Difficulty: Intermediate | Prerequisites: Familiarity with carbonyl group structure, nucleophilic addition to aldehydes and ketones, and basic acid/base chemistry.
Carboxylic acid derivatives (acyl chlorides, anhydrides, esters, amides) are the workhorses of Organic Chemistry I, and nucleophilic acyl substitution (SNAc) is the central mechanism that connects them all. This topic builds directly on your earlier work with nucleophilic addition to aldehydes and ketones, but now the carbonyl carbon carries a leaving group, which changes everything. Understanding the reactivity ladder of these derivatives, and which reagents push a conversion up or down that ladder, is essential for multi-step synthesis problems and will appear heavily on Exam 3.
SNAc lets you swap the leaving group on a carbonyl. More reactive derivatives (acyl chlorides) can be converted down to less reactive ones (amides), but not the other way round without special reagents. The reactivity order is acyl chloride > anhydride > ester > amide > carboxylate, and it tracks how good the leaving group is.
Nucleophilic acyl substitution (SNAc)
A two-step mechanism in which a nucleophile attacks a carbonyl carbon bearing a leaving group, forming a tetrahedral intermediate, which then collapses by expelling the leaving group to regenerate the C=O. Think of it as: the nucleophile kicks the leaving group off the carbonyl.
Acyl chloride (acid chloride)
A carboxylic acid derivative with a Cl leaving group bonded to the carbonyl carbon (RCOCl). The most reactive of the common derivatives toward SNAc. In simple terms, this is the most "eager" carbonyl to react with nucleophiles because Cl is an excellent leaving group.
Anhydride
A derivative formed when two carboxylic acids lose water, giving the structure RCOOCOR'. The leaving group is a carboxylate (RCOO⁻). Second in the reactivity ladder after acyl chlorides.
Ester
A derivative with an alkoxy leaving group (RCOOR'). Less reactive than anhydrides because alkoxide (RO⁻) is a moderate leaving group. Formed by treating acyl chlorides or carboxylic acids with alcohols.
Amide
A derivative with a nitrogen-based group bonded to the carbonyl (RCONR'₂). The least reactive common derivative toward SNAc because nitrogen's lone pair donates strongly into the carbonyl, stabilising the C=O and making the carbon less electrophilic. Think of it as: the nitrogen "holds on" so tightly that almost nothing can displace it.
Carboxylic acid
The parent compound (RCOOH). Sits between esters and amides in electrophilicity, but its reactions often require activation (conversion to a more reactive derivative) before SNAc can proceed usefully.
Thionyl chloride (SOCl₂)
A reagent that converts carboxylic acids to acyl chlorides. The byproducts (SO₂ and HCl) are gases, which drives the reaction to completion.
DCC (dicyclohexylcarbodiimide)
A coupling reagent that activates a carboxylic acid in situ so it can react directly with an amine to form an amide, skipping the need to make the acyl chloride first.
Pyridine
A mild base used as a solvent or additive in acylation reactions. It scavenges the HCl produced when an acyl chloride reacts with an alcohol or amine, preventing unwanted side reactions.
Step 1: Nucleophile attacks the electrophilic carbonyl carbon, breaking the pi bond and forming a tetrahedral intermediate.
Step 2: The leaving group departs, the pi bond re-forms, and you get the new carbonyl product.
This is different from nucleophilic addition (aldehydes/ketones) because the tetrahedral intermediate collapses back to a carbonyl rather than remaining as an alcohol.
Most reactive → least reactive toward SNAc:
Acyl chloride > Anhydride > Ester ≈ Carboxylic acid > Amide
The trend tracks leaving group ability: Cl⁻ (great) > RCOO⁻ (good) > RO⁻ (moderate) > NR₂⁻ (poor).
It also tracks how much the heteroatom donates electron density into the carbonyl. Nitrogen donates the most (its lone pair overlaps well with the C=O pi system), making the carbonyl carbon least electrophilic in amides.
Electron-withdrawing groups on the alpha carbon increase electrophilicity and speed up SNAc.
Example from the exam review (Q9): among CH₃COCH₃, CH₃COCH₂Cl, CH₃COCHCl₂, and CH₃COCCl₃, the trichlorinated ketone (CH₃COCCl₃) is the most reactive toward SNAc because three Cl atoms withdraw the most electron density from the carbonyl carbon.
Carboxylic acid → Acyl chloride: SOCl₂ (thionyl chloride). The go-to first step in many synthesis sequences.
Carboxylic acid → Amide (directly): Use DCC as a coupling agent with an amine (e.g., PhCOOH + CH₃NH₂ with DCC gives PhCONHCH₃). Exam Q13 tests this.
Acyl chloride → Ester: Add an alcohol, often with pyridine as base (e.g., butanoyl chloride + butanol/pyridine → butyl butanoate). Exam Q12 tests this.
Acyl chloride → Amide: Add an amine (e.g., an acyl chloride + methylamine → an N-methylamide).
Carboxylic acid → Ester (Fischer esterification): Carboxylic acid + alcohol with acid catalyst (H⁺). Reversible.
