Hydrohalic

Hydrohalic refers to hydrogen halides and their aqueous acids. The term covers the binary compounds of hydrogen with a halogen (HF, HCl, HBr, HI) and, when dissolved in water, their corresponding hydrohalic acids: hydrofluoric acid, hydrochloric acid, hydrobromic acid, and hydroiodic acid.

In aqueous solution, most hydrohalic acids are strong acids except for HF. HCl, HBr, and HI dissociate nearly completely in water, giving high proton availability. HF is notably weaker as an acid in water due to the unusually strong H–F bond and complex solvation behavior of fluoride. Across the series, acidity generally increases down the group for the aqueous acids, influenced by bond strength and fluoride’s distinctive chemistry in solution.

Preparation methods vary. Direct synthesis by reacting hydrogen gas with halogen gas is common: H2 + Cl2 → 2 HCl; H2 + Br2 → 2 HBr; H2 + I2 → 2 HI. Industrially, HF is often produced by reacting calcium fluoride with concentrated sulfuric acid: CaF2 + H2SO4 → 2 HF + CaSO4. In the laboratory, HCl can also be prepared by acidifying sodium chloride with sulfuric acid: NaCl + H2SO4 → HCl + NaHSO4.

Uses and reactions: Hydrohalic acids are strong mineral acids widely employed as reagents, catalysts, and in pH control. They react with metals to release hydrogen and with carbonates to form carbon dioxide. HF is particularly important for etching glass and silicon dioxide. In organic synthesis, hydrohalic acids add across unsaturated bonds (hydrohalogenation) and serve in various protection and deprotection steps.

Safety: Hydrohalic acids are highly corrosive. HF is especially hazardous due to tissue damage and fluoride toxicity; proper containment and personal protective equipment are essential.