Difficulty: Intermediate | Prerequisites: CHEM 211 or equivalent (nucleophilic substitution, acid-base equilibria, resonance structures, functional group identification).
Organic reactions boil down to electron-rich species (nucleophiles) attacking electron-poor species (electrophiles). This topic explores how you can dial that reactivity up or down by attaching electron-donating groups (EDGs) or electron-withdrawing groups (EWGs) to the molecule. It sits at the heart of Organic Chemistry II because predicting which carbonyl derivative reacts fastest, or which benzene ring is more activated, depends entirely on understanding these electronic effects. You should already be comfortable drawing resonance structures and identifying lone pairs, partial charges, and pi bonds.
Electron-donating groups stabilise electrophiles (making them less reactive) and destabilise nucleophiles (making them more reactive). Electron-withdrawing groups do the opposite. Resonance effects generally outweigh inductive effects, and this principle lets you rank carbonyl derivatives from most to least reactive: acid chloride > anhydride > aldehyde > ketone > ester ≈ carboxylic acid > amide.
Nucleophile
An electron-rich species that donates a pair of electrons to form a new bond. It carries a full or partial negative charge, a lone pair, or a pi bond. Think of it as the molecule that "loves nuclei" and seeks out positive centres.
Electrophile
An electron-poor species that accepts a pair of electrons to form a new bond. Typical examples include carbocations, carbonyl carbons, and alkyl halide carbons. In simple terms, it is the molecule hungry for electrons.
Electron-Donating Group (EDG)
A substituent that pushes electron density toward the rest of the molecule, either through its sigma bonds (inductive donation) or by sharing a lone pair into the pi system (resonance donation). Think of it as a group that makes a nearby atom more electron-rich.
Electron-Withdrawing Group (EWG)
A substituent that pulls electron density away from the rest of the molecule, by electronegativity through sigma bonds (inductive withdrawal) or by delocalising pi electrons toward itself (resonance withdrawal). In simple terms, it starves a nearby atom of electrons.
Inductive Effect
The shift of electron density through sigma bonds caused by differences in electronegativity. It weakens with distance (falls off rapidly after two or three bonds). Think of it as a gentle tug on electrons along the chain.
Resonance Effect
The delocalisation of electron density through the pi system, capable of placing full positive or negative charges on atoms several bonds away. It is typically more significant than the inductive effect because it can redistribute entire electron pairs across a conjugated framework.
Most organic reactions are polar reactions: a nucleophile donates electrons to an electrophile. Every time you look at a new molecule, your first job is to identify potential reactive sites.
Nucleophilic sites: atoms with lone pairs, pi bonds, or negative charge (e.g. the oxygen of an alkoxide, the pi bond of an alkene)
Electrophilic sites: atoms with positive or partial positive charge, or electron-poor bonds (e.g. the carbon of a carbonyl, a carbocation, the carbon of an alkyl halide)
A poor nucleophile can be converted into a good one by deprotonating it with a strong base. For example, methanol (CH₃OH) is a poor nucleophile, but treating it with NaH, NaNH₂, or NaC≡CH generates methoxide (CH₃O⁻), which is a strong nucleophile. The negative charge makes the oxygen far more willing to donate electrons.
A poor electrophile can be activated by protonating it with acid. A ketone on its own is a relatively poor electrophile, but treating it with HCl or H₂SO₄ protonates the carbonyl oxygen. This places a formal positive charge on oxygen, pulling even more electron density away from the carbonyl carbon and making it a much better target for nucleophilic attack.
The carbonyl group (C=O) is the single most important functional group to understand for reactivity. Drawing its resonance structures reveals that oxygen carries partial negative charge (the nucleophilic atom) and carbon carries partial positive charge (the electrophilic atom). When a carbonyl is treated with a nucleophile, the nucleophile attacks the electrophilic carbon. Nucleophilic addition is favoured when the departing oxygen anion (pKₐ of its conjugate acid ≈ 16) is a weaker base than the incoming nucleophile (i.e. the nucleophile's conjugate acid has pKₐ > 16).
These four statements are the backbone of this entire topic. Memorise them and you can reason through almost any reactivity ranking.
Withdrawing groups make electrophiles less stable and more reactive. An EWG pulls electron density away from an already electron-poor centre. The electrophile becomes even more desperate for electrons, so it reacts faster.
Donating groups make electrophiles more stable and less reactive. An EDG feeds electron density into the electron-poor centre, partially satisfying it. The electrophile is calmer, so it reacts more slowly.
Donating groups make nucleophiles less stable and more reactive. An EDG pushes extra electron density onto an already electron-rich species. The nucleophile becomes more eager to offload electrons, so it reacts faster.
Withdrawing groups make nucleophiles more stable and less reactive. An EWG siphons electron density away from the nucleophilic site, making it less willing to donate. The nucleophile is stabilised but sluggish.
