Substitution Comparison

How can we tell which mechanism to use? This question will get more complicated unfortunately, but for now we can use the following factors to answer this question.

Concept: How do we predict if the mechanism is SN1 or SN2

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Video Transcript

Now I've taught you two different mechanisms that we can use to perform a substitution. And it's already hard enough that you have to memorize all these different facts about the rate and about what's favored and what's not favored. But one of the even more confusing parts can just be figuring out when do I use SN2 and when do I use SN1.
So what I want to do is I want to put together a little list of rules, a little list of basically, comparisons so that we can figure out when do we use one mechanism and when do we use another. Just as a heads up, this is actually going to get more complicated later on once you start talking about elimination. But for right now, since we're just in substitution, we can simplify it down to just two variables.
And you're probably going to guess what they are. Let's just go ahead and get started. It's just going to be – we're going to look at nucleophile strength and we're going to look at leaving group substitution. This has to do with the things that I keep saying are different between SN2 and SN1.
So actually, why don't you guys help me fill these in. The first thing we look at is nucleophile strength. What type of nucleophile is favored for an SN1 reaction? Do you guys remember? We said weak. Because remember that weak means that it's not going to start the reaction, it's going to wait for the carbocation to form. So that means what kind of nucleophile's favored for SN2? Strong. Strong is favored because we want it to do a back side attack. Is that cool so far?
What's the other thing? Well, we look at leaving group substitution. Leaving group substitution says that which type of degree – remember alkyl halides are measured in degrees. Which degree is the most favored for SN1? Remember that it goes in order of the best carbocations. Remember carbocations, they're the most stable when they have the most R groups around them. That's just a rule that I told you to memorize that I'll explain more later. That just means that tertiary is going to be more stable than secondary, more stable than primary and that's it. And then methyl is the worst.
So then alternatively for SN2, which one is the most favored? And it turns out it's the opposite trend. For SN2, methyl is the most favored, then primary, then secondary, then tertiary. In fact, just so you guys know, for SN1, primary and methyl don't even happen because they're so bad at making carbocations. For SN2 tertiary doesn't even happen because it's so bad it has a terrible back side.
So I'm just going to put here bad carbocation for this one. And I'm going to put here bad back side. If you understand the mechanisms, this should be too confusing of what I'm saying. That basically the best carbocation is going to be tertiary. The best back side is going to be methyl.
Now for these next questions. I'm going to basically pile everything together. I'm not going to tell you what the mechanisms are. You have to go ahead and determine first of all what the mechanism is using these rules and then you have to draw the final product based on everything I've taught you about these mechanisms. I know that sounds challenging, but I believe in you guys. I think you can at least get close.
So go ahead and try to draw the mechanisms and the products of the following reaction.

When given a substitution reaction, use the following two factors to determine the mechanism:

Nucleophile Strength:              SN1 =   WEAK               SN2 = STRONG

Leaving Group Substitution:    SN1  = 3° > 2°                 SN2 = 0° > 1° > 2°

Problem: Predict the product of the reaction

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Problem: Predict the product of the reaction

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Substitution Comparison Additional Practice Problems

The following reaction sequence gives rise to two isomeric products A and B. Write the structures of A and B and propose a detailed mechanism for their formation.

b. Which one of compounds A and B will be favored under kinetic or thermodynamic control. Explain your answer with one sentence for each compound.

 

 

 

 

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Each reaction shown below is a nucleophilic substitution reaction. Compare the mechanisms of the two reactions. Label each mechanism as Sn1 or Sn2.

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Predict the organic product of the following reaction. When appropriate, be sure to indicate stereochemistry. If more than one product is formed be sure to indicate the major product. Draw your answer in skeletal form. You will be graded on the product your draw from the reaction no other information is needed for this question.

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Put a circle around the statements that relate to an SN mechanism and put a square around the ones that talk about SN1. 

 

CH3X > 1˚ > 2˚                                  No rearrangements

 

Rearranged products                        Rate = k [alkyl halide] [Nuc]

 

Polar protic solvent                            Racemization

 

Inversion at chiral carbon                  Rate = k [alkyl halide]

 

3˚ > 2˚                                                Polar aprotic solvent

 

Strong nucleophile                             Weak nucleophile (may also be solvent)

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The following reaction will not occur. Explain why the reaction will fail. 

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Does the following chart depict and S N1 or SN2 type mechanism?

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What is (are) the organic product(s) of the following reaction?

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Predict the major organic substitution product for the following reaction:

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Which of the following is  NOT a possible step in a substitution reaction?

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Which one of the following correctly describes the reaction below?

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Identify the substitution product(s) in the following reaction.

a) A

b) B

c) C 

d) a mixture of A and B

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Determine the product(s) for the following reaction

a) A

b) B

c) C 

d) a mixture of A and B

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Compare the following reactions and decide which reaction in each group would occur faster. Write your answer and concisely defend your choice.

 

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For each of the following pairs of SN2 reactions, tick the box that corresponds to the SLOWEST reaction of the two:

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Give the rate equation for the reaction occuring between NaCN and CH 3CH2Cl. Write your answer in the box below.

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For a unimolecular nucleophilic substitution (________) reactions, bond breaking and bond formation occur _______, ________ is/are involved in the transition state of the rate-determinining step. However, for biomolecular nucleophilic substitution (_______) reactions, bond breaking and bond formation occur _______. A bimolecular reaction is one in which _______ is/are involved in the transition state of the rate determining step.

  1. SN2; in two distinct steps; One species; SN1; simultaneously; two species
  2. SN1; simultaneously; Two species; SN2; in two distinct steps; one species
  3. SN2; simultaneously; Two species; SN1; in two distinct steps; one species
  4. SN1; in two distinct steps; One species; SN2; simultaneously; two species
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Consider the following reaction. Assuming no other changes, what would happen to the rate of the reaction if the concetration of the nucleophile was doubled and if the concentration of the alkyl halide was tripled?

a. There would be no effect

b. The rate would double

c. The rate would triple

d. The rate would increase 6-fold

e. The rate would increase 9-fold

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