Organic Chemistry : Substitution Mechanisms

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #11 : Substitution Mechanisms

Which of the following molecules would most readily undergo an SN2 mechanism?

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Possible Answers:

I

III

II

V

IV

Correct answer:

IV

Explanation:

 is a better leaving group than  because it is a larger molecule and can distribute the negative charge over a larger area. SN2 works better with better leaving group and with less-substituted carbons (methyl > primary > secondary > tertiary)

Example Question #1 : Help With Sn2 Reactions

Suppose that a chemistry student is trying to run a reaction in the lab. In his solution, he adds , ethanol, and dimethylformamide (DMF) as a solvent. However, no reaction takes place. To solve this problem, the student adds hydrochloric acid to the solution and, in doing so, a reaction takes place that produces the desired product, chloroethane.

What is the most likely reason for why the addition of hydrochloric acid to the solution allowed the reaction to proceed?

Possible Answers:

The addition of hydrochloric acid to the solution protonates the hydroxyl group on ethanol, allowing it be a better leaving group.

The addition of hydrochloric acid increased the energy of the transition state of the reaction, which resulted in enough energy to drive the reaction forward.

The addition of hydrochloric acid protonated the solvent and, as a result, the reaction was able to move forward because DMF is a polar aprotic solvent.

The addition of hydrochloric acid resulted in an increase in the concentration of chloride ion, which increased the reaction rate and drove the reaction forward.

Correct answer:

The addition of hydrochloric acid to the solution protonates the hydroxyl group on ethanol, allowing it be a better leaving group.

Explanation:

In this question, we're presented with a scenario in which a chemistry student is trying to run a reaction. At first, the reaction doesn't work. But after the student adds a strong acid to the mixture, the reaction goes through. We're being asked to determine why this happens.

First and foremost, let's identify which kind of reaction is occurring. The student starts with  and ethanol, and ends up getting chloroethane. So, what has changed? The hydroxyl group on the ethanol has become replaced by a chlorine atom. As a result, we can identify this as a substitution reaction. Furthermore, because we know the hydroxyl group is attached to a primary carbon (a carbon that is only bound to one other carbon), we can categorize this as an SN2 reaction rather than an SN1 reaction. This is because the removal of the hydroxyl group would leave a primary carbocation, which is not likely to occur because this is very unstable.

Before addition of , no substitution reaction occurs. Why is that? What has to happen for the reaction to proceed? The answer is that the hydroxyl group needs to come off as a leaving group and be replaced with chloride. But, hydroxyl groups make very poor leaving groups. Compared to chloride ions, hydroxyl groups floating in solution are much more unstable. Thus, the hydroxyl group would rather stay attached to ethanol than to leave.

But, this all changes once  is added. The reduction in the pH of the solution causes the hydroxyl group to become protonated. Not only does this give the hydroxyl group a positive charge, but it also makes a much better leaving group. This is because when it leaves the hydrocarbon and enters solution, it will exist as , or water. Because water is much more stable than the chloride ion, chloride is able to attack the protonated ethanol and undergo a nucleophilic substitution reaction. Thus, it is the protonation of the leaving group that drives the reaction forward.

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