Halogenation of Alcohols

Types of halogenation of alcohols

If you’ve read the article Nucleophilic Substitution Reactions, you’ll be familiar with how we can turn halogenoalkanes (RX) into alcohols (ROH) using a nucleophilic substitution reaction. In this reaction, a hydroxide ion nucleophile (:OH^-) attacks a halogenoalkane and swaps out its halogen atom (-X) functional group for the hydroxyl (-OH) functional group.

Turning an alcohol back into a halogenoalkane is also a nucleophilic substitution reaction. This is known as halogenation. Howver, the halogenation of an alcohol requires an extra step when compared to going from halogenoalkane to alcohol, despite both being examples of nucleophilic substitution reactions. You see, the hydroxyl group in alcohols is a bad leaving group - it’s quite hard to separate it from the rest of the molecule. The resulting hydroxide ion also reacts rapidly with the halogenoalkane produced to form an alcohol again. In order to halogenate alcohols, we first need to turn the hydroxyl group into a better leaving group. Once we’ve done that, the nucleophilic substitution reaction can go ahead as desired.

There are a few different ways of improving the hydroxyl group's leaving group ability. Because of this, there are multiple ways of halogenating alcohols and so multiple types of alcohol halogenation. Here’s a list of the methods that we’ll look at in this article:

• Halogenation using hydrogen halides (HX).

• Halogenation using phosphorus halides (PX₃).

• Chlorination using thionyl dichloride (SOCl₂; also known as thionyl chloride or sulphur dichloride oxide).

Let’s start with the halogenation of alcohols using hydrogen halides.

Halogenation of alcohols with hydrogen halides

Reacting an alcohol with a hydrogen halide (HX), such as hydrogen chloride (HCl), hydrogen bromide (HBr), or hydrogen iodide (HI) forms a halogenoalkane (RX) and water (H₂O). The reaction takes place under reflux, and the halogenoalkane is separated from the resulting solution using distillation. Like all alcohol halogenation reactions, this is an example of a nucleophilic substitution reaction.

Here's the general equation:

$$ROH+HX\rightarrow RX+H_{2}O$$

Halogenation of primary alcohols: Chlorination with HCl

We can chlorinate alcohols by reacting hydrogen chloride (HCl) with the desired alcohol under reflux. If we want to chlorinate a primary alcohol, we also need to add in some sort of catalyst such as zinc chloride (ZnCl₂).

Alternatively, we can create the hydrogen chloride in situ by adding potassium or sodium chloride (KCl or NaCl) and concentrated sulphuric acid (H₂SO₄) to the reflux mixture. Both methods form a chloroalkane (RCl) and water.

Bromination with HBr

Alcohols are brominated in a reaction with hydrogen bromide (HBr). We typically create the hydrogen bromide in situ, by adding specific reactants to the reflux system. In this case, we mix potassium or sodium bromide (KBr or NaBr) with concentrated sulphuric acid (H₂SO₄). Heated under reflux, this forms a bromoalkane (RBr) and water.

Iodination with HI

In much the same way, alcohols are iodinated by reacting them with hydrogen iodide (HI). Hydrogen iodide also needs making in situ, and we form it by combining potassium or sodium iodide (KI or NaI) with concentrated phosphoric (V) acid (H₃PO₄). Heating with an alcohol under reflux results in an iodoalkane and water.

Halogenation of alcohols with phosphorus halides

Another method of alcohol halogenation involves reacting alcohols with phosphorus halides. This is a further example of a nucleophilic substitution reaction. We'll look at reactions involving phosphorus(V) chloride (PCl₅), phosphorus(III) chloride (PCl₃), phosphorus(III) bromide (PBr₃), and phosphorus(III) iodide (PI₃).

Chlorination with PCl₅

Phosphorus chloride (PCl₅) reacts with alcohols to produce a chloroalkane (RCl), hydrochloric acid (HCl), and phosphoryl chloride (POCl₃). The reaction takes place at room temperature. Here's the equation:

$$ROH + PCl_{5} \rightarrow RCl + HCl + POCl_{3}$$

Hydrochloric acid and phosphoryl chloride are both gases, which means that provided the reaction goes to completion, you should end up with a pure final product.

