Adduct

An adduct is a chemical compound of different entities, usually molecules, which aggregate without any chemical bonds being broken. The classic example is sodium percarbonate (aka Oxi-Clean), which is a binary adduct of hydrogen peroxide and sodium carbonate. It is markedly more soluble than sodium carbonate. It is also much less caustic than hydrogen peroxide of equivalent molarity. Hydrogen peroxide also forms adducts with many other compounds, like urea.

Salts frequently form adducts with water, such as copper(II) sulfate, which crystallizes with five moles of water per mole of the salt itself. Other salts, such as sodium chloride however, are not adducts, as their structure is composed of a lattice of sodium and chloride ions.

Adducts have an important role in organic chemistry. Many weak Lewis acids and Lewis bases can form adducts. It may well be easier to separate the adducts of two similar compounds than to separate the compound itself. If one of the two or more bases will not form an adduct with an acid that the other will, you have a way to isolate it.

Adducts are also useful in making organic compounds more soluble in water, thus making them easier to ingest.

Famous binary adducts:	component	component	formula	notes
sodium percarbonate	hydrogen peroxide	sodium carbonate	2 Na2CO3 · 3 H2O2	Oxi-Clean
acetone sodium bisulfite	acetone	sodium bisulfite	NaHSO3 · (CH3)2CO	insoluble in water!

References

Relevant Sciencemadness threads

• Bisulfite adduct issues
• Bisulfite adducts, iodide adducts, solubility (ketones, aldehydes)