Difficulty: Intermediate | Prerequisites: General chemistry acid-base concepts, Lewis structures, electronegativity trends
Big picture: Nucleophilic substitution and elimination reactions are the backbone of Organic Chemistry I. Before you can predict whether a reaction follows SN1, SN2, E1 or E2, you need to classify the nucleophile, the base and the leaving group. This set of notes covers exactly that: which species are strong nucleophiles, which are strong bases, which groups leave well, and the reasoning behind each ranking. If you can sort these confidently, mechanism prediction becomes far more systematic.
Nucleophiles are electron-rich species that donate electron pairs to form new bonds. Leaving groups are the species that depart with the bonding electrons during a substitution or elimination. Strong bases readily accept protons but are not always good nucleophiles. The central rule connecting all three: weaker bases make better leaving groups.
Nucleophile
An electron-rich compound that donates a pair of electrons to an electron-deficient compound, forming a covalent bond. Think of it as the attacker in a substitution reaction: it brings electrons to the party.
Electrophile
An electron-deficient species that accepts an electron pair from a nucleophile. In simple terms, it is the target the nucleophile attacks.
Leaving group
The atom or group that departs from the substrate during a substitution or elimination reaction, taking the bonding electrons with it. Think of it as the piece that gets kicked out so the nucleophile can take its place.
Strong base
A species that readily accepts a proton (H+) from a substrate, typically promoting elimination over substitution. In simple terms, strong bases are hungry for protons rather than for carbon electrophiles.
Nucleophilicity
A kinetic measure of how rapidly a species attacks an electrophilic carbon. This is distinct from basicity, which measures thermodynamic affinity for a proton.
Basicity
A thermodynamic measure of a species' affinity for a proton. In simple terms, how much a species wants to grab an H+ rather than attack a carbon centre.
Tosylate (TsO⁻)
The conjugate base of para-toluenesulfonic acid. A common excellent leaving group used in organic synthesis because it is a very weak base and therefore stable once it departs.
Mesylate (MsO⁻)
The conjugate base of methanesulfonic acid. Like tosylate, it is an excellent leaving group for the same reason: weak basicity and high stability after departure.
Nucleophiles are ranked by how readily they donate electrons to an electrophilic carbon. The tiers below appear on virtually every Organic Chemistry I exam.
Excellent nucleophiles
N₃⁻ (azide)
CN⁻ (cyanide)
HS⁻ (hydrogen sulfide ion)
RS⁻ (thiolate)
RO⁻ (alkoxide)
HC≡C⁻ (acetylide)
These are all negatively charged with lone pairs on relatively polarisable or small atoms. They attack electrophilic carbon quickly and cleanly.
Good nucleophiles
F⁻, Cl⁻, Br⁻, I⁻ (halides)
RCOO⁻ (carboxylate)
R₃N, R₂NH, RNH₂ (amines)
Charged halides are decent nucleophiles. Amines are neutral but still nucleophilic because nitrogen has a lone pair and is not too electronegative.
Poor nucleophiles
H₂O
R-OH (alcohols)
RCOOH (carboxylic acids)
(CH₃)₃CO⁻ (tert-butoxide)
Water and alcohols are neutral and weakly nucleophilic. Tert-butoxide is an interesting case: it is a strong base but a poor nucleophile because its bulky methyl groups block access to the electrophilic carbon.
Non-nucleophilic species
H⁻ (hydride)
HSO₄⁻ (hydrogen sulfate)
TsO⁻ (tosylate)
These either lack a suitable lone pair for carbon attack or are too stabilised to donate electrons effectively. H⁻ is a powerful base but does not behave as a nucleophile in the usual sense because it abstracts protons rather than attacking carbon.
Strong bases are species with a high affinity for protons. In substitution and elimination chemistry, a strong base favours elimination (E2) over substitution (SN2) because it pulls a proton from the substrate rather than attacking the electrophilic carbon.
The strong bases from this set of notes:
HO⁻ (hydroxide)
RO⁻ (alkoxide, e.g. CH₃O⁻, (CH₃)₃CO⁻)
NH₂⁻ (amide ion)
H⁻ (hydride)
R⁻ (carbanion)
NR₂⁻ (dialkylamide, e.g. LDA)
(CH₃)₃CO⁻ (tert-butoxide)
HC≡C⁻ (acetylide)
Nucleophilicity vs. basicity: the key distinction
Basicity is thermodynamic (how much a species wants a proton). Nucleophilicity is kinetic (how fast a species attacks carbon). Some species are both strong bases and strong nucleophiles (e.g. HO⁻, CH₃O⁻). Others are strong bases but poor nucleophiles because of steric bulk (e.g. tert-butoxide, LDA). This distinction is one of the most commonly tested points in Organic Chemistry I.
The golden rule: weaker base = better leaving group. A species that is stable after departure (i.e. comfortable holding the extra electrons) leaves easily. A species that is a strong base clings to the substrate and resists departure.
