Can you please expain how to make structural formulas for dehydration synthesis and hydrolysis reactions? 

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ncchemist | eNotes Employee

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We are talking about two different chemical reactions here, both of which are prevalent in biological systems.  The first reaction is the dehydration reaction.  This involves the loss of water from a molecule.  It occurs most commonly when the hydroxyl group (OH) of an alcohol is eliminated to produce a double bond.  A hydrogen cation (called a proton) is removed from a carbon atom adjacent to the alcohol.  The resulting negatively charged pair of electrons goes between the carbon atoms to make a double bond, and the OH group is eliminated.  The H+ and OH- groups join to become water (H2O).  A general example of propanol dehydrating to propene is shown below.

CH3-CH2-CH2-OH  ---> CH3-CH=CH2 + H2O

The second reaction is a hydrolysis reaction.  This is where an ester group is reacted with a water molecule to produce a carboxylic acid and an alcohol.  An example is shown below.

Et-C(O)-OCH3 + H2O --> Et-C(O)-OH + CH3OH

The water molecule essentially attacks the ester carbonyl carbon and the alcohol is removed.  So the water molecule becomes part of the carboxylic acid.  This is all a little hard to draw out precisely here, so see the images in the links below.