Inchemistry, ahydrate is a substance that containswater or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understood.
Hydrates are inorganic salts "containing water molecules combined in a definite ratio as an integral part of thecrystal"[1] that are either bound to a metal center or that have crystallized with the metal complex. Such hydrates are also said to containwater of crystallization orwater of hydration. If the water isheavy water in which the constituent hydrogen is theisotopedeuterium, then the termdeuterate may be used in place ofhydrate.[2][3]
_chloride.jpg) | -chloride-hexahydrate-sample.jpg) |
| Anhydrous cobalt(II) chloride CoCl2 (blue) | Cobalt(II) chloride hexahydrate CoCl2·6H2O (pink) |
A colorful example iscobalt(II) chloride, which turns from blue to red uponhydration, and can therefore be used as a water indicator.
The notation "hydrated compound⋅nH2O", wheren is the number of water molecules performula unit of the salt, is commonly used to show that a salt is hydrated. Then is usually a lowinteger, though it is possible for fractional values to occur. For example, in amonohydraten = 1, and in ahexahydraten = 6. Numerical prefixes mostly of Greek origin are:[4]
A hydrate that has lost water is referred to as ananhydride; the remaining water, if any exists, can only be removed with very strong heating. A substance that does not contain any water is referred to asanhydrous. Some anhydrous compounds are hydrated so easily that they are said to behygroscopic and are used as drying agents ordesiccants.
In organic chemistry, a hydrate is a compound formed by the hydration, i.e. "Addition of water or of the elements of water (i.e. H and OH) to a molecular entity".[5] For example:ethanol,CH3−CH2−OH, is the product of thehydration reaction ofethene,CH2=CH2, formed by the addition of H to one C and OH to the other C, and so can be considered as the hydrate of ethene. A molecule of water may be eliminated, for example, by the action ofsulfuric acid. Another example ischloral hydrate,CCl3−CH(OH)2, which can be formed by reaction of water withchloral,CCl3−CH=O.
Many organic molecules, as well as inorganic molecules, form crystals that incorporate water into the crystalline structure without chemical alteration of the organic molecule (water of crystallization). The sugartrehalose, for example, exists in both ananhydrous form (melting point 203 °C) and as a dihydrate (melting point 97 °C).Protein crystals commonly have as much as 50% water content.
Molecules are also labeled as hydrates for historical reasons not covered above.Glucose,C6H12O6, was originally thought of asC6(H2O)6 and described as acarbohydrate.
Hydrate formation is common foractive ingredients. Many manufacturing processes provide an opportunity for hydrates to form and the state of hydration can be changed with environmental humidity and time. The state of hydration of an active pharmaceutical ingredient can significantly affect the solubility and dissolution rate and therefore itsbioavailability.[6]
Clathrate hydrates (also known as gas hydrates, gas clathrates, etc.) are water ice with gas molecules trapped within; they are a form ofclathrate. An important example ismethane hydrate (also known as gas hydrate, methane clathrate, etc.).
Nonpolar molecules such as methane can formclathrate hydrates with water, especially under high pressure. Although there is nohydrogen bonding between water and guest molecules when methane is the guest molecule of the clathrate, guest–host hydrogen bonding often forms when the guest is a larger organic molecule such astetrahydrofuran. In such cases the guest–host hydrogen bonds result in the formation of L-typeBjerrum defects in the clathrate lattice.[7][8]
The stability of hydrates is generally determined by the nature of the compounds, their temperature, and therelative humidity (if they are exposed to air).