Chlorination with PCl₃

Phosphorus(III) chloride (PCl₃) reacts with alcohols to produce a chloroalkane (RCl) and phosphorus acid (H₃PO₃). The reaction requires heating under reflux.

$$3ROH + PCl_{3} \rightarrow 3RCl + H_{3}PO_{3}$$

Bromination with PBr3

Phosphorous(III) bromide (PBr₃) also reacts with alcohols under reflux. You might be able to guess that this reaction forms a bromoalkane (RBr) and phosphorus acid. However, this time we need to make the phosphorus bromide in situ by adding phosphorus (P) and bromine water (Br₂), along with the alcohol, to the reflux vessel.

$$3ROH + PBr_{3} \rightarrow 3RBr + H_{3}PO_{3}$$

Iodination with PI3

We can iodinate alcohols similarly: by refluxing them with phosphorus(III) iodide (PI₃). Like phosphorus bromide, phosphorus iodide needs making in situ by combining phosphorus and iodine (I₂) in the reflux vessel. The reaction produces an iodoalkane (RI) and phosphorus acid.

$$3ROH + PI_{3} \rightarrow 3RI + H_{3}PO_{3}$$

Halogenation of alcohols with SOCl₂

Next up: let's consider how you chlorinate alcohols using thionyl dichloride (SOCl₂; also known as thionyl chloride or sulphur dichloride oxide). Like before, this reaction is an example of a nucleophilic substitution reaction. It takes place at room temperature.

Reacting an alcohol with thionyl dichloride produces a chloroalkane (RCl), hydrochloric acid (HCl), and sulphur dioxide (SO₂). Here's the general equation:

$$ROH + SOCl_{2} \rightarrow RCl + HCl + SO_{2}$$

Halogenation of primary, secondary, and tertiary alcohols

All alcohols can be halogenated in nucleophilic substitution reactions. However, the various reactions differ slightly. Here's how:

• Tertiary alcohols react much faster than secondary alcohols, which in turn react faster than primary alcohols.
• The rate of halogenation also depends on the halide ion used. Iodination is faster than bromination, which in turn is faster than chlorination.
• Secondary and tertiary alcohols react using an SN₁ mechanism, whilst primary alcohols react using an SN₂ mechanism.

Halogenation of alcohols mechanism

Want to find out about the mechanism for the nucleophilic substitution reaction between hydrogen halides and alcohols? You don’t need to know this for your exam, but mechanisms are always fun to learn.

We mentioned earlier on in the article that alcohol nucleophilic substitution requires an extra step, compared to halogenoalkane nucleophilic substitution. In this extra step, the alcohol’s hydroxyl group (-OH) is converted into a better leaving group. The following nucleophilic substitution part of the reaction uses either an SN₁ or SN₂ mechanism, depending on the alcohol classification. However, the nucleophile is always a negative halide ion (X^-). Here’s what happens.

1. In the first step, the hydroxyl group’s oxygen atom attacks the hydrogen halide’s hydrogen atom using one of its lone pairs of electrons. This creates an organic intermediate with a positive -H₂O⁺ group and a negative halide ion (X^-).
2. Primary alcohols then react using an SN₂ mechanism. The negative halide ion attacks one of the organic intermediate’s carbon atoms using its lone pair of electrons, forming a C-X bond.
3. As the new C-X bond forms, the C-H₂O⁺bond simultaneously breaks.
4. Secondary and tertiary alcohols instead react using an SN₁ mechanism. First, the C-H₂O⁺bond breaks, releasing water and leaving a positive carbocation.
5. Next, the halide ion attacks the positive carbocation, forming a new C-X bond.
6. Overall, both mechanisms end up with the same products: a halogenoalkane and water.

Fig. 1: The mechanisms for the nucleophilic substitution halogenation reactions of primary (top), and secondary and tertiary alcohols (bottom).StudySmarter Originals

Summary of halogenation of alcohols

To conclude, let's recap alcohol halogenation. Here's a handy table that should help you summarise and compare the different halogenation reactions of alcohols.