Ch. 7 - Substitution ReactionsWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

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


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°

Practice: Predict the product of the reaction

Practice: Predict the product of the reaction