The dehydration or dehydration refers to the loss of water due to a chemical reaction , so an elimination reaction ( elimination ) but also the elimination of water of crystallization or water of aqua complexes .
In the presence of dehydrating agents such as concentrated sulfuric acid , phosphoric acid or anhydrous zinc chloride , alcohols react in the heat with intramolecular dehydration to form the corresponding alkenes . The tertiary alcohols dehydrate particularly easily. On an industrial scale, the dehydration of the alcohols can be carried out catalytically under pressure in the gas phase . However, due to the ready availability of alkenes from petrochemical sources, the importance of the dehydration of alcohols to alkenes has decreased. Rather, alcohols are obtained on a large scale by hydration of alkenes.
The preferred direction in which the double bond is formed can be found in Saytzeff's rule - according to Alexander Michailowitsch Saizew (1841-1910). It states that the required hydrogen atom is removed from the neighboring carbon atom with the lowest hydrogen content and the most highly substituted (and therefore thermodynamically most stable) alkene is formed.
Another important example of dehydration is the formation of acid anhydrides by elimination of water from the corresponding acids, for example the industrial production of acetic anhydride from acetic acid or of phthalic anhydride from phthalic acid .