Now you’re finally getting ready to start understanding types of chemical reactions. Oh joy!
Concept: What you need to know about types of chemical reactions.1m
In this chapter we’re going to talk about reactions. And what that means is that your professor is going to want you to be able to recognize basic types of reactions. So you’re not going to need to understand all the reactions that happen in Organic Chemistry yet but there are four major types of reactions that your professor is going to expect you to be able to recognize. So maybe you don’t fully understand it but you can say, hey, based on these general features this must be this type of reaction. Alright? And this could be a multiple choice type of question for you on your exam. So let’s just go ahead and get started.
We’re gonna learn about 4 different types of chemical reactions. You don’t actually need to understand these! Just be able to recognize them for now.
Concept: Recognizing Acid-Base Reactions.3m
As I said there are four major types. And the first one is the easiest one. This is going to be called an acid-base reaction and in most textbooks this is will have its own chapter. So usually we’re gonna have to spend an entire chapter just to talk about acid-base reactions. Now the general features of this that you should be aware of are that molecules of opposite charges are going to react together to exchange a proton. Okay? Now, I used the word “proton” earlier in the first chapter when I was explaining just electrons and protons and stuff and what I said what a proton was, was just H+. Okay? Because remember, that’s a hydrogen. It doesn’t have an electron and it doesn’t have a neutron so it’s just a proton. That’s why we call it a proton. Alright? Now notice that I did put the word “usually” next to it because it turns out there’s going to be some special types of acid-base reactions that don’t exchange a proton. Alright? But we’ll get there. I’ll let you guys know what those are.
So I just want to show you guys this example. This is a very common example where I have basically a negative charge, okay, and that would be one of the charges. And I’m looking at the other molecule and I see well there’s no positive charge. So is this really going to be acid-base? Well, if you think about it we’ve already learned about electronegativity, right? And remember that we learned how to draw dipole moments. So if I were to look at this HO bond right there. Is there a dipole on that bond? And the answer is yes. There’s actually a very strong dipole pulling towards the oxygen. So what that means is that more electrons are there at the oxygen and less electrons are at the hydrogen. Are you guys comfortable with that? That just has to do with the partial charges we assign. Imagine that the O is like a sumo wrestler pulling on that rope and pulling all the electrons towards itself and that H is the puny guy that can’t even keep up. Right? So what that means is that we are going to get an exchange. The opposite charge that I am talking about is this oppositely charged hydrogen. So the opposite charge that I’m talking about is oppositely charged hydrogen. So I have a negative interacting with the positive and what’s going to happen at the end is that remember that I said you’re gonna have exchange. So what that means is that I’m going to redraw this first compound and what I’m going to draw is that now this O gets a negative because before, the other O had the negative. And now this OH over here is now attached to the H that came from the other compound. Alright? So basically what happened was that I switched a negative charge and an H. See how that happened? That’s what we call an acid-base reaction, exchanging protons and electrons. Alright? And then we’d get more in-depth with that later.
In this reaction, two molecules of opposite charges react to exchange a proton. (There are some exceptions that don’t, but we’ll get to this later)
Concept: Recognizing Substitution Reactions.2m
So now what I want to do is talk about another type of reaction that's actually very similar and that's the substitution reaction, the substitution action is essentially an acid based reaction really, it's the same thing except that the atom or group of atoms that is exchanged is something other than hydrogen, OK? So, remember that in acid base we're always moving a hydrogen from one compound to another and in a substitution, it might be the same thing where we have the same charge negative positive but instead of moving an H I'm going to move another atom so in this case we would apply the same exact thing I would say OK I have my negative charge here but do I have a positive on the other side? Well if I draw my dipole moment what I would see is that I have a very strong dipole moment pulling towards the floor, do you see that? So, what that means is that there would be a partial negative here and there be a partial positive here and what's going to wind up happening is that we're going to wind up switching places of whatever's here and whatever's out here, OK? So basically the two things that have negative charges are going to wind switching places so like I said you don't need to know this yet because this is going to get an entire chapter to itself, substitution reactions always have their own chapter but I'm just trying to explain that what's going to happen is that now the OH would be up here and now that F negative would be over here so see what happened in a substitution reaction is that we still got things exchanging but they weren't hydrogen it was actually two different molecules, OK?
This is an acid-base reaction where an atom (or group of atoms) other than protons are exchanged.
Concept: Recognizing Elimination Reactions.1m
I just want to talk about elimination really quick, elimination is also a chapter that I mean a type of reaction that we dedicate an entire chapter to and elimination is really easy all you need to know for right now is that you're going to take two single bonds or what we call Sigma bonds, right? And we're going to create one double bond or what we call Pi bond, OK? So, as you can see these Z groups don't even worry about it that just means it could be pretty much anything, OK? And after the elimination reaction takes place I'm going to wind up getting a double bond so I turned 2 Sigmas into a Pi and the reason that we call it elimination is you're going from 2 bonds to 1 so think that you're eliminating a bond pretty easy, right?
Here, two single bonds (σ) are removed to create one double bond (π).
Concept: Recognizing Addition Reactions.1m
Addition is the opposite of an elimination, the same way that you would say like multiplication and division are opposites addition and elimination are opposites, addition also always gets its own chapter so these are things that we're going to spend a lot of time on later in the semester I'm just giving you a glimpse right now and what we do for addition is we take one Pi bond and you guessed it we just do the opposite so we make two Sigma bonds, OK? So, all you need to do for the sake of this chapter is just recognize these, just look at it and say Oh that's an addition reaction that's a substitution, OK? Later on, we'll spend an entire chapter on each of these reactions and you guys will actually be able to understand what's going on.
In these reactions, one double bond generates two new single bonds.
Consider the following reaction (which you have probably not seen before). What type of reaction is it?
Classify the following reactions as either Substiution (S), Addition (A), Elimination (E), or Rearrangement (R).
Classify the following reactions (1-4) as addition, elimination, substitution, or rearrangement from top to bottom: