nucleophilicity

MS 908 - SN1-SN2-E1-E2 Review Submitted by Matt on August 6, 2011.

Textbook and Chapter: Carey and Giuliano 8th Ed. (2010), Chapter 8

Keywords: leaving groups, nucleophilicity, SN1/SN2/E1/E2

Description: Goes through substitution and elimination competition reactions:

Nucleophile strength trends
Leaving group trends
Steric effects
How to tell the difference between SN1, SN2, E1, and E2 reaction conditions

Total Problems: 10

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Problem # 713

When a carbonyl is treated with semicarbazide under acidic conditions an "imine" is produced called a semicarbazone.

Which of the two products below is the correct structure for a semicarbazone? Explain.

Solution

What this problem is really asking is "Which nitrogen on semicarbazide is more nucleophilic?"

The nitrogens bonded to the carbonyl on semicarbazide all have resonance forms, so their lone pairs aren't as available for nucleophilic attack.

The other nitrogen (on the left) is the most nucleophilic because it isn't involved in any resonance forms, so its lone pair isn't shared with another atoms.

Problem Details

MendelSet practice problem # 713 submitted by Matt on July 22, 2011.

Subject: Organic Chemistry

Subject Topic(s): Nucleophilic Acyl Addition (Aldehydes & Ketones, Imines/Enamines, Acetals/Ketals)

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

Keyword(s): nucleophilicity, imines

Problem # 539

Rank the following compounds in order of decreasing nucleophilicity. (1 = most nucleophilic)

Solution

Nucleophilicity increases with electron density; negatively charged molecules are more nucleophilic than neutral molecules. So HS^- is the best nucleophile in this group.

Nucleophilicity also increases with size. This is because the larger an atom, the more polarizable it is. Size increases as we go down the periodic table, so H₂S is a better nucleophile than H₂O.