Written by tutor Heidi R.
When an electron pair donor known as a nucleophile reacts with a SP3 hybridized carbon with a good leaving group attached to it, a reaction will occur known as nucleophilic substitution. There are two main mechanisms which show how this reaction occurs. In this text we will discuss the mechanisms and summarize the main features. The mechanisms are called SN1 (unimolecular) and SN2 (bimolecular).
In the SN1 mechanism, the leaving group will leave first forming the carbocation. The nucleophile is then free to react with the carbocation from either the front or the back. This is why SN1 reactions can lead to racemization. It should also be noted that if the carbocation is not in the most stable place hydride or methyl shifts may occur. This is why SN1 reactions will often lead to a rearranged product.
The term SN2 means that two reactants are involved in the rate determining step. This means the nucleophile will attack the electrophilic carbon at the same time as the leaving group leaves. This leads to a 5 membered transition state. Since the nucelophile is coming into the molecule at the same time as the leaving group leaves it has to attack from the back. If the molecule is initially chiral it will lead to inversion of stereochemistry.
It should be noted in both reactions the leaving group is in competition with the nucleophile. You should therefore understand what makes a good nucleophile and what makes a good leaving group.
Effect of different factors on SN1 and SN2
|Alkyl group (Substrate)||Tertiary preferred and fastest, secondary moderate rate||Primary preferred and fastest, secondary moderate rate|
|Nucleophile||Prefers neutral nucleophile (weak)||Prefers charged nucleophile (strong)|
|Preferred Solvent||Polar protic solvent||Polar aprotic solvent|
|Stereochemistry||Racemization can occur||Inversion|
|Rearrangements||Very common||Rare to never|
|Mechanism||2 steps||1 step|
|Eliminations side reactions||Common with basic nucleophiles||Only occurs with heat and basic nucleophiles|
Nucleophilicity is the ability of the nucleophile to donate its electrons. There are three main factors that allow us to predict nucleophile strength.
1. Nucleophilicity increases as the charge becomes more negative. OH– is a much better nucelophile than H2O.
2. Nucleophilicty increases with base strength, note going from right to left in the periodic table.
CH3– > NH2– > OH– > F–
3. Nucleophilicty increases with size (polarizability). Note as you go down the periodic table elements become bigger.
I– > Br– > Cl– > F–
In polar protic solvents the size (polarizability) wins. So the bigger the molecule the more nucleophilic it is. In polar aprotic solvents the basicity is the more important factor.
Some examples of Nucleophiles for SN1 and SN2
|ROH (alcohol, such as methanol, ethanol)||RO- (alkoxide such as methoxide, ethoxide|
|R- (see Grignard Reactions)|
Some examples of protic and aprotic polar solvents
In an SN1 reaction what happens to the rate of reaction if the concentration of the nucleophile in doubled?
Stays the same
Rate increases but cannot predict by how much
Rate decreases but cannot predict by how much
Because the concentration of the nucleophile does not appear in the equation for the kinetic rate of SN1 is rate=k(substrate), which does not involve the nucleophile.
In an SN2 reaction what happens to the rate of the reactions if the concentration of both the nucelophile and the substrate in increased by a factor of 2?
Rate stays the same
Rate increases by 3 fold
Rate increases by 4 fold
Rate increases but you can not tell by how much
Which would be the best nucleophile in a SN2 reaction?
In a SN1 reaction which would be the best combination of reagents?
Water, primary alkyl substrate
Water, tertiary alkyl substrate
Alkoxide ion, primary alkyl substrate
Alkoxide ion, tertiary alkyl substrate.
Which of the following is a polar aprotic solvent?