In chemistry, a hydrate is a substance that contains water 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.
Chemical nature
Organic chemistry
In organic chemistry, a hydrate is a compound formed by the hydration, i.e. "Addition of water or of the elements of water to a molecular entity". For example: ethanol, CH3−CH2−OH, is the product of the hydration reaction of ethene, 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 of sulfuric acid. Another example is chloral hydrate, CCl3−CH2, which can be formed by reaction of water with chloral, 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. The sugar trehalose, for example, exists in both an anhydrous form and as a dihydrate. 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 as C66 and described as a carbohydrate. Methanol is often sold as "methyl hydrate", implying the incorrect formula CH3OH2, while the correct formula is CH3−OH. Hydrate formation is common for active ingredient. 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 its bioavailability.
Inorganic chemistry
Hydrates are inorganic salts "containing water molecules combined in a definite ratio as an integral part of the crystal" that are either bound to a metal center or that have crystallized with the metal complex. Such hydrates are also said to contain water of crystallization or water of hydration. If the water is heavy water, where the hydrogen involved is the isotopedeuterium, then the term deuterate may be used in place of hydrate. A colorful example is cobalt chloride, which turns from blue to red upon hydration, and can therefore be used as a water indicator. The notation "hydrated compound⋅nH2O", where n is the number of water molecules per formula unit of the salt, is commonly used to show that a salt is hydrated. The n is usually a low integer, though it is possible for fractional values to occur. For example, in a monohydraten = 1, and in a hexahydraten = 6. Numerical prefixes of Greek origin are: A hydrate that has lost water is referred to as an anhydride; 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 as anhydrous. Some anhydrous compounds are hydrated so easily that they are said to be hygroscopic and are used as drying agents or desiccants.
Clathrate hydrates
are water ice with gas molecules trapped within; they are a form of clathrate. An important example is methane hydrate. Nonpolar molecules such as methane can form clathrate hydrates with water, especially under high pressure. Although there is no hydrogen 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 as tetrahydrofuran. In such cases the guest–host hydrogen bonds result in the formation of L-type Bjerrum defects in the clathrate lattice.
Stability
The stability of hydrates is generally determined by the nature of the compounds, their temperature, and the relative humidity.