Carboxylic acid → Methyl ester via acyl chloride: RCOOH → RCOCl (SOCl₂) → RCOOCH₃ (MeOH). Exam Q15 tests this two-step sequence.
You cannot convert an anhydride directly into an acyl chloride by treating it with HCl (Exam Q11). The leaving group on the anhydride is a carboxylate, but HCl is not nucleophilic enough under standard conditions to displace it in a useful way. You would need to go back through the carboxylic acid.
You cannot convert a less reactive derivative into a more reactive one by simple SNAc. Moving "up" the ladder requires special reagents (e.g., SOCl₂ to go from carboxylic acid to acyl chloride).
A SNAc reaction does occur, but not in good yield (Exam Q7). One equivalent of amine is not enough because the reaction produces a carboxylic acid byproduct that can protonate the amine, wasting it.
A dicarboxylic acid with the right geometry (1,4 or 1,5 relationship) can cyclise on heating to form a cyclic anhydride (Exam Q14). Succinic acid and glutaric acid derivatives do this readily.
Students often assume you can convert any derivative into any other by simple SNAc. You cannot move up the reactivity ladder without activating reagents. A less reactive derivative will not spontaneously become a more reactive one.
Students frequently forget that treating an anhydride with HCl does not give an acyl chloride. The conversion requires going through the free carboxylic acid and then using SOCl₂.
It is common to think that DCC "makes an acyl chloride in situ." It does not. DCC activates the carboxylic acid to form an O-acylisourea intermediate, which the amine then attacks directly.
Students sometimes rank reactivity by the electronegativity of the leaving group atom alone (N > O > Cl) and get the order backwards. The trend is about leaving group ability and lone pair donation, not raw electronegativity.
⚠️ Reactivity ranking questions appear on nearly every Org Chem I exam. Know the order cold: acyl chloride > anhydride > ester > amide.
⚠️ Expect at least one question asking you to identify which conversions are possible by direct SNAc and which are not.
⚠️ Electron-withdrawing substituents increasing SNAc reactivity (the alpha-chloro series) is a favourite multiple-choice question.
⚠️ DCC-mediated amide formation from a carboxylic acid and an amine is tested as a one-step transformation. Know the reagent and what it does.
True or False: An amide can be converted to an ester by simple SNAc without special activating reagents.
Answer: False. Amides are less reactive than esters; you cannot move up the ladder by simple SNAc.
Fill in the blank: The reagent used to convert a carboxylic acid to an acyl chloride is ________.
Answer: SOCl₂ (thionyl chloride).
True or False: Adding HCl to an anhydride produces an acyl chloride.
Answer: False. This conversion does not work by direct SNAc.
Fill in the blank: Among the common carboxylic acid derivatives, ________ are the least reactive toward nucleophilic acyl substitution because nitrogen donates electron density into the carbonyl.
Answer: Amides.
True or False: Electron-withdrawing groups on the alpha carbon decrease SNAc reactivity.
Answer: False. They increase it by making the carbonyl carbon more electrophilic.
Q: An acyl chloride is treated with butanol in the presence of pyridine. What is the product?
A: A butyl ester. The alcohol acts as the nucleophile, and pyridine scavenges the HCl produced.
Q: A carboxylic acid is treated with methylamine and DCC. What is the product?
A: An N-methylamide. DCC activates the carboxylic acid so the amine can attack directly.
Q: You need to convert a carboxylic acid to a methyl ester in two steps. What reagents do you use?
A: Step 1, SOCl₂ (gives the acyl chloride). Step 2, MeOH (gives the methyl ester).
Q: Rank the following in order of decreasing reactivity toward SNAc: ester, acyl chloride, amide, anhydride.
A: Acyl chloride > anhydride > ester > amide.
Q: Why does adding more chlorine atoms to the alpha carbon of a ketone increase its reactivity toward nucleophilic attack?
A: Each chlorine is electron-withdrawing (inductive effect), which makes the carbonyl carbon more electron-poor and therefore more electrophilic.
Q: A dicarboxylic acid (HOOC-CH₂-CH₂-COOH derivative) is heated. What product forms?
A: A cyclic anhydride, through intramolecular loss of water.
This material connects directly to nucleophilic addition (aldehydes and ketones from earlier in the course), because SNAc is the same first step with a different second step. It also sets the stage for the reduction chemistry of carboxylic acid derivatives, where reagents like LiAlH₄ and DIBAL-H reduce different derivatives to different oxidation levels. Multi-step synthesis questions will almost always require at least one SNAc interconversion.
Nucleophilic acyl substitution, SNAc, acyl transfer, carboxylic acid derivatives, acyl chloride, acid chloride, anhydride, ester, amide, carboxylate, leaving group ability, SOCl₂, thionyl chloride, DCC, dicyclohexylcarbodiimide, pyridine, Fischer esterification, tetrahedral intermediate, carbonyl electrophilicity, electron-withdrawing groups, alpha substituents, inductive effect, cyclic anhydride, intramolecular cyclisation, reactivity ranking, Org Chem I Exam 3