Alkyl halides are classified as primary, secondary, tertiary, or vinyl. Halogens are substituents that are named with a number and a prefix in front of the parent name of an alkane. Alkyl halides have slightly stronger intermolecular forces than alkanes due to the presence of a permanent dipole-dipole interaction between the halogen and the carbon.
Alkanes are hydrocarbons containing only singly bonded carbon and hydrogen with CnH2n+2 stoichiometry. An example of an alkane is propane, C3H8, or CH3−CH2−CH3. For alkyl halides, one or more hydrogen atoms in the alkane is replaced with a halogen atom—fluorine, chlorine, bromine, or iodine (astatine is a halogen but is not found in normal alkyl halides).
Structurally, alkyl halides differ by the type of carbon bonded to the halide. For primary alkyl halides, the halogen atom is attached to a carbon, which is attached to one other alkyl group, for example CH3−CH2−Cl. For secondary halides, the halogen atom is attached to a carbon, which is attached to two alkyl groups. For tertiary alkyl halides, the halogen atom is attached to a carbon that is attached to three alkyl groups. Vinyl halides are different from alkyl halides in that the halogen atom is attached to an alkenyl group, for example, when the halogen is directly bonded to a double-bonded carbon atom. An example of a vinyl halide is RCH=CHX, where X refers to any halogen such as fluorine, chlorine, bromine, or iodine.
Classification of Alkyl Halides
When naming compounds, start with the longest continuous chain of carbon atoms, and write the name of the parent compound. Then describe the substituents by their locant, or numerical location, using hyphens to separate the words. When naming alkyl halides, locants indicate the positions of the functional groups within the molecules. In compounds where the functional group is the halogen, the prefixes are chloro-, fluoro-, bromo-, and iodo-. The number indicates the carbon on which the halogen is located. For example, CH3−CH2−F is fluoroethane. No number is needed because there are only two carbons. However, F−CH2−CH2CH3 is 1-fluoropropane. The number becomes essential because there is more than one location that can be labeled 1. If two halides are on a chain of carbons, the halide closest to the end will determine the numbering. If there is a tie in the location between halides, alphabetical order determines the numbering.
Nomenclature of Alkyl Halides
Alkanes are nonpolar molecules. Van der Waals forces are a measure of the attraction between molecules. Van der Waals forces are determined by the number of electrons surrounding the molecule and by the molecule's surface area. In general, as the molecular mass of the alkane increases, the boiling point also increases. For alkyl halides, the carbon-halogen bond is polar. The polar bond is a permanent dipole that increases the van der Waals forces, which in turn makes the boiling point of alkyl halides higher than alkanes. Polarity is a weaker force than a hydrogen bond, so alkyl halides have much lower boiling points than an equivalent-weight alcohol or an amine.