Why reactions need attacking reagents
A chemical reaction occurs when one substance is converted into another, and in organic chemistry this is accompanied by the breaking of some bonds and the making of others. The NIOS text frames the four broad reaction types — substitution, elimination, addition and molecular rearrangement — through the lens of reaction mechanism, the detailed sequence of steps by which reactant molecules change into products.
Bond breaking sets the stage. When a covalent bond undergoes heterolytic fission, the two shared electrons are taken unequally, producing ions: a carbon bearing a positive charge is a carbocation and one bearing a negative charge is a carbanion. The charged species generated this way are precisely the ones that initiate reactions, and the text classifies them as electrophiles and nucleophiles. (Homolytic fission, by contrast, splits the electrons equally to give neutral free radicals — covered in the sibling note on bond fission.)
For one of these reagents to attack, the molecule under attack must first develop polarity on some of its carbon atoms, created by the partial or complete displacement of bonding electrons. That polarity is what an electrophile or nucleophile "reads" before it commits. The reagent goes wherever the electron density complements its own need — and that single idea organises everything below.
What is an electrophile
An electrophile is an electron-deficient species, and it may be either positively charged or neutral. Because it is short of electrons, it is an electron-seeking species; the very name means "electron-loving." It therefore attacks a position of high electron density.
The NIOS examples span both charge types. Positively charged electrophiles include the proton $\ce{H+}$, the nitronium ion $\ce{NO2+}$, $\ce{Br+}$, $\ce{Cl+}$, the silver ion $\ce{Ag+}$ and the acylium ion $\ce{CH3CO+}$. Carbocations such as the ethyl cation $\ce{CH3CH2+}$ and the isopropyl cation $\ce{(CH3)2CH+}$ are also electrophiles, since the positive carbon hungers for electrons. Crucially, an electrophile need not carry a charge at all: $\ce{BF3}$ is a neutral electrophile because its boron atom has an incomplete octet and accepts an electron pair.
Is $\ce{AlCl3}$ an electrophile or a nucleophile?
Aluminium in $\ce{AlCl3}$ has only six electrons around it — an incomplete octet — so the molecule readily accepts a lone pair. It is electron-deficient and therefore an electrophile, exactly analogous to $\ce{BF3}$. This is why $\ce{AlCl3}$ serves as the Lewis-acid catalyst in Friedel–Crafts reactions.
What is a nucleophile
A nucleophile is a negatively charged species or an electron-rich neutral species. The name means "nucleus-loving": carrying a lone pair or an excess of electrons, it is drawn to centres of positive charge and attacks a position of low electron density.
The NIOS examples again cover both charge types. Anionic nucleophiles include the hydroxide ion $\ce{OH-}$, the nitrite ion $\ce{NO2-}$, the cyanide ion $\ce{CN-}$, alkoxide ions such as $\ce{C2H5O-}$ and the ethanoate ion $\ce{CH3COO-}$. Neutral nucleophiles carry a usable lone pair: water $\ce{H2O}$ and ammonia $\ce{:NH3}$ both donate, despite having no formal charge. In a substitution reaction a haloalkane is converted into many products by replacing the halogen with nucleophiles such as $\ce{-OH}$, $\ce{-NH2}$, $\ce{-CN}$, $\ce{-SH}$ or $\ce{-OR'}$.
Charge does not decide the label — electron richness does
A common error is to assume "positive = electrophile, negative = nucleophile, neutral = neither." But classification depends on whether the species seeks or donates electrons, not on charge. Neutral $\ce{BF3}$ and $\ce{AlCl3}$ are electrophiles; neutral $\ce{H2O}$ and $\ce{:NH3}$ are nucleophiles. The same species can even be re-read in a new context — water is a nucleophile through its oxygen lone pairs, yet $\ce{H3O+}$ behaves as an electrophile through its proton.
Ask only one question: does it want electrons (electrophile) or supply them (nucleophile)?
Electrophiles vs nucleophiles: master table
Every property below traces back to one root difference — an electrophile is short of electrons and a nucleophile is rich in them. The table collects the definitions, charge possibilities, NIOS examples and the Lewis analogy in one place.
| Feature | Electrophile | Nucleophile |
|---|---|---|
| Electron status | Electron-deficient; electron-seeking | Electron-rich; electron-donating |
| Electron pair | Accepts a pair of electrons | Supplies a pair of electrons |
| Charge possibilities | Positive or neutral | Negative or neutral |
| Attacks at | Position of high electron density | Position of low electron density |
| Charged examples | H+, NO2+, Br+, Cl+, Ag+, CH3CO+, carbocations |
OH-, CN-, NO2-, C2H5O-, CH3COO- |
| Neutral examples | BF3, AlCl3 |
H2O, :NH3 |
| Lewis analogy | Lewis acid (electron-pair acceptor) | Lewis base (electron-pair donor) |
| Typical reaction role | Attacks electron-rich sites (e.g. benzene ring in nitration) | Attacks electron-poor carbon (e.g. C–X in a haloalkane) |
These reagents are born from bond breaking. See Homolytic & Heterolytic Fission for how carbocations, carbanions and free radicals first appear.
