Halides are important for our health. Iodine, in particular, helps with thyroid function. But halides can also react in interesting ways. Let's focus on how they act as reducing agents and how this reaction changes as we move down the periodic table. We'll also explore the reactions of hydrogen halides and organohalides.

What are halides?

Halide ions are negatively charged anions formed from halogen atoms. They play a crucial role in our health, and an iodine deficiency can cause intellectual disability. Almost two billion people worldwide are deficient in iodine, which can reduce the average IQ score by 12 points. Chloride ions are also important for plant growth, and cereal grains contain between 10-20 ppm of chlorine. Hydrogen halides are formed when hydrogen reacts with a halogen atom. When they react with water, alcohols, or ammonia, interesting things happen. Let's explore these reactions further.

Reaction with water

Hydrogen chloride, hydrogen bromide, and hydrogen iodide react with water to form a strong acid. Acids are proton donors, and hydrochloric acid is an example of a strong acid that consists of a hydronium ion and a chloride ion. Hydrobromic acid and hydroiodic acid are also strong acids. However, hydrofluoric acid is a weak acid that only partially ionizes in solution because some of the ions form tightly-bound ion pairs.

The term dissociate means to split into separate parts, and when it comes to acids, they split apart into ions. Weak acids partially dissociate in solution, whereas strong acids fully dissociate. Therefore, hydrofluoric acid is a weak acid because it is only partially ionized.

Reaction with alcohols

Reacting hydrogen halides with alcohols produces alkyl halides, also known as halogenoalkanes. This reaction can also be done using phosphorus halides such as PCl5 or PBr3. For example, reacting ethanol with hydrogen bromide produces chloroethane. The reaction mechanism involves one of the oxygen atom's lone pairs of electrons attacking the partially positive hydrogen atom in hydrogen bromide, adding the hydrogen to the alcohol. Water is then eliminated from the alcohol, leaving behind a carbocation. The negative bromide ion then adds to the carbocation, forming a halogenoalkane.

Reaction with ammonia

Hydrogen halides react with ammonia to produce ammonium halides, such as ammonium chloride when reacting hydrogen chloride with ammonia.

Another important reaction to learn when dealing with halides is their reaction with acidified silver nitrate solution, AgNO3. This is a common way of identifying halide ions in solution. The result of the reaction depends on the halide's reactivity, and the silver halide precipitate's color indicates the type of halide present. For example, silver chloride forms a white precipitate, silver bromide forms a cream-colored precipitate, and silver iodide forms a yellow precipitate. Adding ammonia solution afterward can help confirm the results.

Experiment

To perform the test for halide ions, first add a few drops of nitric acid to the unknown halide solution to react with any soluble carbonate or hydroxide impurities. Nitric acid should be used as it does not form precipitates with silver ions, unlike hydrochloric acid or sulfuric acid.

Next, add a few drops of silver nitrate solution and observe any changes. Chlorine, bromine, or iodine present in the solution should form a precipitate of silver chloride, bromide, or iodide, respectively. Silver fluoride is soluble in water and will not produce any observable results.

Further testing can be done by adding ammonia solution to see if any precipitates dissolve in dilute or concentrated ammonia solution. Recording observations in a table can help keep track of results. If all goes well, the test should produce the expected results.

‍

Equation

In the reaction between a sodium halide and silver nitrate solution, a silver halide (AgX) is formed. The solubility of this compound depends on the solubility product value, which is determined by multiplying the concentrations of the respective ions together. If the product of the two concentrations is less than or equal to the solubility product value, the silver halide will dissolve and no precipitate will form. However, if the product exceeds the solubility value, a precipitate forms.

Silver halides have different solubility product values, with silver chloride having a higher value than silver iodide. When silver halides dissolve in ammonia solution, they form complex ions. Silver is a transition metal, and ammonia can bond to it using a dative covalent bond. Each positive silver ion bonds to two neutral ammonia molecules, forming a complex ion with a positive charge that is attracted to the negative halide ions in solution. This results in the formation of a salt known as a diamine silver halide.

