How to read a reaction mechanism
Each curly arrow means one pair of electrons moved: the tail sits where the pair is now, the head where it goes. Nothing else about the drawing is arbitrary, which is why a mechanism can be checked rather than memorised.
What an arrow is claiming
An arrow starts at electrons — a lone pair or a bond — and never at a positive charge or an atom. It ends where those electrons finish: at an atom that is gaining a lone pair, or between two atoms that are gaining a bond.
Two arrows in the same step mean two pairs moved at once, which is what makes a step concerted rather than sequential. A carbon that gains a bond must lose one in the same step, because it cannot hold five.
Checking one rather than believing it
Count the atoms on each side; they must match. Count the charge; it must balance, though a step can differ by one if a proton was transferred by a base too ordinary to have been drawn. Check that no carbon has five bonds at any point.
Those three checks catch most mechanisms that are wrong, including most that look plausible. They are also the checks this platform runs: a pathway that does not conserve atoms, or does not arrive at the recorded product, is not offered at all.
Where this stops being simple
- A mechanism drawn here is rule-derived: a documented pathway for that reaction class, applied to your structures and checked that it arrives. It is not a measurement of what the electrons did, and where two pathways both arrive, drawing one is not evidence against the other.
- Arrows are a model. They describe bonding changes well and say nothing about timing, energy, or the fact that a real transition state is a continuum rather than a snapshot.
See it happen
Other concepts
What is the difference between SN1 and SN2?
Both replace a leaving group with a nucleophile, and they differ in whether the bond breaks before the new one forms. SN2 does it in one motion and turns the carbon inside out; SN1 lets the leaving group go first, makes a carbocation, and whatever attacks it can attack from either face.
What is a leaving group, and why are some better than others?
A leaving group is whatever leaves taking the bonding electrons with it. How good it is comes down to one question: how comfortable is it holding that negative charge once it has gone?
What is the difference between a nucleophile and an electrophile?
A nucleophile has electrons to give and an electrophile has somewhere to put them. Every curly arrow in every mechanism runs from the first to the second, which is why getting this right makes mechanisms readable rather than memorised.
What is Markovnikov's rule?
Adding HX across an alkene puts the hydrogen on the carbon that already has more of them, and the X on the other. The memorable phrasing hides the actual reason, which is that the reaction goes through whichever carbocation is more stable.
What is a functional group?
A functional group is an arrangement of atoms that behaves roughly the same way wherever it appears. It is why a chemist can look at an unfamiliar molecule with an ester in it and already know several things it will do.
Everything here is open without an account. If you want to try the idea rather than read it, the mechanisms run on structures you supply and prediction will tell you what it thinks a flask would give, with the evidence it is reasoning from attached.