Bromine, symbolized as \(text{Br}\) with atomic number 35, is a highly reactive element. It belongs to Group 17 of the periodic table, known as the halogens, or “salt-formers.” At standard room temperature, bromine exists as a volatile, dense reddish-brown liquid, making it one of only two elements that are liquid under these conditions. Its intense color and pungent odor are immediate indicators of its chemical nature. Bromine’s importance stems from its ability to form compounds called bromides, which are used in everything from flame retardants to pharmaceutical intermediates.
Why Bromine is a Chemical Aggressor
Bromine’s aggressive reactivity is fundamentally dictated by its electronic structure and its position within the halogen group. As a Group 17 element, a bromine atom possesses seven valence electrons in its outermost shell. The element’s strong chemical drive is to achieve a stable octet configuration, meaning it seeks to gain one additional electron.
This intense desire for a single electron is quantified by bromine’s high electronegativity (\(2.96\) on the Pauling scale). High electronegativity allows the bromine atom to strongly attract electrons from other atoms, making it a powerful oxidizing agent. In chemical reactions, bromine typically acts by stripping an electron away from another substance, causing that substance to be oxidized while the bromine itself is reduced to the stable bromide ion (\(text{Br}^-\)).
Bromine is less reactive than chlorine but more reactive than iodine, following the trend down the halogen group. The elemental form of bromine exists as a diatomic molecule (\(text{Br}_2\)), held together by a single covalent bond. The energy required to break this bond is relatively low, which facilitates the initiation of many reactions, especially those involving free radicals.
Reactions with Elements and Inorganic Compounds
Bromine reacts vigorously with a wide array of elements and simple inorganic compounds, primarily through redox reactions where it accepts electrons. Nearly all elements react with bromine to form binary bromides, often with considerable energy release. For instance, when elemental bromine is brought into contact with metals like aluminum or sodium, the reaction is often vigorous and exothermic, immediately forming metal bromide salts such as \(text{AlBr}_3\) or \(text{NaBr}\).
The reaction of bromine with hydrogen gas yields hydrogen bromide (\(text{HBr}\)). This reaction is notably slower at room temperature compared to the corresponding reaction with chlorine, but its speed increases significantly with heat or a catalyst. Hydrogen bromide is a colorless gas that, when dissolved in water, forms hydrobromic acid, a strong acid used in various industrial applications.
When bromine is dissolved in water, a process known as disproportionation occurs, demonstrating the element’s oxidizing nature. This reaction involves the bromine molecule reacting with water to form both hydrobromic acid (\(text{HBr}\)) and hypobromous acid (\(text{HOBr}\)). \(text{HOBr}\) is a strong oxidant responsible for the mild bleaching and disinfecting properties of “bromine water.” This conversion shows that bromine is simultaneously oxidized and reduced, forming compounds with oxidation states of \(-1\) (\(text{HBr}\)) and \(+1\) (\(text{HOBr}\)).
Bromination of Organic Molecules
The interaction of bromine with organic molecules is a distinct area of its chemistry, categorized mainly into two types: addition and substitution. The addition reaction is rapid and characteristic, occurring when bromine reacts with unsaturated hydrocarbons like alkenes or alkynes. In this process, the \(text{Br}_2\) molecule adds directly across a carbon-carbon double or triple bond, breaking the weaker pi bond and forming a new single bond to a bromine atom on each carbon.
This reaction typically proceeds through an intermediate known as a bromonium ion, which forces the two bromine atoms to add to opposite faces of the former double bond, resulting in anti stereochemistry. The instantaneous disappearance of the reddish-brown color of the elemental bromine is routinely used as a simple test for the presence of unsaturation in a molecule.
In contrast, bromine reacts with saturated hydrocarbons, or alkanes, through a substitution mechanism, where a bromine atom replaces a hydrogen atom on the carbon chain. This process is much slower than addition and requires energy, typically ultraviolet (UV) light or heat, to initiate a free-radical chain reaction. The UV light cleaves the \(text{Br}_2\) molecule into highly reactive bromine radicals, which then abstract a hydrogen atom from the alkane.
Bromine substitution reactions are more selective than those involving chlorine, showing a preference for replacing hydrogen atoms attached to more substituted carbons, such as tertiary or secondary positions. This selectivity is a significant feature in synthetic chemistry, allowing for more controlled placement of the bromine atom on the carbon framework.