On this page
Every polar organic reaction is a meeting between something that has an electron pair to give and something that can take it. Name those two partners correctly and the curved arrows almost draw themselves. Name them by charge alone and you will get the alkene, the carbonyl and BF₃ wrong. This post gives a two-question test and applies it to five reactions from your NCERT books.
What do the terms actually mean?
The NCERT Class 11 chapter Organic Chemistry: Some Basic Principles and Techniques defines them by what they do with electrons:
A nucleophile (Nu:) is a reagent that brings an electron pair to the reactive site. The name means "nucleus seeking".
An electrophile (E⁺) is a reagent that takes away an electron pair. The name means "electron seeking".
NCERT's examples of nucleophiles include negative ions with lone pairs, such as HO⁻, NC⁻ and carbanions, and neutral molecules with lone pairs, such as H₂O, R₃N and R₂NH. Its electrophiles include carbocations and neutral molecules with polar groups, such as the carbonyl group and alkyl halides, where the carbon carries a partial positive charge. NCERT also notes that π electrons in a double bond can supply electrons.
The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus list "electrophiles and nucleophiles" with bond fission and carbocations, and the CBSE 2026-27 curriculum includes fundamental concepts in organic reaction mechanism. Check your own year's syllabus.
Why is charge not enough?
Anzovino and Bretz (2015) studied second-semester organic chemistry students and found that they "prioritize structure over function", relying mainly on charges to define and identify nucleophiles and electrophiles. Many needed to know the mechanism before they could say which species was which, the reverse of how chemists use the ideas. The fix is to look for function: where is an electron pair available, and where is there room for one?
Worked examples: five reactions, one method
1. Bromomethane with hydroxide. CH₃–Br + OH⁻ → CH₃–OH + Br⁻
Electron pair: the lone pairs on the negative oxygen of OH⁻. Nucleophile.
Room for the pair: bromine is more electronegative than carbon, so the carbon is δ+, and Br⁻ can leave with the C–Br bonding pair. Electrophile: that carbon.
2. Ethene with hydrogen bromide. CH₂=CH₂ + HBr → CH₃–CH₂Br. NCERT's Hydrocarbons chapter describes this as electrophilic addition in which HBr "provides an electrophile, H⁺".
Step 1: the alkene has no charge and no lone pair, but its π bond is the electron pair. The alkene is the nucleophile, and H⁺ from HBr is the electrophile. A carbocation forms.
Step 2: the carbocation has only six electrons around the positive carbon, so it is the electrophile. Br⁻, with its lone pairs and negative charge, is the nucleophile.
The species derived from the two reactants swap roles between steps. That is why you name partners for each step, not for the whole reaction.
3. Nitration of benzene. NCERT explains that the electrophile is the nitronium ion, NO₂⁺, produced from nitric and sulphuric acids. The benzene ring's π electrons are the nucleophile.
4. Ammonia with boron trifluoride. NH₃ + BF₃ → H₃N–BF₃
Count electrons. Nitrogen in NH₃ has three bond pairs and one lone pair: 8 electrons with a pair to give. Nucleophile.
Boron in BF₃ has three bond pairs: only 6 electrons. NCERT's Problem 8.12 classifies BF₃ as an electrophile for this reason, even though it is neutral.
5. Ethanal with cyanide. NCERT's Problem 8.13 marks the carbonyl carbon of CH₃CHO as an electrophilic centre because of the polarity of C=O. Cyanide, with a negative charge and a lone pair on carbon, is the nucleophile. The π electrons of C=O move onto oxygen as CN⁻ bonds to carbon. Our post on aldehydes and ketones follows this reaction further.
How do you check your answer with NCERT's own list?
NCERT's Problem 8.12 asks you to sort eight species. Apply the two questions before you look at its answer:
HS⁻, C₂H₅O⁻, (CH₃)₃N: and H₂N⁻ each have a lone pair to donate, so all four are nucleophiles.
BF₃, Cl⁺, CH₃–C⁺=O and NO₂⁺ each have an atom with only six valence electrons or a positive centre that can accept a pair, so all four are electrophiles.
Four and four. Notice that (CH₃)₃N is neutral and still a nucleophile, and BF₃ is neutral and still an electrophile.
Which mistakes cost marks?
"Negative means nucleophile, positive means electrophile, neutral means neither." Neutral alkenes, amines and water are nucleophiles; neutral BF₃ and carbonyl carbons are electrophiles.
Labelling the whole molecule instead of the atom. In CH₃Br the electrophile is the carbon, not the bromine.
Drawing arrows from the electrophile. Curved arrows start at an electron pair, on the nucleophile or in a bond that breaks, never at an empty orbital.
Fixing roles for the whole reaction. In multi-step mechanisms, re-ask the two questions at every step.
Nucleophile strength and SN1 versus SN2 build on this; see our guide to haloalkanes and substitution.
When you get stuck partway through a mechanism, Fermi's AI tutor notices where you are stuck and decides whether to explain, ask or show, so you work out which partner has the electron pair yourself.
A practice routine
Day 1: Take ten reagents from your NCERT chapters and sort them with the two questions, writing the reason for each.
Day 2: For five reactions, circle the nucleophilic atom and box the electrophilic atom before drawing any arrow.
Day 3: Two multi-step mechanisms, naming the partners again at each step.
Day 4: Explain to a friend why ethene is a nucleophile in under a minute.
Next time you open a mechanism, cover the answer and name both partners for every step first.
