Chlorination of Alcohols

Creating chloroalkanes can be a very good intermediate step to making other useful organic compounds. One of the methods used to chlorinate alcohols can also be used as a preliminary test to classify unknown alcohol samples as primary, secondary or tertiary.

The -OH group is replaced by a more "nucleus loving" -Cl group. Therefore, this is called nucleophilic substitution.

1. Thionyl Chloride

This reagent (SOCl₂) will react with all alcohols, whether they are primary, secondary or tertiary. Therefore, it is often most commonly used in organic pathways/synthesis. The negative aspect of using this reagent is that it produces sulfur dioxide and hydrogen chloride gases, which are both toxic and highly acidic...

CH₃CH₂CH₂OH + SOCl₂ → CH₃CH₂CH₂Cl + SO₂ +HCl

As an alternative, you can also use PCl₃ or PCl₅. These make POCl and HCl, and POCl₃ and HCl as their products (respectively).

2. Lucas Reagent

Lucas Reagent is made by dissolving anhydrous zinc chloride in concentrated hydrochloric acid. This saturation of chloride ions, and low concentration of water, allows for nucleophilic substitution to occur. However, it only happens readily with tertiary alcohols, and with only up to approximately eight carbons.

SOURCE: By Talos & Andel - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2855381

PRIMARY ALCOHOLS

No reaction observed. A single clear, colourless layer forms as the two solutions mix. Due to the nature of Lucas Reagent and the ability of alcohols to form hydrogen bonds, these two layers are miscible.

SECONDARY ALCOHOLS

The rate of reaction is very low for secondary alcohols. Initially, there is no observable reaction, and a single clear, colourless layer forms. If left for approximately five minutes, two layers should form. One will be cloudy (containing the immiscible chloroalkane), and the other will be clear and colourless (containing the aqueous reactants and products).

TERTIARY ALCOHOLS

The Lucas Test is most effective with tertiary alcohols. Almost immediately, two layers will form. The cloudy layer will be the immiscible chloroalkane, and the clear, colourless layer will be the aqueous species.

Using the Lucas Test in conjunction with oxidation (permanganate is quickest), it can be very simple to classify unknown alcohol samples as primary, secondary or tertiary.