Organic chemistry study guide · Reaction Mechanism MCAT Tutor
Carbonyl reactions: hemiacetal, aldol, and nucleophilic acyl substitution
Carbonyls sit at the center of second-semester orgo: aldehydes and ketones favor addition; carboxylic acid derivatives favor substitution when the leaving group is good. Hemiacetals and acetals form from addition of alcohols; aldol reactions build C–C bonds via enolate attack on another carbonyl. Knowing which pathway wins saves time on mechanism and product questions.
Written for undergraduate orgo, pre-med, and MCAT organic chemistry review — United States exam-style framing.
Short lesson
Nucleophilic addition (aldehydes & ketones)
Nucleophile attacks C=O; π electrons move to oxygen to give a tetrahedral alkoxide. Protonation gives the neutral addition product (alcohol, cyanohydrin, hemiacetal, etc.).
Reactivity: aldehydes > ketones (less steric hindrance, less electron donation from alkyl groups).
Hemiacetal and acetal
One equivalent of alcohol gives a hemiacetal (OH + OR on same carbon); acid catalysis helps. Second equivalent (with acid, remove water) gives an acetal.
Acetals are protecting groups for aldehydes/ketones — stable to base, cleaved with aqueous acid.
Aldol condensation
Base forms an enolate from an aldehyde/ketone with α hydrogens; enolate attacks another carbonyl. Dehydration can give α,β-unsaturated carbonyl (condensation).
Crossed aldol questions test whether you pick the enolate former (usually the less hindered carbonyl) and the electrophilic partner.
Acyl substitution vs addition (preview)
Carboxylic acid derivatives (acid chlorides through amides) usually undergo nucleophilic acyl substitution rather than simple C=O addition. For reactivity order, ester ↔ amide interconversions, and Fischer esterification, see the dedicated carboxylic acid derivatives hub linked below.
Redox at carbonyls and adjacent alcohols
Reduction: NaBH₄ reduces aldehydes and ketones to alcohols but generally does not reduce esters or amides. LiAlH₄ is stronger — it can reduce carboxylic acids and esters to primary alcohols and amides toward amines (course-dependent depth).
Oxidation: PCC stops at aldehydes from primary alcohols; Jones (CrO₃/H₂SO₄) or chromic acid often pushes primary alcohols to carboxylic acids and secondary alcohols to ketones. Tertiary alcohols resist oxidation without C–C cleavage.
Exam pattern: match reagent strength to the functional group in the starting material and the oxidation state you need in the product — especially in multi-step synthesis with carbonyls and alcohols in the same molecule.
Common traps on exams
Using NaBH₄ to reduce an ester
NaBH₄ is for aldehydes/ketones. Ester reduction needs LiAlH₄ (or another strong hydride) unless the question specifies a different reagent.
PCC vs Jones on a primary alcohol
PCC gives the aldehyde; aqueous chromic acid (Jones) over-oxidizes primary alcohols to carboxylic acids. Read the reagent line carefully.
Forgetting dehydration in aldol
Under heat or stronger base, the β-hydroxy carbonyl loses water — product is often the enone, not the aldol alcohol.
Acetal formation under base
Acetal formation is acid-catalyzed; base does not drive the typical protection sequence.
Practice-style prompts
Try these on paper, then check product and mechanism in OrgoMech.
- Draw the hemiacetal from acetaldehyde + one equiv ethanol (acid catalysis).
- Self-aldol of propanal in base — sketch the β-hydroxy aldehyde before dehydration.
- 1-Butanol + PCC vs Jones — what carbonyl or acid product do you expect from each?
- Can NaBH₄ reduce cyclohexanone? Can it reduce methyl benzoate? Explain briefly.
- How would you protect benzaldehyde as an acetal before a Grignard step?
Student questions
Common questions undergraduates ask about this topic before exams.
- Why are aldehydes more reactive than ketones toward nucleophiles?
- Less steric hindrance and fewer electron-donating alkyl groups stabilizing the carbonyl — the electrophilic carbon is more accessible.
- What is the difference between hemiacetal and acetal?
- One equivalent of alcohol gives a hemiacetal (–OH and –OR on the same carbon). Two equivalents with acid catalysis and water removal gives an acetal.
- Why are acetals used as protecting groups?
- They mask aldehydes/ketones as unreactive acetals under basic conditions and are cleaved back with aqueous acid when you need the carbonyl again.
- What is an aldol reaction?
- Base forms an enolate that attacks another carbonyl, giving a β-hydroxy carbonyl. Dehydration can yield an α,β-unsaturated carbonyl (aldol condensation).
- What is the difference between NaBH₄ and LiAlH₄ for carbonyls?
- NaBH₄ typically reduces aldehydes and ketones to alcohols. LiAlH₄ is stronger and can reduce carboxylic acids and esters to alcohols (and reduce amides in many courses). Match reagent to functional group.
- When do I use PCC vs Jones reagent?
- PCC oxidizes primary alcohols to aldehydes and secondary alcohols to ketones without aqueous acid that over-oxidizes primaries to carboxylic acids. Jones (chromic acid) often gives acids from primary alcohols.
- Where should I study esters and amides in depth?
- The carboxylic acid derivatives hub on this site covers nucleophilic acyl substitution, Fischer esterification, and amide formation — the orgo 2 material beyond aldehyde/ketone addition.
- What carbonyl mistake loses partial credit on mechanisms?
- Using addition-only arrows on a carboxylic acid derivative when the leaving group must depart — see the carboxylic acid derivatives hub for acyl substitution.
Further reading (external)
Open textbooks and reference sites for the same topics covered above. We link out to established science education publishers — not affiliate pages.
- Carbonyl chemistry overview (LibreTexts) — LibreTexts
- OpenStax — Aldehydes and ketones — OpenStax
- PubChem — NIH NLM
Continue on this site
Drill this topic in OrgoMech
Predict the major product for your structure, then order the curved-arrow mechanism — the same skills tested on homework, finals, and the MCAT orgo section. Predict the major product, then order the curved arrows. Organic chemistry practice for reactions, mechanisms, and the days left before your final.