Hey guys!
So now we’re going to get into the part that makes this chapter really fun and that’s
the mechanism.
Alright?
So just so you guys know, I’m going to teach this section more in depth than your professor
did for this chapter.
Why is that?
Well, it’s because I realized that mechanisms are such a big part of organic chemistry that
if we can learn them as early as possible it’s going to help you guys so much more
later on.
Okay?
You don’t want to be learning about mechanisms for the first time in Chapter 7 or Chapter
8.
That would be organic suicide.
Alright?
Because you’re going to be very confused and you’re going to have to pick up the
pace a lot.
So what we want to do is we’re want to just use the concepts of acids and bases to introduce
what the mechanism is, alright?
So we’re going to talk about this in two parts.
The first part is I want to talk about the reactivity of atoms.
Why do they even react?
And the second part is I want to show you how they react with electron movement.
So let’s go ahead and begin.
So just so you guys know, the currency of organic chemistry, and I’ve said this before,
is stability.
Okay.
Stability.
Every atom wants to become more stable.
Every molecule wants to become more stable.
Alright?
And it turns out that stability and reactivity, you hear these words all the time, actually
have an inverse relationship.
Inverse.
What I mean by that is opposite.
That means if stability goes up, reactivity is generally going to go down.
If reactivity is up, that means stability generally goes down.
They’re opposites of each other.
Okay?
So how can we tell if a molecule is reactive and if a molecule is unreactive?
Alright.
Again, this is not a section that is usually covered by your textbook.
I have just built this from years of teaching Orgo and realizing that hey, there’s actually
ways we can predict this.
Okay.
So there’s actually four really common way to know that something is going to be reactive.
You guys ready?
And you actually know three of these four already.
Okay?
The first one is formal charges.
Do you guys remember those?
Okay.
Formal charges, what does that mean?
Formal charges mean that an atom is not at its ideal bonding preferences, right?
It has too many or too few valence electrons.
Okay?
Remember the sticks and the dots.
So what that means is that it’s going to try to do everything in its power to try to
go back to its bonding preferences.
So that means it has a very big reason to want to react to something else.
If it can get rid of a few electrons maybe it can go back to the state that it wants
to be in.
Does that make sense?
So that’s the first one.
If you see a formal charge you know that this is going to be reactive.
The second one is net dipoles.
Okay?
So net dipoles have to do with the fact that I have asymmetrical dipoles that are not perfectly
cancelling out and what that means is that I’m going to have partial charges in different
places.
Another way that I could say this is partial charges.
Okay?
So remember that a partial charge is the result of a net dipole.
If I have a net dipole going to the right that means that I would have a partial negative
on one side and a partial positive on the other.
Okay?
So what I’m basically doing here is I’m saying that if you have formal charges—a
full charge—that’s very reactive.
But even if you have a partial charge that’s also very reactive.
Let’s now talk about two other things.
The two other things are π bonds.
Okay?
What we’re going to find out is that π bonds, remember that we went over this in
the first chapter?
π bonds are not as strong as σ bonds, right?
They’re actually much weaker in terms of the amount of energy that is saved.
So what that means is that π bonds are really a good source of electrons.
Okay?
And they’re going to be pretty easy to break.
Remember, σ bonds are almost impossible to break many times.
But π bonds, they’re not as stable so they’re easier to break.
So π bonds are going to also be a lot reactive.
I know I said the word “break” a lot of times.
And finally the fourth one is something you don’t know yet and that is steric effects.
Okay?
Steric effects is something that we’re talking about more when we talk about basically cycloalkanes
and we’re going to talk about a bunch of other stuff a little bit later.
But an example of steric effects, I just want to tell you it would be something like ring
strain.
So imagine if I have a three-member ring and these bond angles are very, very, very tight.
Usually those bond angles would want to be like 109.5° but instead they are like 60°.
That’s an example of ring strain.
That means that these bond angles are not stable because of the fact that they’re,
you know, they’re just not at their ideal position.
Alright?
And that’s also going to make molecules reactive.
Now, do you need to deal with these right now?
No.
But I just want to be comprehensive and let you know like those are the four major things
that make organic molecules reactive.
Cool?
So what I want to do is move on to an exercise and I want you guys to just look at these
eight different compounds.
And just go ahead and look at the first box.
The first box is literally a checkbox.
So I want you to either check off that it is reactive or put an X on it if it is not
reactive.
So I want you to maybe go through A through D, first four, and I want you guys to look
at those first four and see if you can find any of those four reasons why it would be
reactive.
If you can’t then that means it’s not going to be reactive.