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MS 904 - Carbocation Drills

MS 904 - Carbocation Drills

Submitted by Matt on August 6, 2011.

Textbook and Chapters: Carey and Giuliano 8th Ed. (2010), Chapters 4, 5, 6

Keywords: carbocation, carbocation formation, carbocation rearrangement

Description: This mendel set guides you through everything you have to know about carbocations:

Ways carbocations form
Carbocation rearrangements
How carbocations react (elimination or nucleophilic attack)

Also includes some practice problems: addition to an alkene, dehydration (E1), and substitution (SN1).

Total Problems: 8

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Problem # 331

Rank the carbocations below in order of decreasing stability. (1 = most stable)
Solution
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.

Problem Details
MendelSet practice problem # 331 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Carbocation Fundamentals (Stability, Rearrangement, SN1)
Question Type: Concept (explain why this is so..)
Keyword(s): carbocation, stability
Problem # 332

Each of the carbocations below will spontaneously rearrange. Draw the structure of the expected rearrangement product.
Solution
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).

Problem Details
MendelSet practice problem # 332 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Carbocation Fundamentals (Stability, Rearrangement, SN1)
Question Type: Concept (explain why this is so..)
Keyword(s): carbocation, carbocation rearrangement
Problem # 333

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.
Solution

Problem Details
MendelSet practice problem # 333 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Carbocation Fundamentals (Stability, Rearrangement, SN1), Acid-Catalyzed Dehydration of Alcohols
Question Type: Mechanism (show a mechanism using curved arrows..)
Keyword(s): carbocation, carbocation formation
Problem # 334

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.
Solution
3° carbocations are more stable than 2º carbocations, so only the 3º carbocation is formed.

Problem Details
MendelSet practice problem # 334 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Carbocation Fundamentals (Stability, Rearrangement, SN1), Alkene Reactions and Markovnikov's Rule
Question Type: Mechanism (show a mechanism using curved arrows..)
Keyword(s): carbocation formation, Markovnikov's rule
Problem # 335

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.
Solution
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.

Problem Details
MendelSet practice problem # 335 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Carbocation Fundamentals (Stability, Rearrangement, SN1), Alkene Formation: Elimination, Dehydration, and Zaitsev's Rule, Alkene Reactions and Markovnikov's Rule
Question Type: Mechanism (show a mechanism using curved arrows..)
Keyword(s): carbocation, SN1 mechanism, Zaitsev's rule, E1 mechanism
Problem # 336

Predict the product(s) of the reaction below, and used curved arrows to show a mechanism.
Solution
Due to Markovnikov's rule, only the more substituted alkyl halide is formed.

Problem Details
MendelSet practice problem # 336 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Alkene Reactions and Markovnikov's Rule
Question Type: Mechanism (show a mechanism using curved arrows..)
Keyword(s): Markovnikov's rule, alkene addition
Problem # 337

Predict the product(s) of the reaction below, and used curved arrows to show a mechanism.
Solution
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.

Problem Details
MendelSet practice problem # 337 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Carbocation Fundamentals (Stability, Rearrangement, SN1)
Question Type: Mechanism (show a mechanism using curved arrows..)
Keyword(s): SN1 mechanism, carbocation rearrangement
Problem # 341

Predict the product(s) of the reaction below, and used curved arrows to show a mechanism.
Solution
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.

Problem Details
MendelSet practice problem # 341 submitted by Matt on June 7, 2011.
Subject: Organic Chemistry
Subject Topic(s): Alkene Formation: Elimination, Dehydration, and Zaitsev's Rule, Acid-Catalyzed Dehydration of Alcohols
Question Type: Mechanism (show a mechanism using curved arrows..)
Keyword(s): Zaitsev's rule, E1 mechanism, dehydration

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