Difficulty: Intermediate | Prerequisites: CHEM 211 (Organic Chemistry I), familiarity with functional groups, electronegativity, and Lewis structures.
Carbonyl chemistry is the backbone of Organic Chemistry II. Nearly every reaction you will meet this semester involves a nucleophile attacking the electrophilic carbon of a C=O group, so understanding what makes one carbonyl more reactive than another is the single most transferable concept in the course. This material builds directly on the acid-base and functional group knowledge from Organic Chemistry I, and it connects forward to enolate chemistry, aldol reactions, and biosynthetic pathways. If you can rank carbonyl electrophilicity and predict whether a nucleophilic addition or a nucleophilic acyl substitution will occur, you have the framework for most of the semester.
Aldehydes are more reactive than ketones, which are more reactive than esters and other carboxylic acid derivatives, because of steric and electronic differences at the carbonyl carbon. Nucleophilic addition (to aldehydes and ketones) keeps the nucleophile attached; nucleophilic acyl substitution (on acid derivatives) kicks out a leaving group. Strong nucleophiles such as hydrides and carbanions add irreversibly, while weaker ones such as hydroxide and alkoxide add reversibly.
Nucleophilic addition
A reaction in which a nucleophile attacks the electrophilic carbonyl carbon, the pi bond breaks, and the oxygen picks up a negative charge (forming an alkoxide). No leaving group departs. In simple terms, the nucleophile sticks and stays.
Nucleophilic acyl substitution
A two-step process where a nucleophile attacks the carbonyl carbon of a carboxylic acid derivative, forms a tetrahedral intermediate, and then a leaving group is expelled to regenerate the C=O. Think of it as swap rather than stick: one group replaces another.
Tetrahedral intermediate
The sp3-hybridised species formed when a nucleophile adds to a carbonyl carbon. In acyl substitution it collapses by ejecting the leaving group; in simple addition it is protonated to give the product.
Irreversible nucleophile
A nucleophile whose conjugate acid has a very high pKa (roughly 35 to 60), making it an extremely poor leaving group once it has added. Hydride (H-, from LiAlH4 or NaBH4) and carbanions (R-, from Grignard reagents) are the main examples.
Reversible nucleophile
A nucleophile such as hydroxide (HO-) or alkoxide (RO-) that can depart again after adding to a carbonyl, because it is a reasonable leaving group.
Electrophilicity of a carbonyl
The degree of partial positive charge on the carbonyl carbon, which determines how readily a nucleophile attacks. Increased by electron-withdrawing groups and by protonation of the oxygen; decreased by electron-donating groups, resonance donation, and steric bulk.
Saponification
Base-promoted hydrolysis of an ester with a strong base (NaOH or KOH) to produce a carboxylate salt and an alcohol. In simple terms, this is how soap is made from fats.
Acid workup
The addition of aqueous acid (H3O+) at the end of a reaction to protonate the alkoxide intermediate and yield the neutral alcohol product.
Reactivity toward nucleophilic attack at the carbonyl carbon
Aldehydes > Ketones > Esters (and other carboxylic acid derivatives)
Aldehydes have only one alkyl (or H) substituent, so less steric crowding around the carbonyl carbon and fewer electron-donating groups to stabilise the partial positive charge.
Ketones have two alkyl groups, which both donate electron density and create more steric hindrance.
Esters are less reactive still, because the alkoxy group (-OR) donates electron density into the carbonyl by resonance, reducing the electrophilicity of the carbon.
Factors that increase carbonyl electrophilicity
Electron-withdrawing groups (EWGs) nearby: they pull electron density away from the carbon, making it more delta-positive and more attractive to nucleophiles.
Protonation of the carbonyl oxygen: places a full positive charge on the oxygen, dramatically increasing the electrophilicity of the carbon. This is why acid-catalysed additions are faster.
Smaller substituents: less steric shielding of the carbon means the nucleophile can approach more easily.
Factors that decrease carbonyl electrophilicity
Electron-donating groups by resonance (e.g. -NH2 in amides, -OR in esters): they push electron density toward the carbonyl carbon, partially cancelling its positive charge.
Bulky alkyl groups: steric crowding physically blocks the nucleophile's approach.
The reactivity series (most to least reactive toward nucleophilic acyl substitution)
Acid chloride (RCOCl) > Anhydride (RCO-O-COR) > Ester (RCOOR') > Amide (RCONH2)
Why this order holds
Two factors work together: the quality of the leaving group and the degree of resonance stabilisation.
Acid chlorides: Cl- is an excellent leaving group (weak base, stable anion). Chlorine does not donate much electron density by resonance, so the carbonyl carbon stays highly electrophilic.
Anhydrides: the leaving group is a carboxylate (RCOO-), which is a good leaving group. Moderate resonance stabilisation.
Esters: the leaving group is an alkoxide (RO-), a decent but not great leaving group. The alkoxy oxygen donates electron density into the carbonyl by resonance, reducing electrophilicity.
Amides: the leaving group would be NH2-, which is an extremely poor leaving group (very strong base). The nitrogen's lone pair donates heavily into the carbonyl by resonance, making the carbonyl carbon the least electrophilic in the series.
Key mechanistic point
All nucleophilic acyl substitutions proceed through a tetrahedral intermediate. The carbonyl reforms by expelling the leaving group. You can only go downhill in the reactivity series: an acid chloride can be converted to an anhydride, ester or amide, but you cannot easily convert an amide back to an acid chloride under the same conditions.
