Notes on Nucleophiles and Electrophiles

Lecture Overview

  • Topic: Nucleophiles and Electrophiles

  • Course: Organic Chemistry I (LB271), Michigan State University

  • Current Lecture: Lecture 09

Announcements

  • Readings:

  • From Karty: Ch 6 Sections 6.2, 6.3, Sections 7.1–7.3

  • From OCLUE: Chapter 1 Pages 8–28 (focus on nucleophiles and electrophiles)

  • Practice Problems: Karty, specifically exercises 50 (except c), 52, 55-66, Chapters 7: 2, 3, 5 (a-c, e, f)

  • Due Dates:

  • SW HW5 Due Saturday – 02/22 by 11:59pm

  • Complete Recitation WS by next recitation dates

  • Available resources: Lecture 8 slides and video on D2L

Learning Outcomes for Lecture 09

  • Define properties of Lewis acids and bases.

  • Identify Lewis acids and bases in reactions.

  • Explain electron flow in reactions using terms nucleophiles (electron-rich) and electrophiles (electron-poor).

  • Use curved arrow notation to depict electron flow and bond breaking/forming events.

  • Rank leaving group ability based on their structural features.

  • Recognize spectator ions and their inert nature.

  • State the predominant structure of amino acids at various pH levels.

  • Understand techniques for protein and amino acid separation.

Key Concepts

Lewis Acids and Bases

  • Lewis Acid: Electron pair acceptor.

  • Lewis Base: Electron pair donor.

  • Examples of Lewis Acid: BF3, Mg2+, Fe2+ (not Brønsted acids).

  • Real-life examples include biological Lewis Acid-Base interactions (e.g., heme, chlorophyll).

Reactions in Organic Chemistry

  • Acid-base reactions serve as the foundation for organic reactions, emphasizing electron flow from electron-rich sites to electron-poor ones.

  • Stability, bond breaking, and forming, as well as equilibrium, are central themes in understanding reaction mechanisms.

Electrophiles and Nucleophiles

  • Electrophile: Needs electron density; an electron-poor center.

  • Nucleophile: Electrophile seeking more negative charge; an electron-rich center.

  • Leaving Group (LG): An atom/group that departs during bond formation/breaking.

Example Reaction Mechanism

  1. Starting materials: CH3Br + NaOH.

  2. Resulting products: CH3OH + NaBr.

  3. Key change: The bromine (Br) is substituted by hydroxide (OH), demonstrating nucleophilic substitution.

  4. Mechanism details include stabilization and formation of new bonds while breaking existing ones.

Factors Affecting Electrophilic Carbon Centers

  1. Bond polarization of the carbon-leaving group must create a partial positive charge (δ+) on carbon.

  2. The leaving group must stabilize excess electrons.

  3. Accessibility of the reactive center is essential for the reaction to occur.

  4. Hybridization: Carbon being sp3 hybridized is more favorable compared to sp2/sp due to less electron density.

Nucleophiles

  • Characteristics of good nucleophiles:

  • Availability of lone pairs of electrons.

  • Strong bases are usually good nucleophiles (i.e., trends in base strength relate to nucleophilic strength).

  • Trends include:

  • N > O > F across a row due to increasing electronegativity.

  • Comparisons of nucleophiles like –OH > H2O.

  • Strong nucleophilicity also increases down a group (e.g., comparing halogens Cl–, Br–, I–).

Non-Basic Nucleophiles

  • Weak bases can still serve as strong nucleophiles due to polarizability. Examples include iodide (I–) and bromide (Br–).

Summary of Nucleophilic Characteristics

  • Good Nucleophiles: Feature a partial negative charge, availability of lone pairs, and common uncharged nucleophiles (e.g., NH3, CH3OH).

  • Charged Nucleophiles: Full negative charge (e.g., OH–, SH–).

  • Key examples include different compounds and their nucleophilic strengths.

Generic Reaction Dynamics

  • Nuc (Nucleophile) + E (Electrophile) → Product

  • Curved arrows indicate electron movement:

  • Electrons move from nucleophile to electrophile

  • Leaving groups facilitate stabilization of extra electrons resulting from bond changes.