Organic Chemistry I · Lesson 07
Mechanism Fundamentals
Every reaction in the rest of this course is an electron-flow story. Electrons move from electron-rich places to electron-poor places, and a mechanism is just the step-by-step account of that movement. Learn to read and draw it here, and the reaction lessons stop being memorization.
Learning Goals
- Explain what a reaction mechanism shows and why electrons move.
- Identify electron-rich and electron-poor sites in a molecule.
- Draw curved arrows correctly, with the tail on the electrons and the head on the destination.
- Identify nucleophiles, electrophiles, acids, bases, and leaving groups.
- Track formal charge through each step of a mechanism.
- Push arrows for a proton transfer and a nucleophilic attack.
- Tell the difference between an intermediate and a transition state.
- Recognize the four most common mechanism patterns.
The mechanism formula
The one loop I want running in your head
Every mechanism in this lesson, and honestly every mechanism for the rest of the course, comes down to the same five moves in the same order:
Find electrons → find the electron-poor site → draw arrows → track charges → check valence
If you ever feel lost in a mechanism, come back to this line and start at the left.
Key Terms
Tap any term to see its definition.
Section 1
What Is a Mechanism?
A reaction mechanism shows the step-by-step pathway of a reaction. It spells out which bonds break, which bonds form, where the electrons move, which intermediates appear, and why the product forms at all.
A mechanism is the electron-flow story between reactants and products
The framing I want you to keep
A mechanism is simply a drawing of a reaction. Specifically, it is a step-by-step explanation of how electrons move to turn reactants into products. Treat it as a story or a journey of electron movement.
Section 2
Electron-Rich vs Electron-Poor Sites
Before you can push a single arrow, you have to know where the electrons are. These are the electron-rich spots I scan for first in any molecule:
- Lone pairs.
- π bonds, such as the double bond of an alkene.
- Negative charges.
- Partially negative atoms (the δ− end of a polar bond).
Electron-rich: lone pairs, π bonds, negative charges
Electron-poor: positive and δ+ atoms
A polar bond creates a δ+ and a δ− end
The single rule the whole lesson rests on
Electrons move from electron-rich regions to electron-poor regions. That one sentence is the basis of every mechanism you will ever draw, so I want it locked in before we go further.
Section 3
Curved Arrows
This is the most important section in the lesson. A curved arrow shows electron movement, not atom movement. The tail starts where the electrons are, and the head points to where they end up.
Tail on the electrons, head on the destination
There are two kinds of arrowhead. A full arrowhead moves two electrons, and that is what we use almost all the time in intro organic chemistry. A half arrowhead, called a fishhook, moves a single electron and shows up in radical chemistry. For now, focus on the two-electron arrow.
Full arrow moves two electrons, fishhook moves one
Arrows usually start at
- ›A lone pair.
- ›A π bond.
- ›A σ bond that is breaking.
- ›A negative charge.
Arrows usually end at
- ›An atom.
- ›A bond.
- ›A positive or partially positive site.
- ›The direction a leaving group departs.
Common valid starts and ends
The mistake I will not let you make
Do not start an arrow from a positive charge. A positive site is electron-poor, so it accepts electrons, it never donates them. If your arrow tail is sitting on a plus sign, it is pointing the wrong way.
Wrong: an arrow must never start at a positive charge
Section 4
Nucleophiles and Electrophiles
These two words are the heart of every mechanism.A nucleophile is electron-rich and donates electrons. An electrophile is electron-poor and accepts them. The shortest way I can put it: nucleophile = electron-pair donor, electrophile = electron-pair acceptor.
Common nucleophiles
- ›Negatively charged species: OH⁻, RO⁻, CN⁻.
- ›Neutral species with lone pairs: H₂O, NH₃, ROH.
- ›π bonds in alkenes.
Electron-rich donors
Common electrophiles
- ›Positively charged atoms.
- ›Partially positive carbons (the δ+ end of a polar bond).
- ›Carbonyl carbons.
- ›Alkyl halides.
- ›A proton, H⁺.
Electron-poor acceptors
The one pattern under everything
Nucleophile attacks electrophile. Said more carefully, the way I want you thinking about it: an electron pair from the nucleophile forms a new bond to the electrophile. The arrow always runs from the rich site to the poor site.
Electron pair from the nucleophile forms a bond to the electrophile
Section 5
Tracking Formal Charge
Mechanisms fall apart fast if you stop tracking charge. The logic is simple: if an atom gains electrons it becomes more negative, and if it loses electrons it becomes more positive. Every bond you form or break changes a formal charge somewhere.
Charges must be valid before and after every step
The question I ask after every arrow
Does this atom still have a valid charge and a valid number of bonds after the electrons moved? I ask it every single time. This is exactly the formal-charge skill from Lesson 2, now doing real work inside a mechanism.
Section 6
Acid-Base Arrow Pushing
I always teach the acid-base step first, because it is the cleanest possible place to practice arrows. A base uses a lone pair to grab a proton, the H-A bond breaks, and those bonding electrons go back onto A. That is a complete proton transfer.
Base grabs H, the H-A electrons fall onto A
A habit I want you building now
Do not draw H⁺ simply vanishing with no arrow. Even in a proton transfer, those bonding electrons have to go somewhere, and showing where is what separates a real mechanism from a hand-wave.
Section 7
Leaving Groups
A leaving group is an atom or group that takes a bonding electron pair and departs. The ones that leave easily are stable on their own after they go, which usually means they are weak bases.
