---
title: "Anti-Addition Mechanism | Organic Chemistry"
description: "Anti-addition mechanism in Organic Chemistry describes opposite-side addition across a double bond, like oxymercuration hydration that gives anti products."
canonical: "https://fiveable.me/organic-chem/key-terms/anti-addition-mechanism"
type: "key-term"
subject: "Organic Chemistry"
unit: "Unit 8"
---

# Anti-Addition Mechanism | Organic Chemistry

## Definition

Anti-addition mechanism is when two groups add to opposite faces of a carbon-carbon double bond, giving an anti product. In Organic Chemistry, you see it most clearly in oxymercuration hydration.

## What It Is

Anti-addition mechanism is an addition pathway in Organic Chemistry where the two new groups end up on opposite sides of the alkene. If one atom or group adds to the top face of the double bond, the other adds to the bottom face, so the product is anti, not syn.

You usually meet this idea in oxymercuration-demercuration. The alkene first reacts with mercury(II) acetate to form a bridged mercurinium ion, not a free carbocation. That three-membered intermediate blocks one face of the double bond and pushes the next nucleophile attack to the opposite side.

That is why anti addition shows up here so reliably. Water attacks the more substituted carbon of the mercurinium ion, because that carbon carries more positive character, while mercury stays attached to the other carbon. The ring opening happens from the backside relative to the mercury bridge, which gives the anti arrangement.

After that, sodium borohydride replaces the mercury-containing group with hydrogen. The overall result is a Markovnikov alcohol, but the stereochemistry of the addition is still anti because the groups were introduced from opposite faces during the mechanism.

A good way to picture it is to think about access. In oxymercuration, the bridge forces the nucleophile to approach from the side opposite the mercury. You are not looking at two reagents randomly landing on an alkene. You are tracking a specific intermediate that controls where the next bond forms and from which face it can form it.

This is also why anti addition is different from a simple dictionary-style idea of “addition.” In Organic Chemistry, addition reactions are judged by both regiochemistry and stereochemistry. Anti addition tells you the stereochemistry, meaning how the atoms are arranged in 3D after the reaction. For alkene problems, that 3D outcome matters just as much as which carbon gets which group.

A common mistake is to assume all alkene hydration reactions give the same spatial product. They do not. Acid-catalyzed hydration goes through a carbocation and can give different stereochemical outcomes, while oxymercuration gives anti addition because the mercurinium ion forces backside attack. If you are drawing the product, you need to show the substituents on opposite faces, often with wedges and dashes.

So, in this course, anti-addition mechanism means a specific 3D pattern of bond formation, usually tied to a bridged intermediate like the mercurinium ion. It is not just “adding atoms to a double bond.” It is the reason the product comes out with a particular stereochemical arrangement after oxymercuration-demercuration.

## Why It Matters

Anti-addition mechanism matters because it lets you predict the actual structure of the product, not just the molecular formula. In Organic Chemistry, that is the difference between getting a reaction mostly right and getting it completely right. Two products can have the same atoms attached to the same carbons, but if the groups are on opposite faces versus the same face, they are not drawn the same way and they may behave differently.

This term also connects directly to how reaction mechanisms work step by step. Oxymercuration is not memorized as a one-line “hydration of alkenes” label. You need to know that the mercurinium ion intermediate controls the attack, which explains why the product is anti and why rearrangements do not happen the way they can in acid-catalyzed hydration.

It also trains your eye for stereochemistry questions. If a problem asks you to predict the product of alkene hydration, you are often expected to recognize whether the mechanism is syn or anti and to place wedges and dashes correctly. That skill shows up in drawing mechanisms, comparing products, and explaining why one route gives a cleaner result than another.

Anti-addition is especially useful when you are comparing hydration methods. Oxymercuration gives Markovnikov alcohols without carbocation rearrangement, and the anti-addition piece tells you how the groups arrive in 3D. That combination makes the reaction a standard tool in synthesis problems and mechanism quizzes alike.