Adding ammonia to the solution reduces the ion concentration, meaning that the product of ion concentrations is more likely to be lower than the solubility product value. This makes the compound more likely to dissolve. The addition of ammonia also uses up some of the silver ions in solution, decreasing their concentration and further reducing the likelihood of a precipitate forming.

In summary, the addition of ammonia to a solution containing halide ions and silver nitrate reduces the ion concentration and promotes the formation of complex ions, which increases the solubility of the resulting silver halide compound.

Reactions of halides as reducing agents

We have explored how halogens can act as oxidising agents (see Reactions of Halogens).

An oxidising agent oxidises other species and is itself reduced in the process.

Halide ions do quite the opposite - they act as reducing agents.

A reducing agent reduces other species and is itself oxidised in the process.

Do you remember the two acronyms, OIL RIG and RAD OAT? They help you remember the movement of electrons in redox reactions and in reactions involving oxidising or reducing agents.

The acronyms OIL RIG and RAD OAT help you remember electron movement. StudySmarter Originals

This means that a reducing agent donates electrons to another species. The other species gains these electrons and is reduced. The reducing agent loses electrons and so is oxidised.

How do halide ions act as reducing agents? You know that a halide is a negative anion. It contains an extra electron compared to the halogen in its elemental state. Halide ions can react by losing this extra electron to form a neutral halogen atom.

A fluorine atom, left, and a fluoride ion, right. StudySmarter Originals

Let’s consider a general reaction between a halide ion, which we’ll call X-, and another substance, which we’ll call Y:

Note the following:

The halide loses an electron. It is oxidised.The other species gains an electron. It is reduced.The halide reduces the other species. Therefore, the halide is a reducing agent.

Trends in reducing ability

You might remember that halogens become better oxidising agents as you move up the group in the periodic table. However, this trend reverses when it comes to reducing ability. In general, halides become better reducing agents as you move down the group in the periodic table.

Why is this the case? Let’s look at the electronic structures of fluorine and chlorine, by way of an example.

‍

Fluoride ions have the electron configuration 1s2 2s2 2p6. Chloride ions have the electron configuration 1s2 2s2 2p6 3s2 3p6. Chloride is a larger ion than fluoride as it has more electron shells. This means that chloride’s outer shell electron is further from its nucleus than fluoride’s. The attraction between this outer shell electron and the nucleus is weaker and so it is easier to lose the outermost electron - and losing electrons is exactly what reducing agents do.

Reaction of halides with sulfuric acid

All the halide ions react with concentrated sulfuric acid, but the reactions produce a variety of different products. This dependson the halide used. Some halides are able to reduce the sulfur in sulfuric acid, while others are not.

We use sodium halide salts as a source of halide ions. Let’s explore each of the reactions in turn.

Fluoride ions and sulfuric acid

Sodium fluoride reacts with concentrated sulfuric acid to produce hydrogen fluoride and sodium hydrogensulfate:

You’ll see a white solid - sodium hydrogensulfate - and the steamy fumes of hydrogen fluoride.

Notice that this isn’t a redox reaction - fluoride ions are not a strong enough reducing agent to reduce the sulfur in sulfuric acid. All of the oxidation states stay the same. Instead, this is an acid-base reaction.

‍

Chloride ions and sulfuric acid

Sodium chloride reacts in a similar way. Once again, chloride ions aren’t strong enough to reduce sulfur dioxide. The only reaction is an acid-base reaction, producing steamy white fumes of hydrogen chloride and the white solid sodium hydrogensulfate:

Bromide ions and sulfuric acid

We now know that reducing ability increases as you move down the group on the periodic table. This means that bromide ions are a much better reducing agent than fluoride and chloride ions. In fact, bromide ions can reduce sulfuric acid. When sodium bromide reacts with sulfuric acid, we still get the same acid-base reaction that we saw earlier, but we also get an additional redox reaction producing bromine and sulfur dioxide:

Look at the oxidation states in this reaction:

Bromine goes from -1 to +0. Sulfur goes from +6 to +4. Bromide ions lose electrons and are oxidised. Sulfur gains electrons and is reduced.

