First you must understand the mechanism of this reaction:
:
An Explanation Of The Mechanism:
In the first step the nucleophile (-SH) attacks the carbon with the leaving group (Br) attached to it. In the transition state you will see that the nucleophile is beginning to form a bond with that carbon (shown by the dotted line) and the leaving group is beginning to leave (also shown by the dotted line). Notice, how the molecule has almost five bonds at this point. This is very bad and thus this molecule will only last for a very short time. That is why it is called a transition state. In the final product, you will see that the leaving group (Br) has left and that the nucleophile has taken its' place.
*Make sure that you can draw this mechanism from memory.
The Rate Law:
You will see by looking at the mechanism, that the rate of the reaction (how quickly the reaction occurs) depends on two things; how quickly the nucleophile can get in and attack the carbon and how quickly the leaving group can leave. This is the reason why the reaction is called SN2. Additionally since the rate depends on two things the reaction is said to show "second order kinetics".You should be able to draw the rate law on an exam for a SN2 reaction. It looks like the following:
Rate = k [R-X] [Nucleophile]
*R-X represents the original molecule with the leaving group attached to it (molecule (A).
* k is the rate constant (this is all you need to know about this)
:
:
. .
. .
. .
. .
. .
. .
. .
. .
. .
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Nucleophile
Attacks
Final Product
Leaving Group
(A)
(B)
When Molecules Undergo An SN2 Reaction:
Molecules undergo an SN2 reaction when the carbon attached to the leaving group is a primary carbon(meaning it is attached to only one other carbon) or if it has only hydrogen's attached to it. Please note that secondary carbons can undergo a SN2 reaction but the reaction will not occur that quickly. A tertiary carbon can undergo a SN2 reaction but it is very unlikely. Let's make sure that you understand this concept by attempting a practice problem.
Example: Label the following organic compounds with respect to how quickly they will react in an SN2 reaction(#1 being the
best).
(A)
(B)
(C)
Solution:
(A)
(B)
(C)
Notice how I have placed a gold star next to the carbons where the leaving group is attached. These are the carbons you should be focusing on when trying to answer this question. Notice that molecule (A) is the best because the carbon with the star attached to it has only one other carbon directly attached to it. Therefore, it is primary carbon and should react the fastest in an SN2 reaction. In molecule (B), the carbon with the star next to it has two other carbons directly attached to it. Therefore, it is a secondary carbon and is not as fast as molecule (A) in an SN2 reaction. Finally, notice that the carbon in molecule (C) has three other carbons directly attached to it. Therefore it is a tertiary carbon and will react very slowly in an SN2 reaction. Did you get the answer correct? I hope so!
#1(The Best)
#2(Okay)
#3(The Worst)
Let's try a similar problem just to make sure that you understand!
Example: Label the following molecules with respect to how quickly they react in an SN2 reaction.
(A)
(B)
(C)
Solution:
(A)
(B)
(C)
#1
(The Best)
#2
#3
Notice how each carbon with the leaving group attached to it in every molecule is a primary carbon. If this is the case, then you must look at the leaving groups. Since molecule (C) has the best leaving group attached to it, this is the one that reacts the fastest in a SN2 reaction.
You will notice that the starting material has an "R" configuration and the product has an "S" configuration. In all SN2 reactions this phenomenon occurs. This phenomenon is called inversion and means that if you start off with a "R" configuration you end up with an "S" configuration and vice versa.
The reason that inversion occurs is because when the big nucleophile comes in to attack the carbon(remember the big leaving group has not left yet) it must come in from the side where it has the most room. Therefore, it comes in from the backside of the molecule (away from the big leaving group). This is called backside attack and when it occurs it always causes inversion to occur.
Therefore, when answering an SN2 reaction question make sure that you do two things:
1. Replace the leaving group with the nucleophile.
2. Make sure that whatever the configuration is in the starting material that you draw the product in the opposite configuration.
Starting Material
Product
Example:Draw the product of the following reaction.
