---
title: "Substrate Control of Stereochemistry: Models for Facial Selectivity"
description: "When a reagent attacks a carbonyl next to an existing stereocentre, one face is easier to reach than the other. Which face wins is predicted by a small set of conformational models, and each model sta"
canonical: https://lightmysky.com/learn/science/substrate-control-of-stereochemistry-models-for-facial-selectivity-mt_qLBmyLq2ZU
source: https://lightmysky.com/learn/science/substrate-control-of-stereochemistry-models-for-facial-selectivity-mt_qLBmyLq2ZU.md
retrieved: 2026-09-12
---

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# Substrate Control of Stereochemistry: Models for Facial Selectivity

When a reagent attacks a carbonyl next to an existing stereocentre, one face is easier to reach than the other. Which face wins is predicted by a small set of conformational models, and each model states the assumption it rests on.

Subject: Science · Area: Chemistry · Ages 22 to 23
Page: https://lightmysky.com/learn/science/substrate-control-of-stereochemistry-models-for-facial-selectivity-mt_qLBmyLq2ZU

## Ready when they can

- Applies the standard open-chain model to predict the major diastereomer from an alpha-substituted aldehyde
- Says when a chelating metal changes the preferred conformation and reverses the outcome
- Predicts attack direction on a rigid ring from axial and equatorial approach arguments
- Names an experiment whose result would distinguish two competing facial models

## Lesson: Which face gets attacked

When a reagent attacks a carbonyl next to an existing stereocentre, one face is easier to reach than the other. The standard open-chain model lines the biggest neighbour away from the carbonyl, and attack comes from the less blocked face.

**Example.** A hydride meets an aldehyde with a stereocentre next door. The open-chain model predicts one diastereomer as major. Add a chelating metal that grips both oxygens, and the locked shape exposes the opposite face, so the other diastereomer wins. That flip is also the test between the models: run the reaction with and without the metal and see which face wins.

On a rigid ring there is no floppy chain to arrange. The ring's hydrogens point either straight up or down, called axial, or out around the edge, called equatorial. You predict attack from axial and equatorial approach arguments instead: the reagent takes the path with less clash against the ring. One direction is usually clearly more open.

**Recap.** Line up the neighbours, attack the open face, and let chelation or ring shape override the default.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Protecting Group Strategy: Orthogonality and the Order of Removal](https://lightmysky.com/learn/science/protecting-group-strategy-orthogonality-and-the-order-of-removal-mt_-TvfTGZ-_Z)
- [Diastereomers, Meso Compounds and Fischer Projections](https://lightmysky.com/learn/science/diastereomers-meso-compounds-and-fischer-projections-mt_1r5vCBK9zr)
- [Nucleophilic Addition and the Tetrahedral Intermediate](https://lightmysky.com/learn/science/nucleophilic-addition-and-the-tetrahedral-intermediate-mt_N9Eydek3s7)

## Opens up

- [Asymmetric Catalysis: Where Enantioselectivity Comes From](https://lightmysky.com/learn/science/asymmetric-catalysis-where-enantioselectivity-comes-from-mt_vFpTH27iHP)