[507-16-4] · Br2OS · Thionyl Bromide · (MW 207.87)
(brominating agent for alcohols, carboxylic acids, alkenes, and ketones)
Physical Data: mp -52 °C; bp 68 °C/40 mmHg; d 2.688 g cm-3.
Solubility: sol benzene, chloroform, carbon tetrachloride.
Form Supplied in: commercially available.
Handling, Storage, and Precautions: less stable than thionyl chloride and will slowly decompose, even in a stoppered container. Stable if kept dry, but reacts violently with water. Extremely destructive to tissue of mucous membranes, upper respiratory tract, eyes, and skin. Reacts vigorously with acetone to form a lachrymator. Should be handled in a fume hood.
Thionyl bromide is considerably more reactive than Thionyl Chloride and is more difficult to use.2 Unlike thionyl chloride, whose reactions are often carried out in pyridine, thionyl bromide undergoes a reaction with this solvent to form a nonreactive salt (eq 1).3
Although not often used, thionyl bromide can convert a simple organic acid to the acid bromide.4 When the acid is an a,b-unsaturated acid, addition of bromine across the double bond occurs, generally in quantitative yield (eqs 2 and 3).
For simple aliphatic acids, formation of the acid bromide is a high yield reaction (eq 4). Simple aromatic carboxylic acids also form the acid bromides in good yield.
Thionyl bromide can function as a dehydrating agent for suitably positioned dicarboxylic acids (eq 5).
In the ribonucleotide series, specific OH replacement has been noted (eq 6).5 Other examples of the transformation of OH to Br are illustrated in eqs 7 and 8.6,7 Conversion of an alcohol to a dibromo derivative has been reported (eq 9).8
Aromatic ketones with active methylene protons can be brominated with thionyl bromide. There is an example of dibromination of a methyl ketone (eq 10).8 Propiophenone gives a mixture of a-bromopropiophenone and a-bromo-a-methylpropiophenone. The reaction with unsaturated ketones gives addition to the double bond (eq 11).9
A general reaction is given in eq 12.10
Bradford P. Mundy
Colby College, Waterville, ME, USA