The Alkyl halides Halogenalkanes or haloalkanes are chemical compounds in which one or more of the hydrogen atoms of an alkane have been replaced by halogen atoms (usually one or more of fluorine, chlorine, bromine or iodine).
As also applied to alkanes, haloalkanes are saturated organic compounds, meaning that all the chemical bonds that bind the atoms within the molecule are single bonds.
Each carbon atom forms 4 bonds, either with other carbon atoms or with hydrogen or halogen atoms. Each hydrogen atom and halogen is connected to a single carbon atom.
A simple general formula describing many (but not all) of the haloalkanes is:
C N H 2n + 1 X
Where the letter n represents the number of carbon atoms in each molecule of the compound and the letter X represents a particular halogen atom.
An example of a real chemical substance described by this formula is fluoromethane (also known as methyl fluoride), whose molecules have only one carbon atom (so n = 1) and includes halogen-fluorine (where X = F). The formula of this compound is CH 3 F (Haloalkanes, S.F.).
When comparing alkanes and haloalkanes, we will find that the haloalkanes have higher boiling points than the alkanes containing the same number of carbons.
London's dispersal forces are the first of two types of forces to contribute N to this physical property. It should be remembered that London's scattering forces increase with the molecular surface area.
When comparing haloalkanes with alkanes, haloalkanes exhibit an increase in surface area due to the substitution of a halogen by hydrogen.
The dipole-dipole interaction is the second type of force that contributes to a higher boiling point. This type of interaction is a coulomb attraction between the positive partial and partial negative charges that exist between the carbon-halogen bonds in separate haloalkane molecules.
Similar to London's scattering forces, dipole-dipole interactions establish a higher boiling point for haloalkanes compared to alkanes with the same number of carbons (Curtis, 2016).
Types of alkyl halides
The alkyl halides, similar to the amines, may be primary, secondary or tertiary depending on which carbon the halogen is in.
In a primary halogenoalkane (1 °), the carbon bearing the halogen atom is bonded only to another alkyl group. Examples of primary haloalkanes are given in Figure 1.
Figure 1: Examples of haloalkanes, bromoethane (left) chloropropane (cent.) And 2-methyl iodo propane.
In a secondary halogenoalkane (2 °), the carbon with the halogen attached, is directly attached to two other alkyl groups, which may be the same or different. Examples of secondary haloalkanes are illustrated in Figure 2.
Figure 2: Examples of secondary haloalkanes, 2-bromo-propane (left) and 2-chloro butane (der.)
In a tertiary halogenoalkane (3 °), the carbon atom containing the halogen is directly attached to three alkyl groups, which may be any combination thereof or different.
Nomenclature
According to the IUPAC, three standards should be followed to name the alkyl halides:
- The parent chain is numbered to give the substituent found first the lowest number, either halogen or an alkyl group.
- The halogen substituents are indicated by the prefixes fluoro, chloro, bromo and iodo and listed in alphabetical order with other substituents.
- Each halogen is located in the main chain giving a number that precedes the name of the halogen (Ian Hunt, S.F.).
For example if you have the following molecule:
Following the above steps, the molecule is numbered starting at the carbon where the halogen, in this case chlorine, is found, which is at position 1. This molecule will be called 1-chloro butane or chlorobutane.
Another example would be the following molecule:
Note that there is the presence of two chlorine atoms, in this case the prefix di is added to the halogen preceded by the carbon numbers where they are found. In this case the molecule will be called 1,2-dichloro butane (Colapret, S.F.).
Preparation of halogenoalkanes
Halogenoalkanes may be prepared from the reaction between alkenes and hydrogen halides, but are more commonly made by replacing the -OH group in an alcohol with a halogen atom.
The general reaction is as follows:
It is possible to make successful tertiary chloroalkanes of the corresponding alcohol and concentrated hydrochloric acid, but to make primary or secondary it is necessary to use another method since the reaction rates are too slow.
A tertiary chloroalkane can be prepared by stirring the corresponding alcohol with concentrated hydrochloric acid at room temperature.
The chloroalkanes can be prepared by reacting an alcohol with liquid phosphorus (III) chloride, PCl 3.
They can also be prepared by adding solid phosphorus (V) chloride (PCl 5) to an alcohol.
This reaction is violent at room temperature, producing clouds of hydrogen chloride gas. It is not a good choice as a way to make halogenoalkanes, although it is used as a test for -OH groups in organic chemistry (Clark, MAKING HALOGENOALKANES, 2015).
Uses of the alkyl halide
Alkyl halides have various uses, including fire extinguishers, propellants and solvents.
Haloalkanes react with many substances that lead to a wide range of different organic products therefore they are useful in the laboratory as intermediates in the manufacture of other organic chemicals.
Some haloalkanes have negative effects on the environment, such as ozone depletion. The best-known family in this group are chlorofluorocarbons, or CFCs for short.
CFCs are chlorofluorocarbons - compounds containing carbon with chlorine and bound fluorine atoms. Two common CFCs are CFC-11 which is trichlorofluorocarbon and CFC-12 which is dichloro difluorocarbon.
CFCs are not flammable and are not very toxic. Therefore, they were given a large number of uses.
They were used as refrigerants, aerosol propellants, to generate foamed plastics such as expanded polystyrene or polyurethane foam, and as solvents for dry cleaning and general degreasing purposes.
Unfortunately, CFCs are largely responsible for destroying the ozone layer. In the upper atmosphere, the carbon-chlorine bonds are broken to give free chlorine radicals.
It is these radicals that destroy ozone. CFCs are being replaced by compounds less harmful to the environment. Hence, due to the Montreal protocol, the use of most CFCs has been eliminated.
CFCs can also cause global warming. A CFC-11 molecule, for example, has a global warming potential about 5000 times greater than a carbon dioxide molecule.
On the other hand, there is much more carbon dioxide in the atmosphere than CFCs, so global warming is not the main problem associated with them.
Some halogenoalkanes are still used, although simple alkanes such as butane can be used for some applications (eg as aerosol propellants) (Clark, USES OF HALOGENOALKANES, 2015).
References
- Clark, J. (2015, September). INTRODUCING HALOGENOALKANES . Retrieved from chemguide.co.uk: chemguide.co.uk.
- Clark, J. (2015, September). MAKING HALOGENOALKANES . Retrieved from chemguide.co.uk: chemguide.co.uk.
- Clark, J. (2015, September). USES OF HALOGENOALKANES . Retrieved from chemguide.co.uk: chemguide.co.uk.
- Colapret, J. (S.F.). Haloalkanes (Alkyl Halides) . Retrieved from colapret.cm.utexas.edu: colapret.cm.utexas.edu.
- Curtis, R. (2016, July 12). Haloalkanes . Retrieved from chem.libretexts.org: chem.libretexts.org.
- Haloalkanes . (S.F.). Retrieved from ivyroses: ivyroses.com.
- Ian Hunt. (S.F.). Basic IUPAC Organic Nomenclature . Retrieved from chem.ucalgary.ca: chem.ucalgary.ca.