Substitution Unlike typical alkyl halides, aryl halides typically do not participate in conventional substitution reactions. Aryl halides with electron-withdrawing groups in the
ortho and
para positions, can undergo
SNAr reactions. For example, 2,4-dinitrochlorobenzene reacts in basic solution to give a phenol. Unlike in most other substitution reactions, fluoride is the best leaving group, and iodide the worst. A 2018 paper indicates that this situation may actually be rather common, occurring in systems that were previously assumed to proceed via SNAr mechanisms.
Benzyne When treated with strong base, some aryl halides often react via the intermediacy of
benzynes. Benzyne is an intermediate in the reaction of chlorobenzene with strongly basic reagents such as
potassium amide, even at −33 °C. It is also implicated in the conversion of chlorobenzene to
phenol using sodium hydroxide, which requires high temperatures.
Organometallic reagent formation Aryl halides react with metals, generally
lithium or
magnesium, to give organometallic derivatives that function as sources of aryl anions. By the
metal–halogen exchange reaction, aryl halides are converted to aryl lithium compounds. Illustrative is the preparation of
phenyllithium from
bromobenzene using
n-butyllithium (
n-BuLi): : C6H5Br + BuLi → C6H5Li + BuBr Direct formation of
Grignard reagents, by adding the magnesium to the aryl halide in an ethereal solution, works well if the aromatic ring is not significantly deactivated by electron-withdrawing groups.
Other reactions The halides can be displaced by strong nucleophiles via reactions involving radical anions. Alternatively aryl halides, especially the bromides and iodides, undergo
oxidative addition, and thus are subject to
Buchwald–Hartwig amination-type reactions.
Biodegradation Rhodococcus phenolicus is a bacterium that degrades
dichlorobenzene as sole carbon sources. ==Applications==