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Suppose you have three test tubes of unknown organic molecules. You know that they are Alcohols, but beyond that, you're clueless. It would be useful to be able to classify these molecules and determine whether they are primary, secondary, or tertiary alcohols. The Lucas test is an easy means of alcohol classification and is a great example of the halogenation of Alcohols.
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Jetzt kostenlos anmeldenSuppose you have three test tubes of unknown organic molecules. You know that they are Alcohols, but beyond that, you're clueless. It would be useful to be able to classify these molecules and determine whether they are primary, secondary, or tertiary alcohols. The Lucas test is an easy means of alcohol classification and is a great example of the halogenation of Alcohols.
Halogenation of alcohols is the process of swapping an alcohol’s hydroxyl group (-OH) for a halogen atom (-X).
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.
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:
$$ROH+HX\rightarrow RX+H_{2}O$$
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.
Halogenation of alcohols using hydrogen chloride (HCl) forms the basis of the Lucas test, used to differentiate alcohols by their classification. Alcohols react with Lucas' reagent (an equimolar solution of zinc chloride (ZnCl2) and HCl) to produce a halogenoalkane, turning the clear and colourless solution cloudy. However, the necessary conditions and rate of reaction depend on whether the alcohol is primary, secondary, or tertiary:
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.
Write an equation for the reaction between propan-1-ol and hydrogen bromide. Give the names of the reactants used to make hydrogen bromide and state the reaction conditions.
Hydrogen bromide reacts with alcohols to give a halogenoalkane and water. Here we use propan-1-ol and so create 1-bromopropane:
$$CH_{3}CH_{2}CH_{2}OH + HBr \rightarrow CH_{3}CH_{2}CH_{2}Br + H_{2}O $$
The hydrogen bromide is created in situ by mixing potassium bromide (or sodium bromide) with concentrated sulphuric acid alongside the alcohol. The whole reaction takes place under reflux.
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.
We can’t use sulphuric acid to make hydrogen iodide because iodide ions (I-) are a strong enough reducing agent to reduce the sulphur within. This in turn oxidises the iodide ions into iodine atoms (I), rendering them useless in a nucleophilic substitution reaction with alcohol! However, we can use phosphoric acid to make hydrogen bromide.
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).
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.
Write an equation for the reaction between phosphorus(V) chloride and ethanol. Name the halogenoalkane produced.
Phosphorus(V) chloride reacts with ethanol (CH3CH2OH) at room temperature to form chloroethane (CH3CH2Cl), hydrochloric acid and phosphoryl chloride.
$$CH_{3}CH_{2}OH + PCl_{5} \rightarrow CH_{3}CH_{2}Cl + HCl + POCl_{3}$$
Phosphorus(III) chloride (PCl3) reacts with alcohols to produce a chloroalkane (RCl) and phosphorus acid (H3PO3). The reaction requires heating under reflux.
$$3ROH + PCl_{3} \rightarrow 3RCl + H_{3}PO_{3}$$
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.
$$3ROH + PBr_{3} \rightarrow 3RBr + H_{3}PO_{3}$$
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.
$$3ROH + PI_{3} \rightarrow 3RI + H_{3}PO_{3}$$
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:
$$ROH + SOCl_{2} \rightarrow RCl + HCl + SO_{2}$$
Write an equation for the reaction of propan-1,3-diol with an excess of thionyl chloride. Name the organic product produced.
Propan-1,3-diol (CH2OHCH2CH2OH) is a diol and so reacts with thionyl chloride (SOCl2) to produce a dichloroalkane, hydrochloric acid (HCl) and sulphur dioxide (SO2). Here, the dichloroalkane is 1,3-dichloropropane (CH2ClCH2CH2Cl):
$$CH_{2}OHCH_{2}CH_{2}OH + 2SOCl_{2} \rightarrow CH_{2}ClCH_{2}CH_{2}Cl + 2HCl + 2SO_{2}$$
Note that the alcohol in the example above is a diol, meaning that it has two hydroxyl Functional Groups. Both hydroxyl groups can be substituted for a chlorine atom in an excess of SOCl2. As a result, the complete halogenation of this molecule requires two moles of SOCl2. The reaction also produces two moles of HCl and two moles of SO2.
Chlorination using SOCl2 is a common test for the hydroxyl functional group (-OH) because of the tell-tale steamy fumes of HCl it releases. However, HCl and the additional gaseous product (SO2) are both toxic, so this reaction should always be carried out in a fume cupboard.
All alcohols can be halogenated in nucleophilic substitution reactions. However, the various reactions differ slightly. Here's how:
The difference in reactivity between the various alcohol classifications and halide ions explains why chlorination of primary alcohols using HCl requires a ZnCl2 catalyst.
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.
Fig. 1: The mechanisms for the nucleophilic substitution halogenation reactions of primary (top), and secondary and tertiary alcohols (bottom).StudySmarter Originals
Check out Nucleophilic Substitution Mechanism for more about SN1 and SN2 mechanisms, including their similarities and differences.
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.
| Reactant | HX | PCl5 | PX3 | SOCl2 |
| Conditions | Reflux ( + ZnCl2 catalyst if primary alcohol) | Room temperature | Reflux | Room temperature |
| Products | RX + H2O | RX + HCl + POCl3 | RX + H3PO3 | RCl + HCl + SO2 |
Halogenation of alcohols (ROH) is the process of swapping an alcohol’s hydroxyl group (-OH) for a halogen atom (-X).
Alcohols react with HCl to produce a chloroalkane. The reaction takes place under reflux, and primary alcohols also require a zinc chloride (ZnCl2) catalyst.
Alcohols are halogenated in nucleophilic substitution reactions. First, the hydroxyl (-OH) group is turned into a better leaving group. Next, this leaving group is replaced by a halogen atom (-X). The nucleophile is a halide ion (X-).
Primary alcohols react using an SN2 mechanism, in which the leaving group is removed from the molecule at the same time as the nucleophile attacks and forms the new C-X bond. Secondary and tertiary alcohols react using an SN1 mechanism, in which the leaving group is first removed, forming a positive carbocation, before the nucleophile attacks and forms the new C-X bond. Both mechanisms produce the same final products: a halogenoalkane and water.
Alcohols don't react directly with halogens. However, they do react with hydrogen halides to produce halogenoalkanes in an example of a nucleophilic substitution halogenation reaction.
Flashcards in Halogenation of Alcohols15
Start learningWhat is halogenation of alcohols?
Halogenation of alcohols is the process of swapping an alcohol’s hydroxyl group (-OH) for a halogen atom (-X).
Give the general equation for the reaction between a hydrogen halide (HX) and an alcohol.
HX + ROH → RX + H2O
Give the general equation for the reaction between PCl5 and an alcohol.
ROH + PCl5 → RCl + HCl + POCl3
Give the general equation for the reaction between PCl3 and an alcohol.
3ROH + PX3 → 3RX + H3PO3
Give the general equation for the reaction between SOCl2 and an alcohol.
ROH + SOCl2 → RCl + HCl + SO2
Give the conditions for the reaction between a hydrogen halide and an alcohol.
Reflux ( + ZnCl2 catalyst if primary alcohol)
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