The unknown sample is added to an aqueous solution of bromine, which is brown or orange. If the brown color disappears when the specimen is added, it means that there is unsaturation present and if the formation of a white solid occurs, it is a sign that there is a phenol or an aniline.
It is a qualitative test, since quantities are not determined but only the presence of the aforementioned functional groups.
Unsaturations present in alkenes (C = C double bonds) react easily with bromine. The same occurs with phenols and anilines in which the aromatic ring becomes very reactive due to the influence of the –OH and –NH 2 groups respectively.
Rationale for the bromine test
Bromine reacts with double and triple bonds and also with basic nitrogen-containing molecules such as aromatic amines and with aromatic alcohols (phenols). However, olefins are the most reactive sites.
This element (Br) belongs to the group of halogens together with fluorine, chlorine and iodine. Molecular bromine (Br 2 ) is a reddish brown liquid. A saturated aqueous solution of molecular bromine at 20 ° C contains 3.6% (mass / volume ) of this element.
Olefins or alkenes
They are unsaturated compounds of the formula C n H 2n . They are called unsaturated because they have one or more C = C double bonds. The names of these compounds end in – ene , such as ethene (also called ethylene), propene (propylene), butene, pentene, etc.
They are unstable substances when compared to alkanes, so they easily react with bromine and other compounds.
Reaction of bromine with olefins and some alkynes
Molecular bromine reacts with the double bond of alkenes to form dibrominated compounds. This reaction is called addition. Here is an example of adding bromine to a double bond:
When the alkene is added to the brown or orange bromine solution, it reacts with the olefin to form a colorless compound. Alkene discolors bromine water, alkanes do not.
Alkynes have triple bonds (C≡C). The bromination of ethyne (HC≡CH) produces cis- and trans -1,2-dibromoethene and 1,1,2,2-tetrabromoethane. Trans -1,2-dibromoethene is represented in the figure below .
Phenols and reaction with bromine
These are compounds whose general formula is ArOH, where Ar is an aromatic group such as phenyl, substituted phenyl, or any other aromatic group. They differ from alcohols in that the -OH group is directly attached to the aromatic ring.
One of its most important chemical properties, in addition to acidity, is the high reactivity of its aromatic ring towards electrophilic substitution. That is, the replacement of a hydrogen on the ring by an electron-hungry atom.
That is why they react easily with bromine.
Anilines and reaction with bromine
They are aromatic amines, that is, compounds that have an amino group (-NH 2 ) whose nitrogen atom is directly attached to an aromatic ring. The simplest member is aniline:
They have an appreciable basicity. The nitrogen of the amino group has a pair of free electrons that give it a high degree of nucleophilicity or affinity towards compounds that lack electrons and provide high reactivity to the aromatic ring.
For this reason they easily react with bromine, which attacks the aromatic ring. For example aniline with bromine forms 2,4,6-tribromoaniline.
Evidence related to this trial
There are other tests that also take advantage of the reaction of bromine with double bonds. They are qualitative methods for measuring reactive sites in a sample. The tests are the bromine number and the bromine number.
It is the number of grams (g) of bromine that react with 100 g of the sample. It is used in petroleum distillates such as gasoline, kerosene, and some diesel fuels. It is also applied in commercial mixtures of aliphatic monoolefins.
It is the number of milligrams (mg) of bromine that react with 100 g of the sample. It is used in the chemical industry in materials or liquids that have a very low olefin content.
Bromine generation in situ
In these assays, bromine is generated on-site using a bromide and bromate solution.
BrO 3 – + 5 Br – + 6 H + → 3 Br 2 + 3 H 2 O
Once formed, the bromine reacts with the C = C bonds. The reaction is monitored by electrometric titration. The precise procedure has been published by the American Society for Testing and Materials or ASTM (American Society for Testing and Materials).
The bromine number corresponds to the ASTM D1159 method and the bromine number is described in the ASTM D 2710, ASTM D 1492 and ASTM D 5776 methods.
Applications of these tests
The bromine number is a measure of the aliphatic unsaturations in samples of petroleum and its distillates. In commercial aliphatic monoolefins it serves to provide evidence of their purity and identity.
The bromine number applies only to essentially olefin-free hydrocarbons, allowing trace amounts of these to be measured.
Feeding currents to FCC
Both parameters are also measured in heavy materials such as the feed to the fluidized bed catalytic cracking system or FCC ( Fluid Catalytic Cracking ), used in petroleum refining.
Both tests are part of a series of tests that are routinely applied to FCC feed streams, which are important for their characterization in terms of molecular structure, since they allow the development of kinetic models to predict the performance of the desired products.
Reasons for low alkene content
Within FCC processes, olefins can polymerize and generate undesirable products such as sludge and coke. That is why it is desired that the FCC feed streams contain less than 5% (by weight ) olefins.
Hydrotreated feed streams have a bromine number of less than 5, in heavy vacuum gas oil it is 10 and in coking gas oil it is 50. As a general rule it is said that the fraction of olefins in a sample is approximately half that of its bromine number.
In combustion engines, the alkenes present in gasoline can polymerize forming gums and varnishes that settle on the system walls, filters and fine pipes, covering them.
This can result in decreased fuel flow, reduced engine power, and even prevented engine starting.
For these reasons, the determination of olefins in some petroleum derivatives is important, at least qualitatively with these tests.