Intermediate chlorides, bromides and iodides exist. Alkyl halides such as alkyl bromides are more reactive than alkyl chlorides. Analogous compounds are not formed with fluorides, bromides, iodides and cyanides. Alkyl iodides react the fastest, followed by alkyl bromides and then alkyl chlorides. In comparison to aryl chlorides and bromides, aryl fluorides form Grignard reagents only reluctantly. After addition of the acid, chlorides, bromides and iodides will form precipitates with silver nitrate. Examples of such medications include lithium, hydantoin, isoniazid, glucocorticoids, iodides, bromides, and testosterone. Normally, vinyl iodides and bromides are used. Bromides and iodides of rhenium are also well known. Bromides and iodides are confirmed by the "layer test". Chalcogen chlorides and bromides have also been explored. This reaction is mechanistically distinct from the previous transformation of activated bromides and chlorides. The formation of creamy silver bromide precipitate confirms the existence of bromides. Barium bromide is a precursor to chemicals used in photography and to other bromides. The salt forms several crystalline hydrates, unlike the other alkali metal bromides.