6.5 Using Curved Arrows in Polar Reaction Mechanisms

Polar Reaction Mechanisms and Curved Arrow Notation

Electron movement in polar reactions

Curved arrows are the language of organic reaction mechanisms. Each arrow tracks where electrons go as bonds break and form, letting you reconstruct an entire reaction step by step.

There are two types of curved arrows, and the distinction matters:

• Double-headed curved arrows (the most common) show the movement of an electron pair. You'll use these for polar reactions like nucleophilic attacks and proton transfers.
• Single-headed curved arrows (fishhook arrows) show the movement of a single electron. These appear in radical reactions and homolytic bond cleavage, not in typical polar mechanisms.

Drawing the arrows correctly comes down to two rules about placement:

• The tail starts at the electron source: a lone pair, a negative charge, or the middle of a bond (the electrons in that bond).
• The head points to the electron sink: an electrophilic atom, a positive charge, or the space where a new bond will form.

Curved arrows also show electron movement between resonance structures, though in that case no actual reaction is occurring. The arrows just illustrate how you can "push" electrons on paper to draw an equivalent Lewis structure.

Rules for nucleophiles and electrophiles

Every polar reaction involves a nucleophile donating electrons and an electrophile accepting them. Recognizing which is which is half the battle.

Nucleophiles (electron-rich, Lewis bases):

• They have something to give: a lone pair, a negative charge, or a $\pi$ bond
• Examples: $\ce{OH-}$, $\ce{NH3}$, $\ce{CH3O-}$, alkenes
• The curved arrow always starts from the nucleophile

Electrophiles (electron-poor, Lewis acids):

• They have a deficit to fill: a positive charge, a partial positive charge ($\delta+$), or an empty orbital
• Examples: $\ce{H+}$, $\ce{BF3}$, $\ce{AlCl3}$, carbocations
• The curved arrow always points toward the electrophile

When a leaving group is involved, it departs with a bonding pair of electrons. The arrow for this step starts from the bond being broken and points toward the leaving group. Bond polarity is what makes this possible: in a bond like $\ce{C-Br}$, the electrons are already pulled toward the more electronegative bromine, so it's natural for $\ce{Br}$ to leave with that pair.

The key pattern to internalize: electrons always flow from electron-rich to electron-poor. If you can identify the nucleophile and the electrophile, you know which direction the arrows go.

Interpreting curved arrow notation

Reading a mechanism means following the arrows one step at a time and tracking what happens to charges and bonds at each stage. Here are the three fundamental arrow patterns you'll see repeatedly:

Nucleophilic attack:

1. The nucleophile donates an electron pair to the electrophile (arrow from nucleophile to electrophile).
2. A new bond forms between them.
3. The electrophile gains electron density, so its formal charge decreases by one (becomes more negative or less positive).

Proton transfer:

1. A base donates an electron pair to a proton ($\ce{H+}$) on an acid (arrow from base to the proton).
2. The $\ce{H-A}$ bond breaks, and those electrons go to atom A (arrow from bond to A).
3. A new $\ce{B-H}$ bond forms, and the original acid loses its proton.

Notice that proton transfers require two arrows: one to form the new bond and one to break the old bond. This is a common place where students forget the second arrow.

Leaving group departure:

1. The bonding electrons move onto the leaving group (arrow from the bond to the leaving group).
2. The bond breaks, and the leaving group departs with the electron pair.
3. The atom that lost the bond becomes more positive (its formal charge increases by one).

Rearrangements involve intramolecular electron flow, where a group (often a hydride or methyl) migrates to an adjacent atom to form a more stable intermediate, such as converting a secondary carbocation to a tertiary one. The curved arrow shows the electrons in the migrating bond shifting to the new position.

To predict the overall product, combine all the individual arrow steps in sequence. At each step, update the charges and bonds before moving to the next arrow.

Reaction Progress and Intermediates

Many polar reactions don't happen in a single step. Instead, they pass through intermediates, which are real (though often short-lived) species that exist between steps. Carbocations, carbanions, and protonated intermediates are common examples.

Transition states, by contrast, are not real species you can isolate. They represent the highest-energy point along the path between two intermediates (or between reactant and product). You'll often see them drawn in brackets with a double-dagger symbol ($\ddagger$).

Each curved arrow step in a mechanism corresponds to movement from one energy minimum (reactant or intermediate) through a transition state to the next energy minimum. Following the arrows through the full mechanism traces the electron density changes that drive the reaction from start to finish.