Organic Chemistry 1 · Reaction Mechanisms
Mechanisms and Arrow Pushing
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A Mechanism Step-by-step sequence of bond breaking/forming Full entry → is a step-by-step account of how bonds break and form, drawn with curved arrows that track electrons, never atoms. A double-headed arrow shows an electron pair moving (from a lone pair or bond to an atom or between atoms); a single-headed fishhook arrow shows one electron moving (radical reactions). The four basic steps are Nucleophilic attack Pair moves from nucleophile to electrophile Full entry →, Electrophilic attack π-bond electrons attack an electrophile Full entry →, Proton transfer Base removes H⁺; H–X electrons move to X Full entry →, and Leaving-group departure Bond electrons leave with the departing group Full entry →, plus carbocation rearrangements (hydride or alkyl shifts) that convert a less stable cation into a more stable one. Every mechanism must satisfy Mass balance Every atom on the left appears on the right Full entry →, Charge balance Total charge conserved at every step Full entry →, and the Octet rule Second-row atoms hold at most eight electrons Full entry →.
Why this matters
Mechanism drawing predicts and explains drug transformations and metabolic pathways: proton transfer is how enzyme active sites perform acid–base catalysis, and recognizing leaving-group departure helps medicinal chemists judge metabolic stability. Correct arrow pushing also underpins transition-state-analog inhibitors, because a proposed mechanism reveals the highest-energy structure.
The college version
1. Curved Arrows: Pair and Radical Movement
A double-headed curved arrow shows movement of two electrons. Its tail sits on an electron source — a lone pair or a σ/π bond — and its head points to the destination: an atom (new bond or new lone pair) or between atoms (new π bond). A single-headed fishhook arrow shows movement of one electron and is used only in radical reactions such as homolytic cleavage. The rule: arrows move electrons, never atoms; if an atom relocates, the electrons holding it in place moved first.
2. The Four Fundamental Elementary Steps
Every polar mechanism is built from four moves. (1) Nucleophilic attack: a lone pair or π bond attacks an electrophilic atom to form a bond. (2) Electrophilic attack: an electron-rich π bond attacks an electrophile. (3) Proton transfer: a base's lone pair grabs H⁺ while the H–X bond electrons move onto X (two arrows). (4) Leaving-group departure: a σ bond's electrons move onto the leaving group, which departs with them. Bond formation and bond breaking are the two possible arrow outcomes, always shown explicitly.
3. Carbocation Rearrangements and Mechanism Checking
A carbocation rearranges when a Hydride shift 1,2-migration of H with its electron pair to a cation Full entry → (1,2-migration of H with its bonding pair) or an alkyl shift (1,2-migration of a methyl/alkyl group) moves a neighboring group onto the cationic carbon, giving a more stable carbocation — rearrangements happen only when they improve stability (e.g., secondary → tertiary). After drawing, check three things: mass balance (count every element on both sides), charge balance (total charge conserved through every step), and octet compliance (second-row C, N, O, F never exceed eight electrons; carbon never has five bonds).
How it works
- Identify reactive sites (lone pairs, polar bonds, π bonds, positive centers) before drawing any arrow.
- Choose the arrow type: double-headed for a pair, fishhook for one electron.
- Start each arrow at an electron source and end it at an electron destination.
- Draw only the four standard steps — one at a time — stating the intermediate after each arrow.
- Allow a carbocation rearrangement (hydride or alkyl shift) only when it gives a more stable cation.
- Finish by checking mass balance, charge balance, and octets for every species, including intermediates.
Common confusions
| Do not confuse | With | Difference |
|---|---|---|
| Curved arrow (double-headed) | Fishhook (single-headed) | Two-electron (polar) vs. one-electron (radical) |
| Moving electrons | Moving atoms | Arrows track electrons; atoms follow |
| Nucleophilic attack | Proton transfer | Forms a bond to an electrophilic atom vs. moves H⁺ to a base |
| Leaving-group departure | Nucleophilic attack | Breaks a bond vs. forms a bond |
| Hydride shift | Alkyl shift | H migrates vs. an alkyl group migrates |
| Intermediate | Transition state | Real species between steps vs. transient peak within a step |
Memory aids
"N-P-L-R — Nucleophilic attack, Proton transfer, Leaving-group departure, Rearrangement." For arrow types: "two heads, two electrons" (double-headed = a pair) and "one fishhook, one fish" (single-headed = one electron).
Quick review
Topic Recap
Mechanisms use curved arrows to track electron movement — double-headed for electron pairs, fishhook for single electrons — built from four elementary steps (nucleophilic attack, electrophilic attack, proton transfer, leaving-group departure) plus hydride/alkyl carbocation rearrangements. Arrows move electrons, not atoms, and every step must satisfy mass balance, charge balance, and the octet rule.
Knowledge Check
- In a polar reaction, what does a double-headed arrow represent, and where may its tail and head sit?
- Draw the electron movement for OH- attacking CH3I: which two arrows are needed?
- When would a carbocation undergo a methyl shift rather than a hydride shift?
