Nucleophile and electrophile: which is which
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.
Look for the electrons
Nucleophiles have a lone pair, a negative charge, or a π bond — something loosely held and available. Hydroxide, amines, cyanide, an alkene. Electrophiles have an empty orbital, a positive charge, or a bond polarised so one atom is electron-poor. A carbonyl carbon, a carbocation, the carbon of an alkyl halide.
Many species are both, depending on what they meet. Water is a nucleophile toward a carbocation and an electrophile toward nothing much at all; a carbonyl is electrophilic at carbon and nucleophilic at oxygen. The label describes a role in a particular reaction, not a permanent property.
Strong is not the same as basic
Nucleophilicity is about how fast something attacks; basicity is about how strongly it holds a proton. They usually track together and they are not the same axis, which is why they can be traded off deliberately.
Down a group, nucleophilicity generally rises while basicity falls — iodide is a better nucleophile and a weaker base than fluoride. That gap is exactly what lets a chemist pick a reagent that substitutes without eliminating, or eliminates without substituting.
Where this stops being simple
- Nucleophilicity depends heavily on solvent. In a protic solvent the smaller ions are held tightly in a shell of hydrogen bonds and slowed down; in an aprotic one that ordering can reverse entirely.
- Calling something a nucleophile does not predict a rate. It says which role it plays if the reaction happens.
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 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.
What do the curly arrows in a mechanism actually mean?
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.
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.