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Goals

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Types of Arrows in Mechanisms

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The Two Types of Arrows used in Mechanisms.

Representation of electron movement: Representation of electron movement: two-electron (1 bond) processes (Full-arrow →) vs. one-electron (half bond) processes (Half-arrow ↷).

In organic and inorganic mechanisms, arrows are used to show electron movement.

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Full Arrows (→)

A full curved arrow represents the movement of two electrons, usually in bond formation or breaking. This applies to:
  • Nucleophilic attacks
  • Resonance structures
  • Electrophilic and nucleophilic reactions
    • Nucleophiles attack electrophiles by donating an electron pair to form a new bond.
    • Electrophiles receive the nucleophillic attack to accept an electron pair to form a new bond.
Nucleophiles donate electron pairs (electron-rich) vs. Electrophiles accept electron pairs (electron-poor).

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Half Arrows (↷)

A half-arrow represents the movement of one electron, commonly seen in:
  • Radical reactions
  • Homolytic bond cleavage
  • Radical propagation steps
There will be lots of examples in the entries of Reaction Repo of all reactions mentioned

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Applications of the Arrows

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Bond Formation

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Full Arrow

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Half Arrow

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Forming Resonance Structures

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Resonance forms of DMSO.

DMSO is capable of undergoing resonance. The first step (1) towards the Swern Oxidation

DMSO undergoes resonance, which is represented using a full arrow, indicating the movement of two electrons (one bond). This differs from one-electron processes (shown by half-arrows). The electron pair from the S=O double bond shifts toward oxygen, converting it into a single bond. Since oxygen gains an extra lone pair, it takes on a negative formal charge, while sulfur loses electron density, resulting in a positive charge.