Once the bond breaks, the carbocation is formed and the faster the carbocation is formed, the faster the nucleophile can come in and the faster the reaction will be completed. A good leaving group is a weak base because weak bases can hold the charge.
They're happy to leave with both electrons and in order for the leaving group to leave, it needs to be able to accept electrons. Strong bases, on the other hand, donate electrons which is why they can't be good leaving groups. As you go from left to right on the periodic table, electron donating ability decreases and thus ability to be a good leaving group increases. Halides are an example of a good leaving group whos leaving-group ability increases as you go down the column.
The two reactions below is the same reaction done with two different leaving groups. One is significantly faster than the other. This is because the better leaving group leaves faster and thus the reaction can proceed faster.
Other examples of good leaving groups are sulfur derivatives such as methyl sulfate ion and other sulfonate ions See Figure below. How does temperature affect SN1 and SN2 reactions? What conditions favour nucleophilic substitution? Why is acetone used in SN2 reactions? Why do haloalkanes undergo nucleophilic substitution? However, as we make the molecule bulkier sterically hindered , it becomes a weak nucleophile but still a strong base even though the negative charge is still on the same atom: What is going on here?
This is why larger molecules lose their nucleophilicity while retaining the base strength: The effect of a polar protic solvent is similar to this since it forms hydrogen bonds with the nucleophile thus putting it in a cage and making it sterically hindered. As we have seen above, this enlarged species are now weaker nucleophiles, yet, their basicity is largely intact: If we compare this with basicity which is not affected, the result of a polar protic solvent is that the nucleophilicity is revered and it is now opposite to basicity.
Therefore, the basicity and nucleophilicity are not affected and they still go parallel: The chart below is a summary of the solvent effect on nucleophilicity and basicity: The effect of solvent in S N 1 and S N 2 substitution If you need to choose between S N 1 and S N 2, then remember that polar aprotic solvents favor S N 2 , while polar protic solvents favor S N 1 mechanism since the nucleophilicity in this case is decreased.
If strong — you are between S N 2 or E2. If weak -it is either S N 1 or E1. Mark the polar aprotic solvents:.
This content is for registered users only. Remember Me. Forgot Password. Which of the following reactions will go faster? This was very helpful. Thank you! Glad to hear that, Michelle. Thank so much! Thank you, Yasmine. We were studying about nucleophilic substitution reactions.
My professor said that in general SN1 reactions are faster than SN2 reactions. In this case, what I think is that the rate will depend on our reagent, leaving group, solvent, etc and in some cases SN1 will be faster while in some others SN2. What makes a strong Nucleophile? If they bond to a hydrogen atom, we call them bases. If they bond to any other atom especially carbon , we call them nucleophiles.
A good base is usually a good nucleophile. So, strong bases — substances with negatively charged O, N, and C atoms — are strong nucleophiles. Is Solvolysis sn1 or sn2?
Characteristic of SN1 reactions, solvolysis of a chiral reactant affords the racemate. Is ethanol sn1 or sn2? Polar protic solvents hinder SN2 reactions, as they can hydrogen bond to nucleophiles and make them less effective in attacking the substrate. Ethanol is a polar protic solvent though alcohols are only weakly acidic. Why does sn1 favor weak nucleophiles? SN1 reactions nearly always involve weak nucleophiles, because strong nucleophiles are too reactive to allow a carbocation to form.
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