Key Characteristics and Types of Halides in Chemistry
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.
FAQs on Halide: Definition, Properties, and Uses
1. What is a halide in chemistry?
A halide is a chemical compound where one part is a halogen atom (like fluorine, chlorine, bromine, or iodine) and the other part is an element or radical that is less electronegative than the halogen. When a halogen atom gains an electron, it forms a negative ion called a halide ion (X⁻). These compounds can be either inorganic salts, like Sodium Chloride (NaCl), or organic compounds, like Chloroform (CHCl₃).
2. What are the general properties of halides?
The properties of halides largely depend on their chemical bonding. Key properties include:
Melting and Boiling Points: Ionic halides (e.g., NaCl) typically have high melting and boiling points due to strong electrostatic forces in their crystal lattice. Covalent halides (e.g., CCl₄) have lower melting and boiling points due to weaker intermolecular forces.
Solubility: Many ionic halides are soluble in water, while covalent halides are generally soluble in organic solvents.
Electrical Conductivity: Ionic halides conduct electricity when molten or dissolved in water but are insulators in their solid state. Covalent halides do not conduct electricity in any state.
3. What are some important uses of halides in industry and daily life?
Halides are essential in various applications. For example, Sodium Chloride (NaCl) is used for seasoning food and as a preservative. Sodium Fluoride (NaF) is added to toothpaste to prevent tooth decay. Silver Bromide (AgBr) is a key component in photographic film. Chlorides are also widely used in water purification and as disinfectants.
4. How can you provide some examples of common halides?
Common halides are found in both inorganic and organic chemistry. Some examples include:
Sodium Chloride (NaCl): Common table salt.
Potassium Iodide (KI): Used as an iodine supplement.
Silver Chloride (AgCl): A white crystalline solid used in electrochemistry.
Carbon Tetrachloride (CCl₄): An organic halide, previously used in fire extinguishers and as a cleaning agent.
5. What is the fundamental difference between a halogen, a halide, and a haloalkane?
These terms are related but distinct. A halogen is an element from Group 17 of the periodic table (e.g., Chlorine, Cl₂). A halide is the negative ion formed when a halogen atom gains an electron (e.g., Chloride ion, Cl⁻) or a compound containing this ion. A haloalkane is a specific type of organic halide where a halogen atom is covalently bonded to a carbon atom in an alkane chain (e.g., Chloromethane, CH₃Cl).
6. Why do ionic and covalent halides exhibit vastly different physical properties?
The difference in properties stems from their bonding and structure. Ionic halides, formed between a metal and a halogen, create a strong, rigid crystal lattice held by electrostatic forces. This requires a large amount of energy to break, resulting in high melting points. In contrast, covalent halides, formed between non-metals, exist as discrete molecules with weak intermolecular (van der Waals) forces, which are easily overcome, leading to lower melting and boiling points.
7. How can one test for the presence of halide ions (Cl⁻, Br⁻, I⁻) in a solution?
A common laboratory test for halide ions involves adding a dilute nitric acid solution followed by silver nitrate (AgNO₃) solution. The formation of a precipitate indicates the presence of a halide:
A white precipitate (AgCl), soluble in ammonia solution, indicates a Chloride (Cl⁻) ion.
A pale cream precipitate (AgBr), sparingly soluble in ammonia solution, indicates a Bromide (Br⁻) ion.
A yellow precipitate (AgI), insoluble in ammonia solution, indicates an Iodide (I⁻) ion.
8. Why is Silver Chloride (AgCl) insoluble in water, while Sodium Chloride (NaCl) is highly soluble, even though both are ionic halides?
This difference is explained by the balance between two energies: lattice energy and hydration energy. For a substance to dissolve, the hydration energy released when ions interact with water molecules must be greater than the lattice energy holding the crystal together. For NaCl, the hydration energy is sufficient to overcome its lattice energy. For AgCl, the bond between silver and chloride has a more covalent character, resulting in a much higher lattice energy that the hydration energy cannot overcome, making it insoluble in water.
