The halogenated derivatives are all compounds with a halogen atom; that is, any of the elements of group 17 (F, Cl, Br, I). These elements differ from the rest by being more electronegative, forming a variety of inorganic and organic halides.
The image below shows the gaseous molecules of the halogens. From top to bottom: fluorine (F 2 ), chlorine (Cl 2 ), bromine (Br 2 ) and iodine (I 2 ). Each of these has the ability to react with the vast majority of elements, even between congeners of the same group (interhalogens).
Thus, halogenated derivatives have the formula MX if it is a metal halide, RX if it is alkyl and ArX if it is aromatic. The last two are in the category of organic halides. The stability of these compounds requires an energy “benefit” compared to the original gas molecule.
As a general rule, fluorine forms more stable halogenated derivatives than iodine. The reason is due to the differences between their atomic radii (the purple spheres are more voluminous than the yellow ones).
The correct way to name these compounds depends on whether they are inorganic or organic.
Metal halides consist of a bond, ionic or covalent, between a halogen X and a metal M (from groups 1 and 2, transition metals, heavy metals, etc.).
In these compounds all halogens have an oxidation state of -1. Why? Because its valence settings are ns 2 np 5.
Therefore, they need to gain only one electron to complete the valence octet, while metals oxidize, giving them the electrons they have.
Thus, fluorine remains as F – , fluoride; Cl – , chloride; Br – , bromide; and the I – , iodide. MF would be named: (metal name) fluoride (n), where n is the valence of the metal only when it has more than one. For the case of metals of groups 1 and 2, it is not necessary to name the valence.
- NaF: sodium fluoride.
- CaCl 2 : calcium chloride.
- AgBr: silver bromide.
- ZnI 2 : zinc iodide.
- CuCl: copper (I) chloride.
- CuCl 2 : copper (II) chloride.
- TiCl 4 : titanium (IV) chloride or titanium tetrachloride.
However, hydrogen and nonmetal elements – even halogens themselves – can also form halides. In these cases, the valence of the nonmetal is not named at the end:
- PCl 5 : phosphorous pentachloride.
- BF 3 : boron trifluoride.
- AlI 3 : aluminum triiodide.
- HBr: hydrogen bromide.
- IF 7 : iodine heptafluoride.
Regardless of whether it is RX or ArX, halogen is covalently bonded to a carbon atom. In these cases the halogens are mentioned by their names, and the rest of the nomenclature depends on the molecular structure of R or Ar.
For the simplest organic molecule, methane (CH 4 ), the following derivatives are obtained by substituting H for Cl:
- CH 3 Cl: chloromethane.
- CH 2 Cl 2 : dichloromethane.
- CHCl 3 : trichloromethane (chloroform).
- CCl 4 : tetrachloromethane (carbon (IV) chloride or carbon tetrachloride).
Here R consists of a single carbon atom. So, for other aliphatic chains (linear or branched) the number of carbons from which it is linked to the halogen is counted:
CH 3 CH 2 CH 2 F: 1-fluoropropane.
The example above was that of a primary alkyl halide. In the case that the chain is branched, the longest one that contains the halogen is chosen and the counting begins, leaving the smallest possible number:
The same happens for other substituents. Likewise, for aromatic halides the halogen is named and then the rest of the structure:
The top image shows the compound called bromobenzene, with the bromine atom highlighted in brown.
Properties of halogenated derivatives
Inorganic halides are ionic or molecular solids, although the former are more abundant. Depending on the interactions and ionic radii of MX, it will be soluble in water or other less polar solvents.
Non-metallic halides (such as boron halides) are generally Lewis acids, which means that they accept electrons to form complexes. On the other hand, hydrogen halides (or halides) dissolved in water produce what are known as hydracids.
Its melting, boiling or sublimation points fall on the electrostatic or covalent interactions between the metal or non-metal with the halogen.
Likewise, ionic radii play an important role in these properties. For example, if M + and X – are similar in size, their crystals will be more stable.
They are polar. Why? Because the difference in electronegativities between C and halogen creates a permanent polar moment in the molecule. Likewise, this decreases as group 17 descends, from the C – F bond to C – I.
Regardless of the molecular structure of R or Ar, the increasing numbers of halogens have a direct effect on the boiling points, since they increase the molar mass and the intermolecular interactions (RC – X — X – CR). Most are immiscible with water, but can dissolve in organic solvents.
Uses of halogenated derivatives
The uses of the halogenated derivatives could reserve their own text. The molecular “partners” of halogens are a key factor, since their properties and reactivities define the uses of the derivative.
Thus, among the great diversity of possible uses, the following stand out:
- Molecular halogens are used to create halogen bulbs, where it comes into contact with the incandescent tungsten filament. The purpose of this mixture is to react the halogen X with the evaporated tungsten. In this way, its deposition on the surface of the bulb is avoided, guaranteeing it a longer life.
- Fluoride salts are used in the fluoridation of water and toothpastes.
- Sodium and calcium hypochlorites are two active agents in commercial bleaching solutions (chlorine).
- Although they damage the ozone layer, chlorofluorocarbons (CFCs) are used in aerosols and refrigeration systems.
- Vinyl chloride (CH 2 = CHCl) is the monomer of the polymer polyvinyl chloride (PVC). On the other hand, Teflon, used as a non-stick material, consists of polymeric chains of tetrafluoroethylene (F 2 C = CF 2 ).
- They are used in analytical chemistry and organic syntheses for different purposes; among these, the synthesis of drugs.
The upper image illustrates the thyroid hormone, responsible for the production of heat as well as the increase of the general metabolism in the body. This compound is an example of a halogenated derivative present in the human body.
Among other halogenated compounds, the following are mentioned:
- Dichlorodiphenyltrichloroethane (DDT), an efficient insecticide but with serious environmental impacts.
- Tin chloride (SnCl 2 ), used as a reducing agent.
- Chloroethane or 1-chloroethane (CH 3 CH 2 Cl), a topical anesthetic that acts quickly by cooling the skin.
- Dichlorethylene (ClCH = CClH) and tetrachlorethylene (Cl 2 C = CCl 2 ), used as solvents in the dry cleaning industry.