Description: This mendel set is a complement to Carbocation Drills. It is meant to prepare students on how to approach longer and more complicated mechanisms. Reaction mechanisms covered:
Acid-catalyzed elimination (dehydration, E1)
Acid-catalyzed addition to an alkene
Halohydrin formation (halonium ions, which result in anti-additions.)
Let's work through an elimination reaction. Draw the structures for each of the species in the three boxes below (protonated thiol, carbocation, and alkene). Also draw curved arrows to show electron movement.
The first step in this reaciton, like many reactions, is an acid-base reaction- when you see an H+ (acid), the first step is usually something getting protonated. This starting material is a thiol (sulfur), but the same thing would happen with an alcohol or ether (oxygen).
When you have a protonated thiol (or alcohol, ether, etc.), you know the reaction isn't finished yet- products are rarely charged. There are two legal moves to get that positive charge off of the protonated thiol. Either the thiol deprotonates, or it takes off as a neutral leaving group. It can't deprontate because that would be going backwards, so your only "legal move" is for the HSCH2CH3 to act as a leaving group (See problem 518).
But then you're left with a carbocation, which is definitely not the final product. How can we get rid of it? (See problem 335 for general ways carbocations react). If the HSCH2CH3 attacked in an SN1 type reaction that would be OK, but it would be going backwards! So the only way to get rid of the carbocation is to do a beta-elminiation (E1).
MendelSet practice problem # 519 submitted by Matt on June 30, 2011.
For the reaction below, draw the structures of the carbocation intermediate and the final product.
The delta (Δ) in the reaciton arrows means that heat is being added to this reaction, which tends to favor elimination over substitution. Also, the reaction is using a non-nucleophilic acid (H2SO4), which tends to favor elimination reactions (H3PO4 is another common reagent for E1 reactions, while HCl or HBr tend to go SN1).
Because this reaction is taking place in acid, a carbocation is likely to form, so this is an E1 reaction. Since water is lost over the course of the reaction, this is a dehydration, which is a type of elimination reaction.
MendelSet practice problem # 348 submitted by Matt on June 7, 2011.
Predict the product(s) of the reaction below, and used curved arrows to show a mechanism.
You know this is an elimination reaction because no nucleophile is present; H3PO4 and H2SO4 are non-nucleophilic acids. IF the reagent were HCl or HBr on the other hand, this would be a substitution reaction.
Since acid is present, a carbocation will probably form, so we know that this is an E1 mechanism.
The 2º carbocation that is initially formed will undergo a 1,2-hydride shift to become a 3° carbocation, but that doesn't affect the final product of the reaction; with either the 2° or 3° carbocation, the most stable alkene product is 2-methyl-2-butene.
MendelSet practice problem # 341 submitted by Matt on June 7, 2011.
Carbocations aren't very stable and so don't last very long after they are formed.
Use curved arrows to show:
a) how a carbocation reacts with a halide ions to form an alkyl halide.
b) how a carbocation reacts with water to form an alcohol.
c) how a carbocation reacts with a base to form an alkene.
For a), the product is neutral and so you are done after the nucleophile (Cl-) attacks the carbocation.
For b), the the intermediate is a protonated alcohol, and so you must do a proton exchange step (also called a hydrogen exchange or deprotonation) to get the final alcohol product, which is neutral.
For c), there are two different types of beta hydrogens, and so two different alkenes are possible.
The more stable alkene is the one that will form, and this will always be the most highly substituted alkene. This is Zaitsev's rule. The rationale for this is hyperconjugation: neighboring carbon atoms stabilize an alkene.
MendelSet practice problem # 335 submitted by Matt on June 7, 2011.