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For the reaction below, draw the structures of the carbocation intermediate and the final product.
This is a substitution reaction. Because the reaction takes place in acid and the leaving group is on a 2º carbon, it will probably form a carbocation (SN1 mechanism).
-OH is a poor leaving group so the alcohol will protonate first, so it can leave as H2O, which is neutral.
The alcohol below is protonated and contains an oxygen with a positive charge. Using curved arrows, show the two "legal moves" that result in a neutral oxygen.
Protonation is a common first step in mechanisms. When you have a protonated alcohol, it's likely that either:
Either one is possible. If you're not sure which one happens, remember that when writing out mechanisms, never go backwards- don't draw structures you have drawn before.
Rank the carbocations below in order of decreasing stability. (1 = most stable)
The more substituted the carbocation, the most stable it is. This is because of the inductive effect- adjacent carbon atoms donate some of their electron density to neighboring carbocations (which are electron deficient), making them closer to neutral and more stable.
So the 3° carbocation is the most stable.
Each of the carbocations below will spontaneously rearrange. Draw the structure of the expected rearrangement product.
Be on the lookout for a 1,2-shift when you have a carbocation adjacent to a carbon atom that is more substituted (ex: a 2° carbocation next to a 3° or 4° carbon).
Let's go over how a carbocation can form from an alcohol.
Write in the curved arrows to show the formation of the protonated alcohol, and water acting as a leaving group to form a carbocation.
Let's go over how a carbocation can form from an alkene.
Use curved arrows to show the two carbocations that can from from 1-methylcyclohexene.
3° carbocations are more stable than 2º carbocations, so only the 3º carbocation is formed.
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.
Predict the product(s) of the reaction below, and used curved arrows to show a mechanism.
The 2° carbocation formed immediately undergoes a 1,2-methyl shift (a rearrangement) to form the more stable 3° carbocation, so the product is the 3° alkyl chloride instead of the 2º alkyl chloride, which would have formed in the absence of rearrangement.
Using curved arrows, draw a mechanism for the SN1 reaction shown below.
Textbook and Chapters: Carey and Giuliano 8th Ed. (2010), Chapters 4, 5
Keywords: carbocation rearrangement, free radical halogenation, mechanism
Description: Practice mechanisms from the first half of Fall semester orgo. Dehydration (E1), substitution with carbocation rearrangement (SN1), and free-radical bromination.
Total Problems: 3
Textbook and Chapters: Carey and Giuliano 8th Ed. (2010), Chapters 4, 5, 6
Keywords: alkene addition, carbocation
Description: Identify the intermediates (carbocation, radical, borane intermediate, etc.) and products for important reactions dealing with alkenes. Good review for an orgo1 midterm.
Total Problems: 7
Keywords: carbocation, carbocation formation, carbocation rearrangement
Description: This mendel set guides you through everything you have to know about carbocations:
Also includes some practice problems: addition to an alkene, dehydration (E1), and substitution (SN1).
Total Problems: 8