Excellent leaving groups
I⁻ (iodide)
Br⁻ (bromide)
Cl⁻ (chloride)
TsO⁻ (tosylate)
MsO⁻ (mesylate)
H₂O (water, after protonation of -OH)
All of these are weak bases. The halides increase in leaving-group ability going down the periodic table (I⁻ > Br⁻ > Cl⁻) because larger atoms stabilise the negative charge over a greater volume. Tosylate and mesylate are excellent because their conjugate acids are very strong, meaning they are extremely weak bases.
Good leaving groups
R₃N (tertiary amine, as a neutral departing species)
CH₃COO⁻ (acetate)
Poor leaving groups
HO⁻ (hydroxide)
RO⁻ (alkoxide)
F⁻ (fluoride)
Hydroxide and alkoxide are strong bases, so they resist leaving. Fluoride is small and holds its charge tightly, making it a poor leaving group despite being a reasonable nucleophile. This is a frequently tested comparison.
Non-leaving groups
NH₂⁻ (amide)
H⁻ (hydride)
R⁻ (carbanion)
These are all very strong bases. They will not leave under normal reaction conditions. If you see one of these on a substrate, substitution at that position will not happen without converting it to a better leaving group first.
Students often assume that a strong base is automatically a strong nucleophile. It is not. Tert-butoxide ((CH₃)₃CO⁻) and LDA (lithium diisopropylamide) are strong bases but poor nucleophiles because steric bulk prevents them from reaching the electrophilic carbon.
Students frequently rank fluoride as a good leaving group because it is a good nucleophile. Fluoride is a poor leaving group. Its small size means it holds charge tightly, making it reluctant to leave.
A common error is treating H⁻ (hydride) as a nucleophile. In organic chemistry contexts, H⁻ is classified as a strong base and a non-nucleophilic species. It abstracts protons; it does not attack carbon.
Students sometimes forget that -OH is a poor leaving group until it is protonated. Once protonated to H₂O, it becomes an excellent leaving group. This is why acid-catalysed substitution reactions work.
⚠️ Classifying the nucleophile, base and leaving group is the first step in every SN1/SN2/E1/E2 mechanism question. Get this wrong and the entire analysis falls apart.
⚠️ "Strong, bulky base" = E2. "Strong, unhindered nucleophile" = SN2. Examiners test whether you can distinguish the two.
⚠️ Expect a question asking you to rank leaving groups by ability. The answer follows directly from base strength: weaker base, better leaving group.
⚠️ The tert-butoxide trap is a staple exam question: students see a strong base and predict substitution, but tert-butoxide's bulk forces elimination.
⚠️ Protonation converting a poor leaving group (-OH) into an excellent one (H₂O) appears regularly in acid-catalysed mechanism questions.
True or false: A strong base is always a strong nucleophile.
Fill in the blank: The rule for leaving groups is that a ______ base makes a better leaving group.
True or false: Fluoride (F⁻) is an excellent leaving group.
Fill in the blank: Protonating -OH converts it to ______, which is an excellent leaving group.
True or false: Tert-butoxide ((CH₃)₃CO⁻) favours elimination over substitution.
Answers: 1. False (steric bulk can make a strong base a poor nucleophile). 2. Weaker. 3. False (F⁻ is a poor leaving group). 4. H₂O. 5. True.
Q: Rank the following as leaving groups from best to worst: F⁻, I⁻, Br⁻, NH₂⁻.
A: I⁻ > Br⁻ > F⁻ > NH₂⁻. Iodide is the weakest base and therefore the best leaving group. NH₂⁻ is a very strong base and a non-leaving group.
Q: A substrate has (CH₃)₃CO⁻ as the reagent. Will this favour SN2 or E2? Explain.
A: E2. Tert-butoxide is a strong base but a poor nucleophile due to steric bulk. It will abstract a proton rather than attack the electrophilic carbon.
Q: Why is -OH a poor leaving group, and what can be done to improve it?
A: Hydroxide is a strong base, so it resists departure. Protonating the -OH group under acidic conditions converts it to H₂O, which is a weak base and an excellent leaving group.
Q: CN⁻ is listed as an excellent nucleophile. Is it also a strong base?
A: CN⁻ is a moderate base but an excellent nucleophile. Its nucleophilicity is high because carbon and nitrogen both have accessible lone pairs and the ion is not sterically hindered. This is a case where nucleophilicity and basicity do not track together.
Q: Explain why I⁻ is a better leaving group than F⁻, even though fluorine is more electronegative.
A: Leaving-group ability correlates with base strength, not electronegativity alone. I⁻ is a much weaker base than F⁻ because the negative charge is spread over a much larger atom. Weaker base means better leaving group.
This material feeds directly into SN1, SN2, E1 and E2 mechanism selection. Once you can classify the nucleophile, base and leaving group, the decision flowchart for choosing the correct mechanism becomes straightforward. It also connects to acid-base chemistry from general chemistry, since leaving-group ability is fundamentally a question of conjugate-base stability.
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