The underlying logic is consistent: stability and reactivity are inversely related. Anything that stabilises a reactive species slows it down; anything that destabilises it speeds it up.
Both inductive and resonance effects can donate or withdraw electron density, but they work through different mechanisms and differ in magnitude.
Operate through sigma bonds
Driven by electronegativity differences
Fall off rapidly with distance (negligible past two or three bonds)
Relatively weak compared to resonance
Example: Cl is an inductive withdrawer; alkyl groups (CH₃) are weak inductive donors
Operate through the pi system
Can place full formal charges on atoms
Reach across an entire conjugated framework
Typically stronger than inductive effects
Example: –NH₂ and –OCH₃ are resonance donors; –NO₂ and –C(=O)R are resonance withdrawers
Acids are electrophiles (they donate H⁺). An EWG stabilises the conjugate base (making the acid stronger, lower pKₐ), while an EDG destabilises the conjugate base (making the acid weaker, higher pKₐ).
Acid | Substituent | Type | pKₐ |
|---|---|---|---|
Trichloroacetic acid | –CCl₃ | EWG (inductive) | 0.70 |
Formic acid | –H | Neither | 3.75 |
Glycolic acid | –CH₂OH (HO–) | Weak EDG | 3.85 |
Acetic acid | –CH₃ | EDG (inductive) | 4.76 |
Glycolic acid variant | –OH (HO–) | EDG (resonance) | 6.36 |
The resonance donor (–OH directly on the carboxylate framework) raises the pKₐ by roughly 1000-fold compared to the inductive donor (–CH₃). This confirms that resonance effects dominate.
Phenols follow the same logic, and the substituent effects can be cleanly separated into inductive and resonance contributions.
Phenol Derivative | Substituent | Effect Type | pKₐ |
|---|---|---|---|
p-Nitrophenol | –NO₂ | EWG (resonance) | 7.15 |
p-Formylphenol | –CHO | EWG (inductive) | 7.62 |
p-Chlorophenol | –Cl | EWG (inductive) | 9.38 |
Phenol | –H | Neither | 9.98 |
p-Methylphenol | –CH₃ | EDG (inductive) | 10.14 |
p-Methoxyphenol | –OCH₃ | EDG (resonance) | 10.20 |
p-Aminophenol | –NH₂ | EDG (resonance) | 10.30 |
EWGs lower pKₐ (stronger acid) because they stabilise the phenoxide conjugate base. EDGs raise pKₐ (weaker acid) because they destabilise it. The strongest EWG here, –NO₂, is a resonance withdrawer, and the strongest EDGs (–OCH₃, –NH₂) are resonance donors.
The carbonyl carbon is the electrophilic centre. Each carbonyl derivative has a different substituent attached to that carbon, and the substituent's electronic effect determines how reactive the C=O is toward nucleophilic attack.
Rank | Derivative | Substituent on C=O | Effect | Why |
|---|---|---|---|---|
1 | Acid chloride | –Cl | Inductive withdrawer | Cl is electronegative, pulls electron density away from C=O, destabilises it |
2 | Anhydride | –OC(=O)R | Inductive withdrawer | Second carbonyl withdraws inductively |
3 | Aldehyde | –H | Neither donor nor withdrawer | H is the baseline, no significant electronic effect |
4 | Ketone | –R (alkyl) | Weak inductive donor | Alkyl groups donate weakly through sigma bonds, slightly stabilising C=O; also less reactive due to steric bulk |
5 | Ester (≈ carboxylic acid) | –OR | Resonance donor | Oxygen lone pairs donate into the C=O pi system, significantly stabilising the carbonyl |
6 | Amide | –NH₂ / –NR₂ | Resonance donor | Nitrogen is more willing to share its lone pair than oxygen, making it the strongest resonance donor |
The carboxylic acid (–OH) and ester (–OCH₃) have similar electronic effects on the carbonyl carbon because both feature an oxygen atom donating by resonance. In practice, carboxylic acids react primarily as acids (proton transfer) rather than at the carbonyl carbon, so they are often set aside from the electrophilic reactivity ranking.
Two factors are at work. First, the alkyl group is a weak inductive donor, pushing a small amount of electron density toward the carbonyl carbon. Second, the second alkyl group adds steric bulk around the electrophilic carbon, making it harder for a nucleophile to approach. Additional alkyl groups beyond the first (e.g. going from acetone to a bulkier ketone) contribute little electronically because the inductive effect is negligible past the first carbon, but they continue to add steric hindrance.
Students often think "more stable = more reactive." The opposite is true: stability and reactivity are inversely related. A stabilised electrophile is a slower electrophile.
