---
title: "Prelog's Rule | Organic Chemistry II"
description: "Prelog's Rule predicts how a nucleophile approaches a carbonyl in Organic Chemistry II, helping you explain stereochemical outcomes in reduction and addition."
canonical: "https://fiveable.me/organic-chemistry-ii/key-terms/prelogs-rule"
type: "key-term"
subject: "Organic Chemistry II"
unit: "Unit 3"
---

# Prelog's Rule | Organic Chemistry II

## Definition

Prelog's Rule is a stereochemical guideline in Organic Chemistry II that predicts how nucleophiles approach carbonyl compounds. It is used to explain which product forms when sterics control oxidation-reduction reactions.

## What It Is

Prelog's Rule is a stereochemical rule in Organic Chemistry II that predicts the preferred face of attack when a nucleophile approaches a carbonyl compound. In plain terms, it helps you figure out which side of a carbonyl is more likely to be attacked when a reaction is sensitive to shape, crowding, and 3D arrangement.

The basic idea is simple: the incoming nucleophile usually approaches the carbonyl carbon from the less crowded side. If one substituent around the carbonyl is bulkier than the others, the molecule tends to react in a way that keeps that bulky group away from the incoming reagent. That lower-energy approach can influence the stereochemistry of the product, especially when the carbonyl sits next to a chiral center or is part of a complex molecule.

This comes up most often in reduction reactions of carbonyl compounds, where a hydride donor adds to an aldehyde or ketone. Because carbonyl carbon is trigonal planar, attack can happen from either face, but the two faces are not always equally available. Prelog's Rule gives you a way to predict which face is more accessible when steric effects dominate. In a lab or problem set, that usually means deciding whether the product forms with one major stereoisomer over another.

The rule is not just about size, though. Electronic effects and the shape of nearby groups can also change the preferred pathway. Still, in many textbook-style examples, steric hindrance is the main feature you look for. A bulky substituent near the carbonyl blocks one side, so the nucleophile comes in from the opposite side and the product reflects that choice.

You will often see Prelog's Rule discussed alongside carbonyl reductions and other nucleophilic addition reactions. It is especially useful when the substrate already has stereochemistry built into it, because then the new stereocenter formed during addition can be predicted from the face of attack. That makes the rule a practical shortcut for drawing major products instead of guessing between two mirror-image outcomes.

## Why It Matters

Prelog's Rule matters because carbonyl reactions in Organic Chemistry II are not just about turning one functional group into another. They are also about controlling which stereoisomer you make, and that can change the identity, purity, and reactivity of the product.

When you reduce a chiral ketone or an alpha-substituted carbonyl, the new carbon center formed during nucleophilic addition can end up with different spatial arrangements depending on the attack angle. If you can predict the less hindered face, you can predict the major stereochemical outcome instead of treating the product as random.

That shows up in synthesis problems, where chemists want one major alcohol rather than a messy mixture. It also shows up in mechanism questions, where your job is to justify why one face is favored using steric crowding. Prelog's Rule gives you the language for that explanation: less hindered approach, face selectivity, and stereochemical outcome.

It also helps separate steric control from other models you may see in carbonyl chemistry. Not every carbonyl addition follows the same stereochemical logic, so knowing when a rule is based on bulky groups versus when another model is needed keeps your reasoning clean. In other words, this is one of the tools you use when the 3D structure of the substrate matters as much as the reagent.

## Connections

### [Stereochemistry](/organic-chemistry-ii/key-terms/stereochemistry)

Prelog's Rule is really a stereochemistry rule applied to carbonyl reactions. You use it to predict which face of a planar carbonyl gets attacked and, from that, which stereoisomer becomes the major product. If you cannot picture the 3D arrangement, the rule is hard to apply correctly.

### [Hydride Transfer](/organic-chemistry-ii/key-terms/hydride-transfer)

Many Prelog's Rule examples involve hydride transfer to a carbonyl. The hydride is the nucleophile, and its approach from the less crowded side often determines the product's configuration. When you see a reduction reagent, ask which face is blocked before you draw the alcohol product.

### Reduction

Prelog's Rule shows up most clearly in reduction reactions of aldehydes and ketones. Reduction adds electrons and hydrogen across the carbonyl, but the 3D direction of attack can change the stereochemistry. That is why the same starting carbonyl can give different major products depending on its substituents.

### [Felkin-Anh Model](/organic-chemistry-ii/key-terms/felkin-anh-model)

Felkin-Anh Model is a nearby stereochemical idea that also predicts nucleophilic attack on carbonyls, especially when a chiral center is adjacent to the carbonyl. Prelog's Rule focuses more on steric crowding and the favored side of approach, while Felkin-Anh gives a more detailed conformational explanation in many cases.

## On the AP Exam

A problem set or quiz question will usually give you a chiral carbonyl compound and ask for the major product after reduction or nucleophilic addition. You need to spot the bulky substituent, identify the less hindered face, and draw the product with the correct stereochemistry. If the question asks for explanation, use the structure itself as evidence, not just the name of the rule.

You may also be asked to compare two possible products and decide which is favored. In that case, the best answer is a short mechanism-minded justification: the nucleophile approaches from the side with less steric crowding, so that product forms in greater amount. If the substrate has multiple substituents, prioritize the one that most blocks access to the carbonyl face.