The Lewis acid–base link
The electrophile–nucleophile pairing is the organic chemist's restatement of the Lewis acid–base concept. A Lewis acid accepts a pair of electrons; that is exactly what an electrophile does. A Lewis base donates a pair of electrons; that is exactly what a nucleophile does. The molecular rearrangement of 1-chlorobutane to 2-chlorobutane in the presence of $\ce{AlCl3}$, quoted in the NIOS text, is driven by $\ce{AlCl3}$ acting as a Lewis acid — that is, as an electrophilic catalyst.
The two vocabularies are not identical in emphasis, however. The Lewis labels describe the electron transfer in the abstract, whereas "electrophile" and "nucleophile" add the picture of an attacking reagent approaching a reaction centre in a substrate. The underlying electron-pair behaviour is the same; the organic terms simply foreground motion and a target.
Electrophiles are positively charged or electron-deficient species. Nucleophiles are negatively charged or electron-rich species. — NIOS, summary of Section 23.3
Curved arrows: showing the attack
Mechanisms are drawn with curved arrows, and the convention is unambiguous: a curved arrow tracks the movement of an electron pair, starting at the electron source and ending where the pair settles. Because the nucleophile is the electron supplier and the electrophile is the electron seeker, the arrow always runs from the nucleophile to the electrophile. It never points the other way.
A concrete instance is nucleophilic substitution at a haloalkane: $\ce{R-X + Nu^- -> R-Nu + X^-}$. Here the nucleophile donates its pair to the carbon bearing the halogen, the halide leaves with the bonding pair, and one curved arrow runs nucleophile-to-carbon while a second runs carbon-to-halide. The direction is fixed by which partner is rich and which is poor.
Where each reagent attacks
Because each reagent looks for the complementary electron density, the same logic predicts where a reaction happens. A few canonical NIOS reactions make the pattern concrete.
| Reaction | Attacking reagent | Site attacked | Why |
|---|---|---|---|
| Nucleophilic substitution of a haloalkane | Nucleophile (OH-, CN-, …) |
Carbon bearing the halogen | That carbon is electron-poor; the halogen pulls bonding electrons away |
| Nitration of benzene | Electrophile (NO2+) |
The aromatic ring | The ring is electron-rich, so it draws the electron-seeking reagent |
| Addition to an alkene (e.g. $\ce{+ HX}$) | Electrophile (e.g. $\ce{H+}$ from $\ce{HX}$) | The C=C double bond | The π bond is a region of high electron density |
Classify each species and predict its target: $\ce{NO2+}$, $\ce{CN-}$, $\ce{H2O}$, $\ce{BF3}$.
$\ce{NO2+}$ — positively charged, electron-deficient: an electrophile; attacks electron-rich sites such as a benzene ring. $\ce{CN-}$ — negatively charged, electron-rich: a nucleophile; attacks an electron-poor carbon. $\ce{H2O}$ — neutral but carries oxygen lone pairs: a nucleophile. $\ce{BF3}$ — neutral with an incomplete octet on boron: an electrophile. The verdict follows from electron richness in every case.
Classification traps
The NIOS in-text exercise asks you to sort a mixed list, and that list is engineered to expose two recurring mistakes. The first is reading charge instead of electron density; the second is overlooking that one species may attack through more than one atom.
Ambident nucleophiles attack through either of two atoms
An ambident nucleophile carries two different donor sites and can bond through either, giving two different products from the same reagent. The nitrite ion $\ce{NO2-}$ can donate through nitrogen (forming a nitro compound) or through oxygen (forming a nitrite ester). The cyanide ion $\ce{CN-}$ likewise donates through carbon (giving a nitrile) or through nitrogen (giving an isonitrile). The species is still one nucleophile — only the site of attack changes, and so does the product.
In the NIOS list, NO2+ is an electrophile but NO2- is a (ambident) nucleophile — the sign of the charge flips the whole classification.
Worth a separate note: the precise structures of the nitrite-ester versus nitro product, and of the nitrile versus isonitrile, are developed in the dedicated reaction-mechanism material. For NEET-level classification, the load-bearing fact is that an ambident nucleophile is still a nucleophile and that $\ce{NO2-}$ and $\ce{CN-}$ are its standard examples.
Five points to lock in
- Electrophile = electron-deficient, electron-seeking; positive or neutral; attacks high electron density. Examples: $\ce{H+}$, $\ce{NO2+}$, $\ce{BF3}$, $\ce{AlCl3}$, carbocations.
- Nucleophile = electron-rich, electron-donating; negative or neutral; attacks low electron density. Examples: $\ce{OH-}$, $\ce{CN-}$, $\ce{NO2-}$, $\ce{H2O}$, $\ce{:NH3}$.
- Classify by electron richness, never by charge alone — neutral $\ce{BF3}$ is an electrophile, neutral $\ce{H2O}$ is a nucleophile.
- Electrophile = Lewis acid (accepts an electron pair); nucleophile = Lewis base (donates an electron pair).
- The curved arrow always runs nucleophile → electrophile; an ambident nucleophile such as $\ce{CN-}$ or $\ce{NO2-}$ can attack through either of two atoms.