6.1 : 烷基卤化物

Alkyl halides, or haloalkanes, are organic compounds comprised of an alkyl group and a halogen atom bonded to an sp³ hybridized carbon.

Each carbon atom is defined by its proximity to the halogen atom. The carbon bonded directly to the halogen is denoted “alpha-carbon”; the carbon next to it is called “beta-carbon” and so forth.

Depending on the number of alkyl groups on the alpha carbon, alkyl halides are further classified as primary, secondary, and tertiary.

Nomenclature of alkyl halides follows similar IUPAC rules as for alkanes. Halogens are regarded as substituents and named fluoro-, chloro-, bromo-, and iodo-.

First, count the carbons on the longest chain. Here, five corresponds to pentane. Secondly, find and name the substituents. Here it is bromo-, chloro-, and methyl.

Next, number the parent chain and assign a locant to the substituents giving them the lowest possible number. Lastly, assemble the substituents alphabetically. This compound is called 4-bromo-1-chloro-4-methyl pentane.

If the chain contains a double bond, the double bond has precedence and obtains the lowest number giving the compound 3-fluoro-1-butene.

If a molecule is cyclic, chiral, and with multiple substituents, the substituents are likewise assigned the lowest locant possible, and the groups are alphabetically ordered. The stereocenters are indicated at the beginning of the name, giving (1R,2R)-3-butyl-1,2-dichloro-5-ethylcyclohexane.

Alkyl halides are used as building blocks in syntheses because of their reactivity. Going down group 17, the halogen’s atom size and bond length increase, while electronegativity and basicity decrease.

The higher electronegativity of halogens leads to the withdrawal of electron density from the adjacent carbon making the alpha carbon electrophilic and, hence, more reactive.

The alkyl halide’s reactivity is further influenced by the stability of the halogen’s conjugate base. Iodide, bromide, and chloride are stable yet weak conjugate bases of their strong hydrohalic acids. Because of their stability, they can be more easily substituted by a stronger base, making them good leaving groups.