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7.4.4.2.2 - Reaction with phosphorus trihalides

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Introduction to Alcohol Reactions

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Teacher
Teacher

Today, we will discuss the reaction of alcohol with phosphorus trihalides. Can anyone tell me what we understand by phosphorus trihalides?

Student 1
Student 1

I think they are compounds like PCl3, PBr3, and PI3, which contain phosphorus and halogens.

Teacher
Teacher

Excellent! These compounds are commonly used to convert alcohols into alkyl halides. The hydroxyl group is replaced by a halogen.

Student 2
Student 2

So, how does this replacement actually happen in the reaction?

Mechanism of Alcohol to Alkyl Halide Conversion

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Teacher
Teacher

Let's delve into the mechanism now. The first step involves the formation of an alkoxide ion.

Student 3
Student 3

What do we mean by alkoxide ion?

Teacher
Teacher

Good question! An alkoxide ion is formed when the -OH group in alcohol gets replaced, facilitating the nucleophilic attack by halides.

Student 4
Student 4

Do these reactions happen quickly? How do we ensure they proceed to form the halide?

Teacher
Teacher

The reaction usually proceeds effectively due to lower activation energy provided by phosphorus trihalides. The intermediates formed help to push the reaction forward.

Practical Applications of the Reaction

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Teacher
Teacher

Why do you think we often convert alcohols to alkyl halides? What are some applications?

Student 1
Student 1

I suppose we use alkyl halides in many reactions for synthesis of other compounds, right?

Student 2
Student 2

Yes, like in the synthesis of ethers and other organic molecules!

Teacher
Teacher

Exactly! Alkyl halides are versatile intermediates in organic chemistry, helping to extend synthetic pathways effectively.

Introduction & Overview

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Quick Overview

This section discusses the conversion of alcohols to alkyl halides through reaction with phosphorus trihalides, highlighting the chemical mechanisms involved.

Standard

In this section, we learn about the reaction of alcohols with phosphorus trihalides, which leads to the formation of alkyl halides. The process is significant for understanding nucleophilic substitution reactions. The mechanism of this conversion is explored to reveal how alcohols lose their hydroxyl group and form halides effectively.

Detailed

The reaction involving alcohols with phosphorus trihalides (PCl3, PBr3, or PI3) offers an efficient method for converting alcohols into their corresponding alkyl halides. This process is particularly useful in organic synthesis where the -OH group of alcohols is substituted by a halogen. The mechanism of the reaction includes the formation of intermediate alkoxides that are subsequently transformed into alkyl halides. Plus, the use of phosphorus trihalides lowers the activation energy needed to facilitate the substitution reaction, which is essential in practical laboratory setups.

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Audio Book

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Overview of Reactions with Phosphorus Trihalides

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Alcohols are converted to alkyl bromides by reaction with phosphorus tribromide.

Detailed Explanation

Phosphorus trihalides, specifically phosphorus tribromide (PBr3), are reagents used to convert alcohols into alkyl bromides. In this reaction, the hydroxyl group (-OH) of the alcohol gets replaced by a bromine atom, resulting in the formation of an alkyl bromide. This conversion is particularly valuable because it allows for the transformation of alcohols, which are often less reactive, into alkyl bromides that can participate in various substitution and elimination reactions.

Examples & Analogies

You can think of this reaction like swapping an old appliance in your home for a new, more efficient one. Just like an old refrigerator being replaced with a new model that works better and serves your needs, here, the alcohol is replaced by a more reactive alkyl bromide that can take part in further chemical reactions more effectively.

Mechanism of the Reaction with Phosphorus Trihalides

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The mechanism involves the formation of an alkoxyphosphonium intermediate followed by elimination to give the alkyl bromide.

Detailed Explanation

In the mechanism of this reaction, when an alcohol reacts with phosphorus tribromide, the first step involves the alcohol forming an alkoxyphosphonium intermediate. In this step, the -OH group of the alcohol attacks phosphorus, resulting in the displacement of a bromide ion. This intermediate can then lose a bromide ion, resulting in the formation of the desired alkyl bromide. The overall reaction efficiently replaces the -OH group with a bromine atom.

Examples & Analogies

Imagine you’re changing a car's tire. First, you use a wrench to loosen the bolts (similar to the formation of the intermediate), and once they're loose, you can easily remove the flat tire and replace it with a new one (which represents the final alkyl bromide). The tools used to assist in this task represent the roll of phosphorus tribromide in facilitating the replacement of the hydroxyl group.

Types of Alcohols and Their Reactivity in This Reaction

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Primary, secondary, and tertiary alcohols behave differently with phosphorus trihalides. Generally, primary alcohols react more efficiently compared to tertiary alcohols.

Detailed Explanation

The reactivity of alcohols in reactions with phosphorus trihalides varies based on their structure. Primary alcohols typically react more readily because they can easily form stable intermediates that lead to the formation of the alkyl bromide. Secondary alcohols react at moderate speeds, while tertiary alcohols can sometimes form unwanted products due to steric hindrance, which may complicate or hinder the reaction process.

Examples & Analogies

Consider a crowded hallway in a school. If students (representing different types of alcohols) are trying to exit the hallway, primary alcohols have the easiest time getting out quickly, representing their efficiency in reactions. Secondary alcohols can make it through with a little more time, whereas tertiary alcohols might get stuck or delayed because of their bulky structure, making it harder for them to 'get through' the reaction.

Definitions & Key Concepts

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Key Concepts

  • Phosphorus Trihalides: Used to convert alcohols to alkyl halides.

  • Mechanism Details: Involves formation of alkoxide intermediates.

  • Practical Applications: Alkyl halides serve as intermediaries in organic synthesis.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • Conversion of ethanol to ethyl bromide using PBr3.

  • Conversion of isopropanol to isopropyl chloride using PCl3.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • To make a bromide, just take the alcohol, with PBr3, it will answer your call.

📖 Fascinating Stories

  • Imagine a party where alcohols dance away and phosphorus trihalides come to replace them with stylish halides.

🧠 Other Memory Gems

  • Remember A-P-H: Alcohol, Phosphorus, Halide - the sequence of transformation.

🎯 Super Acronyms

APHA

  • Alcohols Transform to Phosphorus Halides for Alkyls.

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: Alcohol

    Definition:

    An organic compound containing one or more hydroxyl (-OH) groups.

  • Term: Alkyl Halide

    Definition:

    A compound derived from an alkane by replacing one or more hydrogen atoms with halogens.

  • Term: Phosphorus Trihalides

    Definition:

    Compounds containing phosphorus and three halogen atoms, commonly used as reagents in organic synthesis.

  • Term: Alkoxide Ion

    Definition:

    An ion formed from an alcohol by the removal of a hydrogen atom from the hydroxyl group.