Irreversible nucleophiles
Hydride (H-) from LiAlH4 or NaBH4: the conjugate acid (H2) has a pKa around 35 to 37. Once delivered, H- is an impossibly poor leaving group.
Carbanion (R-) from Grignard reagents (RMgBr) or organolithiums (RLi): pKa of the conjugate acid is roughly 45 to 60. These add and do not leave.
Because these nucleophiles add irreversibly, the reaction is effectively a one-way street. The initial product is an alkoxide, which requires an acid workup (H3O+) to protonate it and give the final neutral alcohol.
Reversible nucleophiles
Hydroxide (HO-) and alkoxide (RO-): these are reasonable leaving groups and can depart from the tetrahedral intermediate, making the addition reversible.
The role of the acid workup
After a hydride or Grignard addition, the immediate product is a metal alkoxide (e.g. R-O- Li+). The acid workup step provides a proton to the oxygen, converting it from an alkoxide to a stable, neutral alcohol. Without this step, you have the salt, not the free alcohol.
Reactivity order: Aldehyde > Ketone > Ester > Amide
Acyl substitution leaving group ability: Cl- > RCOO- > RO- > NH2-
Acid workup: R-O(-) + H3O(+) --> R-OH + H2O
Saponification: Ester + NaOH --> Carboxylate salt + Alcohol
Saponification is the industrial process behind soap manufacturing: fats (triesters of glycerol) are hydrolysed with NaOH to yield glycerol and the sodium salts of fatty acids, which are soap. Understanding carbonyl reactivity also underpins pharmaceutical synthesis, where selective reduction or addition at one carbonyl in a multifunctional molecule is a routine design challenge.
Students often think esters are unreactive because they are "stable." Esters do undergo nucleophilic acyl substitution; they are simply less reactive than acid chlorides and anhydrides, not inert.
Students sometimes confuse nucleophilic addition (aldehydes/ketones, no leaving group lost) with nucleophilic acyl substitution (acid derivatives, leaving group expelled). The presence or absence of a leaving group is what distinguishes the two.
Assuming that the acid workup is the reaction itself. The acid workup is a separate protonation step after the nucleophilic addition is already complete. The nucleophile adds first under anhydrous conditions; the proton source comes second.
Treating all nucleophiles as irreversible. Hydroxide and alkoxide additions to carbonyls are reversible, which is why hydration and hemiacetal formation are equilibrium processes.
Expect a question asking you to rank carbonyl compounds by reactivity toward nucleophilic attack. Know the order and the reasoning (steric + electronic).
Be ready to identify products of nucleophilic acyl substitution given different nucleophiles and leaving groups.
The distinction between irreversible and reversible nucleophiles is frequently tested. Know which category hydride, carbanion, hydroxide, and alkoxide each fall into.
Saponification and the role of the acid workup are common short-answer topics.
True or False: A ketone is more reactive toward nucleophilic attack than an aldehyde. (False)
Fill in the blank: The least reactive carboxylic acid derivative toward nucleophilic acyl substitution is the ______. (amide)
True or False: Hydride from NaBH4 is considered an irreversible nucleophile. (True)
Fill in the blank: The acid workup step converts the ______ intermediate into a neutral alcohol. (alkoxide)
True or False: Saponification produces a carboxylic acid and an alcohol. (False, it produces a carboxylate salt and an alcohol)
Q: Which of propanal, propanone, and ethyl acetate is the most reactive toward nucleophilic attack at the carbonyl carbon, and why?
A: Propanal (the aldehyde). It has the least steric hindrance (only one alkyl group) and the fewest electron-donating substituents, so its carbonyl carbon carries the largest partial positive charge.
Q: What is the product when an ester is treated with 2 equivalents of a Grignard reagent followed by acid workup?
A: A tertiary alcohol. The first equivalent adds to give a ketone (after loss of the alkoxide leaving group), and the ketone immediately reacts with the second equivalent. Acid workup protonates the alkoxide to give the alcohol.
Q: Which functional group is the least reactive toward nucleophilic acyl substitution: acid chloride, ester, anhydride, or amide?
A: Amide. Nitrogen's lone pair provides strong resonance donation into the carbonyl, and NH2- is a very poor leaving group.
Q: Describe saponification in terms of reactants and products.
A: An ester reacts with a strong base (NaOH or KOH) to produce a carboxylate salt and an alcohol.
Q: What is the role of H3O+ in the acid workup after a Grignard or hydride addition?
A: It protonates the alkoxide intermediate, converting it to the neutral alcohol product.
This material connects directly to oxidation and reduction chemistry, because reducing agents (LiAlH4, NaBH4) are hydride nucleophiles that add to carbonyls. It also connects to organometallic chemistry (Grignard and Gilman reagents), where carbanions serve as the nucleophile. Later in the course, enolate chemistry builds on the concept of alpha-proton acidity adjacent to carbonyls, and aldol and Claisen condensations are extensions of nucleophilic addition and acyl substitution, respectively.
Carbonyl reactivity, nucleophilic addition, nucleophilic acyl substitution, tetrahedral intermediate, electrophilicity, steric hindrance, electron-donating group, electron-withdrawing group, aldehyde vs ketone reactivity, carboxylic acid derivatives, acid chloride, anhydride, ester, amide, leaving group ability, irreversible nucleophile, reversible nucleophile, hydride addition, Grignard addition, acid workup, protonation, alkoxide, saponification, base-promoted hydrolysis, CHEM 212, organic chemistry II