Difficulty: Intermediate to Advanced | Prerequisites: Substitutions and eliminations, basic carbonyl structure, acid-base chemistry

Carbonyl chemistry is the centrepiece of Orgo II. Every carbonyl compound, from aldehydes and ketones to esters and amides, reacts with nucleophiles, but the outcome depends on whether the carbonyl undergoes addition (aldehydes, ketones) or substitution (acid chlorides, anhydrides, esters, amides). Enolate chemistry then opens up the alpha-carbon for new C-C bond formation, which is the foundation of synthesis planning.

TL;DR

Aldehydes and ketones undergo nucleophilic addition (the nucleophile stays, giving an alcohol or alkene), while acid chlorides, anhydrides, esters and amides undergo nucleophilic acyl substitution (the nucleophile replaces the leaving group, giving a new carbonyl derivative). Enolate chemistry uses a strong base to deprotonate next to a carbonyl, creating a nucleophilic carbon that can alkylate or react with another carbonyl in an aldol reaction. These three pillars cover most of the C-C bond-forming reactions in Orgo II.

Key Terms

Grignard reagent (RMgX)

An organomagnesium halide that acts as a strong nucleophile and a strong base. Adds an alkyl or aryl group to a carbonyl: formaldehyde gives a 1° alcohol, an aldehyde gives a 2° alcohol, a ketone gives a 3° alcohol. Think of it as a carbanion delivery system.

Gilman reagent ((R)2CuLi)

A lithium dialkylcuprate. Performs 1,4-conjugate addition to alpha,beta-unsaturated carbonyls rather than 1,2-addition. In simple terms, it adds to the end of the conjugated system instead of directly to the carbonyl.

Wittig reaction

Ketone + Ph3P=CHR (a phosphorus ylide) gives an alkene + triphenylphosphine oxide. Replaces C=O with C=C. One of the few reliable ways to place a double bond at a specific position.

Tetrahedral intermediate

The sp3 carbon formed when a nucleophile adds to a carbonyl carbon. In aldehydes and ketones, this intermediate is the product (after protonation). In acyl derivatives, it collapses by ejecting the leaving group.

Nucleophilic acyl substitution

The mechanism by which acid chlorides, anhydrides, esters and amides react with nucleophiles. The nucleophile adds to form a tetrahedral intermediate, then the leaving group departs. Net result: one group in, one group out.

Saponification

Base-promoted hydrolysis of an ester (ester + NaOH, heat) to give a carboxylate salt and an alcohol. Irreversible because the carboxylate is deprotonated and stabilised by resonance. This is how soap is made from fats.

LDA (lithium diisopropylamide)

A strong, sterically hindered, non-nucleophilic base. Removes the less hindered alpha-proton to give the kinetic enolate. The go-to base when you need the less substituted enolate.

Kinetic enolate

The enolate formed by removing the more accessible (less substituted) alpha-proton, using a strong hindered base (LDA) at low temperature. Forms faster but is less stable.

Thermodynamic enolate

The more substituted enolate, formed under equilibrating conditions (weaker base such as NaOEt, higher temperature, protic solvent). More stable because of greater substitution at the double bond.

Aldol addition

An enolate reacts with an aldehyde or ketone to form a beta-hydroxy carbonyl compound. A key C-C bond-forming reaction.

Aldol condensation

When an aldol product is heated, it loses water to form an alpha,beta-unsaturated carbonyl (an enone). Favoured by conjugation of the new double bond with the carbonyl.

Alpha-halogenation

Treatment of a ketone with Br2 under acidic or basic conditions places a halogen at the alpha position. Under basic conditions, polyhalogenation can occur (haloform reaction).

Core Content: Nucleophilic Addition to Aldehydes and Ketones

Organometallic additions (Grignard and organolithium)

• Aldehyde/ketone + RLi or RMgX, then H3O+ workup: the R group adds to the carbonyl carbon, producing an alkoxide intermediate that is protonated to give an alcohol.

  • Formaldehyde + RMgX gives a 1° alcohol.

  • Aldehyde + RMgX gives a 2° alcohol.