Students frequently confuse the inductive effect of Cl (withdrawing, through sigma bonds) with its resonance effect on a benzene ring (donating, through lone pairs into the pi system). Cl can do both, and which one dominates depends on the system. On a carbonyl, Cl is an inductive withdrawer. On benzene, Cl is a weak activator overall because its resonance donation to the ring slightly outweighs its inductive withdrawal, though it is classified as a deactivator in many textbooks because its net effect is deactivating relative to H.
Students sometimes assume that all EDGs operate the same way. They do not. –CH₃ donates inductively (weak), while –NH₂ and –OCH₃ donate by resonance (strong). The mechanism matters because it determines the magnitude of the effect.
Students often rank carboxylic acids among the carbonyl electrophiles. In practice, carboxylic acids react mainly as acids (proton transfer), not at the carbonyl carbon. Their electrophilic reactivity at C=O is comparable to esters.
⚠️ Expect questions asking you to rank carbonyl derivatives from most to least reactive toward nucleophilic attack. Know the order cold: acid chloride > anhydride > aldehyde > ketone > ester > amide.
⚠️ Be prepared to explain why a given substituent is an EDG or EWG and whether it operates by induction or resonance.
⚠️ pKₐ comparison questions are common: given a set of substituted acids or phenols, rank them by acidity and justify using EDG/EWG logic.
⚠️ The four summary rules (EDG/EWG effects on electrophile and nucleophile stability and reactivity) can appear as fill-in-the-blank or matching questions.
True or False: An electron-donating group makes an electrophile more reactive. (False, it makes it less reactive.)
Fill in the blank: The two types of electronic effects are ____ and ____. (Inductive and resonance.)
True or False: Resonance effects are generally weaker than inductive effects. (False, resonance effects are generally stronger.)
Fill in the blank: In the carbonyl reactivity ranking, ____ is the most reactive and ____ is the least reactive. (Acid chloride; amide.)
True or False: A carboxylic acid has similar electrophilic reactivity at the carbonyl carbon to an ester. (True.)
Q: Rank the following carbonyl compounds from most to least reactive toward nucleophilic attack: amide, acid chloride, aldehyde, ester, anhydride.
A: Acid chloride > anhydride > aldehyde > ester > amide. Acid chloride has the strongest EWG (Cl, inductive withdrawer), making the carbonyl carbon most electrophilic. Amide has the strongest EDG (–NH₂, resonance donor), making it least reactive.
Q: Explain why p-nitrophenol (pKₐ 7.15) is a stronger acid than phenol (pKₐ 9.98).
A: The nitro group (–NO₂) is an EWG that operates by resonance. It withdraws electron density from the ring and stabilises the phenoxide conjugate base by delocalising the negative charge. A more stable conjugate base means the acid gives up its proton more readily, so pKₐ is lower.
Q: A student claims that –CH₃ and –OCH₃ are both EDGs, so they should have the same effect on acidity. What is wrong with this reasoning?
A: –CH₃ is an inductive donor (weak), while –OCH₃ is a resonance donor (strong). Because resonance effects are generally more significant than inductive effects, –OCH₃ raises pKₐ more than –CH₃ does. The mechanism of donation matters, not just the label.
Q: Why does a ketone react more slowly than an aldehyde with the same nucleophile?
A: Two reasons. First, the alkyl group on the ketone is a weak inductive donor that stabilises the carbonyl carbon slightly. Second, the second substituent adds steric bulk, making it harder for the nucleophile to reach the electrophilic carbon.
Q: An acid chloride is treated with a nucleophile. Will the reaction be faster or slower than the same nucleophile attacking an ester? Explain.
A: Faster. The –Cl group is an inductive withdrawer that destabilises the carbonyl (making it more electrophilic), whereas the –OR group of the ester is a resonance donor that stabilises the carbonyl (making it less electrophilic). A less stable, more reactive electrophile is attacked more readily.
This material connects directly to nucleophilic acyl substitution (Chapter 21), where the carbonyl reactivity ranking determines which derivatives can be converted to which. A more reactive derivative (acid chloride) can be converted to any less reactive one (ester, amide), but not the reverse without special activation.
It also links to electrophilic aromatic substitution (EAS), covered in Part 2 of these notes, where EDGs and EWGs on benzene determine both the rate and regiochemistry of the reaction.
The acid-base framework here is the same one used in enolate chemistry (Chapter 22-23): understanding alpha-proton acidity requires the same EDG/EWG reasoning applied to conjugate base stability.
Nucleophile, electrophile, electron-donating group, EDG, electron-withdrawing group, EWG, inductive effect, resonance effect, carbonyl reactivity, acyl chloride reactivity, acid chloride, anhydride, aldehyde, ketone, ester, amide, carboxylic acid, pKₐ ranking, substituent effects, nucleophilic addition, nucleophilic acyl substitution, carbonyl electrophilicity, conjugate base stability, phenol acidity, carboxylic acid acidity, CHEM 212, organic chemistry II, Chapter 22, LT7