Nucleophile, Leaving Group and Solvent

Keep the substrate fixed, then change the leaving group, the nucleophile or the solvent. Each one can change the rate, and sometimes the product.

Leaving groups

A good leaving group is stable on its own, so it is a weak base. The order $I^{-} > B r^{-} > C l^{-} ≫ F^{-}$ matches the fall in acid strength from HI to HF. OH is a poor leaving group until acid turns it into $H_{2} O$ .

Nucleophiles

Charge: $O H^{-}$ is stronger than $H_{2} O$ .
Resonance: spreading the charge weakens the attack, so $O H^{-} > P h O^{-} > C H_{3} C O O^{-} > C l O_{4}^{-}$ . This was asked in January 2026.
Across a period: nucleophilicity falls with basicity, so $C H_{3}^{-} > N H_{2}^{-} > O H^{-} > F^{-}$ .
Bulk: tert-butoxide is a strong base but a poor nucleophile.

Solvent

In protic solvents, small ions are caged by hydrogen bonds.

Type	Examples	Effect
Polar protic	$H_{2} O$ , ROH, $C H_{3} C O O H$	Stabilises ions, so favours $S_{N} 1$ . Cages small anions: $I^{-} > B r^{-} > C l^{-} > F^{-}$ .
Polar aprotic	Acetone, DMF, DMSO	Leaves anions free, so favours $S_{N} 2$ . Order becomes $F^{-} > C l^{-} > B r^{-} > I^{-}$ .

Reactions with common nucleophiles

Reagent	Product from RX
Aqueous NaOH or KOH	Alcohol, ROH
$H_{2} O$	Alcohol, ROH
NaOR'	Ether, ROR' (Williamson synthesis)
NaI	Iodide, RI
$N H_{3}$	Amine, $R N H_{2}$ , and further alkylated amines
KCN	Nitrile, RCN
AgCN	Isonitrile, RNC
$K N O_{2}$	Alkyl nitrite, R-O-N=O
$A g N O_{2}$	Nitroalkane, $R N O_{2}$
R'COOAg	Ester, R'COOR
$L i A l H_{4}$	Alkane, RH

Ambident nucleophiles

Cyanide can attack through carbon or nitrogen, and nitrite through nitrogen or oxygen. Ionic salts free the anion, and the carbon end of cyanide forms the stronger bond. AgCN is largely covalent with Ag bonded to C, so nitrogen is the free end.

Watch the animation: KCN or AgCN? Which end attacks

Catalysis by iodide

A trace of NaI speeds up slow hydrolysis of an alkyl chloride. Iodide displaces chloride, then water displaces iodide faster than it would chloride, and iodide comes back. This needs iodide to be both a good nucleophile and a good leaving group.

Worked example: same halide, six reagents

What does 1-bromobutane give with (a) aqueous KOH, (b) alcoholic KOH, (c) KCN, (d) AgCN, (e) $A g N O_{2}$ , (f) $C H_{3} C O O A g$ ?

Show the solution

(a) Butan-1-ol by substitution.
(b) But-1-ene by elimination.
(c) Pentanenitrile, with one more carbon.
(d) Butyl isocyanide.
(e) 1-Nitrobutane.
(f) Butyl acetate.

Note that (c) and (d) differ only in the metal, and that (a) and (b) differ only in the solvent.

Beyond JEE: neighbouring groups

A lone pair or $π$ bond inside the molecule can push out the leaving group before any outside nucleophile arrives. This can speed up a reaction and can keep the configuration, because two inversions in a row give retention. Clayden covers it under participation. NCERT does not.

In 60 seconds