Halide

The organic compounds where a carbon chain or a single carbon atom is linked with one or more halogen atoms are called alkyl halides. In inorganic chemistry, an alkaline metal combined with a halogen forms a halide. In this article, we will study what halogens are and how they form different halides with the metals in the periodic table. We will also discuss the different properties of metallic halides.

What are Halides?

In the modern periodic table, halogens are referred to as the four elements present in Group 17 (VIIA). These elements are Fluorine (F), Chlorine (Cl), Bromine (Br), and Iodine (I). They are called halogens as they produce salts. The Greek version of ‘hal’ means salt and ‘gen’ means ‘to produce’. Fluorine is the lightest among the halogens and it is a gas. Iodine, on the other hand, is the heaviest of halogens and is solid.

Now that we know what a halogen is, let us define halides. These are binary compounds (atoms of two different elements) where one part is a metal and the other part is a halogen. In our earth’s crust, a majority of the salts are halides. They are quite stable and cannot be dissociated to form other products easily via natural processes.  

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Characteristics of Inorganic Halides

Halides, as mentioned earlier, are inorganic compounds containing two different atoms. The electropositive atoms belong to metal and the electronegative atoms belong to a halogen. These binary compounds are highly soluble in water due to their polarization. The difference in electronegativity between the metals and halogens results in the formation of two poles. The water molecules do the rest to dissociate the constituent atoms to form metallic cations and non-metallic anions.

When silver nitrate is used as a reagent to detect the inorganic halide chemistry then the following reactions happen.

• Silver fluoride formed from the reaction between the metallic halide and silver nitrate will not produce any change in color or precipitation.

• Silver chloride, on the other hand, will create a white precipitate.

• Silver bromide forms pale yellow precipitation in the reaction solution.

• Silver iodide will form green precipitation in the reaction solution.

The difference in the outcome is due to the halides elements. The respective halide ions form a compound with the silver ion in the solution and give the above-mentioned results.

Halide ions are reducing agents in some reactions. On the other hand, halogens are oxidizing agents. This type of chemical property changes when we move or down Group 17. It all depends on the electronic configuration of the elements and their respective polarisability.

Halides can be widely found in different minerals. In fact, the most common inorganic halide we find on earth is sodium chloride. A major part of the halides is found in the water sources. The reason is their excellent solubility in water.

Polarisability of Halide Ions

A natural phenomenon occurs where the halide ions show different values of polarisability. It happens due to their differences in the anionic radii of the atoms. According to halide chemistry, the anionic radius determines the value of polarisability. It has been found that this value is directly proportional to the radius of an anion. The larger the size the higher is the polarisability of a halide ion. In this aspect, we can conclude that the iodide (I-) ion has the highest polarisability value, and the fluoride (F-) ion has the lowest. Thus, the polarisability of halide ions increases in the order of F, Cl, Br, and I.

Alkyl Halides and their Characteristics

Another set of halides exists in organic chemistry. The halogen atoms become a part of the carbon chain compounds and form alkyl or aromatic halides. The hydrocarbon chains containing one or more halogen atoms are classified as alkyl halides.

The classification of alkyl halides is done based on the position of the halide ions in the carbon chain. The types are:

• Primary Alkyl Halide

In this type, the halogen atom will be bonded with a 1° carbon atom in the chain that is attached to only one alkyl group.

• Secondary Alkyl Halide

By definition, the halogen atom will be bonded with a 2° carbon atom in the chain that remains attached to two alkyl groups.

• Tertiary Alkyl Halide

The halogen atom will remain attached to a 3° carbon atom that is attached to three alkyl groups.

As per the description of a number of halogen atoms in one molecule of a hydrocarbon, let us define what are halides.

• Mono Haloalkane – The presence of one halogen atom in one carbon of the chain.

• Dihaloalkane – The presence of two halogen atoms separately in two carbon atoms.

• Tri Haloalkanes – The presence of three halogen atoms in three different carbon atoms.

Properties of Alkyl Halides

Alkyl halides are less soluble in water because of the lack of polarization in the other atoms present in the molecules. Only the halide atoms contain a slight negative charge. The rest of the molecules are nonpolar in nature resulting in their low solubility in water. Alkyl halides perform nucleophilic substitution reactions and elimination reactions. They also react with active metals in particular reaction environments.

This is a brief introduction to halides and their respective chemistry. Inorganic and organic halides differ in terms of physical and chemical properties. For instance, alkyl halides are less soluble in water because of a lack of polarity whereas inorganic halides are highly soluble. Read the properties of these halides in this article.