Heterocycle synthesis — quinoline via condensation
Friedländer synthesis
How the Friedländer synthesis works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: 2-Aminobenzaldehyde, ketone with α-CH₂ (acetone), base (NaOH) or acid. Conditions: EtOH, reflux. Bonds formed: C=N; C–C (aldol); aromatic ring; bonds broken: C=O (×2, as H₂O ×2). Stereochemistry: None; planar quinoline. Rate and selectivity: Intramolecular aldol cyclization or dehydration is rate-limiting.
At a glance
Step 1: Schiff-base (ketimine) formation
2-Aminobenzaldehyde's ketone-reactive partner forms an imine with the aldehyde first — ordinary Schiff-base condensation, positioning the two reactive groups near each other on the same molecule.
Step 2: Enolate (aza-enolate) closes the ring
Base then deprotonates the alpha carbon of the ketone component, and the resulting enolate attacks intramolecularly — an aldol-type cyclization closing the new six-membered ring, the same logic as an intramolecular aldol condensation.
Step 3: Dehydration aromatizes
Dehydration aromatizes the ring, giving a substituted quinoline directly from two simple carbonyl components — a complementary route to the Skraup and Doebner-Miller that builds the ring through carbon-carbon bond formation rather than a conjugate addition.
Abbreviations and terms
- EtOH
- Ethanol. Polar protic solvent.
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
How it compares with related reactions
| Friedländer synthesis | Combes quinoline synthesis | Doebner-Miller reaction | |
|---|---|---|---|
| Reagents | 2-Aminobenzaldehyde, ketone with α-CH₂ (acetone), base (NaOH) or acid | Aniline, 1,3-diketone (acetylacetone); then H₂SO₄ (or PPA) | Aniline, crotonaldehyde (an α,β-unsaturated aldehyde), HCl or Lewis acid, oxidant |
| Conditions | EtOH, reflux | Condensation 25–100 °C; cyclisation in conc. acid, heat | Aqueous HCl, reflux |
| Stereochemistry | None; planar quinoline | None; Z-enaminone is H-bond stabilised | None; 2-methylquinoline is achiral |
| Rate / selectivity | Intramolecular aldol cyclization or dehydration is rate-limiting | Ring closure onto the protonated ketone is rate-determining | Conjugate addition reversible; cyclization/dehydration rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Friedländer synthesis?
2-Aminobenzaldehyde, ketone with α-CH₂ (acetone), base (NaOH) or acid. Conditions: EtOH, reflux.
Which bonds are formed and which are broken?
Formed: C=N; C–C (aldol); aromatic ring. Broken: C=O (×2, as H₂O ×2).
What is the stereochemical outcome?
None; planar quinoline
What controls the rate or the selectivity?
Intramolecular aldol cyclization or dehydration is rate-limiting
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go
- How to answer mechanism questions in an exam