Therefore, bromide ions are a strong enough reducing agent to reduce sulfur.

For more information on oxidation states, check out Redox.

Iodide ions and sulfuric acid

The trend continues down the group - iodide ions are even better at reducing other species than bromide ions! Four separate reactions take place.

Firstly, an acid-base reaction produces hydrogen iodide. Next, iodide ions reduce sulfur from an oxidation state of +6 in sulfuric acid to +4 in sulfur dioxide. Iodide ions then reduce sulfur atoms further to elemental sulfur with an oxidation state of +0.They can also reduce sulfur further still into hydrogen sulfide. In this molecule, sulfur has an oxidation state of -2.

The next table provides an overview of the different reactions, oxidation states involved and what you should expect to see.

‍

In summary:

Fluoride and chloride ions don’t reduce sulfuric acid. Bromide ions reduce sulfur from an oxidation state of +6 to +4. Iodide ions, on the other hand, reduce sulfur from an oxidation state of +6 all the way down to -2!

Reactions of alkyl halides and aryl halides

Alkyl halides and aryl halides are types of halocarbons.

Halocarbons, also known as organohalides, are molecules containing one or more halogen atoms bonded to a carbon atom in an organic compound.

Reactions of alkyl halides

Alkyl halides are also known as halogenoalkanes and contain a halogen bonded to a carbon in an alkane. They are useful because they can be turned into molecules with a variety of other functional groups in the following reactions.

Nucleophilic substitution of halogenoalkanes can produce alcohols, nitriles, and primary amines. Elimination of halogenoalkanes produces alkenes.

For example, eliminating chloroethane using ethanoic sodium hydroxide produces ethene and water:

Check out Nucleophilic Substitution Reactions and Elimination Reactions to find out more about these types of reaction, including mechanisms and examples.

Reactions of aryl halides

Aryl halides are relatively unreactive compared to alkyl halides because the C-X bond in aryl halides is stronger due to two reasons. Firstly, it is shorter than the C-X bond in alkyl halides, making it stronger. Secondly, aryl halides exhibit resonance, meaning the electron bonding cannot be described by a single structure. In chlorobenzene, for example, one of the lone pairs of electrons on the chlorine atom gets involved in resonance, causing the C-X single bond to take on some of the character of a C=X double bond. Double bonds are stronger than single bonds, resulting in an overall increase in the strength of the C-X bond.

This increased bond strength makes it difficult for aryl halides to undergo substitution reactions, where the C-X bond is broken and replaced by a new group. Instead, aryl halides are more likely to undergo metal-halogen exchange reactions, where the halogen atom is replaced by a metal ion, resulting in a metal ion bonded to an aromatic benzene ring.

Halides are negative ions formed from halogen atoms with a charge of -1. When hydrogen halides react with water, they form strong acids except for hydrogen fluoride, which forms a weak acid. Hydrogen halides also react with alcohols to form halogenoalkanes and water. Acidified silver nitrate solution followed by ammonia can be used to test for halide ions in solution. Halide ions can act as reducing agents, meaning they can reduce another species and become oxidised in the process. As you move down the group in the periodic table, halide ions become better reducing agents. All halide ions can react with concentrated sulfuric acid, but only bromide and iodide ions are strong enough reducing agents to reduce it. Alkyl halides react in nucleophilic substitution and elimination reactions, while aryl halides react in halogen-metal exchange reactions.

Reactions of Halides

Are the reactions of halide ions with sulfuric acid reversible?

No. The reactions are irreversible.

What happens when you react potassium and iodine?

Reacting potassium and iodine produces the metal halide potassium iodide.

What type of reaction takes place when you test for halides?

A precipitation reaction occurs, forming an insoluble salt, AgX.

What is the order of reactivity of the halides?

Halides become more reactive as you move down the group in the periodic table.

What are halides?

Halides are halogen atoms that have each gained an electron to form a negative anion with a charge of -1. An example is the chlorine anion.

‍