- After drawing a mechanism you find a carbon with five bonds in one step. What is this error?
- In HBr addition to propene via the carbocation mechanism, what is the net charge of the intermediate step?
Answers and Rationales
- It shows an electron pair moving; the tail starts on a lone pair or bond, and the head points to an atom or between atoms.
- One arrow from the oxygen lone pair to carbon (forming C–O), and a second from the C–I bond to iodine (that pair departs with iodide).
- A methyl shift occurs when hydride migration would not improve stability, but moving an adjacent alkyl group would produce a more stable carbocation.
- An octet-rule violation: second-row carbon cannot hold more than eight valence electrons or form five bonds; the step is invalid.
- Net charge zero overall: a +1 carbocation balanced by a −1 bromide, matching the neutral reactants and product.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Curved arrows are the language of mechanisms: each arrow is a sentence saying "these electrons move from here to there." The tail marks where electrons start (a lone pair or a bond); the head marks where they end (an atom or between atoms). The key rule is that arrows move electrons, not atoms — though when electrons move, atoms often follow.
Comparison: think of electrons as money changing hands in a sequence of small transactions. A double-headed arrow is a two-dollar transfer (a pair of electrons); a fishhook is a one-dollar transfer (a single electron). A mechanism is just the list of transactions taking you from the starting balance (reactants) to the ending balance (products).
Where it stops being exact: a mechanism is a model, not a photograph. The arrows describe the accounting of electrons between stable species, but not the continuous, three-dimensional timing of a real reaction through its transition states.
Simple Example
Hydroxide attacking methyl bromide is drawn with one arrow from the oxygen lone pair to the carbon, and a second from the C–Br bond to bromine. Those two arrows say the O–C bond forms as the C–Br bond pair departs with bromine — without ever treating atoms themselves as the moving objects.
Worked example
Addition of HBr to 2-methylpropene, with electron movement stated before products.
- Identify the players. The alkene π bond is the nucleophile; HBr's proton is the electrophile.
- First arrows (electrophilic attack on the π bond). A double-headed arrow runs from the π bond to the proton, moving the π electrons to form a new C–H bond; a second arrow runs from the H–Br bond to bromine, moving that pair onto bromide.
- State the intermediate before any product. Using the π electrons to make the C–H bond on the less-substituted carbon leaves the other carbon electron-deficient: a tertiary carbocation forms at the more substituted carbon (the more stable choice — Markovnikov's rule as a shortcut, not a substitute for this mechanism).
- Second step (nucleophilic attack). A double-headed arrow runs from a bromide lone pair to the carbocation's empty p orbital, forming the C–Br bond.
- Check the accounting. Mass: all atoms present on both sides. Charge: neutral reactants, a +1 cation balanced by one Br⁻, neutral product. Octets: the carbocation transiently holds six electrons (allowed); no second-row atom exceeds eight.
Key takeaways
- High yield: Curved arrows show electron movement, never atom movement.
- High yield: Double-headed = two electrons; fishhook = one electron (radicals only).
- An arrow's tail must start on a real electron source (lone pair or bond), and its head lands on an atom or between atoms.
- High yield: Proton transfer uses two arrows: one to the proton, one moving the H–X pair onto X.
- Leaving-group departure is a single arrow from the bond to the departing group.
- High yield: Carbocation rearrangements (hydride/alkyl shifts) occur only when they create a more stable cation.
- A carbocation may hold only six electrons (a legitimate transient); no second-row atom may exceed eight.
- High yield: Always verify mass balance, charge balance, and octets.
- Markovnikov and Zaitsev are prediction shortcuts, not substitutes for arrow-pushing analysis.
Study tools & related lessonsYou’ll learn to · Key vocabulary · Related
You’ll learn to
- Use curved arrows to show electron-pair movement, and fishhook arrows for single-electron (radical) movement.
- Recognize the four fundamental elementary steps: nucleophilic attack, electrophilic attack, proton transfer, and leaving-group departure.
- Draw and rationalize carbocation rearrangements via hydride and alkyl (methyl) shifts.
- Check a mechanism for mass balance, charge balance, and octet violations.
Key vocabulary
- Mechanism
- Step-by-step sequence of bond breaking/forming
- Curved arrow (double-headed)
- Shows an electron-pair move
- Fishhook arrow (single-headed)
- Shows a single-electron move
- Nucleophilic attack
- Pair moves from nucleophile to electrophile
- Electrophilic attack
- π-bond electrons attack an electrophile
- Proton transfer
- Base removes H⁺; H–X electrons move to X
- Leaving-group departure
- Bond electrons leave with the departing group
- Bond formation / breaking
- Arrows that make or break a bond
- Hydride shift
- 1,2-migration of H with its electron pair to a cation
- Alkyl (methyl) shift
- 1,2-migration of an alkyl group with its electrons
- Mass balance
- Every atom on the left appears on the right
- Charge balance
- Total charge conserved at every step
- Octet rule
- Second-row atoms hold at most eight electrons
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