Alkyl halides are a functional group in which a halogen is attached to an alkyl group. Fluorine, chlorine, bromine, and iodine are the halogens that are most often seen in alkyl halides. Alkyl halides have dipole-dipole interactions that give them more intermolecular forces than a hydrocarbon of similar molecular weight but fewer intermolecular forces than a compound of similar weight that can form hydrogen bonds. Chloride (Cl–), bromide (Br–), and iodide (I–) are very good leaving groups. Leaving groups are needed for substitution and elimination reactions.
Substitution reactions are reactions where a nucleophile replaces a leaving group. Substitution reactions can be categorized by their kinetics as either SN1 or SN2. Elimination reactions are reactions where a leaving group and a beta hydrogen are removed and replaced with a pi bond. Elimination reactions can be categorized by their kinetics as either E1 or E2.
Substitution (SN1 and SN2) and elimination (E1 and E2) reactions are always in competition with each other. Substrate, reagent, solvent, and temperature are the factors that determine which mechanistic pathway a reaction proceeds through.
Radical halogenation is a special type of substitution reaction that is used for alkanes without a leaving group. In radical halogenation, a hydrogen on an alkane chain is replaced with a halogen via a radical reaction.
At A Glance
- 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.
- Nucleophilic substitution reactions are reactions where a leaving group (a halide) is replaced with a functional group due to the presence of a nucleophilic reagent.
- SN2 reactions are bimolecular, while SN1 reactions are unimolecular. SN1 mechanisms are multistep mechanisms that include the formation of a carbocation intermediate. SN2 mechanisms are concerted mechanisms that have a nucleophilic attack and a loss of leaving group in the same step.
- SN2 reactions involve inversion of the configuration of the carbon with a leaving group, while SN1 reactions involve racemization of the halide carbon. SN2 reactions predominate with methyl and primary alkyl halides. SN1 reactions predominate with tertiary alkyl halides. Secondary alkyl halides can undergo SN1 or SN2 reactions, depending on the solvent used.
- Elimination reactions are reactions where a leaving group (usually a halide) on the alpha carbon and a hydrogen atom on the beta carbon are replaced with a double bond in the presence of a base. E2 reactions are bimolecular, and E1 reactions are unimolecular.
- E1 mechanisms are multistep mechanisms, and E2 mechanisms are concerted mechanisms. E2 reactions form the Zaitsev product with most bases but form the Hofmann product with sterically hindered bases such as KOtBu. E2 mechanisms predominate with all substrates when a base is used. E1 mechanisms are usually only seen with tertiary leaving groups, a solvent, and heat.
- Since SN1, SN2, E1, and E2 reactions all depend on having a leaving group, the reactions are always in competition with each other, and reactions will often form more than one product based on the competing reactions. Substrate, reagents, solvent, and temperature can affect the distribution of products.
- The main reaction of alkanes is radical halogenation, which is the addition of Br2, Cl2, or NBS to an alkane in the presence of heat or light. In radical halogenation, a hydrogen is replaced with a halogen.