Organic Chemistry Reactions - Online Tutor, Practice Problems & Exam Prep

What you need to know about types of chemical reactions.

In this chapter, we're going to be talking 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 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 go ahead and get started.

Recognizing Acid-Base Reactions.

So as I said, there are 4 major types and the first one is the easiest one. This is going to be what we call an acid base reaction, and in most textbooks, this will have its own chapter. So, usually, we will have to spend an entire chapter just talking 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 a proton was is just an H⁺. Okay? Because remember that it's a hydrogen that doesn't have an electron and it doesn't have a neutron, so it's just a proton and 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 that there are going to be some special types of acid base reactions that don't exchange a proton. Alright? But we'll get there. Okay? 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? So that would be one of the charges and then 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 at the oxygen and fewer electrons are at the hydrogen. Are you guys comfortable with that? That just has to do with the partial charges that we assign. Imagine that the O is like the sumo wrestler pulling on that rope and pulling all the electrons towards itself, and the H is like the puny guy that can't even keep up. Alright? So what that means is that we are going to get an exchange. The oppositely charged hydrogen that I am talking about is this oppositely charged hydrogen. So I have a negative interacting with a positive, and what's going to happen at the end is, remember that I said you get an 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 when we're exchanging charges and protons. Alright? And we'll get more into-depth later.

Recognizing Substitution Reactions.

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 reaction is essentially an acid-base reaction. Really, it's the same thing, except that the atom or group of atoms that is exchanged is something other than hydrogen. Okay? So remember that in acid-base, we're always moving a hydrogen from one compound to another. In substitution, it might be the same thing where you have the same charges, 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, okay. I have my negative charge here, but do I have a positive over it 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 fluorine. Do you see that? So what that means is that there would be a partial negative here and there'd 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. Okay? So basically the two things that have negative charges are going to wind up 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 the F- 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 2 different molecules. Okay?

Recognizing Elimination Reactions.

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 2 single bonds or what we call sigma bonds. Right? And we're going to create one double bond or what we call a pi bond. Okay? So as you can see these z groups, don't even worry about it. That just means it could be pretty much anything. Okay? And after the elimination reaction takes place, I'm going to wind up getting a double bond. So I turn 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?

Recognizing Addition Reactions.

Addition is the opposite of 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 1 pi bond and, you guessed it, we just do the opposite so we make 2 sigma bonds. Okay? So all you need to be able 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. Okay? 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.