Leah here from leah4sci.com and in this video we're going to look at the reaction energy
diagram for the SN2 reaction.
First a quick review of the SN2 reaction as I teach in the substitution elimination video
series linked below.
SN2 stands for Nucleophilic Substitution and 2 tells us that it's a bimolecular reaction.
Bimolecular does not mean two steps, Bi means two molecules reacting at the same time.
In this example we'll take a look at the energy diagram for CH3-I reaction with NaSH to give
us CH3 SH + NaI.
The rate of an SN2 reaction is equal to sum constant K times the concentration of the
substrate that would be the CH3I times the concentration of the nucleophile which would
be the SH-.
The substrate and Nucleophile react together in a single step bimolecular reaction.
This is important to help you recognize the energy diagram for an SN2 reaction.
To understand the reaction let's first look at the mechanism where CH3 is attacked by
the SH minus.
To understand the energy diagram let's quickly review the mechanism.
The nucleophile SH- is going to attack the carbon holding the Iodine which will then
kick out the Iodine in a backside attack.
Notice there's no sodium in the mechanism because it's simply a positive spectator ion
that does nothing to the reaction.
Our product is going to have the SH now bound to the carbon with an I minus floating around
somewhere in solution.
Let's move the reaction down so that we can understand what it looks like on the energy
diagram.
An energy diagram simply tells us the energy of a reaction as the reaction is happening.
I like to think of it as we start the reaction and as time passes what happens to the energy
of the molecules?
The Y-axis measures energy.
The X-axis which is called the reaction coordinate can be thought of as time.
At the start of this reaction we have a CH3-I, at some low enough stable energy.
Notice we're not using numbers I'm focusing on the concept of the increase and the decrease.
In this SN2 reaction, the S is going to attack the carbon which kicks off the Iodine which
is not going to happen on its own.
We need to put energy into the system to allow the molecules to adapt this relatively unstable
state because even though the reaction is ideal in the long run the intermediate, the
process of breaking in and making bonds requires a lot of energy and so we need to put enough
energy into the system to almost motivate the reaction to happen.
That big hump of energy that the molecules have to overcome is called the activation
energy or Ea which I think of as energy of activation.
The very top of this hump , that highest point of energy is called the transition state but
to represent with this symbol.
The transition state is not an intermediate, instead it's almost like an in between.
In that transition state we have a very unstable system that looks something like this.
The carbon in the CH3-I is bound to 3 hydrogen atoms and and 1 is bound to an iodine.
But when the sulfur comes in to attack to form a bond to carbon, the bond between carbon
and iodine is going to break, The stronger the bond to sulfur and carbon the weaker the
bond between carbon and iodine until there's a complete carbon to sulfur bond, an iodine
is gone.
That would be the result of this reaction but the in between where the bonds are half
formed and half broken, that is the very unstable, not isolatable transition state which has
a partial negative charge as the negative sulfur is losing the charge by forming a bond
and the partial negative charge on iodine as the neutrally bound iodine is now kicked
out to gain an extra pair of electrons and the negative charge.
Once the bond is fully formed and iodine is fully kicked out, the molecules are happy
and they release all the extra energy so form a very stable final product which is now the
CH3 bound to the SH with an Iodine somewhere off in solution.
A couple of key things to recognize in this graph since we're measuring the energy.
The starting molecule in this case is relatively decent in energy.
But if we compare it to the energy of the final product you'll notice that the final
product is lower in energy because it's more stable and that give us a negative delta-g
for a spontaneous reaction.
But even with this being a spontaneous reaction you still need that high activation energy
because without that energy, without the motivation to kickstart the reaction, nothing's going
to happen.
The key thing to recognize in an SN2 energy diagram is the fact that we have 1 reactant,
1 product, and only 1 transition state.
What we don't have are any intermediates as a reminder an SN2 reaction is a bimolecular
reaction, two molecules reacting at the same time that means there's no intermediate and
as soon as they're done reacting we get our final product.
For a more in-depth review of substitution reactions, first give this video a thumbs
up, and subscribe to my channel make sure you don't miss out on any more videos, then
go watch the substitution elimination series from the beginning which you can find on my
website along with my practice quiz and cheat sheet.
See the link below or go to leah4sci.com/substitution-elimination.