Good leaving groups
Weak leaving groups
The leaving group departs with the electron pair
My guiding rule for leaving groups
Weak bases are usually good leaving groups. It is not an absolute law, and not every weak base always leaves, but it is the rule I lean on first when I am deciding whether a group can take off.
Stable, weak bases leave more easily
Section 8
Bond Breaking and Bond Forming
Two basic moves run through every mechanism. When a bond breaks heterolytically, both electrons go to one atom, which is how you get ions like a carbocation and a leaving group. When a bond breaks homolytically, each atom keeps one electron, which gives radicals and uses fishhook arrows.
Heterolytic: both electrons to one atom
Homolytic: one electron to each atom
Bond formation is the mirror image. A lone pair or a π bond donates electrons toward an electron-poor atom and a new bond appears, like hydroxide donating into a partially positive carbon.
A donor's electrons form the new bond
My non-negotiable for every bond
For every bond formed or broken, show where the electrons came from and where they went. If you cannot point to both, the step is not finished.
Section 9
Intermediates vs Transition States
Keep this simple for now. An intermediate is a real species that forms between steps. Carbocations, carbanions, radicals, and the tetrahedral intermediate are all examples, and you can often draw them as actual structures. A transition state is a temporary, high-energy arrangement during a step, and it is not a stable species you can isolate or draw the same way.
Intermediates sit in valleys, transition states sit at peaks
The distinction to hold for now
An intermediate is real and can often be drawn as a structure. A transition state is not a stable structure. You do not need to draw transition states deeply yet, you just need to know which is which.
Section 10
The Four Common Patterns
Almost every mechanism you meet is built from a small set of repeating moves. Learn these four and you will start recognizing them inside bigger reactions instead of seeing each one as brand new.
Proton transfer
An acid-base step. A base grabs a proton and the bonding electrons fall back onto the atom that held the H.
Nucleophilic attack
A nucleophile donates an electron pair to an electrophile, forming a new bond between them.
Loss of leaving group
A bond breaks and the leaving group departs, taking the bonding electrons with it.
Rearrangement
Atoms or bonds shift to give a more stable intermediate. We will only touch this lightly here.
The mechanism mindset
A weak approach just says draw an arrow from nucleophile to electrophile. I want you asking better questions: where are the electrons, where is the electron-poor site, what bond forms, what bond breaks, where does the charge go, is every atom obeying valence, and is the leaving group reasonable? Ask those and you are thinking like a chemist.
Common Mistakes
Watch Out for These
Practice Set
Try each question before opening the answer.
1. What does a curved arrow represent: the movement of atoms or the movement of electrons?
Answer: Electrons. A full curved arrow shows two electrons moving from a source (lone pair, π bond, or bond being broken) to a destination.
2. Where does the tail of a curved arrow start?
Answer: On the electrons that are moving: a lone pair, a π bond, a σ bond that is breaking, or a negative charge. Never on an atom by itself and never on a positive charge.
3. In one sentence each, what is a nucleophile and an electrophile?
Answer: A nucleophile is an electron-rich electron-pair donor. An electrophile is an electron-poor electron-pair acceptor. The nucleophile's electrons form a new bond to the electrophile.
4. Why should you not draw an arrow starting from a positive charge?
Answer: A positive charge means the site is electron-poor. It accepts electrons, so arrows point toward it, not away from it.
5. In a proton transfer, where do the H-A bonding electrons go?
Answer: They fall back onto A, the atom that was holding the proton. The base takes the proton, and that bond's electrons stay behind on A.
6. Rank these as leaving groups from best to worst: OH⁻, Br⁻, I⁻.
Answer: I⁻ > Br⁻ > OH⁻. The weaker and more stable the base, the better the leaving group. I⁻ is the weakest base of the three, and OH⁻ is a poor leaving group.
7. What is the difference between heterolytic and homolytic bond cleavage?
Answer: Heterolytic cleavage sends both electrons to one atom, making ions. Homolytic cleavage gives one electron to each atom, making radicals, and uses fishhook arrows.
8. What is the difference between an intermediate and a transition state?
Answer: An intermediate is a real species that exists between steps and can often be drawn as a structure. A transition state is a fleeting high-energy arrangement during a step and is not a real, isolable species.
9. A nucleophile forms a bond to a carbon. What happens to that carbon's formal charge bookkeeping?
Answer: You re-evaluate it. Gaining a share of a new bonding pair can change the carbon's formal charge, and the atom that donated electrons may also change. Every step needs valid charges before and after.
10. Name the four common mechanism patterns from this lesson.
Answer: Proton transfer, nucleophilic attack, loss of a leaving group, and rearrangement.
Ready for a bigger set?
Work through the full Mechanism Fundamentals practice page: spotting nucleophiles and electrophiles, drawing curved arrows, fixing broken arrows, tracking charges, and naming the pattern in each step.
Do Mechanism Fundamentals PracticeLesson Summary
A mechanism is the electron-flow story behind a reaction. Electrons move from electron-rich sites (lone pairs, π bonds, negative charges) to electron-poor sites (positive and δ+ atoms), and curved arrows record that movement with the tail on the electrons and the head on the destination. Nucleophiles donate, electrophiles accept, leaving groups depart with their electrons, and formal charge has to stay valid at every step. Master the four patterns (proton transfer, nucleophilic attack, loss of leaving group, rearrangement) and run the same loop every time: find electrons, find the electron-poor site, draw arrows, track charges, check valence.