Nucleophilic substitution at phosphorus
In-line phosphoryl transfer (SN2 at phosphorus)
How the In-line phosphoryl transfer (SN2 at phosphorus) 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: Phosphate triester model, nucleophile (HO⁻, ROH). Conditions: Aqueous / enzymatic, pH 7. Bonds formed: P–Nu; bonds broken: P–OAr (leaving group). Stereochemistry: Inversion at P (in-line, apical entry and exit). Rate and selectivity: Better leaving group (lower pKa) and better nucleophile accelerate.
At a glance
Step 1: In-line attack opposite the leaving group
A nucleophile attacks phosphorus directly opposite the leaving group — 'in-line' attack, geometrically identical in concept to backside attack in carbon SN2, just at a different central atom.
Step 2: Pentacoordinate phosphorane
Because phosphorus can expand its coordination number, the nucleophile and leaving group are both fully bonded at the same time, forming a genuine pentacoordinate phosphorane intermediate — a real species, not just a transition state, unlike carbon's SN2.
Step 3: Leaving group departs: inversion
The leaving group departs from the opposite face, giving inversion of configuration at phosphorus — this exact mechanism governs phosphate ester chemistry throughout biology, from ATP hydrolysis to DNA backbone cleavage.
Abbreviations and terms
- Ar
- Aryl group. Any aromatic ring substituent such as phenyl.
- Nu
- Nucleophile. An electron-rich species that donates an electron pair to form a new bond.
- LG
- Leaving group. The group that departs with the bonding electrons (good ones are weak bases: I⁻, Br⁻, TsO⁻…).
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
- pKa
- Acid-dissociation constant (–log). Lower pKa = stronger acid; used to judge which proton a base removes.
- ATP
- Adenosine triphosphate. The cell’s energy currency; transfers phosphate groups.
- DNA
- Deoxyribonucleic acid. The genetic polymer.
How it compares with related reactions
| In-line phosphoryl transfer (SN2 at phosphorus) | Acid-catalyzed amide hydrolysis | Amide synthesis from an acid chloride and amine | |
|---|---|---|---|
| Reagents | Phosphate triester model, nucleophile (HO⁻, ROH) | Amide, H₃O⁺ (HCl or H₂SO₄), H₂O | Benzoyl chloride, methylamine, aqueous NaOH |
| Conditions | Aqueous / enzymatic, pH 7 | 6 M acid, reflux (hours) | CH₂Cl₂ / H₂O (biphasic), 0–25 °C |
| Stereochemistry | Inversion at P (in-line, apical entry and exit) | No stereocentre created; α-stereocentres retained | No stereocentre; planar amide |
| Rate / selectivity | Better leaving group (lower pKa) and better nucleophile accelerate | Water attack / C–N cleavage rate-limiting; far slower than ester hydrolysis | Very fast; base neutralises HCl so the amine is not lost as its salt |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the In-line phosphoryl transfer (SN2 at phosphorus)?
Phosphate triester model, nucleophile (HO⁻, ROH). Conditions: Aqueous / enzymatic, pH 7.
Which bonds are formed and which are broken?
Formed: P–Nu. Broken: P–OAr (leaving group).
What is the stereochemical outcome?
Inversion at P (in-line, apical entry and exit)
What controls the rate or the selectivity?
Better leaving group (lower pKa) and better nucleophile accelerate
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go