Solution: Notice how the carbon with the leaving group (Cl) attached to
it only has one other carbon directly attached to it. Therefore, it is a primary carbon. This should be your first hint that the reaction should proceed via a SN2 mechanism. SInce the reaction is SN2, inversion must occur (the starting material is "S" so the product must be "R").
*Your answer may look a bit different from the solution above, however, as long as the configuration in you answer is "R" it should be correct.
How To Draw An Energy Diagram For SN2:
You must also be able to draw and interpret the energy diagram for an SN2 reaction. The energy diagram looks like the one below:
TIME
E
N
E
R
G
Y
S.M..
T.S..
PRODUCT.
You should first notice that at the beginning of the reaction you just have the starting material (S.M.). SInce the carbon in the starting material has four bonds, it is happy. When a molecule is happy, it is stable and thus low in energy(remember that a stable molecule is low in energy and that an unstable molecule is high in energy). When the nucleophile attacks and is beginning to form a bond with the carbon, the leaving group is beginning to leave. However, for one brief moment they are attached to the same carbon and in turn the carbon has five bonds. This is very bad,and the molecule becomes very unhappy and in turn very unstable. Therefore the energy of the molecule rises and this is called the transition state (T.S.). Finally the leaving group leaves and the carbon has four bonds again. The carbon becomes happy again (stable) and the energy goes down.
-The End
(Make sure that you can explain the diagram in a way that is similar to this!)
The Solvent:
When running an SN2 reaction it is important to choose the proper solvent. There are two types of solvents that you should consider: protic solvents and aprotic solvents. Protic solvents are solvents that can hydrogen bond and aprotic solvents are solvents that can't hydrogen bond. You can tell if a solvent can hydrogen bond or not by looking at the structure of the solvent molecule. If a molecule has a hydrogen directly attached to an oxygen or a nitrogen then the molecule can hydrogen bond.
Methanol
Dimethyl ether
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. .
. .
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Protic Solvent
Aprotic Solvent
You should notice that methanol has a hydrogen directly attached to an oxygen, therefore it is a protic solvent. Whereas dimethyl ether does not have any hydrogens directly attached to an oxygen or a nitrogen, therefore it is an aprotic solvent.
Below is a list of polar and aprotic solvents that you are responsible for memorizing.
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Common Solvents
Protic
Aprotic
methanol
formic acid
acetic acid
water
diethyl ether
dimethyl formamide
(DMF)
dimethyl sulfoxide
(DMSO)
dimethyl
ether
H3CH2COCH2CH3
hexamethyl-
phosphor-amide
(HMPA)
So which solvent should you use for an SN2 reaction?
The answer is aprotic solvents.
Why should you use aprotic solvents?
Protic solvents tend to surround anything that is positive or negative. Now let's look back at the mechanism:
. .
. .
:
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You will see that the nucleophile has a negative charge. Therefore the protic solvent will surround the nucleophile and form a cage around it. Now, remember that in SN2 reactions that the rate of the reaction depends on two things: how quickly the nucleophile can come in and attack and how quickly the leaving group can leave. If the nucleophile is surrounded by a cage formed by the protic solvent it is going to have a difficult time coming in and attacking the carbon. If the nucleophile can't get in then the reaction will not occur. Therefore we do not want to run a SN2 reaction in a protic solvent. We want a nucleophile that is "bare" also known as a "naked nucleophile". Therefore, always use an aprotic solvent in an SN2 reaction instead.
The Nucleophile
SN2 Reaction in Protic Solvent
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X= Polar solvent Molecules
Help! The nucleophile is blocked by the polar solvent. It can't get to the molecule...
X
SN2 Reaction in Aprotic Solvent
A Naked Nucleophile! I love Naked Nucleophiles...
Oh no! I hate using polar solvent in SN2 reactions...
A NAKED NUCLEOPHILE
ATTACK...
ALWAYS RUN A SN2 REACTION IN AN APROTIC SOLVENT!!!
GOOD JOB!
Let's look at two molecules below and decide if they are protic or aprotic solvents:
