 Okay, so let's check this reaction out This Reaction was first noticed actually by Walden Chemists long time ago. It's called the Walden inversion. So when you take Melaic acid here and add PCl5 to it phosphorus pentachloride and ether of course is just the solvent You're going to get Product that has the Substituent here this one's going forward right the substituent you'll find goes backwards Okay, so that implies that there was a backside attack at the last step Okay, so everybody knows the product so I want you to write it down on your own So hopefully you can see that chlorine is going coming The nucleophile itself is coming in from the backside So that last step is an SN2 reaction So let's go ahead and draw the mechanism of this reaction real quick the Walden inversion Or the first step of the Walden inversion and this is what I was saying earlier whenever you have Atoms that are third Period and below right they can expand their bail so when you're looking at phosphorus here probably with Cross-ides being like wait a second that has more than eight electrons in it It's okay because it's a bigger atom it can actually hold more than eight electrons For our case. It's good for us to just remember PCl5. So we can do this Okay, so what's going to happen here is actually the OH on the The alcohol is more nucleophilic than the OH on the carboxylic acids so PCl5 actually reacts selectively with the alcohol Hydroxyl group as opposed to either of the carboxylic acid hydroxyl groups Okay, so in that case that the alcohol oxygen is reacting as the nucleophile with the phosphorus of PCl5 being the electrophile in this particular instance, okay We're going to kick off one of those chlorine atoms. So I'll just show that mechanism So like that Is everybody okay with what we've just done? Yes And remember our final product is going to be this I'm not showing the other portion of it just the organic So try to write the product on your own before you see me Finish it and see if you Got it right And that way you can identify Where your problems might lie what you put Hopefully essentially is that what you got you might not have finished it with the core part of this Okay, good, and here's the SN2 reaction. Okay, this is the nucleophile Remember in an SN2 we need the nucleophile and electrophile and the nucleophiles going to come in from where? The backside right so it's going to be opposite of the stereochemistry that you see right now So since that oh is coming towards us the chlorine is going to be going away from us So in order to show that it's just we've shown an SN2 reaction before here It is just that classical SN2 reaction. I should have asked you to write it first Did you write it first you get it right? Good job. I should have known that y'all would have written it first. That's good I'm proud of you for taking that initiative And then what did we get as our product so we know what the organic product is of course that SN2 backside of 10 the nucleophile And this whole thing is the electrophile The electrophilic carbon is actually that carbon there and then hopefully you can see right we get that what we call inversion of configuration at that stereocenter Stereocenter So that's because it was pointing forward downs pointing back and then of course I Could see it some people looking at me. Well, where are the other at? And well, we're not as interested in them Because that's not our organic stuff, but that's the other part any questions on this thing This is the first step of that Walden cycle or Walden inversion. We were talking