## Connections

### Oxymercuration

Anti addition is usually discussed through oxymercuration, because the reaction creates the mercurinium ion that controls the face of attack. If you know oxymercuration, you can explain why the nucleophile cannot just approach from any side. The bridge forces a backside opening, which is what makes the stereochemistry anti.

### [Oxymercuration-Demercuration](/organic-chem/key-terms/oxymercuration-demercuration)

The full two-step sequence matters because anti addition is only part of the story. Step one sets the stereochemistry when water opens the mercurinium ion, and step two removes mercury with sodium borohydride. When you draw the final alcohol, you need to keep track of how the first step set the 3D arrangement.

### [Acid-Catalyzed Hydration](/organic-chem/key-terms/acid-catalyzed-hydration)

This is a common comparison because it also converts alkenes to alcohols, but it does not follow the same mechanism. Acid-catalyzed hydration goes through a carbocation, which can rearrange and does not give the same anti-addition pattern. If a problem asks which method gives a cleaner product, this contrast is usually the reason.

### [Electrophile](/organic-chem/key-terms/electrophile)

In oxymercuration, the electrophile is the species that starts the reaction by accepting electron density from the alkene. Mercury(II) acetate acts as the electrophilic partner and helps form the bridged intermediate. Understanding the electrophile step makes it easier to see why the double bond opens the way it does.

## On the AP Exam

A mechanism question may show an alkene and ask you to predict the product of oxymercuration-demercuration. That is where anti-addition shows up: you have to draw the alcohol with the correct Markovnikov placement and the correct 3D relationship from the first addition step. If the prompt gives you wedge and dash choices, you are checking whether the new groups landed on opposite faces of the former double bond.

You may also see it in a compare-and-contrast question with acid-catalyzed hydration. The move is to explain that oxymercuration avoids carbocation rearrangements because of the mercurinium ion, and that the nucleophile opens the bridge from the backside, giving anti addition. If you can trace the intermediate to the product, you usually have the question handled.

## Key Takeaways

- Anti-addition means the two new groups add to opposite faces of a double bond, so the product has a specific 3D arrangement.
- In oxymercuration, the mercurinium ion intermediate forces backside attack, which is why the reaction gives anti addition.
- The overall product is still a Markovnikov alcohol after demercuration, even though the first addition step is anti.
- You should show anti addition with correct wedges and dashes when drawing the product of an alkene reaction.
- Anti addition is different from simple alkene addition because the mechanism controls stereochemistry, not just which atoms are attached.

## FAQs

### What is anti-addition mechanism in Organic Chemistry?

It is an addition process where the two new bonds form on opposite sides of a carbon-carbon double bond. In Organic Chemistry, this is often taught with oxymercuration, where a bridged mercurinium ion makes backside attack possible. The stereochemistry of the product is what makes it anti.

### Why does oxymercuration give anti addition?

Oxymercuration forms a mercurinium ion, which is a three-membered bridged intermediate. That bridge blocks one face of the alkene, so the nucleophile attacks from the opposite side. The result is anti addition rather than syn addition.

### Is anti addition the same as Markovnikov addition?

No, they describe different things. Markovnikov refers to which carbon gets which group, while anti addition refers to the 3D relationship between the new groups. Oxymercuration can give Markovnikov regiochemistry and anti stereochemistry at the same time.

### How do I draw the product of anti addition?

Start with the alkene, then place the two new groups on opposite faces of the former double bond. Use wedges and dashes to show that one bond comes out of the page and the other goes behind it. If the reaction is oxymercuration, also keep the Markovnikov placement of the alcohol in mind.

## Related Study Guides

- [8.4 Hydration of Alkenes: Addition of H2O by Oxymercuration](/organic-chem/unit-8/hydration-alkenes-addition-h2o-oxymercuration/study-guide/FObBVumov9Qwl6sp)

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