## Prelog's Rule vs Felkin-Anh Model

These get mixed up because both describe stereochemical outcomes in carbonyl additions. Prelog's Rule is a simpler steric guideline for which face a nucleophile prefers, while the Felkin-Anh Model gives a more detailed conformational explanation for adjacent chiral centers.

## Key Takeaways

- Prelog's Rule predicts the favored face of nucleophilic attack on a carbonyl when steric crowding controls the reaction.
- The nucleophile usually approaches from the less hindered side, which can determine the major stereoisomer in a reduction or addition.
- This rule is most useful when a carbonyl compound already has bulky substituents or nearby chirality that affects access to the carbonyl carbon.
- When you apply the rule, look at the 3D shape of the substrate, not just the formula, because face selectivity comes from structure.
- If a problem asks for the major product, your job is to connect steric hindrance to the direction of attack and then draw the correct configuration.

## FAQs

### What is Prelog's Rule in Organic Chemistry II?

Prelog's Rule is a stereochemical guideline for predicting how a nucleophile approaches a carbonyl compound. In Organic Chemistry II, you use it most often to decide which face of a carbonyl is attacked during reduction or nucleophilic addition. The less crowded side is usually favored.

### How do you apply Prelog's Rule to a carbonyl reduction?

First, identify the carbonyl carbon and any bulky groups attached to the system. Then decide which face is more blocked and which is more open to attack. The hydride or other nucleophile usually comes from the less hindered side, which helps you draw the major stereoisomer.

### Is Prelog's Rule the same as the Felkin-Anh Model?

No. They are related because both deal with stereochemical outcomes in carbonyl additions, but they are not identical. Prelog's Rule is a simpler steric prediction, while the Felkin-Anh Model explains attack using conformations and the arrangement of groups next to the carbonyl.

### Why does Prelog's Rule matter in synthesis problems?

Because the same carbonyl compound can give different stereoisomeric products depending on which side is attacked. In synthesis, that means the rule helps you predict the major product and explain selectivity. It is especially useful when a carbonyl is part of a larger, crowded molecule.

## Related Study Guides

- [3.3 Oxidation and reduction of carbonyls](/organic-chemistry-ii/unit-3/oxidation-reduction-carbonyls/study-guide/60N9U1uvOQARg2Ul)

## About This Document

Canonical Fiveable pages are available as Markdown at the same path plus `.md`.

- [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns
- [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing
- [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`)
- [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`)

## Structured Data

```json
{"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/organic-chemistry-ii/key-terms/prelogs-rule#resource","name":"Prelog's Rule | Organic Chemistry II","url":"https://fiveable.me/organic-chemistry-ii/key-terms/prelogs-rule","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/organic-chemistry-ii/key-terms/prelogs-rule#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:24:08.525Z","isPartOf":{"@type":"Collection","name":"Organic Chemistry II Key Terms","url":"https://fiveable.me/organic-chemistry-ii/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/organic-chemistry-ii/key-terms/prelogs-rule#term","name":"Prelog's Rule","description":"Prelog's Rule is a stereochemical guideline in Organic Chemistry II that predicts how nucleophiles approach carbonyl compounds. It is used to explain which product forms when sterics control oxidation-reduction reactions.","url":"https://fiveable.me/organic-chemistry-ii/key-terms/prelogs-rule","inDefinedTermSet":{"@type":"DefinedTermSet","name":"Organic Chemistry II Key Terms","url":"https://fiveable.me/organic-chemistry-ii/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is Prelog's Rule in Organic Chemistry II?","acceptedAnswer":{"@type":"Answer","text":"Prelog's Rule is a stereochemical guideline for predicting how a nucleophile approaches a carbonyl compound. In Organic Chemistry II, you use it most often to decide which face of a carbonyl is attacked during reduction or nucleophilic addition. The less crowded side is usually favored."}},{"@type":"Question","name":"How do you apply Prelog's Rule to a carbonyl reduction?","acceptedAnswer":{"@type":"Answer","text":"First, identify the carbonyl carbon and any bulky groups attached to the system. Then decide which face is more blocked and which is more open to attack. The hydride or other nucleophile usually comes from the less hindered side, which helps you draw the major stereoisomer."}},{"@type":"Question","name":"Is Prelog's Rule the same as the Felkin-Anh Model?","acceptedAnswer":{"@type":"Answer","text":"No. They are related because both deal with stereochemical outcomes in carbonyl additions, but they are not identical. Prelog's Rule is a simpler steric prediction, while the Felkin-Anh Model explains attack using conformations and the arrangement of groups next to the carbonyl."}},{"@type":"Question","name":"Why does Prelog's Rule matter in synthesis problems?","acceptedAnswer":{"@type":"Answer","text":"Because the same carbonyl compound can give different stereoisomeric products depending on which side is attacked. In synthesis, that means the rule helps you predict the major product and explain selectivity. It is especially useful when a carbonyl is part of a larger, crowded molecule."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Organic Chemistry II","item":"https://fiveable.me/organic-chemistry-ii"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/organic-chemistry-ii/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 3","item":"https://fiveable.me/organic-chemistry-ii/unit-3"},{"@type":"ListItem","position":4,"name":"Prelog's Rule"}]}]}
```