carbocation rearrangement

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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MS 899 - Beginning Mechanism Practice Submitted by Matt on August 6, 2011.

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

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Problem # 594

A chemist tried to prepare compound A from benzene via Friedel-Crafts alkylation and instead produced compound B.

Why did this happen? How could the chemist prepare compound A?

Solution

Friedel-Crafts alkylation is prone to carbocation rearrangement. In this case, alkylation produced a 1º carbocation which rearranged to a 3º carbocation, leading to compound B.

We can avoid this by instead doing Friedel-Crafts acylation. The intermediate in acylation is the acylium ion, which is stabilized by resonance and so won't rearrange.

But after the acylation reaction we have to get rid of the carbonyl (C=O) group, so we do a Wolff-Kishner reduction (N₂H₄/NaOH, heat).

Problem Details

MendelSet practice problem # 594 submitted by Matt on July 9, 2011.

Subject: Organic Chemistry

Subject Topic(s): Electrophilic Aromatic Substitution

Question Type: Concept (explain why this is so..)

Keyword(s): carbocation rearrangement, EAS mechanism, Friedel-Crafts, Wolff-Kishner

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 # 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).