  • Ketone + RMgX gives a 3° alcohol.

• Ester or lactone + excess RMgX gives a 3° alcohol (two equivalents of R are added; a ketone intermediate forms after the first equivalent).

Conjugate (1,4) addition

• Alpha,beta-unsaturated carbonyl + RLi gives 1,2-addition (direct attack on the carbonyl).

• Alpha,beta-unsaturated carbonyl + (CH3)2CuLi (Gilman reagent) gives 1,4-conjugate addition, placing the R group at the beta position. The cuprate is softer and prefers the softer electrophilic site.

Hydride reductions

• NaBH4 reduces aldehydes and ketones to alcohols. Mild, selective: does not reduce esters or carboxylic acids under standard conditions.

• LiAlH4 is a stronger reducing agent. Reduces aldehydes, ketones, esters (to 1° alcohols, two from one ester), amides (to amines) and carboxylic acids.

Wittig reaction

• Ketone + Ph3P=CHR gives an alkene. The C=O is replaced by C=C. Useful for placing a double bond at a specific location where elimination might give mixtures.

Core Content: Nucleophilic Acyl Substitution

Reactivity order of carbonyl derivatives

Acid chloride > Anhydride > Ester > Amide. The better the leaving group, the more reactive the derivative. You can convert down this series (acid chloride to ester, ester to amide) but not back up without special reagents.

Key transformations

• Carboxylic acid + SOCl2 gives acid chloride. This is the best and most reliable method for making acid chlorides.

• Acid chloride + ROH gives ester.

• Acid chloride + RNH2 gives amide.

• Acid anhydride + nucleophile gives acyl derivative + carboxylic acid (one half of the anhydride leaves as the acid).

• Ester + NaOH (aq), heat gives carboxylate + alcohol (saponification). Irreversible because the carboxylate is resonance-stabilised and cannot be re-attacked.

• Amide + H3O+, heat gives carboxylic acid + ammonium salt (acid hydrolysis of an amide).

• Amide + LiAlH4 gives amine (reduction, not hydrolysis).

• Ester + excess RMgX gives 3° alcohol (the Grignard adds twice: once to open the ester to a ketone intermediate, then again to the ketone).

Core Content: Enols and Enolate Ion Reactions

Forming enolates

• LDA or LiN(iPr)2 (strong, non-nucleophilic base) at low temperature removes the less hindered alpha-proton to give the kinetic enolate (less substituted).

• Alkoxide bases (e.g. NaOEt) under thermodynamic conditions (room temperature, protic solvent) give the thermodynamic enolate (more substituted, more stable).

Alpha-alkylation

• Enolate + alkyl halide gives alpha-alkylated carbonyl via SN2. The enolate carbon is the nucleophile, the alkyl halide is the electrophile. Works best with primary and methyl halides (secondary halides give elimination).

Aldol reaction

• Enolate from a ketone + aldehyde or ketone gives a beta-hydroxy carbonyl (aldol addition product).

• Heating the aldol product causes loss of water to form an alpha,beta-unsaturated carbonyl (aldol condensation). This is thermodynamically favoured because the new double bond is conjugated with the carbonyl.

• Aldol condensation is particularly useful for extending carbon chains and building enone systems.

Alpha-halogenation

• Ketone + Br2 under acidic conditions gives mono-alpha-bromo ketone.

• Under basic conditions, halogenation can proceed multiple times (haloform reaction with methyl ketones).

• Alpha-hydrogens exchange with D2O/OD- (useful for deuterium labelling experiments).

Key Reagent Summary

Common Misconceptions

• Students often confuse nucleophilic addition (aldehydes, ketones) with nucleophilic acyl substitution (acid chlorides, esters, amides). The difference is whether there is a leaving group on the carbonyl carbon. If there is, the tetrahedral intermediate collapses and the leaving group departs. If there is not (as in an aldehyde or ketone), the tetrahedral intermediate is protonated to give the product.

• LiAlH4 and NaBH4 are frequently mixed up. NaBH4 is milder and selective for aldehydes and ketones. LiAlH4 is more powerful and also reduces esters, carboxylic acids and amides. Using the wrong one on an exam changes the product entirely.

