Substitution reactions involve the replacement of the halogen with another molecule—thus leaving saturated hydrocarbons, as well as the halogenated product. Haloalkanes behave as the R synthon, and readily react with nucleophiles.
Hydrolysis, a reaction in which water breaks a bond, is a good example of the nucleophilic nature of haloalkanes. The polar bond attracts a hydroxide ion, OH (NaOH being a common source of this ion). This OH is a nucleophile with a clearly negative charge, as it has excess electrons it donates them to the carbon, which results in a covalent bond between the two. Thus C–X is broken by heterolytic fission resulting in a halide ion, X. As can be seen, the OH is now attached to the alkyl group, creating an alcohol. (Hydrolysis of bromoethane, for example, yields ethanol). Reactions with ammonia give primary amines.
Chloro- and bromoalkanes are readily substituted by iodide in the Finkelstein reaction. The iodoalkanes produced easily undergo further reaction. Sodium iodide is used  as a catalyst.
Haloalkanes react with ionic nucleophiles (e.g. cyanide, thiocyanate, azide); the halogen is replaced by the respective group. This is of great synthetic utility: chloroalkanes are often inexpensively available. For example, after undergoing substitution reactions, cyanoalkanes may be hydrolyzed to carboxylic acids, or reduced to primary amines using lithium aluminium hydride. Azoalkanes may be reduced to primary amines by  Staudinger reduction or lithium aluminium hydride. Amines may also be prepared from alkyl halides in amine alkylation,  Gabriel synthesis and Delepine reaction, by undergoing nucleophilic substitution with potassium phthalimide or hexamine respectively, followed by hydrolysis.
In the presence of a base, haloalkanes alkylate alcohols, amines, and thiols to obtain ethers, N-substituted amines, and thioethers respectively. They are substituted by Grignard reagent to give magnesium salts and an extended alkyl compound.