Inchemistry, thebromine number is the amount ofbromine (Br2) in grams absorbed by 100 g of a sample. The bromine number was once used as a measure ofaliphatic unsaturation ingasoline and related petroleum samples, but this assay has fallen into disuse with the introduction of spectroscopic and chromatographic analyses.
It is based on the following reaction:
Bromine is deeply colored butR2C(Br)−C(Br)R2 is not. Thus, the consumption of bromine can often be gauged visually. Alternatively and more quantitatively, the bromine consumed by a sample can be determined byiodometry.[1]
The bromine number indicates thedegree of unsaturation of a sample. The technique can be subject to a variety of problems, whereby the sample consumes more or less bromine than predicted by the equation above. For example, some alkenes dimerize in the presence of bromine. Allylic bromination is another problem.[1]
The bromine number is similar to theiodine number, which is a similar technique used in evaluating the unsaturation of fats and fatty acids. Iodine is less reactive toward the tri- and tetra-substituted double bonds,[2] found in petroleum-derived samples. The C=C double bonds in fats and fatty acids are exclusively disubstituted.
The bromine number is usually determined by electrochemicaltitration, where bromine is generatedin situ with theredox process ofpotassium bromide andbromate in an acidic solution, using a mercurycatalyst to ensure the complete bromination of all olefins.