Arrow pushing (curly arrows) in organic chemistry

Arrow pushing (curly arrows) in organic chemistry

Lessons

In this lesson, we will learn:

  • To recall the guidelines of arrow-pushing in organic reaction mechanisms.
  • To apply arrow-pushing to suggest organic reaction mechanisms.

Notes:

  • In kinetics, we introduced the idea of a reaction mechanism, talked about what is required for a chemical reaction and why they happen:
    • Particles need to collide with enough energy (the activation energy) and the correct arrangement in order to react. One collision meeting both conditions is unlikely, so most molecular collisions don’t actually lead to a reaction.
    • Reactions are often driven by charge interaction between particles – attraction of opposite charge, either ions or partial charges (like δ\delta+ / δ\delta-).

  • Curly arrows or ‘arrow-pushing’ is widely used in organic chemistry to show the reaction mechanism.
    There are some key ideas behind arrow-pushing that must be followed when arrow pushing in reaction mechanisms.
    • Curly arrows show the movement of electrons. This happens in two ways:
      • Full-headed arrows show a pair of electrons moving. The majority of chemical reactions involve pairs of electrons moving.
      • Half-headed arrows show one electron moving. These are very common in free-radical chemistry but is otherwise rare.
    • Curly arrows are drawn from an area of negative charge. This includes:
      • Formal negative charges (negative ions) such as Cl- or CN-
      • Lone pairs, such as H2O: or : NH3
      • Pi bonds, such as H2C=CH2
      • Sigma bonds between carbon and electropositive atoms, such as R-Li (called organolithium reagents) or R-Mg (Grignard reagents).
      Remember that compounds where curly arrows come from are donating electrons – the examples here are all possible nucleophiles!
    • Curly arrows are drawn going toward an area of positive charge or electron deficiency. This includes:
      • Formal positive charges (positive ions) such as H+ carbocations (R3C+).
      • Partial positive charges (δ\delta+) such as a carbonyl carbon atom C=O, or carbon bonded to a halogen such as C-Cl.
      • Vacant p orbitals, such as boron in BH3
    • Curly arrows must show charge being conserved.
      • When a nucleophile donates an electron pair to form a covalent bond, it effectively loses one electron; the extra electron it had now ‘belongs to’ the electrophile in the covalent bond.
      • Depending on the nucleophile, the outcome can look slightly different.
        See the image below for two examples

  • Worked example:
    The reaction of a general nucleophile and a carbonyl group, with two variations, is shown below.
    The chemistry going on is not important here; the two reactions are just to show more examples of curly arrows being used from electron-rich to electron-poor species.
  • Introduction
    Drawing reaction mechanisms.
    a)
    Rules for drawing curly arrows.

    b)
    A note on curly arrow placement.

    c)
    Example: Curly arrows in nucleophilic addition.

    d)
    Example: Double bonds as nucleophiles.