Alkyl halides (a.k.a. haloalkanes) are molecules with halogens directly bonded to a carbon. They are very useful in a wide gamut of reactions, ranging from organometallic reactions to alkene formation.
An alkyl halide functional group is just like the name suggests: it’s a halide attached to a carbon atom. The “R” represents generic carbon groups and the “X” represents any halogen.
When naming alkyl halides, the halogen doesn’t get any priority over any alkyl substituents. Let’s practice our nomenclature just a bit by naming the following example:
The longest carbon chain of this molecule is five carbons. The substituents are an iodine at position 1 and an ethyl group at position 2. Following our alkyl halide nomenclature rules, we’d get that the name of this molecule is 2-ethyl-1-iodopentane. Notice that our iodine is only at position 1 because that would give us the smallest numbers for the substituents.
Alkyl halides get degrees just like alcohols do! Their degrees are based on the number of carbon atoms their anchoring carbon is attached to. In other words, look at the halogen then look at the carbon it’s attached to. How many carbons is that carbon connected to? If the answer is zero, the degree is methyl or 0º; if it’s attached to three carbons, it’s tertiary or 3º.
From left to right, we have methyl (0º), primary (1º), secondary (2º), tertiary (3º) alkyl halides.
Halides follow the overall electronegativity trend of the periodic table; as we move, up we end up with the most electronegative atom: fluorine. As we move to the right and down the periodic table, our atoms get bigger. Fluorine is more electronegative than the other halogens, and iodine is the biggest of the halogens.
When a halogen is bonded to a carbon, the electronegativities are generally different. For example, a strong dipole exists in the bond between carbon and fluorine since the difference in electronegativity values is 1.5:
The lengths of bonds and their strengths are positively correlated; that is, shorter bonds are stronger and longer bonds are weaker. The bond between carbon and fluorine is much shorter than the bond between carbon and iodine, and the bonds are stronger and weaker, respectively.
Iodine can hold a negative charge much more easily than fluorine can, as is supported by pKa values of the haloacids in the table below:
The polarizability of the halogens and the bond lengths are huge indicators of bond strength.
All things remaining equal, the boiling point of alkyl halides increases with larger halides. Remember your intermolecular forces! Larger atoms means larger Van der Waals forces, which means greater attraction between molecules. The more molecules are attracted to each other, the greater the boiling point will be. Don’t forget that dipole-dipole interactions are also at play here, affecting the boiling point as well.
A nucleophile can replace a halogen through an SN1 or SN2 reaction depending on the type of nucleophile and degree of the alkyl halide. For example, 1-iodopropane reacts with NaOH (sodium hydroxide) through an SN2 mechanism to yield 1-propanol and sodium iodide:
Alkyl halides can be eliminated to form alkenes and alkynes, which are also super useful synthetically. A base removes a beta-hydrogen and kicks off the halide by forming a double bond.
Alcohols can be converted alkyl halides by using either thionyl chloride or phosphorus tribromide. These are super useful reactions when coming up with syntheses!
Adding magnesium or lithium to an alkyl halide will convert it into a Grignard or organolithium, respectively. These are extremely powerful nucleophiles that are often used to make new carbon-carbon bonds.