• The difference between kinetic and thermodynamic enolates trips many students up. LDA at -78°C gives the kinetic (less substituted) enolate. NaOEt at room temperature gives the thermodynamic (more substituted) enolate. The base, temperature and solvent all matter.

• Students often forget that saponification is irreversible. The carboxylate ion formed is resonance-stabilised and deprotonated, so it cannot be re-attacked by the alcohol. This is different from acid-catalysed ester hydrolysis, which is reversible.

Why It Matters / Exam Flags

• ⚠️ "What product forms when this Grignard reacts with this carbonyl?" is a near-certainty on any Orgo II exam. Count the carbons: formaldehyde gives 1°, aldehyde gives 2°, ketone gives 3° alcohol.

• ⚠️ The reactivity order of carbonyl derivatives (acid chloride > anhydride > ester > amide) is tested both as a ranking question and as a synthesis logic question: can you convert an ester to an acid chloride directly? No, you must go through the carboxylic acid first.

• ⚠️ Kinetic vs. thermodynamic enolate: expect a question showing a ketone with two different sets of alpha-protons and asking which enolate forms under which conditions.

• ⚠️ Aldol reactions are a synthesis favourite. Be ready to identify the aldol product, work backwards from an enone to find the starting carbonyl compounds (retro-aldol), and predict whether addition or condensation occurs.

Quick Self-Test

1. Fill in the blank: Formaldehyde + RMgBr gives a ______ alcohol. (1° / primary)

1. True or False: A Gilman reagent performs 1,2-addition to an alpha,beta-unsaturated ketone. (False: it performs 1,4-conjugate addition.)

1. Fill in the blank: The reactivity order of carbonyl derivatives from most to least reactive is acid chloride > ______ > ______ > amide. (Anhydride > Ester)

1. True or False: LDA at -78°C gives the thermodynamic enolate. (False: it gives the kinetic enolate.)

1. Fill in the blank: Saponification of an ester gives a ______ and an alcohol. (Carboxylate salt)

Practice Q&A

Q: An ester is treated with excess methylmagnesium bromide followed by aqueous acid workup. What is the product and why does the Grignard add twice?

A: The product is a tertiary alcohol with two methyl groups from the Grignard. The first equivalent of Grignard opens the ester to a ketone intermediate (the alkoxide leaving group departs), and the second equivalent adds to that ketone.

Q: You need to convert a carboxylic acid to an amide. What is the most reliable two-step route?

A: First convert the acid to an acid chloride using SOCl2, then treat the acid chloride with RNH2. Going directly from acid to amide is not efficient because carboxylic acids are not reactive enough toward nucleophilic acyl substitution.

Q: Draw the product of treating cyclohexanone with LDA at -78°C, then methyl iodide.

A: 2-Methylcyclohexanone. LDA removes the less hindered alpha-proton (kinetic enolate), and the resulting enolate performs SN2 on methyl iodide.

Q: What is the difference between an aldol addition product and an aldol condensation product?

A: The aldol addition product is a beta-hydroxy carbonyl. The condensation product has lost water to become an alpha,beta-unsaturated carbonyl (enone). Heating drives the elimination of water.

Q: Why does NaBH4 reduce a ketone but not an ester, while LiAlH4 reduces both?

A: NaBH4 is a milder hydride source. Esters are less electrophilic than ketones (the lone pair on the ester oxygen donates electron density into the carbonyl), so a stronger reducing agent like LiAlH4 is needed.

Connections to Other Topics

Carbonyl chemistry builds directly on the nucleophile and electrophile logic from substitutions and eliminations. Enolate alkylation is fundamentally an SN2 reaction. Aldol chemistry connects to synthesis planning (multi-step sequences) and to alpha,beta-unsaturated systems that appear again in conjugate addition and in Diels-Alder reactions. The reactivity hierarchy of carbonyl derivatives (acid chloride > ester > amide) also underpins biochemistry: peptide bonds are amide bonds, and their low reactivity is why proteins are stable.

Related Terms / Search Tags

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