Bromine

Bromine is a chemical element that appears as a fuming brown-red liquid. It is a member of the halogen family in the periodic table and is the third-largest halogen. Due to its relatively high vapour pressure, bromine is the only non-metallic element remaining in a liquid state at room temperature.

1.0Introduction

Bromine was discovered by Antoine Jérôme Balard in 1826. In nature, bromine is found only in a combined state. It occurs primarily in seawater as bromides of sodium (Na), potassium (K), and magnesium (Mg). Additionally, bromine is present in carnallite (KCl·MgCl₂·6H₂O) and its bromine-rich variant, Bromo-carnallite (KBr·MgBr₂·6H₂O), which are located in Stassfurt, Germany

2.0Preparation of Bromine

1. From Potassium Bromide (KBr): Bromine can be prepared by heating a mixture of potassium bromide, manganese dioxide (MnO₂), and concentrated sulfuric acid (H₂SO₄).
2. By Chlorine Displacement: Bromine can be obtained by passing chlorine gas (Cl₂) through a bromide salt solution. The chlorine displaces bromine from the bromide compound, releasing bromine gas.
3. From Carnallite:

• Bittern Processing: After the crystallisation of carnallite chlorides, the remaining solution, called bittern, contains sodium (Na), potassium (K), and magnesium bromides (MgBr₂). Bromine can be extracted from this solution through the following process:
• Passing Vapors: Uncondensed vapours are passed through a tower packed with moist iron filings.
• Formation of Ferrosoferric Bromide: This reaction produces ferrosoferric bromide (Fe₃Br₈), which can be used to isolate bromine.

The reaction in the tower can be represented as follows:

Fe+Br₂​→Fe₂​Br_6​

Fe₂​Br_6​ is often referred to in the context of ferrosoferric bromide (Fe₃Br₈).

• Electrolysis: Alternatively, bromine can be produced from the mother liquor through electrolysis. During this process:
• Chlorine Evolution: Electrolysis may cause the decomposition of magnesium chloride (MgCl₂), releasing chlorine gas (Cl₂).

2MgCl_2​ → 2Mg + 4Cl₂ ​+ 2Cl₂​

• Reaction with Magnesium Bromide: The chlorine gas (Cl₂) then reacts with magnesium bromide (MgBr₂) in the solution to liberate bromine (Br₂).

Cl₂ + MgBr_2​ → Br₂​ + MgCl_2​

This process effectively uses chlorine to displace bromine from magnesium bromide, facilitating bromine extraction.

4. From Seawater:

• Concentration of Seawater:
• • Seawater is concentrated to allow salts to separate as crystals. The remaining solution, mother liquor, contains magnesium bromide (MgBr₂).
  • This mother liquor is treated similarly to the process for obtaining bromine from carnallite, resulting in bromine extraction.

3.0Physical Properties

• Bromine is a heavy, dark brown liquid with irritating vapours.
• Density: 3.2 g/ml
• Boiling point: 58.5°C
• Freezing point: -7°C
• Soluble in water, forming bromine water (about 3.6% at 20°C).
• When cooled, a saturated bromine solution forms bromine hydrate (Br₂·8H₂O).

4.0Chemical Properties

• Combination with Hydrogen: Forms hydrogen bromide (HBr).
• Oxidizing Nature: It does not react with water under ordinary conditions but does in the presence of an oxidisable substance, producing HBr. Bromine is a strong oxidising agent and can oxidise various substances. Here’s a summary of its oxidising reactions:
• Sulfur Dioxide to Sulfuric Acid: SO₂ ​+ Br₂​ + 2H₂​O → H_2​SO_4​ + 2HBr
• Sodium Sulfite to Sodium Sulfate: Na₂​SO₃ ​+ Br_2​ + 2H₂​O → Na₂​SO₄  + 2HBr
• Thiosulfite to Sulfate: S_2​O₃^2-​ + Br₂​ + 2H₂​O → 2SO₄⁻²​ + 2HBr
• Arsenite to Arsenate: AsO₃³⁻​+ Br₂​+ 2H_2​O → AsO₄³⁻​+ 2HBr
• Hydrogen Sulfide to Sulfur: H₂​S + Br_2​ → S + 2HBr
• Displacement Reaction: Bromine's higher reactivity allows it to displace other elements from its compounds in a chemical reaction. For example, bromine can displace iodine from potassium iodide (KI) or chlorine from potassium chloride (KCl). This type of reaction is characteristic of halogens, where a more reactive halogen replaces a less reactive one in a compound.

Bromine Displacing Iodine:Br₂​ + 2KI → 2KBr + I_2​

Bromine Displacing Chlorine:Br₂​ + 2KCl → 2KBr + Cl_2​

Action with Organic Compounds:

When bromine reacts with organic compounds, it typically undergoes a substitution or addition reaction, resulting in brominated organic compounds. Here’s how it works:

1. Addition Reactions:

Bromine atoms attach to carbon atoms (e.g., in alkenes or alkynes), forming a brominated product.

C₂​H₄​+Br₂​→C_2​H₄​Br_2​

Ethene reacts with bromine to form 1,2-dibromoethane.

2. Substitution Reactions:

In the presence of a carbon-hydrogen bond (e.g., in alkanes or aromatic rings), bromine can replace hydrogen atoms with bromine atoms. This typically requires the presence of UV light or a catalyst.

CH₄+Br₂​→CH₃​Br+HBr 

Methane and bromine react with to form methyl bromide and hydrogen bromide.

The organic compound is modified in both reactions by adding bromine atoms, creating brominated organic compounds.

5.0Uses of Bromine

• Agricultural Chemicals & Pharmaceuticals: Used to produce agrarian chemicals, dyes, insecticides, and pharmaceuticals. While some uses are being phased out due to environmental concerns, new applications continue to arise.
• Flame Retardants: Added to materials like furniture foam and plastic casings to reduce flammability. Its use in the USA has been discontinued due to toxicity concerns.
• Fire Extinguishers: Organobromides are used in halon fire extinguishers for critical areas like museums and aeroplanes.
• Photography: Silver bromide is essential in traditional film photography.
• Historical Use in Fuels: Previously used to produce 1,2-di-bromoethane, an anti-knock agent in leaded fuels.