides, you need both the alkyl halide and the nucleophile for the very first stages of the... the rate determining step and they determined that this is a nucleophilic substitution reaction SN2 biomolecular second order so it's a second order reaction oh lots of fancy words but what it's telling you is it depends on the concentration of both the alkyl halide and the nucleophile okay whereas if you have a tertiary alkyl halide remember in our general terms the reaction rate is only dependent on the concentration of the alkyl halide. So that means that that very first step, that rate determining step in, has nothing to do with the nucleophile. It's only focused on the alkyl halide.
So something's happening with the alkyl halide for that very first step, and then the reaction goes. So because it only has, it's unimolecular. This one is bimolecular.
You have both that molecule and that molecule involved. This one is unimolecular. That means it's SN.
one. So it's a first order reaction. That's what those terms mean. So that's the end of this component.