---
title: "Phosphonium Salt | Organic Chemistry"
description: "Phosphonium salt is a positively charged phosphorus species used as a precursor to phosphorus ylides in Organic Chemistry, especially the Wittig reaction."
canonical: "https://fiveable.me/organic-chem/key-terms/phosphonium-salt"
type: "key-term"
subject: "Organic Chemistry"
unit: "Unit 19"
---

# Phosphonium Salt | Organic Chemistry

## Definition

A phosphonium salt is a phosphorus compound with four covalent bonds and a positive charge. In Organic Chemistry, it is often the starting material used to make a phosphorus ylide for the Wittig reaction.

## What It Is

A phosphonium salt is a positively charged phosphorus compound in Organic Chemistry, usually made when a phosphine such as triphenylphosphine reacts with an alkyl halide. The phosphorus atom ends up bonded to four carbon groups or substituents, so it carries a formal positive charge.

The most common way to make one is a nucleophilic substitution reaction. A phosphine uses its lone pair to attack an alkyl halide like benzyl bromide, the halide leaves, and you get a stable phosphonium salt. That stability is one reason these compounds are easy to isolate, store, and use later.

The real reason phosphonium salts show up so often in this course is that they are the precursors to phosphorus ylides. If you treat a phosphonium salt with a strong base such as n-butyllithium, a proton next to the phosphorus can be removed. That gives a ylide, which has a carbon atom with a negative charge next to the positively charged phosphorus.

That ylide is the reactive species in the Wittig reaction. It attacks a carbonyl group, forming a new carbon-carbon bond and eventually replacing the C=O with a C=C bond. So the phosphonium salt is not usually the final product you care about, but it is the setup step that makes the alkene synthesis possible.

A good way to think about it is: phosphonium salt first, ylide second, alkene product last. The salt itself is just the charged, isolable form that lets you control when the ylide is generated and what substituents it carries. Those substituents can affect how easily the ylide forms and, in some cases, the alkene stereochemistry you get.

## Why It Matters

Phosphonium salts matter because they connect a simple substitution reaction to one of the most useful carbon-carbon bond-forming methods in Organic Chemistry. Once you recognize the salt, you can predict that the molecule may be one step away from a phosphorus ylide and a Wittig reaction.

This term also shows up when you are tracing reaction mechanisms. You need to know what is being attacked, what leaves, and what the charge on phosphorus means. If you can identify the phosphonium salt, you can usually explain why a strong base is needed next and why the reaction can produce an alkene from a carbonyl compound.

It also helps you sort out reagent roles. Triphenylphosphine, an alkyl halide, the phosphonium salt, the base, and the ylide all do different jobs. On problem sets, that distinction makes it easier to predict intermediates instead of memorizing the whole reaction as a black box.

In synthesis questions, phosphonium salts are a useful hint that the target molecule may have been built by stitching together two smaller fragments. If you see a new alkene where a carbonyl used to be, the phosphonium salt is part of the logic that gets you there.

## Connections

### Phosphorus Ylide

A phosphonium salt is the precursor, and the ylide is the reactive species you get after a base removes a proton next to phosphorus. The ylide is what actually attacks the carbonyl carbon in the Wittig reaction. If you mix these two up, the mechanism gets confusing fast, because the salt itself is not the nucleophile that forms the new C-C bond.

### Wittig Reaction

Phosphonium salts are part of the setup for the Wittig reaction, which converts a carbonyl compound into an alkene. The salt is deprotonated to form a ylide, and that ylide drives the transformation. When you study the mechanism, the salt marks the point before the carbonyl attack begins.

### [Benzyl Bromide](/organic-chem/key-terms/benzyl-bromide)

Benzyl bromide is a common alkyl halide used to form phosphonium salts with phosphines. It is a good example of the substitution step that makes the salt in the first place. If you see benzyl bromide in a synthesis question, it may be there to build the phosphonium salt before the Wittig step.

### [n-butyllithium](/organic-chem/key-terms/n-butyllithium)

n-Butyllithium is one of the strong bases used to turn a phosphonium salt into a phosphorus ylide. It removes the acidic proton next to the phosphorus atom. In mechanism problems, seeing this reagent is a strong clue that the salt is about to become the ylide.

## On the AP Exam

A quiz question may show a reagent set like triphenylphosphine, an alkyl halide, and base, then ask you to name the intermediate or predict the product. You should be able to spot the phosphonium salt, explain how it forms by substitution, and then trace how deprotonation gives the ylide. In mechanism problems, the key move is identifying why phosphorus ends up positively charged and why that makes the next step possible.

You may also see a structure and need to decide whether it is the salt or the ylide. Look for the four-bonded phosphorus cation if it is the salt, and look for the negatively charged carbon next to phosphorus if it is the ylide. That distinction often changes the whole reaction pathway.

## Phosphonium Salt vs Phosphorus Ylide

A phosphonium salt is the protonated, positively charged precursor, while a phosphorus ylide is the deprotonated reactive species. The salt is usually isolated or generated first, and the ylide is formed from it with a strong base. In Wittig mechanism questions, confusing them can make you assign the nucleophilic attack to the wrong species.

## Key Takeaways

- A phosphonium salt is a positively charged phosphorus compound with four covalent bonds.
- It is often made when a phosphine reacts with an alkyl halide by nucleophilic substitution.
- In Organic Chemistry, phosphonium salts are best known as precursors to phosphorus ylides.
- A strong base removes a proton next to phosphorus to form the ylide used in the Wittig reaction.
- If you can identify the salt, you can usually predict the next step in an alkene synthesis.

## FAQs

### What is phosphonium salt in Organic Chemistry?

A phosphonium salt is a phosphorus compound with four covalent bonds and a positive charge. In Organic Chemistry, it is usually made from a phosphine and an alkyl halide, then converted into a phosphorus ylide for the Wittig reaction.

### How is a phosphonium salt formed?

It forms when a phosphine, such as triphenylphosphine, attacks an alkyl halide and the halide leaves. That substitution gives phosphorus four substituents and a formal positive charge. This is a common setup step before ylide formation.

### Is a phosphonium salt the same as a phosphorus ylide?

No. The phosphonium salt is the charged precursor, while the ylide is made after a strong base removes a proton next to phosphorus. The ylide is the species that actually reacts with the carbonyl in the Wittig reaction.

### Why are phosphonium salts used in the Wittig reaction?

They are convenient starting materials for making phosphorus ylides. Because the salt is stable and isolable, you can generate the ylide when you are ready to run the reaction. That gives you a controlled way to form a new carbon-carbon double bond.

## Related Study Guides

- [19.11 Nucleophilic Addition of Phosphorus Ylides: The Wittig Reaction](/organic-chem/unit-19/nucleophilic-addition-phosphorus-ylides-wittig-reaction/study-guide/pph4tD2Ck0sWNewl)

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