Reaction Mechanisms and Arrow Pushing

TL;DR

Reaction mechanisms are step-by-step descriptions of how molecules change during a chemical reaction, showing bond breaking and forming. Arrow pushing is the visual language we use to draw these steps, indicating the movement of electron pairs. Mastering arrow pushing helps you predict reaction products and understand chemical reactivity.

1. The Mental Model

Think of a reaction mechanism as a molecular dance choreography. Arrow pushing is like the dance notation, showing exactly how the electron "dancers" move from one partner (atom) to another, breaking old connections and forming new ones.

2. The Core Material

Reaction mechanisms explain how a reaction happens, not just what happens. They detail each elementary step, including intermediates and transition states. The key tool for visualizing these steps is arrow pushing, which uses curved arrows to represent the movement of electron pairs.

Understanding Curved Arrows

Photo by turek on Pexels

Curved arrows always start from an electron source (a lone pair or a bond) and point to an electron sink (an atom that can accept electrons or where a new bond will form).

There are four main types of elementary steps, each with its own arrow-pushing pattern:

1. Nucleophilic Attack: A nucleophile (electron-rich species) uses a lone pair or pi bond to form a new bond with an electrophile (electron-poor species).

   • Arrow starts from the lone pair/pi bond of the nucleophile.
   • Arrow points to the electrophilic atom.
   • Example: A hydroxide ion attacking a carbonyl carbon.

2. Loss of a Leaving Group: A bond breaks heterolytically, with both electrons from the bond going to one of the atoms, which then departs as a stable leaving group.

   • Arrow starts from the bond.
   • Arrow points to the atom that becomes the leaving group.
   • Example: A halide ion leaving an alkyl halide.

3. Proton Transfer (Acid-Base Reaction): A base removes a proton from an acid.

   • Two arrows involved:
     • One from a lone pair on the base to the proton (H).
     • Another from the H-atom's bond to the atom it was bonded to (usually forming a lone pair on that atom).
   • Example: Water deprotonating an acid.

4. Rearrangement: Electrons or atoms move within the same molecule to form a more stable isomer. These often involve carbocation rearrangements (e.g., hydride or alkyl shifts).

   • Arrow starts from the migrating bond/group.
   • Arrow points to the electron-deficient center (e.g., carbocation).
   • Example: A hydride shift in a carbocation.

These steps often happen in combination. When drawing mechanisms, always keep track of formal charges and ensure that atoms don't exceed their octet (or duet for hydrogen).

graph TD
    A["Nucleophilic Attack"] --> B["New bond forms"]
    C["Loss of Leaving Group"] --> D["Bond breaks; leaving group departs"]
    E["Proton Transfer"] --> F["Acid loses H+, Base gains H+"]
    G["Rearrangement"] --> H["Intramolecular shift for stability"]

    subgraph "Electron Movement"
        A -- Starts from lone pair/pi bond --> I["Points to electrophilic atom"]
        C -- Starts from bond --> J["Points to leaving group atom"]
        E -- Two arrows --> K["Base to H; H-bond to atom"]
        G -- Starts from migrating bond --> L["Points to electron-deficient center"]
    end

3. Worked Example

Let's draw the mechanism for the SN2 reaction of methyl iodide with hydroxide.

Reactants: CH₃I (methyl iodide) and OH⁻ (hydroxide ion)
Product: CH₃OH (methanol) and I⁻ (iodide ion)

Mechanism:

1. Hydroxide (nucleophile) attacks the carbon: The hydroxide ion's oxygen has lone pairs. One of these lone pairs will attack the electron-deficient carbon atom of methyl iodide. This carbon is electrophilic because it's bonded to a more electronegative iodine atom, making it partially positive.

   • Draw a curved arrow starting from a lone pair on the oxygen of OH⁻.
   • Draw this arrow pointing directly to the carbon atom of CH₃I.

2. Iodide (leaving group) departs: As the new C-O bond forms, the C-I bond must break to avoid exceeding carbon's octet. Iodine is an excellent leaving group because it's stable as an iodide ion (I⁻).

   • Draw a second curved arrow starting from the C-I bond.
   • Draw this arrow pointing to the iodine atom. This shows the bonding electrons moving entirely to the iodine, forming I⁻.

Resulting species: Methanol (CH₃OH) and iodide ion (I⁻).

4. Key Takeaways

• Curved arrows show electron movement, not atom movement. They represent the flow of electron pairs.
• Arrows start from electron sources (lone pairs, bonds) and end at electron sinks (atoms, centers).
• Always track formal charges. They change as electrons move and bonds form/break.
• Never exceed the octet rule for second-row elements (C, N, O, F). Hydrogen can only have two electrons.
• Each arrow represents a two-electron movement. (Half-arrows represent one-electron movement in radical reactions, but we're focusing on two-electron for now).
• Mechanisms are a series of elementary steps, not one big leap. Each step must be chemically reasonable.

Common Mistakes to Avoid:
- Drawing arrows starting from a positive charge or an atom, instead of an electron pair/bond. Electrons are what move!
- Drawing arrows ending at a bond, instead of an atom. New bonds form between atoms.
- Ignoring formal charges. Not adjusting charges correctly can lead to impossible intermediates.
- Exceeding the octet for carbon, nitrogen, or oxygen. Carbon can only have four bonds (8 electrons).
- Showing atom movement with curved arrows. Curved arrows are only for electrons.

5. Now Try It

Draw the complete mechanism for the acid-catalyzed hydration of propene (CH₃CH=CH₂). Use H₂SO₄ as your acid catalyst.

Think about:
1. Which part of propene is electron-rich and can act as a nucleophile?
2. What does the acid catalyst do first? (Hint: proton transfer)
3. What intermediate forms after the first step? Is it stable?
4. How does water (a nucleophile) attack?
5. What final step is needed to regenerate the catalyst and form the neutral alcohol product?

Success looks like: A clear, step-by-step mechanism showing all curved arrows, intermediates, and correct formal charges, leading to the formation of propan-2-ol, and regenerating the H₂SO₄ catalyst.