7.4.4.2.2. Reaction with phosphorus trihalides

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

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.

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.

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

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.

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.

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

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.

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.

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