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Dehydration reaction

 

 
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Dehydration reaction
 
 
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In chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, a dehydration reaction is usually defined as a chemical reaction that involves the loss of water from the reacting molecule. Dehydration reactions are a subset of elimination reaction
Elimination reaction

An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism ....
s. Because the hydroxyl group (-OH) is a poor leaving group, having an Brønsted acid catalyst often helps by protonating the hydroxyl group to give the better leaving group, -OH2+.

In organic synthesis
Organic synthesis

Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic_chemistry molecules can often contain a higher level of complexity compared to purely Inorganic_chemistry compounds, so the synthesis of organic compounds has developed into one of the most im...
, there are many examples of dehydration reactions: 2 R-OH ? R-O-R + H2O R-CH2-CHOH-R ? R-CH=CH-R + H2O

for example the conversion of glycerol
Glycerol

Glycerol is a chemical compound also commonly called glycerin or glycerine. It is a colorless, odorless, Viscosity liquid that is widely used in pharmaceutical formulations....
 to acrolein
Acrolein

Acrolein is the simplest saturation aldehyde. It is produced widely but is most often immediately reacted with other products due to its instability and toxicity....
 :
2 RCO2H ? (RCO)2O + H2O RCONH2 ? R-CN + H2O
Some dehydration reactions can be mechanistically complex, for instance the reaction of a sugar with concentrated sulphuric acid to form carbon involves formation of carbon carbon bonds.




C12H22O11 + 98% Sulfuric acid ? 12 C (graphitic foam) + 11 H2O steam + Sulfuric acid/water mixture


The reaction is driven by the strongly exothermic reaction sulfuric acid has with water.






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In chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, a dehydration reaction is usually defined as a chemical reaction that involves the loss of water from the reacting molecule. Dehydration reactions are a subset of elimination reaction
Elimination reaction

An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism ....
s. Because the hydroxyl group (-OH) is a poor leaving group, having an Brønsted acid catalyst often helps by protonating the hydroxyl group to give the better leaving group, -OH2+.

In organic synthesis
Organic synthesis

Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic_chemistry molecules can often contain a higher level of complexity compared to purely Inorganic_chemistry compounds, so the synthesis of organic compounds has developed into one of the most im...
, there are many examples of dehydration reactions:
  • Conversion of alcohol
    Alcohol

    In chemistry, an alcohol is any organic compound in which a hydroxyl Functional group is bound to a carbon atom of an alkyl or substituted alkyl group....
    s to ether
    Ether

    Ether is a class of organic compounds which contain an ether functional group ? an oxygen atom connected to two alkyl or aryl groups ? of general formula R?O?R....
    s
2 R-OH ? R-O-R + H2O
  • Conversion of alcohols to alkene
    Alkene

    In organic chemistry, an alkene, olefin, or olefine is an Saturation chemical compound containing at least one carbon-to-carbon double bond....
    s
R-CH2-CHOH-R ? R-CH=CH-R + H2O

for example the conversion of glycerol
Glycerol

Glycerol is a chemical compound also commonly called glycerin or glycerine. It is a colorless, odorless, Viscosity liquid that is widely used in pharmaceutical formulations....
 to acrolein
Acrolein

Acrolein is the simplest saturation aldehyde. It is produced widely but is most often immediately reacted with other products due to its instability and toxicity....
 :
  • Conversion of carboxylic acid
    Carboxylic acid

    Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the Chemical formula -COH, usually written -COOH or -CO2H....
    s to acid anhydrides
2 RCO2H ? (RCO)2O + H2O
  • Conversion of amide
    Amide

    In chemistry, an amide is one of three kinds of compounds:* the organic chemistry functional group characterized by a carbonyl group linked to a nitrogen atom , or a compound that contains this functional group ; or...
    s to nitrile
    Nitrile

    A nitrile is any organic compound which has a -Carbon=Nitrogen functional group. The -C=N functional group is called a nitrile group....
    s
RCONH2 ? R-CN + H2O
  • in this rearrangement reaction
    Rearrangement reaction

    A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule ....
     called the dienol benzene rearrangement :
Some dehydration reactions can be mechanistically complex, for instance the reaction of a sugar with concentrated sulphuric acid to form carbon involves formation of carbon carbon bonds.


  • Sugar (sucrose) is dehydrated:


C12H22O11 + 98% Sulfuric acid ? 12 C (graphitic foam) + 11 H2O steam + Sulfuric acid/water mixture


The reaction is driven by the strongly exothermic reaction sulfuric acid has with water. (Beware that this reaction produces dangerous sulfuric-acid containing steam, and should only be performed in a fume-hood or well ventilated area.)

Common dehydrating agents; concentrated sulfuric acid
Sulfuric acid

Sulfuric acid, hydrogen2sulfuroxygen4, is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry....
, concentrated phosphoric acid
Phosphoric acid

Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula Hydrogen3PhosphorusOxygen4....
, hot aluminium oxide
Aluminium oxide

Aluminium oxide is an amphoteric oxide of aluminium with the chemical formula 23. It is also commonly referred to as alumina or aloxite in the mining, ceramic and materials science communities....
, hot ceramic.

See also

  • Hydration reaction
    Hydration reaction

    In organic chemistry, a hydration reaction is a chemical reaction in which a hydroxyl group and a hydrogen cation are added to the two carbon atoms covalent bonded together in the carbon-carbon double bond which makes up an alkene functional group....